A firefighter rescued from a house fire has hoarseness, carbonaceous sputum, and confusion. His SpO2 reads 99% on room air and arterial blood gas shows PaO2 of 98 mmHg. Which statement best explains this apparent discrepancy?
- A The pulse oximeter probe is malfunctioning due to soot on the skin
- B Carbon monoxide poisoning lowers PaO2, so a normal PaO2 excludes it
- C Pulse oximetry cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, giving a falsely normal reading ✓
- D Pulse oximetry overestimates saturation only in anaemic patients
Correct answer: C. Pulse oximetry cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, giving a falsely normal reading
Explanation
Conventional pulse oximeters use two wavelengths and read carboxyhaemoglobin as if it were oxyhaemoglobin, so saturation appears deceptively normal in carbon monoxide poisoning. PaO2 is also normal because it measures dissolved oxygen, which carbon monoxide does not displace; only co-oximetry reveals the true carboxyhaemoglobin level. The treatment is 100% oxygen, which shortens the carboxyhaemoglobin half-life from roughly 4 hours on room air to well under an hour.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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