A fire victim brought to the emergency department has SpO2 reading of 98 percent on pulse oximetry yet appears deeply cyanotic with a cherry-red tint and is comatose. Blood gas analysis confirms severe tissue hypoxia. What best explains the discrepancy?

• A Carboxyhemoglobin mimicking oxyhemoglobin on oximetry ✓
• B Methemoglobinemia absorbing light equally at both wavelengths
• C Poor peripheral perfusion causing signal artifact
• D Pulse oximeter calibration error at high altitude

Explanation

Conventional two-wavelength pulse oximeters cannot distinguish carboxyhemoglobin from oxyhemoglobin because their absorption spectra at 660 nm and 940 nm are similar, so the device reports a falsely high saturation in carbon monoxide poisoning. Co-oximetry measuring multiple wavelengths is required for accurate values. Methemoglobin typically pushes readings toward 85 percent rather than elevating them, and perfusion artifact would produce a weak or absent signal, not a confidently high one.

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

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