Which factor lowers MAC for volatile anesthetics?

Which factor lowers MAC for volatile anesthetics?

• A. Serum sodium 151 mEq/L
• B. Red hair
• C. Body temperature 38° C
• D. Acute ethanol ingestion

Answer: (D)

Acute ethanol ingestion lowers MAC because ethanol itself causes CNS depression, so the CNS is already more inhibited and a smaller amount of inhaled anesthetic is needed to prevent movement in response to a surgical stimulus. Ethanol and volatile anesthetics both enhance inhibitory pathways (like GABA) and blunt excitatory pathways (such as NMDA), producing a synergistic reduction in the concentration of volatile agent required to achieve the same immobility effect. By comparison, fever increases MAC since higher body temperature raises cerebral metabolic rate and makes the CNS more resistant to anesthetics, requiring more agent; red hair has been associated with higher, not lower, anesthetic requirements in some studies; and mild hypernatremia doesn’t have a consistent, direct effect to lower MAC. So the factor that lowers MAC in this set is acute ethanol ingestion.

Acute ethanol ingestion lowers MAC because ethanol itself causes CNS depression, so the CNS is already more inhibited and a smaller amount of inhaled anesthetic is needed to prevent movement in response to a surgical stimulus. Ethanol and volatile anesthetics both enhance inhibitory pathways (like GABA) and blunt excitatory pathways (such as NMDA), producing a synergistic reduction in the concentration of volatile agent required to achieve the same immobility effect.

By comparison, fever increases MAC since higher body temperature raises cerebral metabolic rate and makes the CNS more resistant to anesthetics, requiring more agent; red hair has been associated with higher, not lower, anesthetic requirements in some studies; and mild hypernatremia doesn’t have a consistent, direct effect to lower MAC. So the factor that lowers MAC in this set is acute ethanol ingestion.