Canine and Feline Anesthesia PP

Canine and Feline Anesthesia

Scott Wilson, DVM

Mortality Rates with Anesthesia

  • Mortality rates (Brodbelt et al., 2008):

    • Dogs: 0.17%

    • Cats: 0.24%

  • Some studies show higher rates; generally cats have higher rates than dogs.

  • Anesthesia mortality in dogs and cats is significantly higher than in humans (1 out of 100,000 to 1 out of 1,000,000, or 0.001% - 0.0001%).

General Outline of Anesthesia in Dogs

  • History

  • Pre-anesthetic evaluation

  • Breed-specific concerns

  • Premedication

  • Induction

  • Maintenance

  • Complications

Detailed History in Dogs

  • Importance of detailed history, especially for cases with comorbidities.

    • Duration of complaint.

    • Current medications:

    • Heartworm prevention (risk varies by location).

    • Diuretics (risk of hypovolemia, electrolyte imbalances).

    • ACE inhibitors, calcium channel blockers, beta-blockers (risk of unresponsive hypotension).

    • Anti-epileptic agents (may cause additive sedation).

  • Signs of systemic disease:

    • Particularly related to cardiac and pulmonary systems, such as coughing and exercise intolerance.

  • Previous blood transfusions:

    • First transfusion in dogs is “free” with no naturally-occurring antibodies against RBC antigens.

    • Antibodies develop 5-7 days post first transfusion; subsequent transfusions need to be typed and cross-matched.

  • Previous anesthesia:

    • Records should be obtained if previous severe complications are reported.

    • Allergies to anesthesia are unlikely; usually attributed to anesthetic mismanagement.

Signalment in Dogs

  • Age: Special considerations for pediatric and geriatric patients.

  • Gender: Pregnancy is a significant concern.

  • Breed: Many considerations vary by breed.

Breed-Specific Concerns

  • Sighthounds (Greyhound, Italian Greyhound, Whippet, Irish Wolfhound, Basenji, Rhodesian Ridgeback, etc.):

    • Prolonged recovery with thiobarbiturates documented; avoid in these breeds.

    • Longer recoveries also observed with propofol and alfaxalone.

  • Boxers:

    • Possible sensitivity to acepromazine; risk of collapse may be vagally-mediated but has been used safely in many cases.

  • Brachycephalic breeds (English and French bulldogs, pugs, Boston terriers):

    • Careful with sedatives and monitor post-premeds.

    • Intubation may require smaller tube; preoxygenation is essential.

    • Rapid intravenous induction with thorough monitoring during recovery.

  • Small breeds:

    • Risk of tracheal collapse; similar considerations as with brachycephalics.

    • Avoid excitement to prevent issues with intra-tracheal pressure.

  • Several other breed-related concerns include:

    • Cardiomyopathy in Dobermans and Boxers.

    • Sick sinus syndrome in Schnauzers.

    • Mitral valve disease in small breeds.

    • Increased vagal tone in brachycephalics and dachshunds.

    • Von Willebrand disease in Dobermans.

    • Hemophilia in German shepherds.

    • Breed sensitivities may not always be valid but could exist due to unidentified genetic defects affecting anesthesia.

Physical Examination

  • Refer to patient evaluation lecture; key points include:

    • Documentation is critical for avoiding lawsuits based on perceived poor pre-anesthetic evaluations.

    • Not all defects can be detected through physical exam and pre-anesthetic blood work.

    • Pay specific attention to cardiac and pulmonary systems.

    • Additional workup may be necessary based on physical exam findings.

    • Consider temperament in order to minimize patient and personnel injury.

Premedications

  • Reference premedication lectures; typically, an opioid and sedative IM before catheter placement:

    • If an IV catheter is present, premeds can be given IV before induction.

  • Opioid Options:

    • Pure mu agonists, partial mu agonists, agonist-antagonists can be chosen based on patient and procedure.

    • Certain mu agonists (fentanyl, methadone) are less likely to induce vomiting.

  • Non-anesthetic premeds include:

    • H1 antihistamine like diphenhydramine for mast cell tumor removal (to counteract histamine release).

