NAVLE Veterinary Anesthesia and Pain Management 4

Question 1

A 7-year-old horse undergoing colic surgery develops hypotension (MAP 45 mmHg) unresponsive to IV fluid boluses. The anesthetist adds dobutamine at 3 mcg/kg/min with minimal improvement. Which pharmacologic intervention is MOST appropriate as the next step?

Accepted Answer

Add norepinephrine at 0.1–0.5 mcg/kg/min

Answer

Inhalant anesthesia (isoflurane/sevoflurane) in horses causes significant vasodilation and myocardial depression. When dobutamine (a β1/β2 agonist that increases cardiac output but causes vasodilation) is insufficient, norepinephrine is the drug of choice because it provides both α1-mediated vasoconstriction (increases SVR) and β1 inotropic support. Phenylephrine is a pure α1 agonist that raises SVR but can cause reflex bradycardia and reduced cardiac output—problematic in an already-compromised patient. Vasopressin can be used in refractory cases but is not typically a second-line agent. Escalating dobutamine alone will worsen vasodilation.

Question 2

A cat is being anesthetized with an alfaxalone TIVA protocol for a lengthy orthopedic repair. Approximately 90 minutes into the procedure, the cat begins showing signs of awareness (purposeful movement, increased HR/BP) despite what appears to be an adequate infusion rate. Which mechanism BEST explains this phenomenon?

Accepted Answer

Tachyphylaxis to alfaxalone due to GABA-A receptor downregulation

Answer

Alfaxalone acts on GABA-A receptors (specifically the neurosteroid binding site on β subunits). With prolonged, continuous exposure, GABA-A receptor internalization and desensitization (tachyphylaxis) can occur, requiring higher infusion rates over time to maintain the same depth of anesthesia. This is a clinically documented phenomenon with alfaxalone TIVA, particularly in cats during extended procedures. CYP induction is not rapid enough to explain intraoperative changes. Redistribution (option C) would actually deepen anesthesia as context-sensitive half-time increases. Alfaxalone does not have known clinically significant excitatory metabolites.

Question 3

During inhalant anesthesia maintenance in a dog, the end-tidal CO2 (EtCO2) rises to 65 mmHg despite manual ventilation at 15 breaths/min with a tidal volume of 15 mL/kg. Arterial blood gas confirms PaCO2 of 70 mmHg. Which condition is the MOST likely cause of this disproportionately elevated PaCO2 relative to minute ventilation?

Accepted Answer

Increased dead space ventilation from pulmonary thromboembolism

Answer

The key finding is that PaCO2 is disproportionately elevated relative to what should be adequate minute ventilation (15 mL/kg × 15 breaths/min = ~225 mL/kg/min is above normal). Pulmonary thromboembolism significantly increases physiologic dead space—areas that are ventilated but not perfused—so CO2 is not eliminated despite adequate ventilation effort. This creates an elevated PaCO2-to-EtCO2 gradient (normally <5 mmHg; a gradient of 5 mmHg here suggests shunt/dead space). Malignant hyperthermia causes rapid CO2 rise with hyperthermia and rigidity. Exhausted CO2 absorbent would cause rebreathing reflected as elevated inspired CO2 on capnography. Bronchospasm causes hypoxemia via V/Q mismatch but less dramatically impairs CO2 clearance.

Question 4

A 10-year-old dog with severe hepatic insufficiency requires a pain management protocol following abdominal surgery. The clinician wants to avoid drugs with significant hepatic first-pass metabolism and those relying on hepatic glucuronidation. Which opioid is MOST appropriate for this patient?

Accepted Answer

Methadone CRI

Answer

Methadone is the most appropriate choice here. It undergoes N-demethylation (CYP3A4) but has minimal first-pass effect when given parenterally, and does NOT rely on glucuronidation for elimination. Importantly, methadone's large volume of distribution and its partial tissue binding makes it less reliant on single-pass hepatic clearance. Morphine is highly dependent on hepatic glucuronidation (to morphine-6-glucuronide and morphine-3-glucuronide)—these metabolites accumulate in hepatic failure, causing prolonged sedation and respiratory depression. Codeine requires hepatic CYP2D6 conversion to morphine for effect and has high first-pass metabolism. Buprenorphine (OTM/sublingual) bypasses GI first-pass but still requires hepatic metabolism; its very high protein binding is also a concern in hypoalbuminemic hepatic patients.

Question 5

A 3-year-old intact female rabbit is anesthetized for an ovariohysterectomy. Forty minutes into maintenance on 2.5% isoflurane, the rabbit becomes apneic with no response to a sternal rub. SpO2 drops to 82%. Which factor is MOST responsible for this rabbit's increased anesthetic risk compared to dogs and cats?

Accepted Answer

Rabbits have obligate nasal breathing and a relatively small thoracic cavity with diaphragm-dependent ventilation

Answer

Rabbits are obligate nasal breathers with a diaphragm-dependent respiratory pattern and a relatively large abdominal visceral mass that restricts diaphragmatic excursion in dorsal recumbency. They have a narrow thoracic cavity relative to body size, poor respiratory reserve, and a highly sensitive respiratory center that is easily depressed by inhalants. These anatomical features make apnea common at surgical planes of anesthesia and make ventilatory support mandatory. The rabbit MAC for isoflurane (~2.05%) is indeed slightly higher than in dogs (~1.28%) and cats (~1.63%), but this alone doesn't cause apnea at 2.5%. Rabbits do have µ-opioid receptors. While high metabolic rate is real, it does not cause apnea.

NAVLE - North American Veterinary Licensing Examination Practice Test

NAVLE - North American Veterinary Licensing Examination Practice Test

NAVLE Veterinary Anesthesia and Pain Management 4