Which factor among the following increases MAC for volatile anesthetics EXCEPT?

Which factor among the following increases MAC for volatile anesthetics EXCEPT?

• A. Cocaine.
• B. Hyperthyroidism.
• C. Monoamine oxidase inhibitor therapy.
• D. Tricyclic antidepressants.

Answer: (B)

MAC is the concentration of a volatile anesthetic needed to prevent movement in 50% of patients exposed to a standard noxious stimulus. It’s influenced by factors that raise or lower CNS excitability or metabolic demand. Substances that increase sympathetic activity or brain metabolism tend to raise MAC, meaning you’d need more anesthetic to achieve immobility. Cocaine boosts CNS arousal and catecholamine activity, so MAC goes up. Monoamine oxidase inhibitor therapy raises levels of norepinephrine and other monoamines, increasing CNS stimulation and MAC. Tricyclic antidepressants do the same by boosting norepinephrine and serotonin activity, also elevating MAC. Hyperthyroidism, however, is not typically listed as a factor that increases MAC. Its effect on MAC isn’t consistently to raise it, and in the context of this question it’s considered the exception. Thus hyperthyroidism does not increase MAC, while the others do.

MAC is the concentration of a volatile anesthetic needed to prevent movement in 50% of patients exposed to a standard noxious stimulus. It’s influenced by factors that raise or lower CNS excitability or metabolic demand. Substances that increase sympathetic activity or brain metabolism tend to raise MAC, meaning you’d need more anesthetic to achieve immobility. Cocaine boosts CNS arousal and catecholamine activity, so MAC goes up. Monoamine oxidase inhibitor therapy raises levels of norepinephrine and other monoamines, increasing CNS stimulation and MAC. Tricyclic antidepressants do the same by boosting norepinephrine and serotonin activity, also elevating MAC.

Hyperthyroidism, however, is not typically listed as a factor that increases MAC. Its effect on MAC isn’t consistently to raise it, and in the context of this question it’s considered the exception. Thus hyperthyroidism does not increase MAC, while the others do.