A firefighter rescued from a burning building has headache, confusion, and bright cherry-red oral mucosa. His SpO2 reads 98% and arterial PaO2 is 105 mmHg on room air. Carboxyhemoglobin is 28%. Why is his pulse oximetric saturation misleadingly high?

• A Pulse oximetry measures dissolved oxygen directly and ignores bound oxygen
• B Skin pigmentation from soot scatters light and artificially elevates the ratio
• C Carbon monoxide increases the affinity of remaining heme sites, raising true SaO2
• D The device cannot spectroscopically distinguish carboxyhemoglobin from oxyhemoglobin ✓

Explanation

Conventional two-wavelength pulse oximeters read carboxyhemoglobin as if it were oxyhemoglobin at the 660 nm wavelength, so SpO2 remains falsely normal despite reduced functional oxygen-carrying capacity. PaO2 is genuinely normal because carbon monoxide does not affect the amount of oxygen physically dissolved in plasma; it reduces content, not tension. Option C describes the leftward shift caused by CO, which worsens tissue hypoxia rather than raising saturation. Treatment requires 100% oxygen or hyperbaric oxygen.

Reference: Ganong Review of Medical Physiology, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.