A firefighter rescued from a burning building has cherry-red skin, PaO2 of 98 mmHg, SpO2 of 99% by pulse oximeter, but altered sensorium. The most appropriate explanation for the discordance between his saturation reading and clinical state is:
- A Pulse oximeters measure dissolved oxygen rather than bound oxygen
- B Carbon monoxide shifts the oxyhaemoglobin dissociation curve to the right, releasing oxygen prematurely at tissues
- C Pulse oximetry overestimates saturation because carboxyhaemoglobin absorbs light at 660 nm similarly to oxyhaemoglobin ✓
- D Carboxyhaemoglobin selectively absorbs infrared light at 940 nm, mimicking reduced haemoglobin
Correct answer: C. Pulse oximetry overestimates saturation because carboxyhaemoglobin absorbs light at 660 nm similarly to oxyhaemoglobin
Explanation
Standard two-wavelength pulse oximeters cannot distinguish carboxyhaemoglobin from oxyhaemoglobin because both absorb red light at 660 nm similarly, so SpO2 reads falsely high despite severely impaired oxygen carrying capacity. CO also shifts the dissociation curve LEFT, not right, worsening tissue delivery, which eliminates option B. Co-oximetry measuring multiple wavelengths is required to detect the carboxyhaemoglobin fraction.
Reference: Ganong's Review of Medical Physiology, 27th ed.
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Written and medically reviewed by the StethoPrep medical team.