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Closing capacity exceeding FRC under anaesthesia — FRCA Primary MCQ

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HardPhysiologyClosing capacity exceeding FRC under anaesthesiaFRCA Primary

An 82-year-old obese smoker is supine after induction. FRC is reduced, oxygen saturation falls during quiet ventilation and atelectasis is seen at the lung bases. What is the most likely physiological explanation?

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Correct answer: D — Reduced closing capacity preventing airway closure

In older, obese and anaesthetised patients, FRC falls and closing capacity may exceed FRC, causing small airway closure during tidal breathing. This produces atelectasis and V/Q mismatch, particularly in dependent lung. Increased FRC would be protective rather than harmful. The pearl is that preoxygenation, PEEP and recruitment manoeuvres target this physiology.

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