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Restrictive lung function — SCE Respiratory MCQ

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HardRespiratory PhysiologyRestrictive lung functionSCE Respiratory

A hypoxaemic patient receives 100% oxygen but PaO2 rises only minimally. Which mechanism most strongly explains the poor response?

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

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Correct answer: A — A substantial right-to-left shunt

Blood traversing a true right-to-left shunt does not contact ventilated alveoli, so increasing inspired oxygen has limited effect. V/Q mismatch and diffusion limitation generally show a greater oxygen response, while anaemia reduces oxygen content rather than PaO2.

Reference: https://www.brit-thoracic.org.uk/quality-improvement/guidelines/