The potency of a volatile inhalation agent is determined by its what?

• A. Boiling point
• B. Vapor pressure
• C. Minimum alveolar concentration
• D. Blood-gas partition coefficient

Answer: (C)

Potency of volatile anesthetics is determined by the minimum alveolar concentration (MAC) needed to prevent movement in 50% of subjects in response to a painful stimulus. MAC provides a standardized measure of how strong the drug’s immobilizing effect is: the lower the MAC, the more potent the agent because less concentration is required to achieve immobility. Boiling point and vapor pressure describe how readily the agent becomes a vapor and how the delivered concentration is achieved, which influence how we administer the drug but not the inherent strength of its anesthetic effect. The blood–gas partition coefficient affects how quickly the agent is taken up and eliminated—impacting onset and recovery—rather than the dose needed to produce immobility. So the key factor that sets potency is MAC.