The MAC-awake of an inhalational agent — the concentration at which 50% of patients open their eyes on command — is approximately what fraction of MAC?

• A 1.5 × MAC
• B 0.5 × MAC
• C 0.3 × MAC ✓
• D 1.0 × MAC

Explanation

MAC-awake is the alveolar concentration below which 50% of patients regain consciousness (open eyes to command during emergence). It is approximately 0.3–0.4 × MAC for volatile agents. For example, MAC-awake for isoflurane is ~0.5% (MAC = 1.17%), for sevoflurane ~0.7% (MAC = 2.0%). This is clinically relevant for understanding the concentration at which recall and awareness become possible during emergence, explaining why patients can respond before they are fully alert from volatile anaesthesia. MAC-bar (blocking autonomic response to intubation) is 1.6–2.0 × MAC.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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