A firefighter rescued from a house fire has confusion, tachypnoea, and singed nasal hairs. His standard pulse oximeter reads 98% on room air. Which statement best explains this reading?

• A The reading confirms that oxygen delivery to tissues is adequate
• B Carboxyhaemoglobin absorbs light almost identically to oxyhaemoglobin at the wavelengths used by pulse oximetry ✓
• C Pulse oximeters overestimate saturation only when methaemoglobin is present
• D The reading is falsely low and should prompt supplemental oxygen

Explanation

Conventional two wavelength pulse oximeters cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so saturation is falsely reassuring in carbon monoxide poisoning. The correct action is to measure co-oximetry for actual carboxyhaemoglobin level and give 100% oxygen, which shortens the half life of carboxyhaemoglobin from around four hours on room air to under one hour. Methaemoglobinaemia also confounds oximetry, but it typically drives readings toward 85%, unlike carbon monoxide.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.