Chapter 5
Anesthesia in patients with concurrent disease

Sharon Tenenbaum Shih

This chapter addresses anesthetic considerations and protocols for patients with specific diseases. The pathophysiology of the disease, its influence on anesthesia, recommended drug protocols, expected or common complications, and preventive or treatment methods for these complications are reviewed. While some drugs may be ideal for a particular disease, the anesthetic protocol is often based on the anesthetist’s familiarity, availability, and cost‐effectiveness of these drugs. As with other chapters, many of these recommendations are a reflection of the author’s personal preference.

I. Cardiovascular disease

Cardiovascular disease involves either a disturbance of the conduction system (which may manifest as an abnormal rhythm) or an alteration of the structure of the heart itself. Structural abnormalities are broadly categorized into systolic, diastolic, and obstructive dysfunction. Cardiac patients usually compensate for their disease and may display only limited clinical signs (CS), such as exercise intolerance, coughing, or an increased RR while sleeping. Unfortunately, anesthesia disrupts the physiologic balance even in a healthy patient. Therefore, the compensated patient stands a reasonable chance of decompensation during an anesthetic event.

To minimize the impact of this occurring, a thorough understanding of the patient’s current disease is necessary. This is obtained by a preanesthetic physical exam (PE), blood work (BW), a thorough history of which cardiac medications the patient receives, and a cardiovascular workup (ECG and echocardiogram). A preoperative blood pressure (BP) is also obtained as a baseline. This is important both because drugs such as ACE inhibitors (e.g., enalapril or benazepril, alone or in combination with aldosterone receptor antagonists such as spironolactone) are commonly prescribed to these patients, and because of the growing awareness of cardiovascular–renal axis disorders [1]. For example, dogs which receive enalapril prior to their anesthetic event are more likely to experience moderate to severe hypotension than those which do not [2]. Ultimately, that hypotension may negatively impact renal function.

Most cardiac protocols share common goals: use of reversible drugs, avoiding hypoxemia, maintaining cardiac output (CO) (by maintaining a normal heart rate and blood pressure), avoiding extreme changes in vascular tone (i.e., vasoconstriction or vasodilation), minimizing hypothermia to avoid postoperative shivering, minimizing anesthesia time, and creating a stress‐free recovery.

A. Conduction and rhythm disturbances

The conduction system (electrical component) of the heart triggers the heart’s mechanical actions (contraction component, i.e., forward flow of blood). When conduction disruptions occur while a patient is anesthetized, this may result in a worsened forward flow versus when a patient is awake and compensates appropriately. Arrhythmias are classified in many ways, but a broad category would be whether it is a bradyarrhythmia (a slow abnormal rhythm) or a tachyarrhythmia (a fast abnormal rhythm). A thorough discussion of cardiac arrhythmias is found in Chapter 3.

1.Anesthesia concerns/common complications

(a) Bradyarrhythmias: Because CO is equal to heart rate (HR) multiplied by stroke volume (SV), a slow HR will decrease CO. Due to the reduction of CO secondary to the use of general anesthesia (inhalants, as well as other injectable agents), there is a risk of severely compromising perfusion to vital tissues including the brain, kidney, and liver.

(b) Tachyarrhythmias: Perfusion to the heart occurs during diastole. When a tachyarrhythmia occurs, the heart spends very little time in diastole, resulting in compromised myocardial perfusion. This significantly worsens ventricular arrhythmias as the ventricle is the largest heart muscle body to perfuse.

2. Anesthesia protocol

A patient which is scheduled for an elective procedure but has an arrhythmia on PE must have at minimum an ECG analyzed before anesthesia proceeds. An echocardiogram is highly recommended to rule out any structural abnormalities. In emergent cases where the procedure and anesthesia are unavoidable, the goals are to stabilize and/or reduce the impact of the arrhythmia. This means using cardiac‐friendly drugs for premedication (i.e., opioids and benzodiazepines), induction (i.e., alfaxalone or etomidate with a benzodiazepine, or neuroleptic anesthesia [fentanyl and a benzodiazepine]), and reducing the amount of inhalant administered (i.e., through the use of opioids for their MAC‐sparing effect, as well as local blocks if possible). Adding some local lidocaine on the larynx will aid with intubation. Additionally, with bradyarrhythmias, it is necessary to increase HR, either through the use of anticholinergics, isoproterenol, or a temporary pacemaker. In cases of tachycardia, it is essential to know whether the tachycardia is ventricular (below the AV node) or supraventricular (above the AV node) in origin. Ventricular arrhythmias may slow in response to lidocaine; see Chapter 3 for treatment of arrhythmias.

B. Structural abnormalities

1. Systolic dysfunction

Systolic dysfunction includes diseases such as valvular insufficiencies (mitral regurgitation [MR] or tricuspid regurgitation [TR] and dilated cardiomyopathy [DCM]). One of the most common complications secondary to this disease, under anesthesia, is hypotension. In patients with DCM, arrhythmias are a common sequela (ex. atrial fibrillation) and some necessitate treatment. Pimobendan is a drug with a unique mechanism of action that includes positive inotropy (an increase in contractility of the heart) and is also an inodilator (which causes vasodilation by calcium sensitization and phosphodiesterase inhibition) [3]. Studies suggest that dogs administered pimobendan demonstrate a significantly longer life span when they have myxomatous mitral valve disease [4].

(a) Anesthesia concerns/common complications:

Maintain HR with normal heart rate as the target goal (and within 25% of resting HR as a point of absolute treatment), with an anticholinergic as needed (atropine or glycopyrrolate); if the anesthetist must choose between slightly below or slightly above resting HR, these cases often benefit from a slightly elevated HR. In the case of mitral or tricuspid regurgitation, this slight tachycardia will help with forward flow and prevent back flow into the atria. However, one must also try to avoid severe tachycardia.
Avoid peripheral vasoconstriction as this will increase afterload and decrease SV, both of which compromise CO. Slight vasodilation may help improve forward flow and decrease flow back into the atria.
Maintain myocardial contractility by ensuring the patient receives pimobendan prior to their procedure if they are currently prescribed this drug, and by using inotropic agents, such as dopamine or dobutamine (see Table 10). Dopamine and dobutamine are also used to correct hypotension.
Avoid volume overload with a conservative fluid therapy plan (usually 2–3 mL/kg/h for canine patients or 1–2 mL/kg/h for feline patients). Do not administer fluid boluses for hypotension unless there is true volume loss or evidence of hypovolemia.
Minimize stress during the perioperative period as well as the postoperative recovery period.
Keep inhalant to minimum alveolar concentration (MAC) or less during anesthesia. For example, end‐tidal isoflurane concentration would stay below 1.25%.
Prevent hypothermia to decrease postoperative shivering, which will increase myocardial oxygen requirement.

(b) Anesthetic protocol (Table 5.1): Premedications facilitate IV catheterization with minimal stress and reduce drug and inhalant requirement. When selecting drugs for the anesthetic protocol, reversible drugs are ideal in the cardiac patient. Opioids, given in higher doses, will help reduce the need for sedatives that may impact the cardiovascular system. However, high doses of opioids may cause bradycardia; this is usually treated with an anticholinergic (which may be included in the premedication). Patients with mild MR might benefit from a low dose of acepromazine (0.01 mg/kg) to reduce afterload and provide additional sedation. Benzodiazepines are also suitable based on the patient’s temperament, but may cause agitation and excitement at times. The use of eutectic mixture of local anesthetic (EMLA) cream helps reduce stress during catheter placement [5–7]. Dexmedetomidine is contraindicated in these patients due to severe vasoconstriction and reflex bradycardia. Preoxygenation for a minimum of 5–10 minutes prior to induction is appropriate until the patient is intubated. Monitoring equipment is placed on the patient prior to induction if possible. A preoperative ECG and BP are obtained at this time as a baseline.

Table 5.1 Systolic dysfunction.

Premedication (mg/kg)		Induction (mg/kg)	Maintenance	Intraoperative analgesia a (mg/kg)	Postoperative analgesia ^a (mg/kg)
Opioid(select one) (a) Methadone 0.3–0.5 IM, 0.2–0.3 IV OR (b) Hydromorphone 0.1 IM, 0.05 IV OR (c) Fentanyl 0.002–0.005 IV OR (d) Buprenorphine 0.01–0.03 IM or IV OR (e) Butorphanol 0.1–0.4 IM or IV	Sedative (if needed) (a) Midazolam 0.1 IM OR (b) Acepromazine 0.005–0.02 IV (MR only) +/− (c) Alfaxalone 1–3 IM	(a) Etomidate 1–2 IV +/− midazolam 0.1–0.2 IV OR (b) Alfaxalone 1–2 to effect IV +/− midazolam 0.1–0.3 IV OR (c) Propofol 2 + Ketamine 2 (“ketofol”) to effect +/− midazolam 0.2 OR (d) Fentanyl 0.01 and midazolam 0.2	(a) Sevoflurane OR (b) Isoflurane AND CRI for reduction of inhalant requirement	(a) Appropriate local regional block (b) MAC‐sparing CRI (mg/kg/h) (i) Fentanyl 0.012–0.042 OR (ii) Hydromorphone 0.03 OR (iii) Remifentanil 0.012–0.042 +/− (iv) Ketamine 0.6 mg/kg/h +/− (v) (dogs only) Lidocaine 1–3 mg/kg/h, loading dose 1–2 mg/kg IV required Fluids: Crystalloids 2–3 mL/kg/h (canine) or 1–3 mL/kg/h (feline)	(a) CRI: (i) Fentanyl 0.002–0.005 mg/kg/h OR (ii) Hydromorphone 0.01 mg/kg/h OR (b) Intermittent bolus: (i) Methadone 0.3 IV q 4–6 h OR (ii) Hydromorphone 0.05–0.1 IV q 4–6 h AND (c) NSAID where not contraindicated +/− (d) Liposomal encapsulated bupivacaine infiltrative block 5.3 mg/kg

“AND” means that multiple choices from this column are selected, “+/−” means it is up to the anesthetist if they would like to combine multiple choices, “OR” means that the anesthetist will choose one or the other selection. For example, the postoperative analgesia column would be interpreted as including an opioid and an NSAID, with an optional long‐acting local anesthetic block. An anticholinergic IM is ideal if HR is low following premedication, prior to induction. Lidocaine is avoided in cats.

^a Analgesia requirements should adequately address the level or degree of anticipated pain and are dependent on the procedure being performed.

CRI, constant‐rate infusion; IM, intramuscular; IV, intravenous; MAC, minimum alveolar concentration; MR, mitral regurgitation; NSAID, nonsteroidal antiinflammatory drug.

Historically, etomidate (after an effective premedication and typically with a benzodiazepine) was considered the ideal induction agent for these cases, as it minimally affects HR and BP. However, alfaxalone is cardiac friendly [8–10]. In the author’s experience, it also produces a smoother induction than etomidate. Premedications are used to reduce the dose of the induction agent needed and to decrease inhalant requirement. A benzodiazepine is also considered as part of the induction to reduce the amount of induction agent needed [11]. Inhalants are routinely used to maintain anesthesia but constant‐rate infusions (CRIs) such as fentanyl, or other opioids, are used to reduce the MAC requirement of inhalants, thus minimizing negative cardiovascular side‐effects. Local blocks are included where appropriate. Positive inotropes such as dopamine or dobutamine are used to support BP by increasing contractility and therefore increasing the cardiac output. Dopamine at higher doses causes vasoconstriction; these higher doses are avoided.

Monitoring includes an ECG, CO₂, temperature, pulse oximetry, Doppler and ideally invasive blood pressure (IBP), if obtainable. In extremely critical cardiovascular patients, central venous pressure (CVP) is performed to closely monitor preload (venous return and fluid status). Fluid therapy is conservative (2–3 mL/kg/h) with minimal to no administration of fluid boluses.

In recovery, patients may require supplemental oxygen, depending on the procedure and severity of cardiovascular disease. Continuous pulse oximetry monitoring is important during the recovery phase. Patients with mild to moderate cardiovascular disease tolerate anesthesia with minimal complications, provided the protocol is appropriate for them [12].

(c) Key points: Dilated cardiomyopathy is one of the most difficult cardiac diseases to manage for the anesthetist. Knowing how severely the patient’s ejection fraction (i.e., the patient’s ability to provide forward flow) is impacted will give the anesthetist an idea of how seriously the patient is compromised; an echocardiogram is necessary for this information. In addition to the above goals, the best possible management of these patients is to minimize anesthesia time, and only undergo elective procedures when they are “stable.” This often includes management of any concurrent arrhythmias as well as the use of drugs such as pimobendan (a positive inotrope). A dobutamine CRI is also considered in DCM patients to help increase myocardial contractility; however, it is not without increases in myocardial O₂ consumption, increased coronary blood flow requirements and decreased myocardial mechanical efficiency in these patients [13]. However, in the face of hypotension (which has its own deleterious effects), it is a suitable choice.

2. Diastolic dysfunction

Diastolic dysfunction includes diseases such as hypertrophic cardiomyopathy (HCM) and pericardial disease, where there is limited filling of the ventricle due to small chamber size. HCM is associated with a higher all‐cause mortality in cats due to the increased cardiovascular death and its contributions to noncardiovascular death, so special care is taken with these cats [14].

(a) Anesthesia concerns/common complications:

Avoid excessive peripheral vasodilation or constriction.
Maintain HR within 25% of baseline; if the anesthetist must choose between slightly below or slightly above resting HR, these cases often benefit from a slightly decreased HR to allow more filling time.
Avoid increases in myocardial contractility and myocardial oxygen consumption. This includes avoiding drugs such as ketamine and positive inotropes. Atropine is avoided due to the potential for tachycardia. Glycopyrrolate is used instead to help increase HR, when needed, as the heart rate increase is more often moderate, rather than profound. Severe hypothermia is avoided, since it causes shivering postoperatively and increases myocardial oxygen demand significantly.
Maintain adequate circulating volume without causing fluid overload (conservative fluid rates of 1–2 mL/kg/h).
If BP is compromised, one should assess the depth of anesthesia and try to reduce the inhalant concentration to help decrease vasodilation. The use of local blocks also helps to reduce the inhalant concentration. If this does not correct the BP, phenylephrine is used at 0.03–0.06 mg/kg/h to tighten vascular tone [15]. The goal is to return vascular tone back to its baseline prior to the impact of inhalant anesthesia (which reduces SVR). Phenylephrine works directly on alpha receptors to promote vasoconstriction; this can become profound at higher doses. It is difficult to know if this occurs with routine monitoring, but invasive cardiovascular monitoring equipment and measuring of lactate levels (which should remain below 2.0 mmol/L) provides general information on adverse effects due to the use of phenylephrine. If lactate levels increase, it is possible there is compromised perfusion secondary to vasoconstriction. If this occurs, phenylephrine is decreased or discontinued. It is most prudent, as blood pressure returns to normal, for phenylephrine to be gradually but expeditiously removed from the support measures.

(b) Anesthetic protocol (Table 5.2): If sedation is needed in patients with HCM, low doses of dexmedetomidine facilitate the goals of decreasing HR to allow more time in diastole as well as reducing MAC of inhalants. An opioid is included to help reduce the dose of dexmedetomidine and also provide analgesia. Alfaxalone can be administered IM in situations where a patient is extremely fractious. In a study on cats with HCM, systolic anterior motion and left ventricular outflow tract obstruction, medetomidine (an alpha‐2 agonist similar to dexmedetomidine) was used for sedation and helped improve forward flow [16]. Benzodiazepines may be used as part of the premedication but may result in agitation. For cardiac patients, it is good practice to administer drugs that are reversible and which have minimal cardiovascular effects.

Table 5.2 Diastolic dysfunction.

Premedication (mg/kg)		Induction (mg/kg)	Maintenance	Intraoperative analgesia a (mg/kg)	Postoperative analgesia^* (mg/kg)
Opioid (select one) (a) Methadone 0.3–0.5 IM, 0.2–0.3 IV OR (b) Fentanyl 0.002–0.005 IV OR (c) Buprenorphine 0.01–0.03 IV or IM OR (d) Butorphanol 0.1–0.4 IM or IV	Sedative (if needed) (a) Midazolam 0.1 IM OR (b) Dexmedetomidine 0.001–0.003 IM or IV +/− (c) Alfaxalone 1–3 IM	(a) Etomidate 1–2 IV +/− midazolam 0.1–0.2 IV OR (b) Alfaxalone 1–3 IV to effect +/− midazolam 0.1 IV	(a) Sevoflurane OR (b) Isoflurane AND CRI for reduction of inhalant requirement	(a) Local regional block AND (b) Opioid CRI (mg/kg/h) (i) Fentanyl 0.012–0.042 Fluids: Crystalloid 1–2 mL/kg/h (cats) or 2–3 mL/kg/h (dogs)	(a) CRI: (i) Fentanyl 0.002–0.005 mg/kg/h OR (ii) Hydromorphone 0.01 mg/kg/h OR (b) Intermittent bolus: (i) Methadone 0.3 IV q 4–6 h OR (ii) Hydromorphone 0.05–0.1 IV q 4–6 h AND (c) NSAID where not contraindicated +/− (d) Liposomal encapsulated bupivacaine infiltrative block 5.3 mg/kg

^a Analgesic selection should be appropriate for the duration of procedure and level of anticipated pain.

