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Alveolar-arterial oxygen gradient — FRCA Primary MCQ

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HardPhysiologyAlveolar-arterial oxygen gradientFRCA Primary

An anaesthetised patient has PaO2 13.0 kPa on FiO2 0.5 with PaCO2 5.3 kPa. The alveolar oxygen tension is much higher than arterial oxygen tension. What is the most likely physiological explanation?

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Correct answer: C — Ventilation-perfusion mismatch

A large alveolar-arterial oxygen gradient implies impaired transfer from alveolus to arterial blood, most commonly V/Q mismatch in perioperative practice. Anaemia lowers oxygen content but does not increase the A-a gradient. Changes in 2,3-DPG alter haemoglobin affinity, not the PaO2 gradient. The teaching point is to separate oxygen tension from oxygen content and haemoglobin saturation.

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