Patient preparation

The information from the history, physical examination, and blood testing is used to establish a plan for patient preparation and make a selection of premedication drugs. Interventions to consider include volume resuscitation (crystalloids, colloids, blood components), pain control (narcotics, adjunctive therapies), antiarrhythmic drugs, positive inotropes, vasopressors, and specific steps to normalize metabolic, electrolyte, and acid–base derangements. Patients at risk for adverse cardiovascular events will benefit from having cardiovascular drug doses (such as continuous rate infusion doses of vasopressor agents or positive inotropes) precalculated for rapid intervention if required.

Patients with traumatic injuries, gastric dilation‐volvulus, airway disease, respiratory dysfunction or cardiovascular disorders can decompensate rapidly or respond negatively to the anesthetics or other medications administered. The anesthesia monitoring devices, including blood pressure, ECG, and pulse oximetry, should be placed on the animal during the patient preparation phase of anesthesia to allow continuous monitoring throughout all phases.

Catheter placement is essential in critically ill patients both for monitoring and therapy. At least one intravenous or intraosseous catheter is necessary prior to induction of anesthesia. A second catheter should be placed if blood administration or rapid, large‐volume fluid administration is anticipated during anesthesia. Placement of catheters such as a central line, direct arterial line, urinary catheter, nasal oxygen and nasogastric tube prior to induction will minimize anesthetic time. However, the procedures may be more challenging and uncomfortable in the awake patient, making placement shortly after induction in these animals another option.

Patients with intravascular volume deficits should have the deficit corrected through resuscitation with the appropriate fluid(s) prior to induction of anesthesia. Additional monitoring procedures such as urinary output and central venous pressure can help assess and maintain an adequate fluid balance during the anesthetic period. Administration of crystalloids is considered standard of care for patients throughout general anesthesia, and is recommended at a dose of 5–10 mL/kg/h. However, fluid choices and infusion rates may need to be altered for short or prolonged anesthetic procedures depending on the fluid balance, cardiac function, and blood pressure of the patient.

Examples of patients that may require preoxygenation include those with pulmonary, pleural or chest wall disease, neurological disease, obese patients, and patients with abdominal distension (including ascites, pregnancy, and pyometra). Those patients with preexisting increase in work of breathing, brachycephalic breeds and patients with anemia would benefit as well. Preoxygenation for 3–5 minutes can increase the alveolar concentration of oxygen. This will provide a slightly longer time for hemoglobin desaturation than without preoxygenation. Preoxygenation can serve as a safeguard during rapid induction and intubation or during patient transfer into the surgery suite [4].

Arrhythmias should be treated prior to anesthetic induction when one or more of the following criteria are met: (1) the arrhythmia is impacting cardiac output and compromising tissue perfusion, (2) if the high heart rate is negatively impacting coronary perfusion, and potentially causing further arrhythmias or (3) prefibrillatory rhythms are noted (such as torsades de pointes, multiform ventricular beats or R‐on‐T phenomenon). If any of these are present, preoxygenation is instituted and specific pharmacologialc intervention is indicated (see Chapter 11). For bradyarrhythmias, atropine or glycopyrrolate is usually the first drug of choice. For ventricular tachyarrhythmias, lidocaine, procainamide or magnesium sulfate are common drug options.

Anesthesia can reduce GI motility and delay gastric emptying. An appropriate time of fasting prior to anesthesia is desired to decrease the amount of gastric contents. When this is not possible, the administration of antiemetics and prokinetics in the perianesthetic period may decrease the risk of aspiration pneumonia. A nasogastric tube can be placed and used to remove gastric fluid and air to reduce the stimulus for vomiting.

Medical management for an elevation in intracranial pressure should be considered prior to anesthetic induction in any patient with altered mentation, head trauma or a recent history of seizures (see Chapter 12). Interventions can include preoxygenation, maintaining a normal blood pressure, maintaining PaCO₂ between 35 and 45 mmHg and ensuring adequate glucose levels. The head and shoulders can be elevated up to 15–20° and jugular vein occlusion is avoided. Hypoventilation leading to hypercapnia will alter cerebral vasomotor tone and predispose to altered intracranial pressures. Drugs such as opioids, dissociative anesthetics, and phenothiazines can induce hypoventilation. Some inhalant anesthetics, particularly halothane and nitrous oxide, can disrupt cerebral autoregulation and increase cerebral metabolic rate and oxygen consumption [4].