CRI, constant‐rate infusion; IM, intramuscular; IV, intravenous; NSAID, nonsteroidal antiinflammatory drug.

Patients are preoxygenated before induction and up to the point of intubation. Inductions with etomidate or alfaxalone with the addition of midazolam are ideal. If the patient is cooperative, monitoring equipment is placed prior to induction. Isoflurane or sevoflurane at low concentrations are used routinely for maintenance.

Recovery should occur in a stress‐free environment. Supplemental oxygen should be available if necessary and continuous pulse oximetry is used.

3. Obstructive dysfunction

Obstructive dysfunction includes diseases such as fulminant heartworm disease or valvular stenosis (pulmonic or aortic). Cardiac diseases with obstructive dysfunction have reduced CO and limited cardiac reserve. In addition, the ventricular chambers begin to hypertrophy in response to pushing volume against an obstruction. As with other types of cardiovascular disease, these patients often compensate until heart failure develops. Anesthetizing these animals carries the risk of decompensation.

(a) Anesthesia concerns/common complications:

Maintain HR within 25% of resting HR; if the anesthetist must choose between slightly below or slightly above resting HR, these cases often benefit from a decreased HR, allowing hypertrophied cardiac muscles time in diastole to perfuse.
Avoid severe peripheral vascular tone changes, such as anesthesia‐induced peripheral vasodilation. Depending on the disease, different inotropes and vasopressors are considered for BP interventions. Phenylephrine CRI is the agent of choice. Caution is exercised when using this pressor, and lactate levels are measured if this drug is used at higher dose intervals for an extended period of time.
Avoid volume overload by using a conservative fluid therapy (usually 1–2 mL/kg/h in cats or 2–3 mL/kg/h in dogs) [17].
Avoid severe increases in myocardial contractility and oxygen consumption by avoiding drugs like ketamine and positive inotropes.
Minimize stress during the perioperative and postoperative recovery periods.
A propofol or alfaxalone CRI is used for patients that are heartworm positive as total intravenous anesthesia. An opioid CRI is used in addition to help reduce the dose of propofol or alfaxalone needed.

(b) Anesthetic protocol (Table 5.3): In general for cardiac patients, it is good practice to administer drugs that are reversible and have minimal cardiovascular effects (i.e., an opioid and benzodiazepine). The goal of premedication is to provide sufficient sedation to minimize stress, facilitate IV catheterization and reduce the amount of drugs necessary for induction and maintenance. If the patient is cooperative, IV catheterization is performed with the assistance of EMLA cream in the nonsedated patient [7,18]. It is important to preoxygenate these patients. Placement of monitoring equipment prior to induction is ideal (Doppler, ECG, noninvasive blood pressure [NIBP] and, when obtainable, IBP). Rapid access to the airway is important to minimize oxygen desaturation. Neuroleptic anesthesia (opioid + benzodiazepine), etomidate with midazolam, or alfaxalone with midazolam are ideal for induction. Low doses of propofol may be used following bolus of opioid, benzodiazepine, and/or lidocaine.

MAC‐sparing CRIs and local blocks are indicated to reduce inhalants; extremely critical canine patients may require minimal to no inhalant when placed on high doses of opioid CRIs. Monitoring includes at least IBP, ECG, Doppler, SpO₂, and capnography. In recovery, be prepared to provide oxygen supplementation. Recovery should occur in a stress‐free environment where the patient is closely monitored.

Table 5.3 Obstructive heart disease.

Premedication (mg/kg)		Induction (mg/kg)	Maintenance	Intraoperative analgesia a (mg/kg)	Postoperative analgesia ^a (mg/kg)
Opioid^a (select one) (a) Methadone 0.3–0.5 IM, 0.2–0.3 IV OR (b) Hydromorphone (canine) 0.1 IM, 0.05 IV OR (c) Fentanyl 0.002–0.005 IV OR (d) Buprenorphine 0.01–0.03 IV or IM OR (e) Butorphanol 0.1–0.4 IV or IM	Sedative (a) Midazolam 0.1 IM +/− (b) Alfaxalone 1–3 IM	(a) Etomidate 1–2 IV to effect +/− midazolam 0.1–0.2 IV OR (b) Alfaxalone 1–2 IV to effect +/− midazolam 0.1–0.2 IV OR (c) Fentanyl 0.005–0.01 and midazolam 0.2 (canine)	(a) Sevoflurane OR (b) Isoflurane OR (c) TIVA: (i) Alfaxalone 4–6 mg/kg/h (ii)Propofol (for use with patients with heartworm disease) 6–24 mg/kg/hr AND CRI for reduction of inhalant requirement	(a) Local regional block AND (b) Opioid CRI (mg/kg/h) (i) Fentanyl 0.012–0.042 OR (ii) Hydromorphone 0.01–0.03 OR (iii) Remifentanil 0.012–0.042 Fluids: Crystalloid 1–2 mL/kg/h (cats) or 2–3 mL/kg/h (dogs)	(a) CRI: (i) Fentanyl 0.002–0.005 mg/kg/h OR (ii) Hydromorphone 0.01 mg/kg/h OR (b) Intermittent bolus: (i) Methadone 0.3 IV q 4–6 h OR (ii) Hydromorphone 0.05–0.1 IV q 4–6 h AND (c) NSAID where not contraindicated +/− (d) Liposomal encapsulated bupivacaine infiltrative block 5.3 mg/kg

^a Analgesic selection should be appropriate for the duration of procedure and level of anticipated pain.

CRI, constant‐rate infusion; IM, intramuscular; IV, intravenous; NSAID, nonsteroidal antiinflammatory drug; TIVA, total intravenous anesthesia.

II. Endocrine diseases

A. Diabetes insipidus

Diabetes insipidus (DI) is rare in veterinary patients. This disease occurs either centrally or at the level of the kidney (nephrogenically); the patient either does not secrete or does not respond to (respectively) antidiuretic hormone (ADH). The lack of ADH effect results in an animal that urinates large volumes and thus must drink large volumes to accommodate its polyuria. It is important to stabilize these patients (especially correcting dehydration) prior to anesthesia. Animals with confirmed DI are not anesthetized for elective procedures until their DI is managed, through the use of desmopressin. For nonelective procedures, volume status is of critical importance prior to anesthesia. Generally speaking, these patients have a free water deficit that is corrected as best as possible prior to anesthesia.

1. Anesthesia concerns/common complications

Water is not restricted at any point for these patients. These patients are considered dehydrated, both systemically and cerebrally.
Monitor electrolytes (especially sodium) throughout anesthesia and into recovery; use of fluid therapy assists with keeping sodium in the patient’s target range, which should match their sodium on blood work. See Chapter 3 p.80 for management of hypernatremic patients, as rapid changes in sodium have severe neurologic consequences.
Fluid choice is based on matching the sodium in the fluid to as close to the patient’s blood sodium as possible. For example, if the patient’s sodium is 160 mEq/L, 0.9% NaCl is the closest match at 154 mEq/L.

2. Anesthetic protocol

No anesthetic is specifically contraindicated for patients with DI; however, it is important to select drugs based on patient presentation and preanesthetic assessment. Judicious fluid therapy is warranted. In order to minimize the time to return of a patient’s full function (and thus drinking on its own), it is wise to use reversible drugs whenever possible. Premedication is tailored to level of invasiveness of the procedure, with invasive procedures warranting full mu agonist opioids and minimally invasive procedures warranting mildly depressive drugs such as butorphanol. Induction with propofol is smooth and the drug is metabolized quickly. Alfaxalone is also considered but has a longer metabolism. Maintenance anesthesia with inhalants also includes the use of an opioid CRI, such as fentanyl or remifentanil, which reduces MAC of inhalant. Postoperative analgesia is tailored to the level of invasiveness of the procedure.

B. Diabetes mellitus

Diabetes mellitus (DM) is characterized by either relative or absolute insulin deficiency. DM is classified into two main categories. Type 1 diabetes is more common in dogs and is a true lack of insulin production. Type 2 diabetes is more commonly seen in cats and results in an insulin resistance. However, the cat (unsurprisingly as it is a cat) may have transient diabetes, noninsulin‐dependent diabetes, or traditional insulin‐dependent diabetes. Most commonly, in anesthesia cases, the patient is insulin dependent. Ultimately, the body’s inability to regulate glucose with insulin leads to osmotic diuresis and excessive urination. If insulin is not exogenously administered, the body will attempt to utilize other energy sources, which results in ketone formation and, especially with any perpetuating factors (such as stress), ketoacidosis.

Ketoacidosis is a life‐threatening emergency that is addressed before anything else but the most crucial of surgeries. Therefore, the anesthetist is typically confronted with the managed (although infrequently regulated) diabetic. Routine BW including a complete blood count (CBC) and chemistry is performed; knowing electrolytes is essential. A urinalysis to assess for ketonuria is warranted. Special attention is paid to the hydration status of the patient. Diabetic patients may have an enlarged liver and weakened abdominal muscles, which will contribute to inadequate ventilation during anesthesia. Pancreatitis is also seen in DM patients. Therefore, gastroprotectants (such as maropitant, pantoprazole, and famotidine) are often included in the anesthetic protocol.

Phacoemulsification is a common procedure performed on patients with DM for the removal of cataracts. Due to the nature of the procedure, the eye must remain in a central position using neuromuscular blocking agents such as atracurium and rocuronium. While some studies have shown that these patients may require higher rocuronium infusion dosing than patients that do not have DM [19], others show no difference when using atracurium [20].

Overall, the diabetic patient may be quite difficult to manage under anesthesia, especially when the diabetes is not controlled. Hypotension is moderate to severe in patients that have DM undergoing phacoemulsification. This may be attributed to the hypovolemia that these patients may exhibit due to the hyperglycemia [21].

1. Anesthesia concerns/common complications

Timing: The focus of the anesthetic protocol is minimizing time out of the patient’s routine feeding schedule. Elective procedures are scheduled in the morning so the patient recovers by the evening, and the patient is offered food as soon as they are fully recovered from anesthesia. During that time of fasting, glucose is monitored and supplemented if needed. Additionally, short‐acting, reversible drugs are used where appropriate to minimize the “hangover” effects.
Fasting: In managed diabetics, patients are fasted for 2–4 hours after a pâté consistency food is given, with half dose of their regular insulin given in the morning [22]. BG is measured at admission to ensure the patient is not hypoglycemic.
Blood glucose monitoring: Monitor BG every 30–60 min, beginning immediately after induction. A sampling line (either an arterial line or a large‐bore catheter) is convenient for this purpose. Normal glucose is 80–120 mg/dL but knowing a diabetic patient’s level of regulation will indicate their “normal.” A poorly regulated patient, stress from hospitalization, fasting, and certain drugs that may be administered (ex. dexmedetomidine) will alter this blood glucose state. Typical diabetic patients have a BG of 200–250 mg/dL; a glucose drop below 200 mg/dL may prompt intervention. If glucose is less than 120 mg/dL in any diabetic patient, give dextrose 2.5–5% at regular surgical rate of 5 mL/kg/h in the canine and 3 mL/kg/h in the feline. This is often accomplished with a buretrol or smaller bag of fluids, as the patient’s requirements for dextrose may fluctuate over the anesthetic period. If a dextrose bolus is needed, 0.5–1 mL/kg of 50% dextrose is given slowly over 10 minutes after it is diluted with saline in a 1:3 ratio.

If glucose levels exceed the patient’s “normal” BG based on BW (or more generically, over 300 mg/dL), regular insulin at 0.1–0.2 IU/kg IV or SQ is administered. Alternatively, a CRI of regular insulin at 0.05 IU/kg/h, if continually monitored, may assist in regulation. It is prudent for the anesthetist to remember, however, that the patient is better off “sweet than sour” and therefore aggressively lowering BG is inappropriate.
Fluid resuscitation: Dehydration is common in these patients; this is corrected prior to anesthesia if possible. Dehydration may contribute to hypotension (see below).
Hypotension: Diabetic dogs were more likely than their nondiabetic counterparts to experience moderate to severe hypotension, which may be due to hypovolemia secondary to osmotic diuresis [21]. Monitoring blood pressure is necessary in these dogs, and it is prudent to begin with a fluid bolus to correct hypotension, when it becomes evident.
Electrolyte abnormalities: It has been reported that life‐threatening hyperkalemia could occur in an uncontrolled diabetic patient under anesthesia (possibly due to an insulin deficiency and blood hyperosmolarity) [23]. It is prudent to have access to blood sampling, through a second IV catheter or an arterial line, in order to evaluate blood electrolytes intraoperatively.

2. Anesthetic protocol (Table 5.4)

Dexmedetomidine results in a transient hyperglycemia and is avoided when possible. Selection of premedication and induction drugs is based on patient presentation with a preference given to titratable drugs (i.e., propofol, alfaxalone, etomidate). Most induction agents are relatively short‐acting and will be metabolized quickly. Inhalants are routinely used. CRIs are used to provide analgesia and reduce inhalant requirements. The patient receives continued glucose checks at 30–60 min intervals until they resume eating, including time spent in recovery. Glucose supplementation (2.5–5%) in crystalloid fluids may be required throughout the recovery phase at 2–5 mL/kg/h. Appropriate and balanced postoperative analgesia is necessary in cases of invasive procedures so the patient resumes normal function as quickly as possible. Local blocks are contraindicated if there is peripheral neuropathy present.

C. Hyperadrenocorticism (Cushing disease)

Hyperadrenocorticism is an increase in corticosteroids, either exogenously due to supplementation or from the adrenal glands themselves (secondary to a primary adrenal mass or pituitary gland tumor causing stimulation of the adrenal gland). While the term “Cushing disease” actually indicates the pituitary form of hyperadrenocorticism, it is often used interchangeably for all forms of hyperadrenocorticism. Hyperadrenocorticism is confirmed or suggested based on a variety of methods, including history of supplementation, ACTH stimulation, low‐dose dexamethasone suppression test, high‐dose dexamethasone suppression test, and urine cortisol to creatinine ratio.

Table 5.4 Diabetes mellitus.

Premedication (mg/kg)		Induction (mg/kg)	Maintenance	Intraoperative analgesia (mg/kg/h)	Postoperative analgesia (mg/kg)
Opioid (a) Methadone 0.3–0.5 IM, 0.2–0.3 IV OR (b) Hydromorphone 0.1 IM, 0.05 IV OR (c) Buprenorphine 0.01–0.03 IV, IM OR (d) Butorphanol 0.1–0.4 IV, IM	Other drugs: (a) Sedative (if needed) (i) Acepromazine 0.01–0.02 OR (ii) Midazolam 0.1 IM OR (iii) Alfaxalone 1–3 IM AND (b) GI protection: (i) Famotidine 1 mg/kg IV or SC AND (ii) Maropitant 1 mg/kg IV or SC	(a) Propofol 1–4 to effect +/− midazolam 0.2 OR (b) Alfaxalone 1–3 IV +/− midazolam 0.1–0.2 IV OR (c) Propofol 2 + Ketamine 2 (“ketofol”) to effect +/− midazolam 0.2	(a) Sevoflurane OR (b) Isoflurane +/− CRI for reduction of inhalant requirement	(a) Opioid CRI (select one) (i) Fentanyl 0.012–0.042 OR (ii) Hydromorphone 0.03 OR (iii) Remifentanil 0.012–0.042 +/− (b) Adjunctive CRI (i) Lidocaine CRI 1.5–3 +/− (c) Local regional block Fluids: Crystalloids +/− 2.5–5% dextrose 2–5 mL/kg/h	(a) CRI: (i) Fentanyl 0.002–0.005 mg/kg/h OR (ii) Hydromorphone 0.01 mg/kg/h +/− (iii) Lidocaine 1.5 mg/kg/h OR (b) Intermittent bolus: (i) Methadone 0.3 IV q 4–6 h OR (ii) Hydromorphone 0.05–0.1 IV q 4–6 h AND (c) NSAID where not contraindicated +/− (d) Liposomal encapsulated bupivacaine infiltrative block 5.3 mg/kg

“AND” means that multiple choices from this column are selected, “+/−” means it is up to the anesthetist if they would like to combine multiple choices, “OR” means that the anesthetist will choose one or the other selection. For example, the postoperative analgesia column would be interpreted as including an opioid from the options (either as a CRI or intermittently given) and an NSAID, with an optional lidocaine CRI and optional long‐acting local anesthetic block.

In patients who received pantoprazole for greater than 36 hours, pantoprazole will replace famotidine.

Lidocaine is contraindicated in cats; alternatively, lidocaine patches are an option alongside incision for both the cat and dog.

CRI, constant‐rate infusion; GI, gastrointestinal; IM, intramuscular; IV, intravenous; NSAID, nonsteroidal antiinflammatory drug; SC, subcutaneous.

