skip to main content

Preoxygenation and denitrogenation — FRCA Primary MCQ

Instant feedback + full explanation. One question, done properly.

EasyPhysiologyPreoxygenation and denitrogenationFRCA Primary

A healthy adult breathes 100% oxygen through a tight-fitting mask before induction. After apnoea begins, oxygen saturation remains stable for longer than it would after breathing air. Which mechanism best explains this?

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

Reveal the answer and explanation

Correct answer: E — Denitrogenation increases the alveolar oxygen reservoir

Preoxygenation replaces alveolar nitrogen with oxygen, increasing the oxygen reservoir in the functional residual capacity. Haemoglobin is already nearly saturated on air in a healthy adult, so the main gain is alveolar oxygen store rather than haemoglobin concentration. Oxygen consumption continues during apnoea. The clinical pearl is that a tight seal and adequate time are essential for effective denitrogenation.

Reference: RCoA Primary FRCA Syllabus 2026; Ganong's Review of Medical Physiology; West's Respiratory Physiology