NAVLE Anesthesia | Drug Classes, Monitoring, and Protocols

Anesthesia questions on the NAVLE typically test drug contraindications, monitoring parameter interpretation, and species-specific differences. The most common traps involve ketamine contraindications, acepromazine restrictions, and Greyhound barbiturate sensitivity.

Premedication Drug Classes

Alpha-2 Agonists

Dexmedetomidine and xylazine produce dose-dependent sedation, analgesia, and muscle relaxation. Cardiovascular effects: initial hypertension (increased SVR) followed by bradycardia and decreased cardiac output. Reversal agent: atipamezole (specific for alpha-2 agonists). Species sensitivity: cattle are 10× more sensitive to xylazine than horses (xylazine dose 0.05–0.1 mg/kg in cattle vs 0.5–1.1 mg/kg in horses).

Opioids

Mu agonists (morphine, hydromorphone, fentanyl): full analgesia, respiratory depression, nausea/vomiting. Butorphanol: mu antagonist/kappa agonist, moderate analgesia, less respiratory depression, suitable for cats. Buprenorphine: partial mu agonist, slow onset (20–30 min), long duration (6–8 hr), excellent sublingual bioavailability in cats. Reversal: naloxone (reverses all mu effects).

Acepromazine

Dopamine antagonist—provides sedation but NOT analgesia. Multiple contraindications: bleeding disorders (inhibits platelet function), hypovolemia/shock, epilepsy, Boxer dogs (vagal sensitivity → profound bradycardia), brachycephalic breeds (respiratory compromise), laminitic horses. In colic horses: CONTRAINDICATED due to vasodilation in a potentially hypovolemic patient.

Classic NAVLE TrapAcepromazine is CONTRAINDICATED in: hypovolemic patients, bleeding disorders, Boxers, laminitic horses, epileptic patients, and brachycephalic breeds. It provides NO analgesia. It is a sedative only. Do not use it as the sole premedication for painful procedures.

Induction Agents

Drug	Key Properties	Contraindications
Propofol	Fast onset/offset; good intubation; dose-dependent apnea	Repeated use in cats (Heinz body anemia risk from phenol)
Ketamine	Dissociative; maintains cardiovascular function; bronchodilator; does NOT relax muscles (causes rigidity)	Head trauma (↑ICP), glaucoma (↑IOP), HCM in cats, urethral obstruction cats
Alfaxalone	Neurosteroid; similar to propofol; less pain on injection; minimal Heinz body risk	Few absolute contraindications
Etomidate	Minimal cardiovascular depression; ideal for ASA IV–V	Adrenocortical suppression (single-dose, transient); pretreat with benzodiazepine to prevent myoclonus

NAVLE PearlKetamine INCREASES intracranial pressure (ICP) and intraocular pressure (IOP). It is CONTRAINDICATED in head trauma, glaucoma, and procedures where elevated ICP is dangerous. It is also contraindicated in cats with urethral obstruction (causes muscle rigidity making catheterization impossible, and is renally excreted in cats). Always combine ketamine with a benzodiazepine for muscle relaxation.

Inhalant Anesthetics

MAC (Minimum Alveolar Concentration) = the concentration that prevents movement in 50% of patients in response to a surgical stimulus. Higher MAC = more potent agent needed (more potent = lower MAC value).

Agent	Dog MAC	Cat MAC	Key Notes
Isoflurane	1.28%	1.63%	Standard; minimal metabolism; cardiovascular depression dose-dependent
Sevoflurane	2.36%	2.58%	Faster induction/recovery; less pungent; good for mask induction; more expensive
Nitrous oxide	~200% (no MAC alone)	~255%	Second gas effect; speeds induction; CONTRAINDICATED in pneumothorax, GI obstruction (diffuses into gas spaces)

Monitoring Parameters

ETCO2: Normal 35–45 mmHg. >45 = hypoventilation (too deep, airway obstruction, rebreather). <35 = hyperventilation. Also confirms endotracheal tube placement. SpO2: Normal >95%. <90% requires immediate intervention. Blood pressure: Mean arterial pressure >60 mmHg; systolic >90 mmHg; treat hypotension with dopamine or dobutamine CRI and/or fluid bolus. Temperature: hypothermia risk especially in small animals, rabbits; use warm IV fluids and forced-air warming.

Greyhound and Sighthound Sensitivity

Greyhounds and other sighthounds (Whippets, Salukis, Afghan Hounds) are highly sensitive to barbiturates (thiopental) due to: low body fat (thiopental is redistributed to fat, so less redistribution = prolonged CNS effect) and lower plasma protein levels (more free drug). Recovery from thiopental in Greyhounds can take hours. Use propofol or alfaxalone instead. Propofol is also somewhat prolonged in sighthounds.

Species-Specific Notes

Cats: Laryngospasm risk—always spray larynx with lidocaine before intubation. Do NOT fast cats before anesthesia... wait, cats ARE fasted 6–8 hours (unlike rabbits which should NEVER be fasted). Rabbits: NEVER fast before anesthesia (no vomiting reflex, fasting causes GI stasis and hypoglycemia). Horses: IPPV (assisted ventilation) is routine for surgeries >30–60 min; hypoxemia and myopathy are major concerns in dorsal recumbency. Ruminants: Bloat risk in recumbency; intubate or position in sternal if heavily sedated.

NAVLE TipMalignant hyperthermia: triggered by halothane or succinylcholine (triggering agents); rapid temperature rise, muscle rigidity, severe hypercapnia, metabolic acidosis. Specific treatment: dantrolene sodium IV. Remove triggering agent, provide supportive cooling. Rare but classic NAVLE pharmacology question.

NAVLE Anesthesia Guide: Drug Classes, Monitoring, and Species-Specific Protocols