Clinical signs of this disease include polyuria and polydipsia (PU/PD), polyphagia, thin hair and skin, especially on the flank and abdomen, along with pyoderma, a “pot belly” appearance, panting, muscle weakness, and lethargy. On PE, hepatomegaly is present. Potential concurrent disease processes which merit investigation include hypertension, hypercoagulability, renal disease, and diabetes mellitus. Characteristic BW abnormalities include a stress leukogram, increases in RBC counts, increase in platelets, BG, ALP, ALT, triglycerides, and cholesterol, and a decrease in phosphate. Urinalysis reveals a decreased urine specific gravity (USG), proteinuria, and possibly bacterial infection. In one study, the mortality rate was as high as 14.6% for patients undergoing an adrenalectomy to address the primary adrenal mass resulting in Cushing disease [24]; the anesthetic protocol is carefully planned as the outcome of the procedure has been associated with the anesthetic management [24].

1. Anesthesia concerns/common complications

Fragile skin: Care is taken when clipping these patients. Their skin is prone to bruising and lacerations. The hair that is clipped does not grow back as quickly or aesthetically as the hair of a normal patient, so minimizing the clipped areas is indicated.
Concurrent medications: As with all patients, a thorough understanding of all medications or supplements a hyperadrenocortical patient receives is important for the anesthetist. A patient which has iatrogenic adrenocortical disease (i.e., is on glucocorticoid supplementation) is not abruptly taken off glucocorticoids prior to anesthesia, as an Addisonian crisis may result (see p.157). Some patients with the pituitary form of Cushing disease are treated with selegiline HCl (Anipryl,® Zoetis). This drug is an irreversible monoamine oxidase inhibitor (MAOI), meaning that until new MAOs are formed, monoamines are not oxidized and therefore increase in circulation. While this is useful in certain disease states, too much monoamine may result in serotonin syndrome, characterized by hyperthermia, tachycardia, hypertension and muscle tremors, which range in severity from mild to fatal. Additional drugs that increase the availability of monoamines are contraindicated (e.g., tramadol, metoclopramide, trazodone). During the anesthetic period, meperidine, fentanyl, and pentazocine are avoided for patients receiving MAOIs. It appears morphine is the most suitable opioid, although neurologic depression may accompany administration of morphine in a patient on selegiline HCl. Some authors suggest avoiding ketamine as it may exacerbate the symptoms of a serotonergic crisis, but there is little, if any, nonanecdotal information for this recommendation.
Hypoventilation: This results from muscle weakness and abdominal distension secondary to hepatomegaly and fat redistribution; mechanical ventilation is warranted.
Management of concurrent diseases: The possibility of a pulmonary thromboembolism (PTE) secondary to hypercoagulability is something the anesthetist should be vigilant to monitor for; this is suggested by a large difference between EtCO₂ and arterial CO₂ (see Chapter 3, p.69, “Pulmonary thromboembolism”). The use of reversible drugs is recommended to ensure the patient is able to wake up quickly. Most clinicians also request the patient has frequent walks in the postoperative period to reduce the formation of blood clots.
Survival: In patients undergoing an adrenalectomy to address their Cushing disease, the survival rate was better statistically when patients received hydrocortisone intraoperatively [24]. The anesthetist should discuss this with the surgeon prior to the procedure.

2. Anesthetic protocol (Table 5.5)

Premedication reduces stress and facilitates IV catheterization; oftentimes, a sedative is unnecessary, but the anesthetist can include acepromazine in low doses or midazolam as necessary. Care is taken when clipping the site of the IV catheter to prevent abrasions and bruising. Morphine, hydromorphone, and methadone are safe choices that provide appropriate analgesia for invasive procedures. Following IM premedication, patients are watched for respiratory depression. Preoxygenation is recommended for at least 5–10 minutes. If the patient is cooperative, monitoring equipment such as Doppler, ECG, and NIBP are placed prior to induction. Induction is accomplished with many options depending on the patient’s anesthetic assessment and drug availability. Etomidate results in adrenocortical suppression, although whether this is beneficial is unknown. Inclusion of a benzodiazepine helps reduce the amount of etomidate necessary; however, even small amounts of etomidate will result in adrenocortical suppression [25]. A neuroleptic induction may work well for extremely critical patients. Propofol and alfaxalone are also acceptable choices for induction in combination with a benzodiazepine to reduce the amount of true induction agent needed. Inhalants are used to maintain anesthesia. A CRI of opioids (i.e., fentanyl, hydromorphone) and/or lidocaine helps provide analgesia and reduce MAC requirements of the inhalant.

Diligent monitoring of ventilation, invasive blood pressure, acid–base status, and electrolyte analysis is performed throughout the procedure, and possibly into recovery. Patients are extubated only when they are able to adequately ventilate themselves and monitored after extubation for trouble ventilating. Supplemental oxygen is provided to patients which need it.

D. Hypoadrenocorticism (Addison’s disease) [26]

A deficiency of glucocorticoid and/or mineralocorticoid from the adrenal glands is called hypoadrenocorticism (Addison disease). The atypical Addisonian will only have deficiency in glucocorticoids, not mineralocorticoids. The adrenal gland’s outermost layer, the zona glomerulosa, is responsible for secreting mineralocorticoids (with aldosterone as one of the most important). When the zona glomerulosa no longer secretes aldosterone, electrolyte excretion and water regulation become dysfunctional. The subsequent layer, the zona fasciculata, is responsible for cortisol (glucocorticoid) production and when this is impaired, many normal functions of the body (such as metabolism, cardiovascular stability, and response to stress) are compromised.

Table 5.5 Cushing disease.

Premedication (mg/kg)		Induction (mg/kg)	Maintenance	Intraoperative analgesia (mg/kg/h)	Postoperative analgesia (mg/kg)
Opioid (select one) (a) Methadone 0.3–0.5 IM, 0.2–0.3 IV OR (b) Hydromorphone 0.1 IM, 0.05 IV OR (c) Morphine 0.1–0.5 IM OR (d) Buprenorphine 0.01–0.03 IV, IM OR (e) Butorphanol 0.1–0.4 IV, IM	Sedative (if needed) (a) Acepromazine 0.01–0.02 IV, IM OR (b) Midazolam 0.1 IM, IV	(a) Propofol 2–4 IV to effect +/− midazolam 0.2 IV OR (b) Alfaxalone 1–3 IV +/− midazolam 0.1–0.2 OR (c) Propofol 2 + Ketamine 2 (“ketofol”) to effect +/− midazolam 0.2 OR (c) Etomidate 1–2 IV +/− midazolam 0.1–0.2 IV	Sevoflurane OR Isoflurane AND CRI for reduction of inhalant requirement Fluids: Balanced, isotonic electrolyte solution	(a) Opioid CRI (i) Fentanyl 0.012–0.042 OR (ii) Hydromorphone 0.03 OR (iii) Remifentanil 0.012–0.042 +/− (b) Lidocaine CRI 1.5–3 +/− (c) Local regional block	(a) Opioid CRIs: (i) Fentanyl 0.002–0.005 mg/kg/h OR (ii) Hydromorphone 0.01 mg/kg/h OR (b) Intermittent bolus: (i) Methadone 0.3 IV q 4–6 h OR (ii) Hydromorphone 0.05–0.1 IV q 4–6 h +/− (c) Lidocaine 1.5 mg/kg/h +/− (d) Liposomal encapsulated bupivacaine infiltrative block 5.3 mg/kg

“AND” means that multiple choices from this column are selected, “+/−“ means it is up to the anesthetist if they would like to combine multiple choices, “OR” means that the anesthetist will choose one or the other selection. For example, the postoperative analgesia column would be interpreted as including an opioid from the options (either as a CRI or intermittently given), with an optional lidocaine CRI and optional long‐acting local anesthetic block.

Lidocaine is contraindicated in cats; alternatively, lidocaine patch can be placed alongside incisions in cats and dogs. Preoxygenate patients. In patients on Anipryl®, meperidine is avoided, as are fentanyl and remifentanil.

CRI, constant‐rate infusion; IM, intramuscular; IV, intravenous.

Addison disease is often referred to as “the great pretender” because the clinical signs of this disease are vague; they include weakness, lethargy, inappetence, chronic vomiting and diarrhea, pre‐renal azotemia (sometimes mistaken for renal failure and not present with atypical Addison), and occasionally PU/PD. The patient may be hypotensive with abnormalities related to hyperkalemia (bradycardia and ECG changes including loss of P waves, wide and bizarre QRS waves, and tall, tented T waves). Suspected Addison disease is confirmed by testing the adrenocortical reserve with ACTH stimulation. Elective procedures in an unmanaged Addisonian patient are postponed until the patient is stabilized.

Even in the managed Addisonian patient, it is recommended that a CBC and chemistry are performed. Common abnormalities on the CBC include absence of stress leukogram – this is unusual for an animal presenting as ill or “not doing right.” Anemia is likely present but may be masked by hypovolemia, resulting in a relatively normal PCV. Abnormalities on the chemistry include a host of electrolyte abnormalities, such as hypoglycemia, hyperkalemia with hyponatremia (ratio Na:K <27, although this may vary by lab analyzer), hypochloremia, and hypercalcemia, as well as azotemia and acidosis (which are secondary to hypovolemia). A urinalysis may reveal a low USG. When Addison disease is confirmed, traditional management usually includes desoxycorticosterone pivalate (DOCP) for mineralocorticoid supplementation and a corticosteroid for glucocorticoid (such as prednisone) supplementation.

In the managed Addisonian patient, it is the author’s preference to schedule elective procedures after recent administration of DOCP, rather than close to the next date of administration (as it is administered every 25 days per the manufacturer). Additionally, it is imperative that any maintenance glucocorticoid is given on schedule, and in some cases, an additional dose is administered preemptively.

1. Anesthesia concerns/common complications

Addisonian crisis: An Addisonian crisis manifests as life‐threatening hypovolemic shock and hyperkalemia in patients acutely removed from glucocorticoids. Patients must receive large volumes of 0.9% NaCl for volume resuscitation and may require treatment for hyperkalemia (see Chapter 3, p.78). When recognized, supplementation with dexamethasone after volume resuscitation will benefit the patient without interfering in an ACTH stimulation test for confirmation of hypoadrenocorticism.
Effective circulating volume: Fluid stabilization with 0.9% NaCl is necessary prior to any procedure; aggressiveness of stabilization is based on how well managed the Addisonian patient is. Unstable cases may require up to 90 mL/kg (shock dose/blood volume of a dog) of 0.9% NaCl for the first hour. Goal‐directed therapy is important; giving aliquots of fluid and assessing the patient will help better titrate the correct amount and avoid volume overload.
Hyperkalemia: Preinstrumentation, especially an ECG, is warranted if the patient allows. Arrhythmias associated with hyperkalemia, and their treatments, are discussed in Chapter 3. Hyperkalemia is one of the most life‐threatening electrolyte abnormalities. It requires aggressive and timely treatment.
Hypoglycemia: Hypoglycemic patients may require a 2.5–5% dextrose supplementation. A dextrose bolus may be needed. A dose of 0.5–1 mL/kg of 50% dextrose may be given slowly and diluted in a 1:3 ratio with saline.
Hypotension: Obtain a BP measurement prior to induction. Hypotension is difficult to manage in these cases, but heavily relies on volume support. IBP measurement is optimal. Inotropes, such as dobutamine, might also be useful for BP support as well as helping to lower potassium concentration, once volume status is optimized.
Stress: These patients compensate poorly for stress; of all patients, minimizing stress is a top priority in the Addisonian patient.
Supplementation: Ensure any managed Addisonian has received their corticoid supplementation for the day, and their mineralocorticoid supplementation within the last month (DOCP). Unmanaged cases receive hydrocortisone 2–4 mg/kg IV prior to induction. It is best to increase their glucocorticoid supplementation during their stay in the hospital.

2. Anesthetic protocol (Table 5.6)

These patients usually require lower drug dosages than the standard patient; when possible, use reversible, short‐acting drugs. Premedication with an opioid and application of EMLA cream will reduce the stress associated with IV catheterization. A neuroleptic anesthesia premed/induction will often facilitate intubation. Propofol or alfaxalone to effect, with midazolam 0.2 mg/kg to reduce the amount of primary induction agent necessary, is used as well. Etomidate is contraindicated in these patients due to its inhibition of steroidogenesis and decreasing cortisol production for 4–6 hours postoperatively. Inhalant anesthetics are routinely used for anesthesia maintenance. Opioid CRIs are used to reduce the MAC requirement of inhalant anesthetics. Provide MV if necessary; however, the anesthetist must be cognizant of the negative impacts MV will have on BP and discontinue it in the face of hypotension. These patients require aggressive fluid therapy. It is ideal to monitor CVP and IBP. Additionally, EtCO₂, blood gas analysis, BG, and electrolytes are closely monitored throughout anesthesia and recovery.

E. Hyperthyroidism

Hyperthyroidism results in excessive thyroid hormone production, increasing a patient’s metabolic state. While relatively rare in dogs (generally secondary to a thyroid tumor which is functional; most are not), hyperthyroidism is extremely common in the aging cat. Clinical signs consist of weight loss (often in the face of increased appetite), vomiting, increased activity, and PU/PD. On PE, the patient is thin/has poor body condition with muscle wasting and at least one palpably enlarged thyroid gland. These patients are often agitated and easily stressed; stress evokes respiratory changes such as dyspnea and hyperventilation. Cardiovascular changes include hypertension, tachycardia, arrhythmias, and increased oxygen requirements. Often times, a gallop murmur is auscultated. Further investigation of the cardiac system may reveal cardiac enlargement and heart failure, which in cats manifests as pulmonary edema or pleural effusion.

While azotemia may be present, an absence of renal problems is not uncommon; even if the beginning stages of renal failure exist in the cat, the kidney is heavily perfused secondary to the hypermetabolic state so that renal values remain normal. This is important information for the anesthetist, because it is unwise to assume these patients have adequate kidney reserve. This is also important to note in any patients that have received radioiodine treatment for their hyperthyroidism. A decrease in blood volume and velocity at the level of the kidney may occur in patients post treatment, as shown by contrast enhanced ultrasound [27]. Changes in BW include an elevated T4 level, increased RBC count, and increased liver enzymes (ALT, ALP, AST).

Table 5.6 Addison’s disease.

Premedication (mg/kg)		Induction (mg/kg)	Maintenance	Intraoperative analgesia (mg/kg/h)	Postoperative analgesia (mg/kg)
Opioid (select one) (a) Methadone 0.3 IM, 0.2 IV OR (b) Hydromorphone 0.05 IM, 0.05 IV OR (c) Morphine 0.1–0.5 IM OR (d) Buprenorphine 0.01–0.03 IV, IM OR (e) Butorphanol 0.1–0.4 IV, IM	Sedative (if needed) (a) Midazolam 0.1 IM (b) Acepromazine 0.01–0.02 IM	(a) Alfaxalone 1–3 +/− midazolam 0.2 OR (b) Propofol 2 + ketamine 2 (“ketofol”) to effect +/− midazolam 0.2 OR (c) Propofol to effect +/− midazolam 0.2	(a) Sevoflurane OR (b) Isoflurane +/− CRI for reduction of inhalant requirement Fluids: Balanced, isotonic crystalloid solution	(a) Opioid CRI (i) Fentanyl 0.012–0.042 OR (ii) Hydromorphone 0.03 OR (iii) Remifentanil 0.012–0.042 +/− (b) Lidocaine CRI 1.5–3 (dogs only) +/− (c) Local regional block	(a) Opioid CRIs: (i) Fentanyl 0.002–0.005 mg/kg/h OR (ii) Hydromorphone 0.01 mg/kg/h OR (b) Intermittent bolus: (i) Methadone 0.3 IV q 4–6 h OR (ii) Hydromorphone 0.05–0.1 IV q 4–6 h +/− (c) Lidocaine 1–3 mg/kg/h +/− (d) Liposomal encapsulated bupivacaine infiltrative block 5.3 mg/kg

“AND” means that multiple choices from this column are selected, “+/−” means it is up to the anesthetist if they would like to combine multiple choices, “OR” means that the anesthetist will choose one or the other selection. For example, the postoperative analgesia column would be interpreted as including an opioid from the options (either as a CRI or intermittently given), with an optional lidocaine CRI and optional long‐acting local anesthetic block.

Preoxygenate. Give hydrocortisone 2–4 mg/kg IV at induction if unsure of glucocorticoid supplementation. Lidocaine is contraindicated in cats; alternatively, lidocaine patch is used alongside incision in both dogs and cats.

CRI, constant‐rate infusion; IM, intramuscular; IV, intravenous.

