A patient receiving intravenous dapsone for a skin condition is found cyanotic on the ward. Co-oximetry confirms methaemoglobinaemia. What reading would a standard two-wavelength pulse oximeter most likely display?

• A A fixed SpO2 of approximately 85% irrespective of the true arterial saturation ✓
• B A falsely elevated SpO2 near 100%
• C An accurate SpO2 that tracks the true saturation
• D Complete failure of the monitor to display any value

Explanation

Methaemoglobin absorbs light equally at both the red (660 nm) and infrared (940 nm) wavelengths used by standard pulse oximeters, so the ratio of absorption is fixed at a value corresponding to roughly 85% saturation, whatever the true PaO2. Carboxyhaemoglobin behaves differently: it mimics oxyhaemoglobin at 660 nm and produces a falsely high reading. Treatment of severe methaemoglobinaemia is methylene blue.

Reference: Miller's Anesthesia, 9th ed.

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