A 62-year-old man becomes centrally cyanotic shortly after topical benzocaine spray was applied for upper gastrointestinal endoscopy. He is breathing 100% oxygen via facemask. The pulse oximeter reads 85% and remains at 85% despite a rise in inspired oxygen. Arterial blood gas shows PaO2 of 480 mmHg on 100% oxygen. What is the most likely cause of the discrepancy between the pulse oximeter reading and the arterial sample?
- A Methaemoglobin absorbs light equally at both pulse oximeter wavelengths, fixing the displayed saturation near 85% ✓
- B Carboxyhaemoglobin is being read as oxyhaemoglobin, giving a falsely high saturation
- C Poor peripheral perfusion has caused signal dropout and a falsely low saturation
- D Venous pulsation at the probe site has lowered the calculated saturation
Correct answer: A. Methaemoglobin absorbs light equally at both pulse oximeter wavelengths, fixing the displayed saturation near 85%
Explanation
Methaemoglobinaemia from benzocaine produces a functional anaemia: MetHb absorbs red and infrared light almost equally, so the ratio approaches that of 85% desaturation regardless of the true saturation. The high PaO2 confirms adequate oxygenation of dissolved gas while delivery capacity is impaired, and treatment is methylene blue. Carboxyhaemoglobin behaves oppositely, absorbing infrared like oxyhaemoglobin, so it yields a falsely HIGH saturation near 100%, which kills distractor B.
Reference: Miller's Anesthesia, 9th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.