A 28-year-old man is brought unconscious after being found in a closed garage with a running car engine. His skin appears bright cherry pink. Pulse oximeter shows saturation of 99 percent, but arterial blood gas analysis reveals severe tissue hypoxia. The most likely explanation for the discrepancy between pulse oximetry and true oxygenation is:
- A Carboxyhaemoglobin is misread as oxyhaemoglobin because it absorbs light like HbO2 at 660 nm ✓
- B Methaemoglobinaemia absorbing light equally at both wavelengths
- C Pulmonary shunt causing venous admixture
- D Cyanide poisoning blocking cytochrome oxidase
Correct answer: A. Carboxyhaemoglobin is misread as oxyhaemoglobin because it absorbs light like HbO2 at 660 nm
Explanation
Carbon monoxide binds haemoglobin with an affinity 200 to 300 times that of oxygen, forming carboxyhaemoglobin. Conventional pulse oximeters use two wavelengths (660 nm and 940 nm) and cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so they report a spuriously high saturation despite profound hypoxia. Methaemoglobinaemia typically produces readings around 85 percent, not 99. Cyanide poisoning does not alter pulse oximetry and does not cause cherry pink skin. Treatment of choice in severe cases is hyperbaric oxygen.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.