A 45-year-old man rescued from a house fire arrives in the emergency department. He is alert with soot around the mouth. Pulse oximeter shows SpO2 of 99% on room air, yet blood gas analysis reveals significant carboxyhaemoglobinaemia. Why does standard pulse oximetry fail in this patient?
- A Carboxyhaemoglobin absorbs light identically to oxyhaemoglobin at both wavelengths used by the pulse oximeter ✓
- B Carbon monoxide abolishes the plethysmographic signal
- C Soot on the skin scatters the emitted light
- D The pulse oximeter switches off above a PaCO2 of 50 mmHg
Correct answer: A. Carboxyhaemoglobin absorbs light identically to oxyhaemoglobin at both wavelengths used by the pulse oximeter
Explanation
Standard pulse oximeters use two wavelengths, 660 nm (red) and 940 nm (infrared). Carboxyhaemoglobin has an absorption spectrum at these two wavelengths that is nearly identical to oxyhaemoglobin, so the device reads COHb as oxyhaemoglobin and displays a falsely reassuring, often normal or high, saturation. The plethysmographic signal is unaffected, and skin soot may cause a poor trace but does not explain a falsely normal reading. Diagnosis requires multiwavelength co-oximetry on an arterial sample.
Reference: Morgan and Mikhail's Clinical Anesthesiology, 5th ed.
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Written and medically reviewed by the StethoPrep medical team.