A fireman rescued from a house fire has 30% TBSA burns, singed nasal vibrissae, soot in the oropharynx and confusion. His oxygen saturation by pulse oximeter reads 98% on room air. The most important reason this oximetry reading is unreliable in this patient is:
- A Pulse oximeters fail when peripheral perfusion is poor in shock
- B Methaemoglobinaemia caused by combustion gases falsely lowers readings
- C The device cannot distinguish carboxyhaemoglobin from oxyhaemoglobin ✓
- D Pulse oximetry overestimates saturation in patients with cyanide toxicity
Correct answer: C. The device cannot distinguish carboxyhaemoglobin from oxyhaemoglobin
Explanation
Standard pulse oximetry uses two wavelengths and reads carboxyhaemoglobin identically to oxyhaemoglobin, so a victim of smoke inhalation may show a falsely reassuring 98% while actual tissue oxygen delivery is severely impaired. The diagnosis requires co-oximetry measuring the carboxyhaemoglobin level, and treatment is 100% oxygen, which shortens the half-life of carboxyhaemoglobin from about 4 hours to under an hour. Hyperbaric oxygen is considered when levels exceed roughly 25% with neurological features.
Reference: Sabiston Textbook of Surgery, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.