1. Anesthesia concerns/common complications

Client communication: If it is necessary to anesthetize an uncontrolled hyperthyroid cat, the owner must be thoroughly counseled regarding the increased anesthetic risk, as well as intraoperative and postoperative risks (including sudden death). The CPR status of the cat is confirmed with the anesthetist. In cats that are euthyroid, this risk is greatly diminished (although not eliminated). It is recommended that a patient receives treatment (methimazole) prior to undergoing an elective procedure. A T4 analysis is performed prior to the procedure to confirm normal levels.
Cooperativeness: Hyperthyroid patients cope even more poorly than a normal cat with restraint, putting both personnel and the cat at risk. Gabapentin 100 mg orally on the morning of the procedure is ideal to reduce stress.
Adequate premedication is a necessity. Cardiac arrest has been experienced clinically due to stress induced from restraint. It is not uncommon for these cats to exhibit dyspnea and panting from even minimal handling and restraint.
Hypertension: The vast majority of these cats are hypertensive. A BP is obtained prior to induction, if possible, without causing too much stress to the patient. If it is not possible to obtain a BP without a premedication, a BP is attempted when the patient is more sedated, knowing that this may be altered by the medications. The anesthetist should be aware of any drugs that are used to control the hypertension (ACE inhibitors, Ca+ channel blockers, etc.). Depending on the response to the medication, these drugs may be withheld on the day of surgery (i.e., if the patient is very responsive to BP medication, it may be withheld to prevent intraoperative hypotension; see next) (see Appendix F).
Hypoventilation: If respiratory muscles are weak, hypoventilation will be evident under anesthesia. As long as cardiovascular parameters are within normal limits, mechanical ventilation is initiated for these patients.
Multiorgan involvement: Given the number of organ systems impacted by hyperthyroidism, minimizing time under anesthesia and maintaining adequate tissue perfusion (i.e., preventing hypotension) are critical to success in these patients.
Tachycardia: Patients who are severely hypertensive and tachycardic are stabilized on beta‐blockers prior to anesthesia; this may also decrease the incidence of arrhythmias. Intraoperatively, cats with heart rates over 220 bpm may benefit from esmolol at an initial bolus of 0.1–0.5 mg/kg IV slowly followed by CRI of 6–12 mg/kg/h [28].
Hypothermia: In an underweight patient, thermoregulation is difficult due to increased body surface area and use of a nonrebreathing system in small patients.
Thyroid storm: This complication is life‐threatening and is characterized by rapid release of thyroid hormones into systemic circulation (usually in response to stress). Clinical signs include tachycardia and hyperthermia; a patient which experiences this can develop pulmonary edema and decompensate rapidly. While this occurs most commonly postoperatively, it can occur intraoperatively. Treatment involves controlling the symptoms. Cool the patient with cool IV fluids and high fresh gas flow rates, and use abdominal lavage for open abdominal procedure.
Surgical planning: If a subtotal thyroidectomy is performed, damage to the recurrent laryngeal nerves and swelling can contribute to airway obstruction at extubation. The proximity of the thyroid gland to the parathyroid glands makes hypocalcemia a possible postoperative complication if a bilateral thyroidectomy is performed. Close attention needs to be paid to calcium levels postoperatively, with supplementation provided as necessary. The patient is placed on a seizure watch postoperatively and monitored for any neurologic signs.

2. Anesthetic protocol

It is important the anesthetist thoroughly understands all systems affected and the level of involvement of each system, and whether all of these conditions are managed. Premedication with opioids alone in the hyperthyroid cat tends to make most cats euphoric; this may or may not alleviate agitation to facilitate IV catheterization, although inclusion of eutectic mixture of lidocaine and prilocaine cream may help in this regard [29]. High doses of opioids will result in dysphoria [30,31]. Alfaxalone IM is considered as a premedication in fractious cats. In the case of significantly unmanageable patients, inclusion of dexmedetomidine may render the patient more manageable and decrease the HR. Smooth, short‐acting induction agents such as propofol or alfaxalone IV given slowly to effect are warranted. The anesthetist includes a benzodiazepine to reduce the amount of induction agent needed. Cats which show structural cardiac changes are induced with etomidate or alfaxalone and a benzodiazepine. Anesthetics causing sympathetic stimulation are avoided (i.e., ketamine). Unfortunately, there are very few options to allow for a reduction in MAC in cats, but an opioid CRI will provide analgesia even in light of a vaporizer setting that is not reduced. Local blocks, such as an epidural, if warranted, are considered.

An ECG is vital to detect arrhythmias, and an arterial line gives continuous information on blood pressure. Caution is taken when placing arterial lines in the feline patient, due to a decrease in collateral blood supply and a higher risk of necrosis occurring after placement. It is also important to monitor body temperature closely.

F. Hypothyroidism

Hypothyroidism, manifested as a deficiency in thyroid hormone, is one of the most common diseases seen in older dogs. The function of the thyroid gland is to maintain a normal metabolic state, and therefore hypothyroidism results in a decreased metabolic state. Common clinical signs include lethargy, depression, intolerance of cold, and obesity. Cardiovascular changes secondary to hypothyroidism include bradycardia and decreased CO due to decreased myocardial contractility. A decreased metabolic rate makes patients more sensitive to anesthetics, as the drugs are not metabolized as quickly. Common BW abnormalities include a nonregenerative anemia, elevation in cholesterol, triglycerides, and a decrease in total T4. For a full discussion on diagnosis of hypothyroidism utilizing thyroid assays, the reader is referred to other texts.

1. Anesthesia concerns/common complications

Delayed gastric emptying: With the overall slowdown in metabolism, the GI tract does not empty in a timely manner. Even with fasting, the anesthetist is prepared for regurgitation. Gastroprotectants are included in the anesthesia protocol to help decrease the risk of vomiting and nausea.
Drug dosages: Use lower drug doses, drugs to “effect” or reversible, short‐acting drugs when possible.
Hypotension: Patients which have not returned to normal T4 levels have multiple cardiovascular issues, but hypotension frequently manifests itself under anesthesia, due to the increased drug sensitivity coupled with an already decreased CO and decrease in intravascular volume. These patients are minimally responsive to traditional management of hypotension with positive inotropes due to alteration in beta‐adrenergic receptors. This leads to a decrease in response to catecholamines and anticholinergics. It is advised that hypothyroid patients are managed for their disease prior to elective procedures, because of the uncertainty in blood pressure management.
Hypothermia: Core temperature is poorly maintained in these patients due to a decrease in metabolism. Heat support is provided perioperatively.
Prolonged recovery: Slow drug metabolism and hypothermia both prolong recovery in these patients.

2. Anesthetic protocol

The modifications to a routine anesthesia protocol depend on how well a patient’s hypothyroid disease is controlled; in patients that are fully stabilized, minimal changes are necessary. It is recommended that a patient be on treatment for several weeks prior to anesthesia for an elective procedure and a T4 checked prior to anesthesia.

Patients with a degree of unmanaged hypothyroidism will present the anesthetist with some challenges. Lower drugs doses are generally given due to the decrease in metabolic rate. Preferentially, the anesthetist selects drugs that are short‐acting and reversible. Often times, only a low dose of an opioid (no sedative) is required for premedication, although this is often combined with an anticholinergic to prevent opioid‐induced bradycardia. Preoxygenation and preinstrumentation are important for these patients.

Ketamine is considered the induction agent of choice, if not otherwise contraindicated, to increase HR and stimulate release of any catecholamines present. This will also help with a transient increase in BP. This agent is combined with a benzodiazepine for muscle relaxation. A CRI of ketamine will balance the anesthesia technique. Gas inhalants are used but kept as low as possible due to the patient’s sensitivity to anesthetic drugs; studies in patients with induced hypothyroidism did not show a decreased requirement in isoflurane MAC concentration [32]. A short‐acting opioid CRI such as remifentanil or fentanyl is included to allow the anesthetist to keep the inhalant levels low. High doses of opioids are avoided due to the bradycardia that might ensue which may be less responsive to interventions with anticholinergics and catecholamines. Ephedrine is useful in management of intraoperative hypotension and bradycardia.

Close monitoring of the patient is warranted throughout the prolonged recovery period.

G. Pheochromocytoma

Pheochromocytomas arise from the chromaffin cells of the adrenal medulla and are catecholamine‐producing adrenal tumors. Often these tumors are found incidentally. Patients frequently have indistinct signs and it is not until a patient is anesthetized that the ramifications of these tumors come to light. Clinical signs include lethargy and weakness, inappetence and weight loss, vomiting, and PU/PD. On PE, panting, pale mucous membranes (secondary to vasoconstriction), tachycardia, and a possibly a fever are present. BW changes include anemia, a stress leukogram, increased liver enzymes (ALP, ALT, AST), azotemia, and electrolyte alterations. If there is a suspicion of an adrenal tumor, a complete workup before anesthesia is indicated, including an abdominal ultrasound and/or CT to determine the degree of vascular involvement before surgical planning.

1. Anesthesia concerns

Hemorrhage: This tumor is highly invasive and any attempts to remove it may result in massive hemorrhage. It is prudent to blood type and cross‐match these patients and ensure that blood products are available. Two large‐bore IV catheters are placed in case blood products are necessary.
Hypertension and tachycardia: The secretion of epinephrine (canine) or norepinephrine (feline) results in profound vasoconstriction (via stimulation of the alpha‐adrenergic receptors), as well as tachycardia. The body compensates for this unrelenting increase in vascular tone by decreasing circulating volume. The only relief for the vascular system is stimulation of the beta receptors by catecholamine release, which results in some small degree of vasodilation. When these patients are anesthetized, the ensuing vasodilation in light of the decreased effective circulating volume may result in cardiovascular collapse. To reduce this possibility, a patient with a pheochromocytoma is administered alpha‐adrenergic‐blocking drugs such as phenoxybenzamine prior to their anticipated anesthesia, with the intention of allowing them relief of vasoconstriction in order to restore effective circulating volume. This has significantly improved outcome [7]; ideally, this is done for 14 days prior to anesthesia. It is inappropriate to administer a beta‐blocker (i.e., esmolol) to reduce tachycardia if the patient is not alpha‐blockaded, as the beta‐blockade may worsen hypertension by blocking the beta smooth muscle effects.
Impaired venous return: This tumor is very invasive into surrounding vasculature, including the vena cava. In addition to possible intraoperative blood loss, there is also concern regarding the degree of impairment on venous return to the heart and compounding compromise to CO.

2. Anesthesia protocol

Premedication with a full mu agonist opioid (hydromorphone, methadone) will provide sedation, preoperative analgesia and a reduction in the drugs necessary for induction and maintenance of anesthesia. If a sedative is necessary, benzodiazepines have the lowest number of cardiovascular side‐effects. As much monitoring and support equipment as possible is pre‐placed in the awake patient, using things such as EMLA cream to facilitate arterial line placement and a large‐bore second catheter. Minimizing time under anesthesia is critical.

The only induction agent contraindicated is ketamine. Due to its very safe profile, a neuroleptic induction with fentanyl and a benzodiazepine is often chosen in these cases. Additional alfaxalone or propofol are titrated only to effect in order to intubate the patient. Lidocaine is used on the larynx to help with intubation. Maintenance with inhalant and a balanced technique with an opioid and lidocaine (dogs only) CRI is suitable. Intraoperatively, in a patient which has received phenoxybenzamine, esmolol is used to address tachycardia, and nitroprusside is administered to reduce hypertension. Magnesium is also considered for tachycardia and blood pressure management as nitroprusside may be extremely expensive and is somewhat difficult to dose in smaller patients. Acepromazine is avoided due to the alpha‐blockade already in place with phenoxybenzamine. Blood for transfusion must be available.

The patient will continue to have high levels of circulating catecholamines for several days, so the postoperative period is a critical time for these patients; as much monitoring as possible is continued (i.e., arterial line is left in). Glucose is monitored postoperatively as hypoglycemia is a possible complication. Continuous ECG and blood pressure monitoring is advised.

H. Insulinoma

Insulinoma is a functional tumor of the beta cells of the pancreas that secrete insulin. In a normal patient, a glycemic feedback mechanism allows for proper secretion of insulin due to a higher concentration of glucose in the body. An insulinoma secretes insulin regardless of this glycemic feedback mechanism, therefore hypoglycemia will ensue. This usually occurs in middle‐aged to older dogs.

Signs are usually neurologic due to the severe hypoglycemia, including trembling, confusion, hunger, and nervousness to more extreme neurologic signs, such as seizures. Mild signs usually resolve without intervention. With extremely severe hypoglycemia, cardiac and respiratory arrest can occur. There are usually no major abnormalities noted on PE, but BW will show hypoglycemia. A fasting hypoglycemia with inappropriate high serum insulin concentrations will diagnose an insulinoma. A full workup includes abdominal ultrasound, thoracic radiographs, and possibly a CT for surgical planning.

Insulinomas have a high risk of metastasis at the time of diagnosis. Therefore, hypoglycemia and neurologic signs often continue even after surgery or several months postoperatively. Surgery is rarely curative and long‐term medical treatment is usually necessary to prevent hypoglycemic crises from occurring.

1. Anesthesia concerns

Hypoglycemia: Avoiding severe hypoglycemia is extremely important. These patients should not be fasted overnight; instead, small, frequent meals are given until 6 hours before surgery.
Additional medication: Prednisone is administered to antagonize insulin and therefore decrease hypoglycemia. Diazoxide, a benzothiadiazide, may also be administered as it prevents K‐ATP channels on beta plasma cell membranes from closing. This prevents the cells from depolarizing and reduces the amount of insulin secreted.
Target blood glucose: These patients have usually adapted to lower BG concentrations that run between 40 and 60 mg/dL, while the patient remains asymptomatic. It is not recommended to push these patients to “normal” blood glucoses (>80 mg/dL) with dextrose or dextrose CRIs as this may result in the release of insulin, therefore making hypoglycemia worse. Dextrose is reserved if neurologic signs are noted, or if BG drops below the normal insulin level for the specific patient undergoing anesthesia (i.e., there is no “one size fits all” guideline). Glucose is monitored frequently in these patients.

2. Anesthesia protocol

Glucose monitoring intraoperatively occurs every 30–60 minutes. Any balanced isotonic crystalloid fluid is used; dextrose at 2.5–5% is added only if the patient has a fall in BG compared to their resting BG. The goal is to maintain the BG roughly at 50–60 mg/dL, keeping in mind some patients tolerate BG as low as 30–40 mg/dL. Glucagon CRI at 5–40 ng/kg/min (bolus of 50 ng/kg IV) is another treatment option. It is important to measure the BG when surgical manipulation of the tumor is occurring as this may cause release of insulin and therefore worsen hypoglycemia. Drugs that increase cerebral metabolic rate are avoided.

Premedication with full mu opioids are a reasonable choice and will help decrease sympathetic stimulation (which can cause an increase in BG, therefore releasing insulin, thus leading to a further decrease in BG). Gastroprotectants (maropitant and famotidine or pantoprazole) are also included. Induction with propofol or alfaxalone combined with a benzodiazepine or neuroleptanesthesia are all acceptable for these patients.

If possible, monitoring equipment is placed while the patient is preoxygenated. A sampling line is also appropriate due to frequent BG monitoring. An opioid CRI will help to reduce inhalant requirement. Some anesthetists will also include an alpha‐2 agonist as it may suppress insulin secretion and increase plasma glucose [33]. Adequate perfusion to the pancreas throughout the procedure is important to help prevent pancreatitis. The MAP is kept over 60 mmHg and hypoxemia is avoided. BG monitoring should continue postoperatively, and the patient is monitored for neurologic signs and seizures.

III. Hepatic function diseases

Hepatic function is of critical importance to the anesthetist. It is responsible for protein synthesis, drug metabolism, glycogen storage, and production of coagulation factors. Certain disease states will alter hepatic leakage enzymes; these disease states must be very advanced to alter hepatic function, however. Therefore, the focus of this section is on disease that impairs hepatic function.

Portosystemic shunts (PSS) are prototypical examples of such alterations, but things like end‐stage cirrhosis or liver damage secondary to drug (i.e., NSAID) overdose result in similar changes. On PE, the patient with PSS is dull, small in size and may have a history of seizures. Dullness is often due to hepatic encephalopathy; medical management is warranted prior to anesthesia to resolve hepatic encephalopathy. On BW, anemia, an increase in ammonia and bile acids, and a decreased BUN, BG, albumin, cholesterol, and prolonged coagulation times are present. Urinalysis may reveal ammonium biurate crystals, which result in uroliths.

1. Anesthetic concerns/common complications

Congenital abnormalities: In any animal presenting with a congenital abnormality such as a PSS, a particularly thorough PE is warranted to ensure there are no congenital abnormalities of other body systems, such as the cardiovascular, respiratory, and renal systems.
Drug metabolism: Select drugs that are extrahepatically metabolized or short‐acting/reversible. Drugs dependent on the liver for metabolism have a more profound effect for a longer duration in patients with insufficiencies. Barbiturates are contraindicated. If the patient has neurologic hepatic encephalopathy due to a PSS, benzodiazepines are controversial, due to an increase in endogenous benzodiazepines and altered central nervous system receptors to benzodiazepines. By giving more benzodiazepines, the patient may become more dysphoric or even comatose for a prolonged duration. Lidocaine may rapidly accumulate in patients with extreme liver insufficiencies.
Hemorrhage (see Chapter 3 “Hemorrhage,” p.64): The liver is extremely vascular. Liver biopsies and PSS ligations all have the potential for significant hemorrhage. This is compounded by the concurrent coagulation disorders of PSS. Two IV catheters (one large gauge) are recommended; the patient is blood typed and/or cross‐matched before surgery. Coagulation times may be performed based on clinician preference; many cases of PSS are presumed to have coagulopathies.
Hypoglycemia: Monitor BG every 30–60 min. Supplement fluids with 2.5–5% dextrose as necessary.
Hypotension: Maintain hepatic profusion (i.e., blood pressure). These patients are commonly hypotensive when maintained under general anesthesia. Monitoring IBP is ideal, but as clotting times are increased, an arterial catheter may be contraindicated. Hypotension from anesthetics is compounded by the reduction in oncotic pressure secondary to the decrease in protein production; often, plasma is the fluid of choice for patients where albumin is low (give plasma if albumin is less than 2.0 g/dL). Alternatively, a hetastarch bolus of 2–5 mL/kg is an option. However, hetastarch may worsen coagulopathies if given at higher doses; a total dose of 0.8 mL/kg/h is not exceeded. Positive inotropes and reducing MAC requirements with fentanyl or remifentanil CRIs will help reduce the risk of hypotension.
Hypothermia: The small size of these patients and the use of a nonrebreathing circuit make thermoregulation very difficult for the anesthetist. Circulating warm water blankets, forced air warming devices, and plastic wrap may reduce the degree of hypothermia, but the patient is unlikely to be normothermic if anesthetized for a procedure involving an open abdomen. Warming the patient after premedication is extremely important in these cases and warm abdominal lavage is utilized.
Mentation: Patients which present clinically with hepatic encephalopathy are medically managed prior to anesthesia. If the patient is not medically managed, recovery in these patients is often markedly prolonged and may require mechanical ventilation. If the patient has seizures, antiseizure medication (such as levetiracetam) is recommended prior to surgery.