    • NK1 receptor antagonist (Maropitant) can reduce the risk of vomiting if given 30 minutes pre-op.

Sedative Options

  • Acepromazine:

    • Provides mild-moderate sedation but can cause hypotension, especially when followed by inhalant anesthesia.

    • Recommended for systemically healthy patients; low doses should be used.

  • Dexmedetomidine:

    • Produces marked sedation, associated with hypertension and reflex bradycardia.

    • Should only be used in systemically healthy patients.

  • Benzodiazepines:

    • Provide minimal sedation in healthy dogs and may cause excitement.

    • More sedating effects observed in younger or older patients.

    • Cardiovascular and respiratory effects are minimal.

Premedications for Aggressive Dogs

  • Pre-anesthetic evaluation (PE and bloodwork) may not be feasible.

  • Requires informed consent from owners.

  • Options include:

    • Pole syringe or “door restraint”.

    • IM combinations of ketamine, telazol, or alfaxalone with an alpha-2 agonist and opioid.

    • Caution: Alpha-2 combinations without anesthetic drugs can lead to unexpected arousal and injure personnel.

  • Intubation should be ready if laryngeal reflexes are lost.

Anticholinergics

  • Used for animals with high vagal tone or specific issues (e.g., brachycephalics, ophthalmic disease, puppies).

  • Important for procedures that may provoke vagal stimulation (laryngeal, GI, urogenital surgeries).

  • Not recommended with alpha-2 agonists unless hypotension is documented as it can induce bradycardia.

Induction

  • Refer to induction drug lecture; choice based on patient's cardiovascular stability and not solely the procedure.

  • Common induction agents:

    • Propofol, Alfaxalone, Ketamine (+ benzo).

  • Titration to effect is crucial:

    • Administer half the calculated induction dose over 10-15 seconds and evaluate effects, then administer small boluses until the animal is intubatable.

  • Induction Drug Characteristics:

    • Propofol and Alfaxalone: Administer cautiously and titrate to effect.

    • Ketamine: High therapeutic index; ideal for rapid induction but slower onset than others.

    • Benzos must be used to enhance the safety of the induction process.

Intubation

  • Selecting the right laryngoscope and ET tube:

    • Generally: 10mm tube for 20kg dog, 8mm tube for 10kg dog.

    • Bring three tube sizes for versatility.

  • Intubation Technique:

    • ETT should pass easily; use a laryngoscope placed at the base of the tongue.

    • Use the largest tube fitting to reduce resistance, ensuring it does not obstruct airflow.

    • Apply sterile lubricant to ETT cuff for better seal and decrease aspiration risk.

  • Considerations for Brachycephalics:

    • Intubation may be more challenging due to the anatomical structure.

    • Use appropriate techniques to handle the long soft palate.

Maintenance of Anesthesia

  • Common maintenance drugs include:

    • Inhalants (Isoflurane, Sevoflurane) and injectables for CRI (Propofol, Alfaxalone).

    • Avoid etomidate and administer adjunct drugs like opioids (most commonly fentanyl), lidocaine, ketamine, and benzodiazepines during maintenance.

Cardiovascular Support During Anesthesia

  • Must titrate IV fluids carefully due to profound vasoconstriction.

    • Blood volume: ~90 mL/kg for dogs, ~60 mL/kg for cats.

    • Balanced isotonic crystalloid fluids recommended (LRS, Plasmalyte-A, Norm-R).

    • AAHA consensus for intra-op rate is 5 mL/kg/hr.

  • Treat hypotension primarily with drugs such as dopamine, dobutamine, or ephedrine; norepinephrine or vasopressin may be required in critical patients.

Regurgitation in Dogs

  • Common occurrence in dogs (5% at University of Georgia).

  • More prevalent in large dogs and during orthopedic procedures.

  • Vomiting post-premedication does not exacerbate the risk of regurgitation.

  • Goals of management include preventing esophageal damage or aspiration.

  • Esophageal Damage Considerations:

    • Issues can include esophagitis leading to dysphagia, gagging, and pain or esophageal stricture.

  • Aspiration Risks:

    • Silent aspiration can lead to chemical irritation and pulmonary edema (aspiration pneumonitis).

