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Diffusion Hypoxia — FRCA Primary MCQ

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ModeratePharmacology - Inhalational AgentsDiffusion HypoxiaFRCA Primary

At the end of an anaesthetic using a high inspired concentration of nitrous oxide, the nitrous oxide is discontinued and the patient begins breathing air. Which mechanism best explains the resulting transient diffusion hypoxia?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: A — Rapid movement of nitrous oxide from blood into the alveoli dilutes alveolar oxygen

Explanation lettering: C = shown as A · E = shown as B · D = shown as C · B = shown as D · A = shown as E

C is correct. After nitrous oxide is discontinued, its low blood–gas solubility permits rapid elimination of a large volume from blood into the alveoli. If the patient breathes air, this transiently dilutes alveolar oxygen, reducing PAO2 and potentially PaO2—the Fink effect. Alveolar carbon dioxide is also diluted, which may reduce ventilatory stimulation and compound the fall in oxygen tension, but residual central respiratory depression is not the defining mechanism. Nitrous oxide does not cause diffusion hypoxia by altering haemoglobin–oxygen affinity or increasing metabolic oxygen consumption. Nitrogen absorption is also not responsible. Administration of supplemental oxygen during early emergence prevents clinically important alveolar oxygen dilution.

Reference: Fink BR. An early example of evidence-based medicine: hypoxemia due to nitrous oxide. Anesthesiology. 2007;106(1):1–2. https://pubmed.ncbi.nlm.nih.gov/17197861/