2. Anesthetic protocol (Table 5.7)

Agents metabolized by the liver (e.g., barbiturates and phenothiazine tranquilizers), highly protein‐bound agents (e.g., diazepam and barbiturates), and hepatotoxic agents (e.g., halothane) are avoided because of poor hepatic function and hypoalbuminemia. Use reversible, short‐acting, extrahepatically metabolized drugs when possible. An opioid appropriate for the level of pain anticipated +/− an anticholinergic (as necessary) are administered as a premedication. Inductions with propofol or alfaxalone are smooth and both have extrahepatic metabolism [34]. Although isoflurane is considered the inhalant of choice in patients with liver disease due to its low liver metabolism, in reality sevoflurane is only marginally more metabolized by the liver and the two are likely comparable. Remifentanil, if available, is metabolized by plasma esterase and therefore is not dependent on the liver for termination of effect, making it a suitable choice to reduce inhalant levels and provide analgesia. In practicality, fentanyl CRIs are often used but this drug does require liver metabolism. Check blood glucose periodically throughout surgery and supplement with dextrose as needed.

The patient’s mentation and degree of hypothermia will affect recovery, which is likely to be prolonged.

IV. Neurological disorders: Intracranial disease

The calvarium is a fixed space and as such, there is a balance of tissue (the brain itself) and fluid (i.e., blood and cerebral spinal fluid [CSF]) within that space. If one of the components increases, another must decrease or an increase in intracranial pressure (ICP) will result. The concern for patients with intracranial disease is the increase of ICP causing herniation of brain tissue, resulting in death. Unfortunately, anesthetizing these patients disrupts cerebral blood flow, one of the three major components in the calvarium, and this becomes critical in patients which already have a disrupted balance (i.e., intracranial disease).

A thorough history and neurologic exam as part of a complete PE helps to identify the presence, frequency and duration of seizures, trauma or a neurotoxin in cases of intracranial disease of unknown cause. The following are signs of increased ICP: altered levels of consciousness, miosis, mydriasis, anisocoria, decreased pupillary reflex, papilledema, and Cushing reflex which includes bradycardia, hypertension, and breathing disturbances. CBC, chemistry, and urinalysis may be normal but will help to identify any contributing comorbidities.

1. Anesthetic goals/special considerations

Autoregulation: Inhalant anesthesia will disrupt the brain’s normal ability to autoregulate its own perfusion pressure; inhalants allow for a dose‐dependent increase in cerebral blood flow (CBF) by causing vasodilation, which is detrimental to these patients. Thus, a reduction in inhalant requirement is a primary goal.

Cerebral metabolic rate (CMRO ₂): One of the anesthetist’s goals for these patients is to reduce CMRO₂. In general, anesthesia does this. The anesthetist, however, is careful to avoid dissociative anesthetics (ketamine) which may increase the CMRO₂. Allowing the patient to become mildly hypothermic (95–96.8 °F) will also reduce CMRO₂.

Table 5.7 Hepatic disease.

Premedication (mg/kg)		Induction (mg/kg)	Maintenance	Intraoperative analgesia (mg/kg/h)	Postoperative analgesia (mg/kg)
Opioid (select one) (a) Methadone 0.3 IM, 0.2 IV OR (b) Morphine 0.5 IM OR (c) Hydromorphone 0.05–0.1 IV, IM OR (d) Fentanyl 0.002–0.005 IV OR (e) Butorphanol 0.1–0.4 IM, IV OR (f) Buprenorphine 0.01–0.03 IV, IM	Other medications: Avoid sedatives, if possible, reduce doses if necessary (a) GI protection: (i) Famotidine 1 IV or SC AND (ii) Maropitant 1 IV or SC	(a) Propofol 2–4 IV to effect OR (b) Alfaxalone 1–3	(a) Isoflurane OR (b) Sevoflurane AND Opioid CRI for reduction of inhalant requirement	(a) Opioid CRI (i) Fentanyl 0.012–0.042 OR (ii) Remifentanil 0.012–0.042 +/− (b) Local regional block	(a) Fentanyl CRI 0.002–0.005 mg/kg/h OR (b) Intermittent bolus: (i) Methadone 0.5 IM q 4–6 h OR (ii) Buprenorphine 0.01–0.03 IV, IM +/− (c) Liposomal encapsulated bupivacaine infiltrative block 5.3 mg/kg

“AND” means that multiple choices from this column are selected, “+/−” means it is up to the anesthetist if they would like to combine multiple choices, “OR” means that the anesthetist will choose one or the other selection. For example, the postoperative analgesia column would be interpreted as including an opioid from the options (either as a CRI or intermittently given), with an optional long‐acting local anesthetic block.

In patients who received pantoprazole for greater than 36 hours, pantoprazole will replace famotidine.

CRI, constant‐rate infusion; GI, gastrointestinal; IM, intramuscular; IV, intravenous; SC, subcutaneous.

Glycemic control: Hyperglycemia worsens neurologic outcomes in people, but hypoglycemia is not beneficial either [35]. Little information on animals is available in this regard. It is best for the anesthetist to monitor BG, with a target goal of maintaining euglycemia through the use of regular insulin if necessary. The BG range should be maintained between 80 and 180 mg/dL.
Hypoxemia: PaO₂ below 60 mmHg will trigger more blood flow to the brain in order to maintain perfusion. If hypoxemia is present, careful steps are taken to determine the underlying cause and begin treatment (see Chapter 3 “Hypoxemia,” p.66).
Cushing reflex: This reflex occurs in response to significant increases in ICP, and heralds impending brain herniation; as such, it is a life‐threatening emergency. It is easily recognized by profound bradycardia and significant hypertension. Any anesthetic utilized is immediately discontinued. Treatment involves reducing ICP by giving 4 mL/kg hypertonic saline over 2–5 min or mannitol 0.5–1.5 g/kg over 10–20 min. The patient is repositioned with the head raised at 30° above the heart, and the head is packed with ice packs. Ventilation targets an EtCO₂ of 30–35 mmHg throughout anesthesia. Severe hypocarbia (<30 mmHg) is also avoided as this reduces perfusion to the brain which leads to ischemic injury. Any compression around the neck (e.g., positioning devices) is removed. Anticholinergics are avoided as a decrease in HR is the physiologic response to the massive increase in mean arterial blood pressure. Giving anticholinergics will cause a tachycardia (and possibly worsen hypertension) that will mask if a true Cushing reflex is occurring. A neurologist may also choose to include corticosteroids in the treatment plan and is consulted in this regard.
ICP: The target goal of maintaining as normal ICP as possible is achieved through several practical steps.

‐ Avoid vomiting. This is done by choosing opioids such as methadone, and avoiding opioids such as hydromorphone or morphine, as well as alpha‐2 agonists. Gastroprotectants (such as maropitant) are included in the protocol to help decrease the risk of vomiting. An increased risk of aspiration pneumonia has been associated with neurosurgery and neurologic disease in dogs [36]. This is also similar in cats, with 7% of cats with neurologic disease developing aspiration pneumonia [37]. The risk of pneumonia in patients which underwent multiple anesthetic procedures for radiation treatment was reduced by several changes in the anesthetic protocol including decreased use of anticholinergics, decreased use of pure mu opioids, changing the patient’s position (both during intubation and during recovery), when the cuff of the ET tube was inflated, as well as using an aseptic technique with the equipment used for intubation [38].
‐ Avoid jugular compression.
‐ Avoid coughing or gagging, especially when the patient is intubated. Lidocaine is applied topically to help decrease the sensitivity of the larynx.
‐ Avoid fluid overload.
‐ Position patient with the head elevated (ideally 30° degrees above heart).

Seizures: Seizures are detrimental in that they increase CMRO₂. Seizures may occur at any time during anesthesia and will go unnoticed in the adequately anesthetized patient. Diazepam or midazolam 0.5–1 mg/kg IV are used as treatment.
Ventilation: Changes in EtCO₂ will also impact CBF. As CO₂ increases, so does CBF. Unfortunately, too low CO₂ may result in cerebral ischemia. Maintain PaCO₂ between 32 and 38 mmHg and EtCO₂ between 30 and 35 mmHg. Mechanical ventilation is often necessary for this maneuver.

2. Anesthetic protocol (Table 5.8)

Sedation is often unnecessary for IV catheterization; however, if IM premedication is needed, avoid drugs that cause vomiting and extreme respiratory depression. Methadone 0.2–0.5 mg/kg IV or IM (lower doses needed for IV dosing) with midazolam 0.1 mg/kg if needed are used for premedication. If an IV catheter is placed, a fentanyl bolus at 0.0025–0.005 mg/kg is given IV and a fentanyl CRI is continued throughout the procedure. Preoxygenate all patients prior to induction.

The induction drugs of choice are propofol or alfaxalone given slowly IV to effect, but etomidate will reduce CBF as well. However, etomidate is avoided in patients with a seizure history. Anesthesia is maintained with a propofol or alfaxalone CRI with/without lidocaine CRI. Alfaxalone CRIs are considered for patients with cardiac disease or in cats. A rougher recovery has been noted with patients recovering from alfaxalone CRIs [39,40]. If mechanical ventilation is utilized, an opioid CRI such as remifentanil or fentanyl will reduce the amount of drug necessary to maintain immobility. If it is not possible to maintain the patient solely on an opioid and propofol or alfaxalone CRI, inhalants are added but kept at less than 1 MAC. It is ideal to monitor blood gas analysis to ensure PaCO₂ is within optimal range (32–38 mmHg); however, it is often more practical to monitor EtCO₂. IBP is preferred so a Cushing reflex is detected early and reliably.

3. Key points

In cases of traumatic brain injury, complications arise both from the direct injury itself and the brain’s response to this injury (swelling and inflammation, vasospasm, etc.). These cases are not anesthetized unless absolutely necessary; indeed, a large portion of things that would normally require anesthesia are performed without anesthesia due to the degree of obtundation of the patient. Mannitol is avoided in these patients as they often have a disrupted blood–brain barrier and therefore mannitol may worsen cerebral edema; hypertonic saline may be a better choice for these patients. Oxygen support is provided to these patients.

V. Renal insufficiencies

From the perspective of the anesthetist, the kidney serves to eliminate fluids and drugs after they are metabolized by the liver; however, the kidney’s role physiologically is much broader and the reader is referred to other texts for a more thorough discussion on the many functions of the kidney.

Renal disease is a broad term indicating dysfunction of the kidney; the scope of this dysfunction ranges from renal insufficiency to true renal failure. Indeed, clinical signs (CS) and BW changes are slow to manifest because of the kidney’s incredible reserve. By the time renal failure is diagnosed, over 75% of the kidney’s functional capacity has been lost (which is when BUN and creatinine begin to elevate). Renal failure results from a number of causes, including toxins, parathyroid disease and concurrent hypercalcemia, infectious disease, sepsis, and idiopathic renal failure. Disturbingly, feline patients which have undergone anesthesia within the previous year have an increased risk of developing CKD [41,42], highlighting the need to perform high‐quality anesthesia in our feline patient population.

Table 5.8 Intracranial disease.

Premedication (mg/kg)		Induction (mg/kg)	Maintenance (mg/kg/h)	Intraoperative analgesia (mg/kg)	Postoperative analgesia (mg/kg)
Opioid (select one) (a) Methadone 0.3 IM, 0.2 IV OR (b) Butorphanol 0.2–0.4 IV, IM OR (c) Buprenorphine 0.01–0.03 IV, IM	Other drugs: Sedative (if needed) (a) Midazolam 0.1 IM, IV AND (b) GI protection: (i) Famotidine 1 IV or SC AND (ii) Maropitant 1 IV or SC +/− (c) Mannitol 0.5–1.5 g/kg over 15–20 min	(a) Propofol 2–4 IV +/− Midazolam 0.1–0.2 IV OR (b) Alfaxalone 1–3 IV +/− Midazolam 0.1–0.2 IV	(a) Propofol CRI 12–24 OR (b) Alfaxalone CRI 4.2–9 OR (c) Isoflurane <1 MAC OR (d) Sevoflurane <1 MAC +/− CRI for reduction of inhalant requirement	(a) Intermittent bolus: (i) Methadone 0.3 IV q 4–6 h OR (ii) Fentanyl, 0.005 IV as needed OR (b) Fentanyl or remifentanil CRI 0.01–0.02	(a) Intermittent opioid bolus: (i) Methadone 0.1–0.5 IM q 4–6 h OR (ii) Buprenorphine 0.01–0.03 IM, IV OR (c) Fentanyl CRI 0.002–0.005

“AND” means that multiple choices from this column are selected, “+/−” means it is up to the anesthetist if they would like to combine multiple choices, “OR” means that the anesthetist will choose one or the other selection. For example, the postoperative analgesia column would be interpreted as including an opioid from the options either as a CRI or intermittently given.

In patients who received pantoprazole for greater than 36 hours, pantoprazole will replace famotidine.

Continuously observe patient after premedication. Because steroids may be necessary to reduce swelling, avoid NSAIDs. Preoxygenate patients. Lidocaine CRI is contraindicated in cats.

CRI, constant‐rate infusion; GI, gastrointestinal; IM, intramuscular; IV, intravenous; MAC, minimum alveolar concentration; SC, subcutaneous.

Renal failure may be acute or chronic. It is unusual for the anesthetist to be presented with a patient in acute renal failure, as this condition is possibly reversible and thus anesthesia (which will worsen renal perfusion) is contraindicated in these patients. The one exception to this is the patient with a ureteral obstruction, which is discussed under anesthetic considerations.

This section will focus on chronic renal failure, a permanent and ultimately fatal disease. Clinical signs of chronic renal failure include depression, weight loss, vomiting, and PU/PD. As uremia (a systemic disease resulting from the accumulation of toxins) develops, mentation becomes dull (uremic encephalopathy) and there is an odor to the pet’s breath. On PE, kidneys are palpably small and oral ulcerations are often present.

On auscultation, cardiac arrhythmias are present in some of these patients; indeed, veterinary medicine is beginning to appreciate the pathologic interplay between the two organ systems [1]. This connection between these systems occurs in both healthy patients and those with disease. In human medicine, the pathophysiology of these two systems is named the “cardiorenal system” [1]. For both systems, it is important that blood pressure is evaluated prior to inducing anesthesia. BW often reveals an anemia, azotemia (elevations in BUN and creatinine), acidosis, hypoproteinemia, hyperamylasemia, electrolyte changes (including hypokalemia, hyperphosphatemia, hypocalcemia, hypermagnesemia), and a possible hyperglycemia.

