A fireman rescued from a burning building has confusion and cherry-red skin discoloration. His pulse oximeter reads 98% and arterial PaO2 is 95 mmHg. The most appropriate explanation for the discrepancy between the oximeter reading and his clinical state is:

• A Cherry-red skin indicates very high tissue PO2 despite low arterial content
• B Dissolved oxygen contributes most of the oxygen content in carbon monoxide poisoning
• C The oximeter underestimates saturation because carboxyhaemoglobin absorbs infrared light strongly
• D Pulse oximetry cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so it overestimates true saturation ✓

Explanation

Conventional two-wavelength pulse oximeters read carboxyhaemoglobin as if it were oxyhaemoglobin because both absorb light similarly at 660 nm, so the displayed SpO2 is falsely reassuring while actual oxygen carrying capacity is severely reduced. PaO2 remains normal because it reflects dissolved oxygen, which is unaffected. Treatment decisions therefore rely on co-oximetry and blood carboxyhaemoglobin levels, not SpO2.

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

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