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Nitrous oxide diffusion hypoxia — FRCA Primary MCQ

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ModeratePharmacologyNitrous oxide diffusion hypoxiaFRCA Primary

A 30-year-old woman receives nitrous oxide during a short anaesthetic. On emergence, she breathes room air and rapidly becomes hypoxaemic despite adequate ventilation. Which explanation is most likely?

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Correct answer: B — Diffusion of nitrous oxide into alveoli diluting oxygen

Diffusion hypoxia occurs when nitrous oxide rapidly leaves blood for alveoli at emergence, diluting alveolar oxygen if supplemental oxygen is not given. Nitrous oxide is not significantly metabolised to nitric oxide in this setting. Closing capacity may affect oxygenation but does not explain the immediate classic phenomenon. The pearl is to administer oxygen during early recovery after nitrous oxide use.

Reference: RCoA Primary FRCA Syllabus 2026; Peck, Hill & Williams Pharmacology for Anaesthesia