A firefighter rescued from a burning building has confusion and cherry-red skin discolouration. His oxygen saturation measured by standard pulse oximetry reads 98% on room air, yet arterial blood gas analysis is sent. Why does conventional pulse oximetry fail in this setting?

• A Pulse oximeters measure dissolved oxygen rather than bound haemoglobin
• B Carbon monoxide abolishes peripheral perfusion and eliminates the plethysmographic signal
• C Carboxyhaemoglobin absorbs light at wavelengths indistinguishable from oxyhaemoglobin ✓
• D Methaemoglobinaemia caused by smoke inhalation saturates the sensor

Explanation

Standard two-wavelength pulse oximetry cannot distinguish carboxyhaemoglobin from oxyhaemoglobin because their absorption spectra overlap at 660 and 940 nm, so saturation is falsely reported as normal. Diagnosis requires co-oximetry on an arterial blood sample, which measures carboxyhaemoglobin directly. Treatment is 100% oxygen to shorten the half-life of carbon monoxide from roughly 4 hours to under 1 hour, with hyperbaric oxygen considered for neurological features. Peripheral signal loss is not the mechanism, and methaemoglobinaemia is a separate entity.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.