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Alveolar gas equation in hypoventilation — FRCA Primary MCQ

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ModeratePhysiologyAlveolar gas equation in hypoventilationFRCA Primary

An obese patient in recovery is drowsy after opioids. ABG on air shows PaCO2 8.0 kPa and PaO2 7.5 kPa, with a near-normal A-a gradient. What is the most likely explanation?

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Correct answer: C — Alveolar hypoventilation

A raised PaCO2 with hypoxaemia and a near-normal A-a gradient is most consistent with alveolar hypoventilation. Shunt and pulmonary embolism usually widen the A-a gradient because alveolar gas and arterial gas diverge. Diffusion limitation is uncommon at rest and would not primarily raise PaCO2. The alveolar gas equation explains why PaO2 falls when PaCO2 rises.

Reference: West Respiratory Physiology; RCoA Primary FRCA Syllabus