Principles of Anesthesia Flashcards

Which of the following factors would most likely DECREASE the Minimum Alveolar Concentration (MAC) of an inhalant anesthetic?

Answer: Administration of an opioid premedication

Minimum Alveolar Concentration (MAC) is a measure of anesthetic potency. Factors that decrease MAC make the patient more sensitive to the anesthetic, requiring a lower concentration. Co-administration of other CNS depressants, such as opioids, has a MAC-sparing effect. Factors that increase the metabolic rate or CNS activity, such as hyperthermia and stimulants, tend to increase MAC.

During a canine exploratory laparotomy under general anesthesia, the capnograph reading abruptly drops from 40 mmHg to 5 mmHg, accompanied by sudden hypotension and tachycardia. Which of the following is the most likely cause?

Answer: A massive pulmonary thromboembolism

A sudden, severe drop in end-tidal CO2 (ETCO2) indicates a failure of CO2 to be transported to the lungs, which is a hallmark of a massive increase in alveolar dead space (ventilation without perfusion). A pulmonary thromboembolism (PTE) would cause this, along with obstructive shock leading to hypotension and reflex tachycardia. Esophageal intubation would result in a near-zero ETCO2 reading from the beginning. Exhausted CO2 absorbent would cause a gradual rise in inspired and end-tidal CO2. A machine leak would make it difficult to maintain anesthetic depth but would not typically cause such a specific and drastic ETCO2 drop.

Anesthetized hypotension in a healthy patient, defined as a mean arterial pressure (MAP) below 60 mmHg, is most commonly caused by which of the following?

Answer: Peripheral vasodilation and myocardial depression from anesthetic agents

The most common cause of hypotension in healthy anesthetized patients is the dose-dependent effect of inhalant and many injectable anesthetics. These agents cause peripheral vasodilation (decreasing systemic vascular resistance) and direct myocardial depression, both of which lower blood pressure. Inadequate stimulation might lead to a lighter plane and higher blood pressure. Excessive fluid can cause overload but isn't the primary cause of hypotension. Mild to moderate hypercarbia often causes an initial increase in blood pressure due to sympathetic stimulation.

A veterinarian needs to provide standing sedation for a 650 kg dairy cow. When choosing to use xylazine, an alpha-2 adrenergic agonist, which of the following statements is most accurate?

Answer: Cattle are highly sensitive to xylazine, requiring approximately one-tenth the dose used in horses.

Ruminants, especially cattle, are exceptionally sensitive to the sedative and analgesic effects of xylazine. The dose required for sedation is approximately 1/10th of the dose needed for a horse. This species-specific sensitivity is a critical safety consideration. Tolazoline is the more commonly used and approved reversal agent in cattle, though others may be used off-label.

What is the primary function of the soda lime granules within the canister of a rebreathing anesthetic circuit?

Answer: To chemically absorb exhaled carbon dioxide.

Soda lime is a chemical absorbent, primarily composed of calcium hydroxide, whose function is to remove carbon dioxide from the exhaled gases in a rebreathing circuit. This allows the other gases (oxygen and anesthetic agent) to be safely rebreathed by the patient, conserving heat, humidity, and anesthetic agent. Waste gas is removed by the scavenger system, and the vaporizer is responsible for turning liquid anesthetic into gas.

Which of the following best describes the mechanism of action for local anesthetics such as lidocaine and bupivacaine?

Answer: They block voltage-gated sodium channels, preventing nerve impulse propagation.

The primary mechanism of action for local anesthetics is the reversible blockade of voltage-gated sodium channels on the intracellular side of the nerve membrane. By preventing the influx of sodium, they block the depolarization phase of the action potential, thereby inhibiting the generation and conduction of nerve impulses.