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V/Q mismatch response to oxygen — FRCA Primary MCQ

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HardPhysiologyV/Q mismatch response to oxygenFRCA Primary

A smoker develops hypoxaemia after abdominal surgery. CXR shows basal atelectasis, PaCO2 is normal and PaO2 improves markedly with 60% oxygen. What mechanism best explains the hypoxaemia?

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Correct answer: E — Ventilation-perfusion mismatch

Ventilation-perfusion mismatch is the commonest mechanism of post-operative hypoxaemia and usually improves with supplemental oxygen. A pure shunt is more resistant to oxygen because perfused units receive no ventilation. Anaemia reduces oxygen content but not PaO2, and cyanide toxicity impairs tissue use rather than arterial oxygenation. Basal atelectasis often produces a mixture of low V/Q units and shunt.

Reference: West Respiratory Physiology; Nunn's Applied Respiratory Physiology