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Nitrous oxide expansion of closed gas spaces — FRCA Primary MCQ

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HardPhysics and Clinical MeasurementNitrous oxide expansion of closed gas spacesFRCA Primary

A ventilated patient receives 70% nitrous oxide in oxygen. A compliant air-filled cuff on a supraglottic airway becomes progressively more inflated during the case. Which physical principle best explains this?

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

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Correct answer: C — Nitrous oxide diffuses into closed gas spaces faster than nitrogen leaves

Nitrous oxide is more soluble than nitrogen and diffuses into closed gas spaces faster than nitrogen leaves, increasing volume or pressure. This can raise cuff pressure, expand pneumothoraces or bowel gas, and affect middle ear pressure. Carbon dioxide absorption is not the mechanism in an airway cuff. The pearl is to monitor cuff pressure or avoid nitrous oxide in vulnerable closed-space situations.

Reference: RCoA Primary FRCA Syllabus 2026; Davis and Kenny Basic Physics and Measurement in Anaesthesia