MAC-Awake — FRCA Primary MCQ

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ModeratePharmacology - Inhalational AgentsMAC-AwakeFRCA Primary

For potent volatile anaesthetic agents such as sevoflurane and isoflurane, MAC-awake is approximately what fraction of age-adjusted MAC?

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Correct answer: B — One-third of MAC

MAC-awake is the alveolar concentration of an inhaled anaesthetic that suppresses an appropriate response to verbal command in 50% of subjects. For potent volatile agents commonly considered in FRCA pharmacology, including sevoflurane and isoflurane, it is approximately 0.33–0.35 MAC; therefore, the best answer is B, one-third of MAC. Standard MAC instead describes immobility in 50% of subjects exposed to a defined noxious stimulus, so MAC-awake is substantially lower than 1 MAC. One-half and two-thirds are too high for sevoflurane or isoflurane, although gaseous agents can have different ratios. One-tenth is below the usual MAC-awake range.

Reference: Katoh T, Suguro Y, Nakajima R, Kazama T, Ikeda K. Blood concentrations of sevoflurane and isoflurane on recovery from anaesthesia. British Journal of Anaesthesia. 1992;69:259–262. https://pubmed.ncbi.nlm.nih.gov/11046204/