1. Anesthesia concerns/common complications

Acidosis: A venous or arterial blood gas provides the patient’s current pH. If pH is below 7.1, therapy is warranted. Acidosis alters many of the anesthetic drugs administered, changing their availability due to the level of ionized versus nonionized forms. In order to provide treatment, the anesthetist must first identify if the acidosis is primarily metabolic (common in chronic renal failure) or respiratory in nature; please see Chapter 3, p.72 for more information.
Drugs selection: Drugs dependent on renal elimination for termination of their effect are minimized or avoided in these patients. This includes ketamine in cats (which excrete norketamine, an active metabolite, unchanged) and some muscle relaxants (e.g., vercuronium). Midazolam does not have active metabolites reliant on renal excretion and so may be a more suitable choice than diazepam. Etomidate results in hemolysis, which is difficult for the kidney, and is therefore avoided in renal failure patients unless absolutely necessary due to severe cardiac disease. Regardless of which drug is selected, it is prudent to decrease the dosage or titrate the drug to effect, as many of the anesthetic agents are protein bound and renal failure patients are often hypoproteinemic. Uremic patients, possibly due to their neurologic changes, also appear particularly sensitive to anesthetic drugs.
Fluid balance: These patients often do not tolerate changes in volume well. Placement of a urinary catheter allows for measurement of urine output, which is anticipated to decrease with anesthesia. It is often helpful to hospitalize patients undergoing anesthesia for an elective procedure the night prior to anesthesia for diuresis with maintenance IV fluids.
Renal function: General anesthesia is likely to result in a transient decrease in renal function via indirect mechanisms such as changes in renal perfusion and sympathetic stimulation secondary to surgery, regardless of agent and management. Appropriate management limits the impacts and hopefully prevents permanent damage.
Renal perfusion: While several drugs are utilized to support the kidney, arguably the best strategy for preventing further kidney deterioration is maintaining renal perfusion. Because the kidney’s ability to autoregulate its own perfusion is lost in response to anesthesia, maintaining normal BP is of the utmost importance in these animals. Dopamine or dobutamine may help maintain renal perfusion by providing inotropic support. Mannitol and fenoldopam are also considered for patients with renal disease. Fenoldopam is a selective DA‐1 receptor agonist which can help increase renal blood flow (RBF) and natriuresis and possibly provide renal protection against acute renal injury [43, 44].
Vascular access: Cats with chronic renal failure (CRF) have vessels that are friable, often bleeding profusely when the tip of an IV catheter comes into contact with them. It is prudent for experienced personnel to attempt catheterizing these challenging vessels.
Urethral obstruction: This is not considered a true renal insufficiency or chronic renal disease, but rather a postrenal azotemia. However, if not treated, it can lead to acute or chronic renal impairment [45]. This is an emergency that may present most commonly in male cats (and sometimes dogs). These patients must be sedated or anesthetized (after they are stabilized) in order to unblock the urethra. These patients usually present with some degree of azotemia, electrolyte abnormalities (hyperkalemia being the most life‐threatening), and a firm bladder on palpation with the inability to excrete urine. Other presenting signs may include cardiovascular collapse, dehydration, hypovolemia, and a metabolic acidosis [46]. These patients require immediate intervention, especially the correction of abnormal electrolytes (see Chapter 3, “Treatment of hyperkalemia,” p.79). Both surgical and medical interventions have a high morbidity and mortality rate in cats. An increase in the patient’s age, a higher ASA status, preoperative bicarbonate administration, hyperkalemia, and an increase in creatinine were associated with increased anesthetic risk. The most common anesthetic complications included bradycardia, hypotension, and hypothermia [46]. These patients are not given any sedative or cardiodepressive drugs until they are stabilized and their potassium is within normal range.

2. Anesthetic protocol

Avoiding sedatives as part of the premedication is appropriate in these patients. In the patient where an opioid alone will not facilitate handling, midazolam (if the temperament is appropriate) does not rely on the kidney for elimination of active metabolites and is added. Acepromazine is considered for sedation in small doses since slight vasodilation may increase RBF in the canine patient. Induction drug selection is often based on the lesser of all evils; alfaxalone is probably the best choice, but slowly titrating the drug is key to preventing worsened kidney disease. If midazolam was not used for a premedication, it is used as part of the induction to help decrease the dose of alfaxalone needed. This helps decrease the hypotension seen with induction agent administration.

Isoflurane is the inhalant of choice for maintenance of anesthesia. The breakdown of sevoflurane results in the release of inorganic fluoride ions that are toxic in high levels. Compound A, another renal toxic degradation product of sevoflurane (when in the presence of soda lime), is present in laboratory animals (rats). However, other species failed to demonstrate significant production. Therefore, if only sevoflurane is available, this is an acceptable alternative. Reducing the amount of inhalant necessary through the use of an opioid, such as fentanyl, CRI is warranted. Opioid epidural is controversial due to the possibility of urine retention. The author finds performing an opioid epidural is appropriate for painful abdominal procedures, including those that involve patients with questionable renal function, and inclusion of a local anesthetic in the epidural is rarely contraindicated. Epidurals greatly reduce MAC requirements of inhalant anesthetics. Alternatively, a transversus abdominis plane (TAP) block is performed without concerns for urinary retention and still provides sufficient analgesia for abdominal procedures.

The choice anesthetic for fluid maintenance is a balanced isotonic solution, which is buffered and should not worsen acidosis. Due to skeletal muscle weakness and sensitivity to respiratory depressants, the anesthetist supports ventilation. Fenoldopam may have a renal protective effect, but currently is an expensive agent for routine use [44]. Use of dopamine may help renal perfusion in our canine patients; there is controversy about the presence of dopamine receptors in the cat kidney [47], although dopamine for positive inotropic support is often utilized in this species.

VI. Respiratory/pulmonary disease

A. Upper airway disease

Upper airway disease involves the nasal passages, pharynx, and larynx. The most classic example of upper airway disease is the dog with brachycephalic airway syndrome, but laryngeal paralysis, tumors of the upper airway, foreign bodies, and swelling from trauma or snake bite all result in upper airway disease. See Chapter 4 for specific concerns for procedures involving the upper airway.

1. Anesthesia concerns/common complications

Minimize stress: These patients are often maximally compensating for their airway disease; additional stress will significantly worsen the obstruction and may lead to respiratory arrest.
Obstruction: An obstruction of the airway can occur preoperatively or postoperatively (Figure 5.1). In certain patients (i.e., brachycephalic patients), the animal may live their whole life with an obstruction. However, premedication and sedation worsen this obstruction. Postoperatively, while these patients are recovering, sedation contributes to their inability to maintain a patent airway. Obstruction increases work of breathing, which will eventually exhaust respiratory muscles. Hypoxemia also results from airway obstruction. Preoxygenation is always warranted in these cases, as is prolonged extubation.
Securing an airway: Depending on what type of upper airway disease is present, routine intubation is a challenge for the anesthetist (i.e., in the patient with a tumor of the pharyngeal region). If this is suspected prior to anesthesia (usually based on stridorous noises), having a variety of ET tube sizes, stylets, retrograde intubation equipment (Table 5.9) and possibly an endoscope may assist the anesthetist in obtaining a difficult airway.

Photo depicts airway obstruction. — **Figure 5.1** Airway obstruction. Source: Courtesy of Anderson da Cunha.

Table 5.9 Retrograde intubation.

Materials:Clippers, scrub, sterile gloves, 18 G needle, guidewire, endotracheal (ET) tubes, laryngoscope
Technique:
1. Clip and prep an area of the skin over the trachea, approximately halfway between the mandible and thoracic inlet. Once the anesthetist is appropriately gloved, ensure the guidewire will fit through the 18 G needle. Another assistant positions the patient and opens their mouth appropriately.
2. Induce the patient.
3. Insert an 18 G needle between tracheal cartilages, directing the bevel cranially.
4. Advance the guidewire through the needle; the wire should course cranially and exit between the arytenoids.
5. Advance the wire out of the mouth and place an ET tube on the wire; the anesthetist will need enough length protruding from the mouth so the guidewire exits the connection on the ET tube.
6. Advance the ET tube down the guidewire. When the ET tube is successfully placed, remove the needle and guidewire. Place a light wrap if desired.

Once the ET tube is placed, presence of EtCO₂ is used to verify accurate placement. A tracheotomy may also be indicated in certain cases.

2. Anesthesia protocol

The objective of premedication is to provide enough sedation to minimize restraint for IV catheterization, while avoiding excessive relaxation that may predispose the patient to airway obstruction. Alpha‐2 agonists are avoided for this reason (e.g., sedation is too profound). Low doses of acepromazine are beneficial to facilitate IV catheterization: however, an opioid without a sedative is a suitable option in manageable patients, provided appropriate antiemetic medication has been administered (i.e., maropitant or ondansetron). Hydromorphone may increase panting and cause vomiting so methadone is often a better choice. Gastroprotectants (such as famotidine 60 min prior to the procedure, or pantoprazole administered regularly for at least 36 h prior to the procedure) are added to reduce the risk of vomiting and the possible consequences if vomiting occurs (i.e., aspiration pneumonia). Preoxygenate the patient prior to induction for at least 5–10 min. Apply ECG and other monitoring equipment if the patient will allow prior to induction.

The goal of induction is rapid airway access (rapid‐sequence induction). Proper use of a laryngoscope and having several sizes of ET tubes readily available assist with this. Inhalants (isoflurane or sevoflurane) are routinely used, although propofol or alfaxalone CRIs (forms of total intravenous anesthesia [TIVA]) are suitable in cases where extubation is anticipated. Opioid, ketamine, and lidocaine CRIs decrease MAC and provide analgesia when appropriate. These animals may require mechanical ventilation.

Recovery takes place in a quiet location where the patient is continuously monitored for respiratory distress. Respiratory distress is more likely to occur in brachycephalic breeds due to bronchospasm, laryngospasm, airway obstruction by the soft palate or swelling laryngeal tissue from traumatic intubation. Corticosteroids are beneficial in treating postoperative complications due to swelling and edema, so NSAIDs are often withheld until after the patient is completely recovered (i.e., the following day), and removed from the plan altogether if corticosteroids were included. Patients are placed in sternal recumbency with head and neck comfortably supported and extended. Leaving the ET tube in place as long as possible (note: this may be hours) is advised with continuous pulse oximetry monitoring. Only after the patient is strongly protesting, able to lift and support the head independently, and can swallow is the ET tube is removed. Continue to monitor SpO₂ and respiratory effort. Be prepared with laryngoscope, smaller ET tubes than originally used, additional induction agent and 100% oxygen to reintubate. In some cases supplemental oxygen via mask is all that is needed.

For extremely prolonged recoveries, reversal of drugs (i.e., opioids, benzodiazepines) given during the procedure is required. Usually, patience and time is the best method of recovery for brachycephalic breeds.

3. Key points

Brachycephalic patients without corrective surgery for brachycephalic syndrome (see Chapter 4, p.104) present an upper airway management challenge for the anesthetist, regardless of the reason for anesthesia (Figure 5.2).

Photo depicts brachycephalic patient. — **Figure 5.2** Brachycephalic patient. Source: Courtesy of Anderson da Cunha.

Brachycephalic patients have a higher perianesthetic and postanesthetic risk compared to nonbrachycephalic breeds. Factors that increased brachycephalic complications perioperatively included a long duration of anesthesia, soft tissue procedures (as opposed to orthopedic or diagnostic procedures), use of ketamine plus a benzodiazepine (as opposed to propofol) for anesthetic induction, and invasive procedures [48].

B. Lower airway disease

Lower airway disease encompasses structures below the level of the larynx (i.e., trachea, bronchi, lungs). Tracheal and large bronchial disease compromise the delivery of oxygen to the lungs; this includes diseases such as tracheal collapse or trauma, foreign bodies, and tumors. Gas exchange takes place in the lungs; diseases resulting in V/Q mismatch or diffusion barrier impairment (e.g., pulmonary edema, pneumonia, and asthma) are lower airway diseases which concern the anesthetist, as hypoxemia and increased work of breathing result. As is true for most patients with compromising disease, a complete PE and diagnostic workup are recommended. Diagnostic workup involves thoracic radiographs, pulse oximetry, and possible blood gas analysis. Depending on the severity of the disease, the patient may need supplemental oxygen even to complete a physical exam.

1. Anesthesia concerns/common complications

Bronchodilation: Anticholinergics are simple bronchodilators the anesthetist already has at their disposal. Anticholinergics result in smooth muscle relaxation through muscarinic antagonism and therefore a degree of bronchodilation [49]. Additionally, bronchodilation is achieved with beta‐2 agonists, which are administered either aerosolized or as an injection. Albuterol or terbutaline are two routinely used bronchodilators. Isoflurane is considered a potent bronchodilator but sevoflurane has mixed results in patients with asthma [50]. Dobutamine (and dopamine at higher doses) are beta‐2 agonists that may exhibit some degree of bronchodilation, though more research needs to be performed in this area.
Minimize stress: Although this is ideal for all patients, patients with respiratory disease are maximally compensating. Additional stress may result in decompensation and respiratory arrest.
NSAIDs: NSAIDs are avoided (unless directly indicated) in patients with airway disease, as steroids may be necessary to address inflammation that is present or iatrogenically induced.
Elective procedures: All elective procedures are postponed until the lower airway disease is managed, or resolved.
Mechanical ventilation (MV): MV reduces the work of breathing, which reduces the respiratory fatigue these animals are experiencing. When setting up the ventilator, it is important to allow for adequate duration of inspiration to maximize time for gas exchange.
Recovery: The patient is recovered in a location where continuous monitoring is possible, and in a quiet, stress‐free environment. Sadly, these two requirements are often mutually exclusive. Supplemental oxygen is provided and the pulse oximeter is continually monitored until the patient maintains a reading of 94–96% or greater without oxygen support.

2. Anesthesia protocol

Premedication includes an opioid and an anticholinergic. If the patient is severely stressed, acepromazine is added to the premedication to help with sedation. While it is prudent to avoid opioids that result in histamine release, opioids as a class decrease tracheal sensitivity, reduce stress, and are antitussive.

All patients with respiratory disease are continually observed after administration of premedication. If the animal is tolerant, begin preoxygenation at least 5–10 minutes prior to induction, and throughout the course of induction. If the patient is cooperative, instrumentation is placed prior to induction (i.e., ECG, NIBP, Doppler). Propofol or alfaxalone are the induction agents of choice for a smooth, controlled induction; however, there are no injectable induction drugs specifically contraindicated in these cases. The addition of a benzodiazepine will reduce the amount of induction agent necessary. Additional induction agent is kept available by the anesthetist, as are the laryngoscope and spare ET tubes of appropriate size, in case the patient requires reintubation during recovery. Maintenance of the patient on inhalants, +/− opioid CRIs, is common. These patients are not extubated unless absolutely required, and if so, are rapidly reintubated if pulse oximeter drops below 94%.

Specialized monitoring equipment includes spirometry loops for volume loop assessment. Otherwise, monitoring for these patients includes monitoring ventilation and oxygenation closely with SpO₂ and EtCO₂ as well as arterial blood gases if possible. Monitoring equipment is left on the patient until it is completely recovered and oxygenating well on its own.

3. Key points

Pulmonary edema in small animal patients is often secondary to cardiovascular disease (i.e., congestive heart failure [CHF]); these patients must have their cardiac disease thoroughly worked up, as well as CHF managed, prior to anesthesia for any elective procedure.

Diuretics: Diuretics, such as furosemide, reduce circulating volume and therefore reduce the amount of pulmonary edema present. Although the patient may already receive diuretics as part of its maintenance therapy, additional diuretics are occasionally necessary intraoperatively due to changes in volume status subsequent to anesthesia. If the patient is already on diuretics such as furosemide, BW to assess electrolytes (particularly potassium) is warranted prior to anesthesia.
Fluid therapy: Fluid therapy is kept to no more than 3 mL/kg/h during anesthesia, to prevent volume overload from worsening this disease.
Ventilation: Intermittent positive pressure ventilation (IPPV) is indicated in these patients, as it might help move fluid out of the tracheobronchial tree as well as recruit compromised alveoli, ultimately increasing lung volume and the area for gas exchange. Sigh breaths (manually administered supramaximal pressures of 25–30 cmH₂O every 4–6 breaths) also assist with this recruitment. Patients in which alveolar recruitment maneuvers are used (increase tidal volume, sigh breaths, positive end‐expiratory pressure [PEEP]) had an increase in PaO₂ and improved lung compliance [51].

Tracheal rupture appears more commonly in the cat than in the dog, and is often associated with previous intubations with a high‐pressure, low‐volume cuffed ET tube. This, coupled with negligent handling (rotating the cat without disconnecting it from the circuit, etc.), may result in a tracheal tear [52]. These patients often present with subcutaneous emphysema after an anesthetic procedure. Further diagnostics to rule out a pneumothorax or pneumomediastinum is warranted; in case of pneumothorax, a chest tap is warranted (see Table 3.10, p.71).

Often times, these patients are medically managed, but if the case is surgical, there are several management steps.

Intubation: The level of the tear is important for the anesthetist, who will preferentially intubate past this tear. Unfortunately, this is sometimes difficult in cases of intrathoracic tracheal rupture. The key lesson here is to never intubate beyond the thoracic inlet in your own patients. When a tracheal tear knowingly occurs, communication between the anesthetist and surgeon is critical as well as quick intervention to correct the positioning of the ET tube [53].
Mechanical ventilation: Leakage of air from the tracheal rupture will alter respiratory compliance, significantly increasing work of breathing and chance of hypoxemia for these patients. MV reduces the work of breathing, and offers recruitment options for compromised alveoli.

C. Space‐occupying respiratory disease

Air, fluid (e.g., chyle, blood, pus, or transudates) or tissue (see Chapter 4 “Thoracotomy,” p.123) accumulation affects the ability of the lungs to expand within the thoracic cavity. Thoracic masses (primary lung tumors such as carcinomas or mediastinal masses such as thymomas or lymphoma) also affect the ability of the lungs to expand, resulting in patients showing signs of respiratory distress. Underlying causes include a traumatic event (e.g., hit by car [HBC]), pathology (e.g., cancer or infectious agents) or idiopathic disease. This is also iatrogenically induced in procedures such as thoracoscopy or a thoracotomy (see Chapter 4, p.123). A thorough PE includes evaluation of respiratory effort and auscultation of the chest for lung sounds; however, in the case of acute respiratory disease, such as HBC, there may not be time to perform such an exam before respiratory arrest ensues.

In the case of trauma, if an animal appears to have difficulty oxygenating (pale, often blue‐gray mucous membrane color [MMC], increased RR, and obtunded mentation), a thoracic focused assessment with sonography in trauma (T‐FAST) scan is performed, if an ultrasound unit is available. This information helps guide a thoracentesis (“chest tap”) with a simple butterfly catheter and large‐volume syringe (see Table 3.10). If a T‐FAST scan is not available, it is far better to attempt to tap air off the chest and find none than to try to perform radiographs in a patient with a pneumothorax. A repeat of the T‐FAST and a more thorough auscultation are performed after the tap. A small amount of air may be introduced into the thorax in the event of negative tap; while this is clinically of little consequence, radiographs may reveal this air and the primary clinician may be misled if they are not informed about a tap.