Altered drug pharmacokinetics, including drug absorption, distribution, metabolism, and excretion, can occur in critical patients associated with changes in fluid balance, cardiovascular function, albumin concentration, hepatic function, and renal function. Depending on the drug and the pharmacokinetic changes, drug doses or intervals may need to be altered or drug combinations used. Changes in the magnitude and duration of drug effects should be anticipated and adjustments made as indicated.

A flow sheet should be prepared for every patient undergoing anesthesia with precalculated doses of emergency, anesthetic, and vasoactive drugs available for that specific patient in case urgent intervention is required. The clinician should be familiar with side‐effects of all medications administered. A list of common sedatives, analgesics, anesthetics, drug reversal agents, and cardiopulmonary resuscitation drugs, their recommended dose and common side‐effects is provided in Table 22.2.

One of the overall goals of the anesthetic plan is to minimize the time that the patient is required to be under anesthesia. Methods to consider for saving anesthetic time for patients are listed in Table 22.3. During the premedication phase of anesthetic preparation, the critical care team must prepare the anesthesia and monitoring equipment as well as the procedural or surgical suite. An example of a preanesthetic check‐off list is provided in Table 22.4. Equipment, drugs, and materials should be set up for immediate use prior to inducing anesthesia in the patient.

Anesthetic induction

Rapid‐sequence anesthetic induction and endotracheal intubation techniques are employed in patients with critical illness. The purpose is to minimize the time between loss of consciousness and endotracheal intubation, allowing airway and ventilation. The patient is preoxygenated and injectable anesthetics such as propofol, etomidate‐midazolam, ketamine‐midazolam or alfaxolone are titrated until the patient loses the gag reflex. The injectable agents are often used in combination for several reasons: (1) when combining different mechanisms of action, analgesic effects can be increased, (2) adding drugs that work by different mechanisms can allow a lower dose of some anesthetics, including volatile agents, and (3) lower doses will decrease unwanted side‐effects of any single agent (see Table 8.12).

Endotracheal intubation is immediately performed, followed by cuff inflation, oxygenation, ventilation, and securing of the endotracheal tube with ties. A complete description of how to perform endotracheal intubation and techniques to assure proper tube placement within the trachea is provided in Chapter 8, with tube sizes in Table 8.8 and Table 8.9. The endotracheal tube should have a thin coat of sterile water‐based lubricant applied to the tip prior to placement, confirmed with auscultation, visualization and ETCO₂ measurement and secured with a tie. A back‐up plan for intubation should be in place in case standard tracheal intubation is not possible (such as brachycephalic abnormalities, intraoral or pharyngeal disease, or unable to open mouth). Using a stylet or guidewire, endoscopic visualization, transtracheal catheter or tracheostomy may be required. In addition, suction should be available to clear regurgitated GI contents, large volumes of tracheal fluid or salivary secretions. An oropharyngeal and laryngeal exam is performed in every patient at the time of intubation to evaluate for any masses, polyps or laryngeal dysfunction.

When the airway is confirmed and secured, the patient is ready for maintenance of anesthesia with volatile anesthetics or constant rate infusion of injectable anesthetic and sedative drugs. Monitoring devices should be immediately connected to the patient and assessed to ensure an adequate level of anesthesia is achieved and the initial vital parameters are within acceptable limits. Careful patient monitoring is required throughout the entire anesthetic preparation and maintenance period.

Anesthetic maintenance

Balanced anesthesia involves the administration of different drugs to target anesthesia, analgesia, and neuromuscular relaxation. Often opioids are given during induction for analgesia with benzodiazepines for neuromuscular relaxation and volatile anesthetic agents used to maintain the desired plane of anesthesia.

Anesthesia drugs are a contributing factor to negative physiological changes in the animal since certain drugs, e.g. inhalant anesthetics, phenothiazines, and propofol, can cause vasodilation. This will decrease preload and can cause hypotension. Anesthetic agents will also depress the respiratory drive. Manual or mechanical ventilation should be employed for any critical patients undergoing general anesthesia.

Analgesic drugs in combination with sedatives can obtain neuroleptanalgesia. When used in combination with anesthetic drugs, neuroleptanalgesics can decrease the required dose for all medications since many of the drugs are synergistic. Local anesthetic techniques can also minimize the need for systemic analgesia and reduce the depth of anesthesia required.

The five stages of anesthesia are listed in Box 22.1 along with questions that the anesthetist should consider when assessing the anesthetized patient in Box 22.2 [7]. The physical monitoring parameters that aid in determining the plane of anesthesia are listed in Table 22.5. Physical parameters such as jaw tone, palpebral and corneal reflexes, presence or absence of voluntary movement to surgical stimulation and eye position are used to assess the plane or depth of anesthesia.