Anesthesia of the Critically Ill Patient

Chapter 163 Anesthesia of the Critically ill Patient



Jane Quandt, DVM, MS, BS, DACVA, DACVECC



KEY POINTS



• Stabilization of the critically ill animal before anesthesia is imperative to minimize anesthetic complications.

• Anticipate problems and have an appropriate and efficient treatment and therapeutic plan before anesthesia is begun.

• Consider using a balanced anesthesia technique to minimize deleterious effects of single-use drug therapy.


INTRODUCTION


In the critically ill patient, a thorough preoperative assessment is necessary to define what type of trauma or compromise the patient is undergoing. The critically ill patient has altered physiology and decreased reserves that will affect the pharmacokinetic and pharmacodynamic behavior of anesthetic drugs. These patients benefit from minimizing stress levels and optimizing oxygen delivery.


Stabilization of the critically ill patient before anesthetic drug exposure is essential, because the risks associated with anesthesia in an unstable patient increase the risks of anesthetic complications.



STABILIZATION


Thorough diagnostic tests should be performed before administering anesthesia, including serial physical examinations, radiographs, blood chemistry, complete blood count, coagulation profile, acid-base status, and blood glucose and lactate levels. A dehydrated or hypovolemic state along with fluid, acid-base, and electrolyte abnormalities should be corrected before anesthesia is begun.


Venous access is imperative in managing and anesthetizing the critically ill patient, because anesthesia-associated hypotension is not uncommon. Intravenous administration of medications is usually preferred, because absorption may be delayed with intramuscular or subcutaneous administration, particularly when the patient is dehydrated, hypovolemic, poorly perfused, or hypothermic. Critically ill patients often benefit from having more than one IV catheter, so that multiple agents and fluids can be given during and after the anesthetic period. Either peripheral or central placement can be used; however, if fluids need to be given at a rapid, shock bolus rate, the shortest, widest bore catheter will allow for the most rapid administration (i.e., peripheral cephalic catheter).


Venous access is also important to provide warm IV fluids before and during anesthesia, to help maintain organ perfusion and body temperature. An IV catheter will provide a port for drug administration, antibiotic delivery, vasopressor and inotropic support, and fluid therapy. A minimum of two intravenous catheters should be placed before anesthesia in unstable patients to accommodate different fluid rates and incompatibilities of various agents, such as vasopressors, sodium bicarbonate, and blood products.


Blood products should be given through a dedicated catheter; no other fluids or drugs should be administered in that line during the transfusion because of concerns for possible contamination and potential for bacterial growth. This is also true for the catheter used for total parenteral nutrition, a dedicated line that should never be disconnected or have any other fluid running through it concurrently because of the risk of sepsis. Total parenteral nutrition contains approximately 70% to 80% free water (depending on the formula). As a consequence only 20% to 30% of the infused volume should be accounted for as part of the crystalloid fluid volume.


An arterial catheter should be inserted once the animal is under general anesthesia. An arterial catheter will allow for direct arterial blood pressure measurement and can be used to collect blood samples for blood gas analysis.


A packed cell volume (PCV) greater than 25% is necessary for adequate oxygen carrying capacity and oxygen delivery.1 During anesthesia the PCV can decrease by 3% to 5%; therefore even a small volume of blood loss may be significant and may warrant a blood transfusion.1 Similarly, hypoproteinemic patients (total protein 3.5 g/dl or less and/or an albumin 2 g/dl or less, or both) may benefit from colloids to help maintain normal colloid osmotic pressure (COP) (normal is 18 to 24 mm Hg) and to prevent edema formation or vascular leak.2,3


Measurement of COP before anesthesia is helpful in determining the need for colloid support and to help determine when to terminate colloid therapy. If patients are hypoproteinemic, options include hydroxyethyl starch, dextran-70, 25% human serum albumin, or even Oxyglobin. If the patient is small, hypocoagulable, or hypoalbuminemic, fresh frozen plasma (FFP) given at 6 to 20 ml/kg is warranted.


