A firefighter rescued from a burning building has cherry-red skin, headache, and confusion. Pulse oximetry reads 98% SpO2, but he is visibly hypoxic. Arterial blood gas shows a normal PaO2. The discrepancy between the pulse oximeter reading and the patient's clinical state occurs because:

• A Standard pulse oximetry uses only two wavelengths that cannot distinguish carboxyhemoglobin from oxyhemoglobin ✓
• B Carbon monoxide increases PaO2 by displacing oxygen from hemoglobin
• C Carboxyhemoglobin absorbs light at the same wavelengths as oxyhemoglobin
• D Carbon monoxide dissolves in plasma and falsely elevates the calculated saturation

Explanation

Conventional pulse oximeters use two wavelengths (660 nm red and 940 nm infrared) and cannot differentiate carboxyhemoglobin from oxyhemoglobin, so SpO2 reads falsely high despite severe tissue hypoxia. Co-oximetry, which uses multiple wavelengths, is required to measure carboxyhemoglobin directly. PaO2 reflects dissolved oxygen and remains normal because carbon monoxide does not alter dissolved gas tension; it only blocks hemoglobin binding and transport capacity.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.