Blood work, including a CBC, chemistry, arterial blood gas (when possible), chest/abdominal radiographs, and a full thoracic ultrasound, is performed to work up the disease in stabilized patients. A CT scan is also recommended in cases requiring surgical planning. These diagnostics indicate the type of space‐occupying disease present, help rule out possible causes, and quantify the degree of severity.

When air or fluid is the cause of the space‐occupying disease (e.g., pneumothorax or pyothorax), thoracocentesis or chest tube placement is performed immediately following diagnosis. Supplemental oxygen is available to the patient at all times.

1. Anesthesia concerns/common complications

Atelectasis: Space occupation results in lung atelectasis and subsequent hypoxemia. Monitoring at minimum includes frequent blood gas analysis (q 30–60 min), EtCO₂ and SpO₂. IBP, Doppler, and ECG are recommended.
Elective procedures: As with all respiratory diseases, elective procedures are not performed until the underlying disease is addressed and stabilized.
Hypoventilation: Hypoventilation, defined by an increase in CO₂, results from space occupation. Respiratory acidosis is evident as well. In the stable patient prior to anesthesia with gas or fluid present in the chest, a thoracocentesis is performed prior to induction of anesthesia (see Table 3.10). Because hypoventilation results from most anesthetics, it is imperative that any gas or fluid is removed from the thoracic cavity. The anesthetist must not allow residual volume to compound anesthetic hypoventilation.
Hypovolemia: Hemothorax will result in hypovolemia and a decrease in effective circulating volume as well as space occupation. In cases of hemothorax, a cross‐match and/or blood type is performed and blood is available for administration.
Mechanical ventilation: Defining the underlying etiology in cases of pneumothorax is necessary before MV is safely started. For example, in case of trauma, MV may rupture bullae that are currently stable. Therefore, in cases of trauma, it is best to cautiously, manually assist ventilation with a second set of hands rather than place the patient on a ventilator. In other cases of space occupation, MV is safe and often necessary. EtCO₂ target range is 30–35 mmHg. High positive inspiratory pressure (PIP; 20–25 cmH₂O) provides effective ventilation due to the space‐occupying disease (i.e., increased resistance). PEEP valves may need to be used in certain cases, especially those with severe atelectasis and hypoxemia.

2. Anesthesia protocol

Intravenous catheterization is usually accomplished without sedation in compromised patients. However, premedication involving an opioid with minimal respiratory effects (i.e., methadone) in combination with a sedative (i.e., midazolam or low dose of acepromazine) is administered IM to facilitate catheterization and reduce stress when necessary, or IV after catheter placement to reduce the amount of induction drug necessary and provide preemptive analgesia. A fentanyl bolus and a subsequent fentanyl CRI are typically incorporated for these patients, and may be used as premedication. The patient is continuously monitored after premedication when respiratory disease exists. All patients are preoxygenated. When possible, prior to the administration of drugs, place monitoring equipment (i.e., ECG, Doppler, NIBP) on the patient.

A smooth, rapid‐sequence induction allows quick access to the airway so assisted manual ventilation is started immediately. Drug selection will depend greatly on the degree of compromise to the patient. In severely compromised patients, the premedication IV may allow intubation. If that is not sufficient, propofol or alfaxalone are the induction agents of choice, combined with benzodiazepines to reduce the dose needed.

Maintenance with inhalant anesthetics is routine. Often CRIs are necessary in critical patients to reduce MAC of inhalant and provide other benefits (i.e., analgesia). Supporting ventilation is important in these cases. If chest tubes are not present prior to induction, the anesthetist must have all equipment needed to tap the chest (see Tables 3.9, 3.10) readily available in case suspect pneumothorax occurs. In patients with pulmonary contusions or possible bullae, conservative IPPV or manual ventilation with low PIP (10–12 cmH₂O) is recommended. Chest tubes, if placed, are left in place for recovery. Check for negative pressure with the patient in multiple positions (i.e., lateral and sternal recumbency). If negative pressure is not achieved, a chest drain system, such as a Pleur‐evac®, for continual suction is necessary. The arterial line is maintained (if possible) for continued blood gas analysis and IBP.

The patient remains intubated as long as possible; the anesthetist must have the supplies on hand to reintubate, if necessary. Supplemental oxygen is provided either in an oxygen cage with 40–60% O₂ or via nasal cannulas or facemask (see Chapter 3, p.69). Try to create a quiet, calm environment during recovery. Provide adequate analgesia. Continue monitoring oxygenation using pulse oximetry.

VII. Other conditions that influence anesthesia

A. Age

Patients who are young or old have different physiology from the average adult patient.

1. Anesthesia concerns/common complications

Anesthetic risk: When assessing a large number of patients, studies indicate age does not increase anesthetic risk in a patient 11 years or younger [54,55]. Breed is likely to play some role in this, as a Great Dane is unlikely to ever reach 11 years of age and a toy poodle may live well beyond that.
Inhalant requirement: Inhalant requirement (i.e., MAC) is highest at puberty, and steadily declines after that in humans [56]. If this is true in dogs, it is likely the geriatric patient has a low MAC requirement, whereas the patient approaching puberty may require more inhalant to prevent movement.
Altered drug disposition: Reduced liver and renal function, due to organ immaturity in the very young and decrease in organ mass in the aged, will result in a reduction in the amount of drug that is metabolized and excreted. In addition to drug metabolism, the liver is responsible for many other important functions, including production of proteins. Hypoalbuminemia may be present in both the young and the elderly [57]. Because many of our anesthetic drugs are highly protein bound, hypoalbuminemia results in a higher free fraction of circulating drug, leading to a greater effect than intended. These factors combine to create the possibility of relative overdosing of a patient. Reduce drug dosage and use short‐acting, reversible drugs (i.e., opioids or benzodiazepines) when possible.

a. Geriatrics

These animals are characterized by those patients which have reached 75% of their expected lifespan. As patients age, physiologic changes (as listed in Table 5.10) may affect general anesthesia; additionally, concurrent systemic diseases may manifest over time. A thorough preanesthetic exam and history (with a focus on systemic disease and current medications) is evaluated and addressed prior to anesthesia. This includes CBC, chemistry, urinalysis, and ECG. Newly made diagnoses such as chronic renal disease, Cushing disease, and neoplasia have arisen from preanesthetic BW in geriatric patients [58]. This may or may not delay the anesthetic procedure to a later time based on the findings. Thoracic radiographs and abdominal ultrasound are also considered as part of a full workup.

Table 5.10 Geriatric changes.

Cardiovascular system	Decreased arterial compliance Decreased myocardial compliance Decreased maximal heart rate Decreased maximal cardiac output Blunted beta‐adrenergic receptor activity
Respiratory system	Reduced gas exchange efficiency Reduced vital capacity Increased work of breathing Decreased thoracic compliance Decreased lung elasticity Increased closing volume
Nervous system	Altered sympathetic activity and outflow Downregulation of beta‐adrenergic receptors Decreased parasympathetic activity Decreased central neurotransmitter activity
Renal and hepatic systems	Decreased drug clearance Decreased glomerular filtration rate Decreased capability to handle water and sodium loads
Body composition	Decreased skeletal muscle mass Increased lipid fraction Decreased perfusion and organ blood flow Decreased tissue mass

(1) Anesthetic goals/considerations for geriatrics:

Hypothermia: As animals age and become geriatric, unless they have underlying disease (i.e., hypothyroidism), they tend to lose both fat and muscle mass. This decrease in weight often leads to difficulty thermoregulating when anesthetized. Circulating warm water blankets, forced air warming units, and fluid warmers are necessary to reduce the hypothermia these patients experience under anesthesia.
Patient positioning: Osteoarthritis is an insidious disease occurring in our geriatric patients that, because of its slow progression, owners may not notice in their pet. Tension on joints and pressure points on muscles during anesthesia result in considerable pain postoperatively, so care is taken to pad the patient and position them appropriately. Gentle handling when moving patients between tables (i.e., from the prep table, to the gurney, on to the OR table) is considered good practice.
Physiologic reserve: Physiological parameters are important due to geriatrics’ limited functional reserve of organ systems.

‐ Cardiovascular: As a patient ages, in addition to decreased organ mass, there is also stiffening of cardiac and vascular tissues, which increases afterload and may manifest as hypertension. A patient’s response to catecholamines decreases as well. Age‐related cardiovascular structural changes (e.g., mitral and tricuspid regurgitation) occur and progress over time.
‐ Neurologic: A generalized cerebral atrophy occurs as a patient ages; this manifests as an increase in anxiety and possible cognitive dysfunction. Anesthetic drugs (e.g., sedatives) may enhance these behaviors.
‐ Respiratory: As thoracic wall compliance and lung capacity decrease, a decrease in functional lung capacity occurs. These patients are at increased risk for hypoxemia and hypoventilation, especially when administered anesthetics.
‐ Renal: Renal function decreases over time due to a decrease in functioning glomeruli and decreased tubular function. This is present in some patients in spite of laboratory changes (see this chapter, “Renal insufficiencies,” p.170); anesthesia goals for geriatric patients include supporting BP to maintain perfusion to the kidney.

(2) Anesthetic protocol for geriatrics (Table 5.11): Premedication with an opioid, without a sedative, is suitable for many geriatric patients. If a sedative is required and the patient’s temperament is appropriate, a benzodiazepine such as midazolam is included in the premedication. If additional sedation is required, low‐dose acepromazine is used. While multiple induction agents are suitable in the elderly, titrating the drug to effect is appropriate, as less drug is generally required. Inclusion of a benzodiazepine, if not administered as part of the premed, is appropriate to reduce the amount of induction drug. Maintenance with inhalant anesthesia is suitable, but an opioid CRI is almost always included in an effort to reduce the amount of inhalant necessary. Local blocks are performed if appropriate to reduce inhalant requirement and provide analgesia. Maintain HR within 25% of baseline rate and manage hypotension as necessary (see Chapter 3, “Hypotension,” p.61).

Table 5.11 Geriatric protocol.

Premedication (mg/kg)		Induction (mg/kg)	Maintenance	Intraoperative analgesia (mg/kg/h)	Postoperative analgesia (mg/kg)
Opioida (select one) (a) Methadone 0.3 IM, 0.2 IV OR (b) Hydromorphone 0.05 IM, IV OR (c) Buprenorphine 0.01–0.03 IM, IV OR (d) Butorphanol 0.1–0.4 IM, IV	Sedative (if needed) (a) Midazolam 0.1 IM OR (b) Acepromazine 0.005–0.02 IM, IV +/− (c) Alfaxalone 0.5–2 IM	(a) Alfaxalone 1–3 IV +/− midazolam 0.1–0.2 IV OR (b) Propofol 2 + ketamine 2 (“ketofol”) to effect +/− midazolam 0.2 OR (c) Propofol to effect +/− midazolam 0.2 IV OR (d) Etomidate 1–2 IV +/− midazolam 0.1–0.2 IV OR (e) Fentanyl 0.005–0.01 and midazolam 0.1 (canine)	(a) Sevoflurane OR (b) Isoflurane AND Opioid CRI for reduction of inhalant requirement	(a) Opioid CRI (i) Fentanyl 0.012–0.042 OR (ii) Remifentanil 0.012–0.042 +/− (b) Lidocaine CRI 1–3 (dogs only) +/− (c) Ketamine 0.6 +/− (d) Local regional block	(a) Intermittent opioid bolus (select one): (i) Methadone 0.3 IV q 4–6 h OR (ii) Hydromorphone 0.05–0.1 IV q 4–6 h OR (iii) Buprenorphine 0.01–0.03 IM, IV q 6–12 h OR (b) Fentanyl CRI 0.002–0.005 AND (c) NSAID where not contraindicated +/− (d) Liposomal encapsulated bupivacaine infiltrative block 5.3

“AND” means that multiple choices from this column are selected, “+/−” means it is up to the anesthetist if they would like to combine multiple choices, “OR” means that the anesthetist will choose one or the other selection. For example, the postoperative analgesia column would be interpreted as including an opioid from the options (either as a CRI or intermittently given), an NSAID, and optional long0acting local anesthetic block.

^a Opioid selection is based on anticipated degree of pain the procedure will involve. Preoxygenate patient.

CRI, constant‐rate infusion; IM, intramuscular; IV, intravenous.

b. Neonatal and pediatric patients

A dog or cat is considered a neonate in the first 4 weeks of life (Figure 5.3). A pediatric patient is 5–20 weeks of life. Differences impacting neonatal and pediatric anesthesia are listed in Table 5.12. Minimal laboratory tests include PCV, total proteins (TP), and BG; care is taken to minimize the blood collected from these animals, as too much blood collected results in a decrease in effective circulating volume. Patients less than 6 weeks of age are not fasted. Patients less than 8 week or 2 kg are fasted for no longer than 1–2 hours [22]. Do not withhold water.

Photo depicts neonate patients. — **Figure 5.3** Neonate patients. Source: Courtesy of Anderson da Cunha.

Table 5.12 Differences impacting neonatal and pediatric anesthesia.

Cardiovascular system	Low myocardial contractility Low ventricular compliance Low cardiac reserve Cardiac output dependent on heart rate Increased cardiac index Poor vasomotor control
Respiratory system	High oxygen consumption High minute volume (higher respiratory rate) Low pulmonary reserve
Renal and hepatic system	Immature glomerular filtration rate Inability to metabolize drugs due to immature liver
Body composition	Limited thermoregulation Low body fat and muscle ratio Hypoalbuminemia Low hematocrit High total body water content Large extracellular fluid compartment Fixed and centralized circulating fluid volume
Nervous system	Increased permeability of blood–brain barrier Immature sympathetic nervous system

(1) Anesthetic goals/considerations for neonatal and pediatric patients:

Blood–brain barrier: The blood–brain barrier is more permeable in young patients, and therefore drugs that cross into the CNS have a more profound effect.
Cardiovascular: Cardiac output is highly dependent on HR, as SV is relatively fixed in the young (reminder: CO is equal to HR × SV). Because systemic vascular resistance (SVR) is naturally low due to the immature sympathetic system in the young, it is imperative that HR is maintained at or above resting rate. This is usually achieved by the avoidance of drugs such as alpha‐2 agonists and with the use of anticholinergics.
Hypoglycemia: If a patient’s liver has not fully matured (usually between 4 and 6 months), glycogen storage is imperfect. Young patients may experience hypoglycemia during an anesthesia procedure. Monitoring of BG every 30–60 min and supplementing with 2.5–5% dextrose in crystalloid solution, as needed, is warranted.
Hypothermia: Because these patients have a very large body surface area to body mass ratio, they often become hypothermic during a procedure. Minimizing heat loss through the use of bubble wrap over the animal, fluid warmers, circulating warm water blankets, and forced air warming devices is warranted. Prewarming these patients is extremely important and effective at maintaining a normal body temperature. Continuous temperature checks are taken to ensure that the patient is not overheating.
Respiratory: Until the bones further calcify, the thorax of these young patients remains very compliant, and at least in the first 6 weeks of life, lungs are still relatively stiff. While overall this results in normal ventilation, anesthesia disrupts this balance. Mechanical ventilation is not often warranted in these patients, but the anesthetist may need to hand ventilate the patient in periods of relatively minimal stimulation.
Venous access: In small patients IV access is difficult to obtain. Interosseous (IO) catheters are useful in emergency situations. An 18 G or 20 G needle works nicely as an IO catheter but requires more advanced experience to place and may be difficult to stabilize in place.

(2) Anesthetic protocol for neonatal and pediatric patients (Table 5.13): Neonate and pediatric patients present challenges to the anesthetist due to their size. Premedication with an opioid, without a sedative, is suitable for most young patients. If a sedative is required and the patient’s temperament is appropriate, a benzodiazepine such as midazolam is included in the premedication. Reversal drugs for the premed are at least calculated if not drawn up. Oxygen is administered for at least 5–10 minutes before induction occurs. Drug dose volumes are often so small that dilutions are necessary to ensure adequate dose delivery.

Table 5.13 Neonate/pediatric protocol.

Premedication (mg/kg)		Induction (mg/kg)	Maintenance	Intraoperative analgesia (mg/kg/h)	Postoperative analgesia (mg/kg)
Opioid (select one) (a) Methadone 0.1–0.5 IM OR (b) Hydromorphone 0.05–0.15 IM OR (c) Morphine 0.1–0.5 IM OR (d) Buprenorphine 0.01–0.03 IM OR (e) Butorphanol 0.1–0.4 IM	Sedatives (if needed) (a) Midazolam 0.1 IM +/− (b) Alfaxalone 0.5–2 IM	(a) Ketamine 5 + midazolam 0.25 IV OR (b) Propofol 2 + ketamine 2 (“ketofol”) to effect +/− midazolam 0.2 OR (c) Alfaxalone 1–3 to effect IV +/− midazolam 0.2 OR (d) Propofol 2–4 to effect IV +/− midazolam 0.2	(a) Sevoflurane OR (b) Isoflurane Fluids: 10 mL/kg/h balanced crystalloid solution + 2.5–5% dextrose additive	Local blocks as necessary	(a) Intermittent opioid bolus: (i) Methadone 0.3 IV q 4–6 h OR (ii) Hydromorphone 0.05–0.1 IV q 4–6 h OR (iii) Buprenorphine 0.01–0.03 IV, IM q 6–12 h AND (d) NSAID where not contraindicated

Glycopyrrolate is included in premedication at 0.01 mg/kg IM, or atropine at 0.02 mg/kg.

