MAC-Awake Sevoflurane — FRCA Final MCQ

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ModerateGeneral AnaesthesiaMAC-Awake SevofluraneFRCA Final

A 45-year-old man (ASA II, 80 kg) undergoes inguinal hernia repair under GA with an LMA. Anaesthesia is maintained with sevoflurane 2% in O2/air. His end-tidal sevoflurane is 1.8%. What is the approximate MAC-awake value for sevoflurane (the concentration at which 50% of patients open their eyes to command)?

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Correct answer: A — 0.3%

MAC-awake (MAC for return of consciousness) is approximately 0.33 × MAC for all volatile agents. For sevoflurane (MAC = 2.0% in adults): MAC-awake ≈ 0.66%. At end-tidal sevoflurane 1.8% (0.9 MAC), the patient is well above MAC-awake (0.66%) so awareness is extremely unlikely. MAC-awake is the concentration at which 50% of subjects respond to verbal command – it represents the threshold between consciousness and unconsciousness. BIS values of 60-70 approximate MAC-awake. This concept is important for understanding awareness risk during the transition between maintenance and emergence.

Reference: RCOA – 2023 – Final FRCA curriculum: MAC variants; NAP5 – 2014 – Accidental awareness