A firefighter rescued from a closed-space house fire is confused with a hoarse voice and singed nasal hairs. His peripheral pulse oximetry reads 98 percent on room air, yet he appears cyanotic. Why is the pulse oximeter value misleading in this setting?
- A Smoke inhalation causes methaemoglobinaemia that spuriously lowers readings
- B Conventional pulse oximetry cannot distinguish carboxyhaemoglobin from oxyhaemoglobin ✓
- C Pulse oximeters underestimate saturation when peripheral perfusion is poor
- D Pulse oximetry measures dissolved oxygen rather than bound oxygen
Explanation
Standard two-wavelength pulse oximeters read both carboxyhaemoglobin and oxyhaemoglobin as saturated species, so the displayed saturation is falsely normal despite severe functional anaemia from carbon monoxide poisoning. Arterial blood gas shows a normal PaO2 with a low calculated oxygen saturation, and co-oximetry gives the true carboxyhaemoglobin level. Treatment is 100 percent oxygen, with hyperbaric therapy considered for severe poisoning or pregnancy. Methaemoglobinaemia affects readings differently, and poor perfusion causes signal dropout rather than a consistently reassuring number.
Reference: Sabiston Textbook of Surgery, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.