Unfortunately, size, dosing, and cost become limiting factors for the use of FFP to treat hypoalbuminemia in larger patients, because a dose of approximately 45 ml/kg of FFP is required to increase the albumin by 1 g/dl.4 Hydroxyethyl starch and dextran-70 can both cause a dosage-dependent coagulopathy. Administration of these products should be limited or avoided in patients with known coagulation defects, and the total amount given to any one patient should ideally be limited to less than 20 ml/kg per 24 hours.


Human serum albumin (HAS) 25% has been utilized in veterinary medicine; however, its use has not been well researched and side effects such as polyarthritis, future transfusion reaction, glomerulonephritis, and other immune-mediated effects warrant further investigation.3 If used, this product should be treated as a transfusion, knowing that subsequent reactions may occur.


Finally, patients should be evaluated carefully for underlying metabolic disease before anesthesia, because this may affect the anesthetic protocol. Patients with renal insufficiency may require a higher fluid rate to maintain renal perfusion, and urine output (UOP) should be monitored carefully during anesthesia.5 In addition, renal drug excretion may be delayed, so these agents should be used cautiously (e.g., ketamine in cats).


In patients with liver disease, anesthesia protocols and monitoring may be affected by decreased glucose production, decreased albumin production, altered drug metabolism via cytochrome P-450 enzymes, and decreased production of clotting factors.5 Patients with heart disease may be less able to compensate under anesthesia, and fluid overload should be avoided. Blood pressure should also be carefully monitored, because anesthesia-induced hypotension may result in decompensation. Finally, one should always consider preexisting drug therapy, such as nonsteroidal antiinflammatory drugs (NSAIDs), diuretics, anticonvulsants, and cardiac medications.



PREMEDICATION


Premedication may not be necessary unless the animal is in extreme pain or is vicious. If the critically ill patient would benefit from premedication, opioids such as morphine, hydromorphone, or oxymorphone in combination with a tranquilizer such as midazolam or low-dose acepromazine can be given intramuscularly to provide analgesia and sedation. In the animal that is in extreme pain or is vicious, the μ-agonist narcotic can be combined with the α2-agonist, medetomidine (5 to 10 μg/kg IM) for enhanced analgesia, sedation, and restraint. Table 163-1 lists drugs used for anesthesia.


