High-flow oxygen in a patient with COPD and chronic CO₂ retention can worsen hypercapnia and acidosis, through ventilation–perfusion mismatch and the Haldane effect. In the ambulance and the resuscitation bay, the reflex to push saturations to 100% causes harm.
Use controlled oxygen through a Venturi mask, target 88 to 92%, and check a blood gas within an hour. A pH below 7.35 with a raised CO₂ despite initial treatment is the trigger to start non-invasive ventilation.
- Target SpO₂ 88–92% in known or suspected CO₂ retainers
- Use a Venturi mask (24–28%) rather than a non-rebreather
- Repeat a blood gas within 30 to 60 minutes
- pH <7.35 and raised PaCO₂ after initial treatment: start NIV
Why it matters
Over-oxygenation is one of the commonest avoidable causes of worsening acidosis in COPD exacerbations.
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