A patient receiving prolonged nitric oxide therapy for acute respiratory distress syndrome develops increasing methaemoglobinaemia. The conventional two-wavelength pulse oximeter will display a saturation closest to:

• A Rising toward 100%
• B Zero, because MetHb absorbs equally at both wavelengths
• C Accurate values until MetHb exceeds 20%
• D Approximately 85%, regardless of the actual arterial saturation ✓

Explanation

Methaemoglobin absorbs light almost equally at both the 660 nm and 940 nm wavelengths used by standard pulse oximeters, causing the device to read a fixed saturation of approximately 85% irrespective of true arterial oxygenation. This occurs because MetHb has similar absorbance ratios to a mixture containing roughly 85% oxyhaemoglobin. Co-oximetry measuring multiple wavelengths is required for accurate assessment.

Reference: Miller's Anesthesia, 9th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.