Table 163-1 Anesthesic Agents and Their Dosages


















































































































































































































Drugs Comment Dosage in mg/kg
Anticholinergic agents May make secretions more viscous Atropine 0.04 IM, 0.02 IV
Increase anatomic dead space Glycopyrrolate 0.01 IM, IV
Increase heart rate Glycopyrrolate does not cross BBB or the placenta
Can increase myocardial work and oxygen consumption  
Opioids Complete reversal with naloxoneimage Morphine 0.2 to 2 IM, SC
Analgesic CRI 0.1 to 0.3 loading dose, then 0.1 mg/kg/hr
Respiratory depression Oxymorphone 0.05 to 0.2 IM, IV, SC
Bradycardia Meperidine 2 to 11 IM, SC
Minimal effect on CV performance Hydromorphone 0.1 to 0.2 IV, IM, SC
  CRI 0.025 to 0.05 IV loading dose, then 0.01 to 0.04 mg/kg/hr
image Fentanyl 0.005 to 0.08 IM, IV, SC
  CRI loading dose for dog 5 to 10 μg/kg, then 0.7 to 1 μg/kg/min
  CRI loading for cat 5 μg/kg, then 0.3 to 0.4 μg/kg/min
  Give anticholinergic drug before starting CRI
Partial μ-agonist Buprenorphine 0.005 to 0.02 IM, IV
Partial reversal of μ-agonist with butorphanol Butorphanol 0.1 to 0.8 IM, IV, SC
CRI 0.1 to 0.2 IV loading dose, then 0.1 to 0.2 mg/kg/hr
Complete reversal with naloxone Naloxone 0.002 to 0.02 IM, IV
Dissociative agents Salivation Ketamine 4 to 11 IV, IM
  CRI 0.5 IV loading dose, then 0.1 mg/kg/hr
Increase heart rate Tiletamine and zolazepam (Telazol) 2 to 4 IM, 2 IV
Increase ICP and intraocular pressure  
Do have analgesic effects  
Renal elimination in cat  
Benzodiazepines Can decrease other drug dosages Diazepam 0.2 to 0.5 IM, IV
CRI 0.1 to 0.5 mg/kg/hr
Mild sedation and muscle relaxation Midazolam 0.07 to 0.4 IM, IV
CRI 0.1 to 0.5 mg/kg/hr
Anticonvusant  
Not analgesic  
Diazepam has propylene glycol  
Phenothiazines Vasodilation Acepromazine 0.01 to 0.2 IM, IV
  No more than 3 mg total dose
Long duration  
Not analgesic  
Barbiturates Cardiovascular depression Thiopental 4 to 20 IV
Respiratory depression Methohexital 4 to 10 IV
Rapid induction  
Decrease ICP and intraocular pressure  
Effects may be potentiated by concurrent acidosis or hypoproteinemia  
Can use with lidocaine Lidocaine 2 to 4 IV with thiopental 4 to 8 IV
Propofol Rapid acting with short duration 2 to 8 IV
  CRI 0.1 to 0.4 mg/kg/min
Respiratory depression  
Decreases ICP and intraocular pressure  
Not analgesic  
Caution with volume depletion or cardiovascular compromise; can be significant depression  
Peripheral vasodilation  
Myocardial depressant  
Heinz body anemia in cats  
Etomidate Maintain cardiovascular stability 0.5 to 4 IV
Not used alone  
Suppresses adrenocortical function for 2 to 6 hours following a single bolus dose  
α2-Agonists Cardiovascular depression Xylazine 0.55 IM, IV
Vomiting Medetomidine 10 to 80 μg/kg IM, IV
Good sedation and analgesia CRI 1 μg/kg IV loading dose, then 1 to 3 μg/kg/hr
  Can combine with butorphanol or ketamine
Reversible with atipamezole Atipamezole 0.04 to 0.3 IM, IV
Neuroleptanalgesic Analgesic Combination of opioid and tranquilizer
Noise sensitive  
Maintain cardiovascular stability  
Inhalants All inhalants will produce a dose-dependent cardiovascular depression and peripheral vasodilation —
Anesthesia depth adjusted rapidly  
Isoflurane: Rapid uptake and recovery  
Nitrous oxide: Caution with closed gas spaces  
Potential for hypoxemia  
Sevoflurane: Rapid uptake and recovery  

BBB, Blood-brain barrier; CRI, constant rate infusion; CV, cardiovascular; ICP, intracranial pressure; IM, intramuscular; IV, intravenous; SC, subcutaneous.


Critically ill patients are often depressed, lethargic, and require minimal drug therapy for induction. Anticholinergic agents are not used routinely unless there is a need to treat bradycardia. Protocols should be implemented to minimize the amount of time the animal is under anesthesia; therefore preparations such as preclipping the surgical site while the animal is still awake should be performed if possible.


Preoxygenation will allow for additional time to intubate the animal; this is especially helpful for those animals in respiratory distress or those with an airway that may be difficult to intubate. Finally, electrocardiography (ECG) and blood pressure monitoring should be in place before induction to detect arrhythmias, hypotension, or cardiovascular collapse that may occur during induction.

Related posts:

  1. Deteriorating Mental Status
  2. Hyperthermia and Fever
  3. Ventilator-Associated Lung Injury
  4. Allergic Airway Disease in Dogs and Cats and Feline Bronchopulmonary Disease

Stay updated, free articles. Join our Telegram channel
Tags:
Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Anesthesia of the Critically Ill Patient

Full access? Get Clinical Tree
Get Clinical Tree app for offline access