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Oxygen-induced hypercapnia — SCE Respiratory MCQ

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HardCOPDOxygen-induced hypercapniaSCE Respiratory

A 69-year-old man with COPD is admitted with an infective exacerbation. He is placed on 15 L/min oxygen via reservoir mask in triage, after which he becomes somnolent. ABG shows pH 7.19, PaCO2 12.2 kPa, PaO2 17.0 kPa and HCO3− 36 mmol/L. Chest radiograph shows hyperinflation without pneumothorax. What is the most likely underlying cause?

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Correct answer: A — Excess oxygen worsening ventilation-perfusion mismatch and hypercapnia

The high PaO2 with severe hypercapnic acidosis after uncontrolled oxygen is typical of oxygen-induced hypercapnia in COPD. Pulmonary embolism usually causes hypoxaemia with low or normal PaCO2 unless late and severe. Controlled oxygen with repeated blood gases is a core safety intervention in patients at risk of hypercapnic respiratory failure.

Reference: BTS Emergency Oxygen Guideline; BTS/ICS acute hypercapnic respiratory failure guideline