    • Efforts to mitigate risk include positioning and ensuring ETT cuff inflation.

Drugs for High-risk Patients

  • Proton Pump Inhibitors (PPIs) such as omeprazole or esomeprazole can be administered to raise gastric pH pre-surgery.

  • Prokinetic drugs (metoclopramide, cisapride) show efficacy in lowering incidence of gastroesophageal reflux in anesthetized dogs.

Anesthesia Considerations for Cats

  • Generally more challenging than for dogs:

    • Higher mortality rates; smaller body size complicates intubation.

    • More likely to experience hypotension during anesthesia.

    • Differences in metabolism; heightened sensitivity to adverse effects of drugs also noted.

History in Cats

  • Similar to dogs but cats may exhibit less obvious signs; specific questioning techniques required.

    • Symptoms to inquire about: Activity, sleeping patterns, respiratory rate, and appetite.

    • Grouping outdoor vs. indoor cats aids in assessing infectious disease and trauma risks.

Breed-Specific Concerns in Cats

  • Maine Coon:

    • Hypertrophic cardiomyopathy (HCM) poses anesthetic risks including potential arrhythmias and congestive heart failure post-anesthesia.

  • If murmur detected, a cardiology consult is advisable for risk assessment and anesthetic planning.

  • Outcomes indicate that HCM may not always be detectable pre-anesthetically.

Premedications in Cats

  • Similar options as for dogs with preference for certain mu-agonist opioids (oxymorphone, methadone) over morphine.

  • Use of fentanyl as CRI and buprenorphine transmucosally for effects.

  • Caution as all opioids can result in post-operative hyperthermia.

Sedatives for Cats

  • Acepromazine and dexmedetomidine provide effective sedation; higher dose requirements observed than in dogs.

  • Benzodiazepines: Generally ineffective; often leading to excitement instead.

IM Anesthesia for Cats

  • Use combinations of alfaxalone or ketamine for immobilization.

  • Essential to monitor for response post-injection; intubation as soon as laryngeal reflexes diminish.

  • Careful monitoring and provision of 100% O2 necessary.

Induction in Cats

  • Chamber or mask is inadvisable due to stress responses and potential hypotension.

  • Recommended agents: Propofol, alfaxalone, ketamine, and etomidate.

  • Specific caution with repeated use of propofol due to potential for Heinz body anemia in cats.

Intubation in Cats

  • Technique similar to dogs but often requires more caution due to potential for damaging delicate tissues.

  • Laryngospasm can occur; application of lidocaine may alleviate this.

  • Over-inflation of cuffs can lead to damage; moderate inflation necessary.

Maintenance in Cats

  • Similar drugs as in dogs, yet the MAC for inhalants is higher for cats (Iso: 1.4-1.6).

  • Adjunct drugs during maintenance are important: Opioids (fentanyl) and benzodiazepines are commonly used with caution against IV lidocaine due to associated cardiovascular risks.

Complications in Cats

  • Hypovolemia and Hypotension: Critical monitoring required due to limited blood volume (60 mL/kg).

    • AAHA consensus recommends crystalloid rates of 3 mL/kg/hr for the first hour.

    • Dopamine cited as effective for hypotension treatment.

  • Blood Transfusions: Understanding of blood types (A, B, AB) crucial for safe transfusions; cross-matching is necessary.

    • Type B cats receiving Type A blood can experience fatal reactions.

    • Majority of domestic cats are Type A, while certain breeds carry higher risk for Type B.

  • NSAID Usage: Increased incidence of adverse effects, particularly renal.

    • Long-term meloxicam under debate but shown safe in CKD cats.

Normal Blood Pressure Ranges

Table 1: Normal Arterial Blood Pressure Values in Adult Dogs & Cats

  • Systolic Arterial Pressure: 90-140 mm Hg (Dogs), 80-140 mm Hg (Cats)

  • Diastolic Arterial Pressure: 50-80 mm Hg (Dogs), 55-75 mm Hg (Cats)

  • Mean Arterial Pressure: 60-100 mm Hg (both Dogs and Cats)