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Oxygen-induced hypercapnia — SCE Acute Medicine MCQ

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EasyAcute Respiratory PresentationsOxygen-induced hypercapniaSCE Acute Medicine

A 73-year-old man is assessed on the acute medical unit. He has COPD and becomes drowsy after high-flow oxygen. ABG shows PaO2 18 kPa, PaCO2 11 kPa and pH 7.19. Initial assessment includes relevant observations, bedside tests and urgent blood results. What is the most likely underlying cause/mechanism?

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Correct answer: E — Ventilation-perfusion mismatch with increased physiological dead space

Ventilation-perfusion mismatch with increased physiological dead space is the best answer because excess oxygen in COPD can worsen hypercapnia through V/Q effects, loss of hypoxic pulmonary vasoconstriction and Haldane physiology. Acute right ventricular pressure overload is plausible, but it is less appropriate for the physiology or timing described in this stem. The other distractors either fit a neighbouring presentation, delay definitive treatment, or miss the key immediate risk. Clinical pearl: Target saturations of 88-92% reduce the risk of oxygen-induced hypercapnia.

Reference: BTS oxygen guideline