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Intrapulmonary shunt in ARDS — FRCA Primary MCQ

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HardPhysiologyIntrapulmonary shunt in ARDSFRCA Primary

A patient with severe ARDS is ventilated with FiO2 1.0. PaO2 remains 7.9 kPa despite high inspired oxygen, and chest imaging shows dependent consolidation. What is the most likely mechanism of hypoxaemia?

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Correct answer: A — Diffusion limitation across normal alveoli

Consolidated, perfused but non-ventilated lung units create an intrapulmonary shunt that responds poorly to increased FiO2. Anaemia reduces oxygen content but does not usually cause low PaO2. Diffusion limitation typically improves more with supplemental oxygen than true shunt. The teaching point is that refractory hypoxaemia suggests shunt physiology.

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