MAC Nitrous Oxide — FRCA Primary MCQ
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Correct answer: B — 104%
The correct answer is B: approximately 104%. MAC is the steady-state alveolar concentration that prevents movement in response to a standard noxious stimulus in 50% of subjects. Direct human measurement gave a nitrous oxide MAC of 1.04 atmospheres absolute; educational tables commonly quote approximately 101–105%. Because this exceeds 100% at normal atmospheric pressure, one MAC of nitrous oxide cannot be administered under normobaric conditions, particularly when an adequate inspired oxygen concentration must also be provided. Nitrous oxide therefore cannot produce reliable surgical immobility as the sole general anaesthetic at sea level, although it may be used at lower concentrations for analgesia or as an adjunct. Concentrations of 40%, 60% and 80% are sub-MAC, while 120% is higher than the accepted approximate value.
Reference: Royal College of Anaesthetists, e-Learning Anaesthesia, comparative characteristics of inhalational anaesthetic agents, Table 1, accessed 2026. https://rcoa.ac.uk/media/25671