IM, intramuscular; IV, intravenous; NSAID, nonsteroidal antiinflammatory drug.

Induction drugs are carefully titrated to minimize cardiopulmonary side‐effects, using the smallest volume syringe suitable (e.g., 1 mL of propofol is drawn up in a 1 mL syringe, NOT a 3 mL syringe). Alfaxalone can be diluted with saline for easier titration and delivery. A ketamine and midazolam induction is also considered as ketamine may help with maintaining heart rate, causing transient tachycardia and an increase in BP. However, this may not occur with patients that have an immature nervous system. Some authors suggest that neuronal degeneration may occur when using NMDA antagonists (ketamine), although other studies show these same drugs as neuroprotective [59]. This suggests we should reserve general anesthesia only for patients where it is an absolute necessity. While use of heparinized saline for IV catheter flush is no longer routine, one should pay attention to how much flush is administered to the patient [60], especially if heparin is included. Small volumes of just several milliliters are often the patient’s hourly fluid dose.

Crystalloid fluid therapy with 2.5–5% dextrose at 10 mL/kg/h due to a higher fluid requirement is recommended. Use a syringe pump or buretrol for accurate hourly delivery and monitor blood glucose every 30–60 min. With today’s injectable profile of medications (i.e., alfaxalone [61]), mask induction with inhalants is rarely required but is an option when injectable options are exhausted or unavailable. The addition of an opioid is also recommended for analgesia. Eutectic mixture of lidocaine and prilocaine cream applied topically facilitates IV catheterization. Inhalant anesthesia maintenance is recommended for procedures of more than 15 min. Intubate immediately and use a nonrebreathing circuit or small pediatric circle/universal circuit system, depending on the patient’s weight.

Monitoring of cardiopulmonary function is mandatory. Normal vital parameters for neonates and pediatrics can be found in Table 5.14. Shivering postoperatively increases oxygen consumption, requiring supplemental oxygen during the recovery period.

B. Body condition

An animal’s body condition score is assigned as part of a routine PE. Body condition score reflects overall health and is also related to anesthetic risk. Research supports obesity being classified as a chronic inflammatory condition, which may alter other body systems including the cardiovascular, endocrine, and respiratory system [62].

Table 5.14 Normal vital parameters for neonates and pediatric patients.

Vital parameter	Low	High
Heart rate	140 bpm	240 bpm
Respiratory rate	24 breaths/min	40 breaths/min
Mean arterial pressure	50–60 mmHg	–
Temperature	100 °F	103 °F
Glucose	70 mg/dL	120 mg/dL

1. Anesthesia concerns/common complications

Anesthesia risk: In a recent study examining factors associated with increased anesthetic morbidity and mortality, it was found that underweight canine patients had an increase in anesthetic mortality compared to their normal‐weight counterparts [63]. Interestingly, both under‐ and overweight felines had an increase in anesthesia‐related mortality [64].
Drug dosage: Because fat serves as a depot for lipid‐soluble anesthetic drugs, it is appropriate to calculate a patient’s anesthetic drug dosage based on ideal body weight in obese animals; emergency drugs are calculated for actual body weight.
Thermoregulation: Patients at either extreme of body condition will thermoregulate differently from their normal‐weight counterparts, because of changes in body surface area to mass ratios. Underweight patients will experience a worsened degree of hypothermia. Obese patients are hard to rewarm once hypothermic but are also prone to hyperthermia.

2. Anesthesia protocol

No drugs are specifically contraindicated or indicated; a protocol is selected based on the patient’s temperament, signalment, comorbidities, and anticipated procedure. Temperature is diligently monitored.

3. Key points

Obesity predisposes patients to hypoventilation, which results in a respiratory acidosis and difficulty maintaining anesthetic plane due to the decreased uptake of inhalant agents. It is often necessary to mechanically ventilate obese patients.

C. Pregnancy

The heavy focus on neutering animals in veterinary medicine makes obstetric anesthesia relatively uncommon in modern practice. However, some breeds, such as the bulldog, require a cesarean section (C‐section) for successful delivery. Pregnancy leads to significant physiological changes (see Chapter 4, “C‐section,” p.119).

1. Anesthetic goals/considerations

Drug disposition: Pregnancy activates the endogenous endorphin system; in combination with the mild sedative effect of progesterone, there is a decrease in MAC requirement and an increase in pain threshold for these dogs. There is also a reduction in epidural volume. This is likely due to decreased epidural space from engorged vessels.
Electrolyte imbalances: If the mother has been in labor for an extended period of time, hypocalcemia is a concern. All electrolytes, including ionized calcium, are evaluated prior to induction. Calcium is supplemented if required.
Fetal viability: It is imperative the anesthetist know whether the owner prioritizes general fetus health (which may compromise the mother) or the mother’s systemic health (which may compromise the fetus). While every effort is made to deliver healthy fetuses, in cases of dystocia it is difficult to prioritize both mother and neonates. If fetuses are the top priority, fetal viability is assessed before anesthesia is performed, usually by ultrasound. If the fetuses are distressed (i.e., HR is decreased), atropine will cross the placental barrier and may increase fetal HR for long enough to deliver the pups. To prevent drug‐induced sedation and increase viability of the young, minimal drugs are administered to the mother; selection of drugs which are cardiovascular and respiratory friendly, reversible, and minimally metabolized is ideal. Morphine is often the opioid of choice, as it is very hydrophilic [65]. However, it is not usually given until all the fetuses have been delivered or removed from the uterus, as opioids cause respiratory depression in the puppies. Some form of analgesia must be provided for the patient experiencing an invasive abdominal procedure, such as a C‐section. An epidural with lidocaine and preservative‐free morphine is ideal in patients where systemic opioids are minimized initially (although they are administered after puppies are no longer present in the uterus). Liposomal encapsulated bupivacaine incorporated into the incision at closure may provide the mother with prolonged analgesia, with minimal impacts on the young.
Hypotension: CO is increased during pregnancy and there is also a rise in maternal blood volume. However, inhalant anesthetics are negative inotropes and therefore still result in some degree of hypotension. This is concerning because there is no autoregulation of blood flow to the uterus, and therefore uterine BP is determined in large part by maternal BP. Maintaining BP is often challenging in that the anesthetist strives to minimize drugs administered to the patient (which ultimately go to the fetuses). Keeping the vaporizer setting low in this situation is therefore reliant on MAC decrease secondary to pregnancy. Providing analgesia to the mother therefore assists in reducing inhalant requirements, helping to maintain maternal BP and thus uterine perfusion (i.e., oxygen to the fetus). Fluid boluses and glycopyrrolate, to maintain HR, are other means of improving BP; glycopyrrolate does not cross the placental barrier. In the human medical field, ephedrine is the drug of choice for maintaining BP during pregnancy. Little work is available for the dog or cat but work in sheep suggests a beneficial effect [66]. Drugs that cause vasoconstriction are avoided (i.e., phenylephrine) as they could reduce uterine perfusion and thus oxygen to the fetus.
Patient positioning: Patients in late pregnancy are not placed in dorsal recumbency if avoidable to reduce the negative impact on hypoventilation and venous return. Consider a dorsal oblique position with the cranial aspect of the mother elevated above the abdomen to assist with ventilation and venous return.
Physiologic changes.

– Cardiovascular changes include increased CO, decreased SVR, increased intravascular volume, and mild increase in HR.
– Hepatic/renal: There is a mild and transient increase of some liver enzymes including ALP, cholesterol and LDH. Both organs positively benefit from the increase in CO secondary to pregnancy, in that blood flow increases to these organs as well.
– RBC volume: In a possible physiologic attempt to reduce blood viscosity, there is a so‐called “anemia of pregnancy.” In essence, this is a dilutional anemia resulting from increasing plasma volume to a greater extent than RBC volume (although RBC volume is increased as well) [].
– Respiratory: Changes associated with pregnancy include increased minute ventilation, RR and V_T, and decreased functional residual capacity (FRC) from atelectasis and lung compliance. Overall, this leads to an increase in PaO₂ and a decrease in PaCO₂. In spite of the increase in PaO₂, these patients, at the end of pregnancy, desaturate approximately twice as fast as nonpregnant animals, making preoxygenation critical for the mother as well as the young.

2. Anesthetic protocol

For “C‐section” or dystocia procedures, see Chapter 4, p.119. Obtaining IV access is usually accomplished without sedation, as high progesterone levels present in the pregnant animal facilitate handling. If IM sedation is required to obtain IV access, opioids are preferred as they are reversible. Crystalloid fluids, ideally containing calcium (e.g., LRS), are started.

The patient is provided supplemental oxygen via oxygen mask during preoperative preparation. Preparation includes clipping the surgical area and epidural area, as well as a dirty scrub before induction (try to allow patient to remain in sternal/standing position). Induce with propofol or alfaxalone IV to effect (ideally in the OR, with the patient already on the OR table). Apgar scores were higher for the first 60 min after delivery in patients receiving alfaxalone versus propofol. However, overall survival was the same [68]. Additional work found that, in large‐breed dogs, alfaxalone provided higher Apgar scores vs propofol after an elective C‐section [69], so there is evidence for both drugs having utility in C‐sections. If no IM sedation was used, a higher dose of induction agent (ex. propofol at 4–8 mg/kg) may be needed to intubate the patient.

It is the helpful for one anesthetist to perform the epidural while another anesthetist induces the patient, so there is a rapid transition to surgery. Inhalants are used for maintenance anesthesia. Without premedication, inhalant requirements are high. Regional anesthetic techniques will minimize inhalant requirement. Epidurals (morphine 0.1 mg/kg and bupivacaine 0.5–1 mg/kg or lidocaine 0.5 mg/kg) are administered if the anesthetist is experienced and administration is quick. If an epidural was not placed during induction or immediately following, low dose of opioids with a preference for morphine, buprenorphine or bolus of fentanyl (0.002–0.005 mg/kg) IV will provide analgesia and reduce inhalant requirement. Opioids will cross the placental barrier and may result in respiratory depression in the puppies/kittens, but these effects are reversible with naloxone. If an epidural was performed, opioids are administered once the puppies are removed from the uterus. An incisional block with bupivacaine (preferably liposomal encapsulated bupivacaine due to its 3‐day duration) is also performed. Recovering the mother involves providing adequate analgesia without significant sedation; NSAIDs are often utilized.

The anesthetist focuses on the mother during anesthesia, while other people provide care for the puppies. An organized team with supplies ready will optimize neonatal outcome.

D. Shock/trauma patient

Shock refers to inadequate delivery of oxygen to tissues. There are several forms of shock: hemorrhagic, anaphylactic, cardiogenic, and septic shock, amongst others. Ultimately, shock is a life‐threatening condition requiring immediate intervention, and therefore the anesthetist commonly sees the patient after stabilization. However, in some cases, such as septic shock and hemorrhagic shock, surgical intervention is necessary to address the nidus of the disease. This section will focus on these two forms of shock.

1. Anesthesia concerns/common complications

Decreased effective circulating volume (ECV): A decrease in ECV is due to either true volume loss (hemorrhagic shock) or ineffective circulating volume due to vasodilation (septic shock). In either case, improving effective circulating volume is critical. Aggressive volume resuscitation with crystalloids, colloids (hypertonic saline, hetastarch, plasma), and other blood products is indicated to improve BP and perfusion. A CRI of positive inotropic agents such as dopamine or dobutamine also improves BP and supports myocardial function. Vasopressors such as epinephrine, norepinephrine, and phenylephrine are often necessary as well (see Table 10). Norepinephrine is the vasopressor of choice for septic patients as it may help with capillary beds that may be vasodilated due to sepsis [70]. When vasopressors are used, monitoring of lactate is recommended. Physiologic doses of steroids, such as dexamethasone‐SP, are also considered if other treatments have not successful in raising BP.
Management of underlying disease: Shock is a symptom of a severe underlying disease, rather than a disease itself. In order to resolve, not just manage, shock, the underlying disease must be addressed.
Tissue hypoxemia: Tissue hypoxemia results in lactic acidosis (see Chapter 3 “Metabolic acidosis,” p.74); supplemental oxygen may not restore the delivery of oxygenation, but it will improve the concentration of oxygen available.
Circulatory collapse/cardiac arrest: As multiple organ systems become starved for oxygen and appropriate perfusion, circulatory collapse and cardiac arrest become evident (see Chapter 3, p.58 and Appendix B for CPR).

2. Anesthesia protocol

Multiple large‐gauge IV catheters are placed; no premedication is necessary. Preoxygenate the patient for as long as possible. IV drug administration is preferred as IM injections are compromised due to poor peripheral blood flow. Stabilization with appropriate fluid therapy is ideal before anesthesia and a normalized BP is preferred. Large volume replacement is indicated (shock doses 50–60 mL/kg in cats, 90 mL/kg in dogs). Goal‐directed therapy is important with aliquots of fluids given with reevaluation of blood gases and BP. The start of norepinephrine and possibly positive inotropes is considered before inducing anesthesia to obtain a normal BP.

When patients require anesthesia, drugs are selected based on short duration and reversibility. This makes fentanyl, midazolam, and CRI loading doses (i.e., lidocaine in dogs) the ideal combination as a neuroleptic induction. If required, low doses of alfaxalone given very slowly to effect are useful to further facilitate intubation. Prompt placement of the ET tube (rapid‐sequence induction) and inflation of the cuff are recommended to guard against aspiration. Lidocaine is used on the larynx to help smooth intubation.

Either isoflurane or sevoflurane is used to maintain anesthesia; the most important factor is using low vaporizer settings. Supplement with opioid or lidocaine CRIs to maintain a suitable depth and analgesia level; note, lidocaine CRIs are limited to the canine patient due to the cardiovascular disruption cats experience with lidocaine CRIs. The patient is kept as light as possible to optimize hemodynamic function. Monitoring is critical and preplaced to the fullest extent possible. IBP measurement and blood gas analysis are indicated, beginning preoperatively and continuing into recovery. The patient must recover where 24‐h care, pressure support, and oxygen supplementation are available.

Tags: Small Animal Anesthesia Techniques

May 13, 2023 | Posted by admin in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off

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Anesthesia in patients with concurrent disease

I. Cardiovascular disease

A. Conduction and rhythm disturbances

1.Anesthesia concerns/common complications

2. Anesthesia protocol

B. Structural abnormalities

1. Systolic dysfunction

(a) Anesthesia concerns/common complications:

2. Diastolic dysfunction

(a) Anesthesia concerns/common complications:

3. Obstructive dysfunction

(a) Anesthesia concerns/common complications:

II. Endocrine diseases

A. Diabetes insipidus

1. Anesthesia concerns/common complications

2. Anesthetic protocol

B. Diabetes mellitus

1. Anesthesia concerns/common complications

2. Anesthetic protocol (Table 5.4)

C. Hyperadrenocorticism (Cushing disease)

1. Anesthesia concerns/common complications

2. Anesthetic protocol (Table 5.5)

D. Hypoadrenocorticism (Addison’s disease) [26]

1. Anesthesia concerns/common complications

2. Anesthetic protocol (Table 5.6)

E. Hyperthyroidism

1. Anesthesia concerns/common complications

2. Anesthetic protocol

F. Hypothyroidism

1. Anesthesia concerns/common complications

2. Anesthetic protocol

G. Pheochromocytoma

1. Anesthesia concerns

2. Anesthesia protocol

H. Insulinoma

1. Anesthesia concerns

2. Anesthesia protocol

III. Hepatic function diseases

1. Anesthetic concerns/common complications

2. Anesthetic protocol (Table 5.7)

IV. Neurological disorders: Intracranial disease

1. Anesthetic goals/special considerations

2. Anesthetic protocol (Table 5.8)

3. Key points

V. Renal insufficiencies

1. Anesthesia concerns/common complications

2. Anesthetic protocol

VI. Respiratory/pulmonary disease

A. Upper airway disease

1. Anesthesia concerns/common complications

2. Anesthesia protocol

3. Key points

B. Lower airway disease

1. Anesthesia concerns/common complications

2. Anesthesia protocol

3. Key points

C. Space‐occupying respiratory disease

1. Anesthesia concerns/common complications

2. Anesthesia protocol

VII. Other conditions that influence anesthesia

A. Age

1. Anesthesia concerns/common complications

a. Geriatrics

(1) Anesthetic goals/considerations for geriatrics:

b. Neonatal and pediatric patients

(1) Anesthetic goals/considerations for neonatal and pediatric patients:

B. Body condition

1. Anesthesia concerns/common complications

2. Anesthesia protocol

3. Key points

C. Pregnancy

1. Anesthetic goals/considerations

2. Anesthetic protocol

D. Shock/trauma patient

1. Anesthesia concerns/common complications

2. Anesthesia protocol

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