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Oxygen-induced hypercapnia in COPD — SCE Acute Medicine MCQ

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HardAcute Respiratory FailureOxygen-induced hypercapnia in COPDSCE Acute Medicine

A 71-year-old man with COPD is admitted with breathlessness. He is placed on 15 L oxygen before review and becomes increasingly somnolent. ABG shows pH 7.25, PaCO2 10.8 kPa, PaO2 19.2 kPa and HCO3 36 mmol/L. Chest radiograph shows hyperinflation without consolidation. What is the most likely underlying mechanism of the deterioration?

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Correct answer: D — Worsening ventilation-perfusion mismatch and loss of hypoxic pulmonary vasoconstriction

High-concentration oxygen can worsen hypercapnia in COPD mainly by worsening ventilation-perfusion mismatch, with additional contributions from the Haldane effect and reduced hypoxic drive in some patients. The ABG shows oxygen-induced hypercapnic respiratory failure rather than respiratory alkalosis. PE usually causes hypoxaemia with low or normal PaCO2 early. Controlled oxygen with a target saturation of 88 to 92 percent is the safer approach in patients at risk of CO2 retention.

Reference: BTS guideline for oxygen use in adults, NICE NG115