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Pulse oximetry in carbon monoxide poisoning — FRCA Primary MCQ

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HardPhysics and Clinical MeasurementPulse oximetry in carbon monoxide poisoningFRCA Primary

A burns patient rescued from a house fire has headache, cherry-red lips and pulse oximetry of 99% on oxygen. ABG PaO2 is high but co-oximetry shows significant carboxyhaemoglobin. Why is the pulse oximeter misleading?

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Correct answer: A — Carboxyhaemoglobin absorbs light like oxyhaemoglobin

Standard two-wavelength pulse oximeters cannot reliably distinguish oxyhaemoglobin from carboxyhaemoglobin, so saturation may appear falsely normal. The probe does not measure PaO2 directly. Methaemoglobin tends to drive readings toward about 85%, not 99%. Suspected smoke inhalation requires co-oximetry and treatment based on exposure and symptoms.

Reference: Dorsch and Dorsch Understanding Anesthesia Equipment; Primary FRCA physics texts