A fireman rescued from a burning building has cherry-red skin, headache, and confusion. His arterial PaO2 is 98 mmHg and standard pulse oximetry shows SpO2 of 97%. The most appropriate interpretation of these findings is:
- A Pulse oximetry overestimates true oxyhaemoglobin saturation because it cannot distinguish carboxyhaemoglobin from oxyhaemoglobin ✓
- B Pulse oximetry accurately reflects oxygen content because dissolved oxygen is adequate
- C PaO2 is falsely elevated because carbon monoxide dissolves in plasma
- D Co-oximetry would show a lower methaemoglobin fraction than pulse oximetry suggests
Correct answer: A. Pulse oximetry overestimates true oxyhaemoglobin saturation because it cannot distinguish carboxyhaemoglobin from oxyhaemoglobin
Explanation
Carbon monoxide binds haemoglobin with affinity about 250 times that of oxygen, forming carboxyhaemoglobin. Conventional two-wavelength pulse oximeters read carboxyhaemoglobin as if it were oxyhaemoglobin, so SpO2 appears falsely reassuring while actual oxygen-carrying capacity is severely reduced. PaO2 measures dissolved oxygen and remains normal, which explains why ABG PaO2 is misleading here. Diagnosis requires co-oximetry measuring carboxyhaemoglobin directly.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.