A fire victim presents with confusion. His pulse oximetry reads 98%, but his measured arterial oxygen content is severely reduced. Which mechanism best explains this discrepancy?

• A Carbon monoxide binding increases oxygen affinity, preventing tissue unloading
• B Carboxyhemoglobin is measured as oxyhemoglobin by the pulse oximeter, but it cannot carry oxygen ✓
• C Cyanide poisoning shifts the oxygen-hemoglobin dissociation curve to the left
• D Methemoglobin falsely lowers pulse oximetry readings while actual oxygen content remains normal

Explanation

Standard pulse oximeters use two wavelengths of light and cannot differentiate carboxyhemoglobin from oxyhemoglobin. They read carboxyhemoglobin as oxyhemoglobin, yielding a falsely normal SpO2 reading. However, because carbon monoxide occupies hemoglobin binding sites, the actual oxygen carrying capacity and arterial oxygen content are severely reduced. While CO does shift the curve leftward, the immediate reason for the discrepancy is the falsely elevated pulse oximetry reading.

Reference: West's Respiratory Physiology, 10th ed.

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