During awake fibreoptic intubation, topical airway anaesthesia is being applied. Benzocaine spray used on the oro- and hypopharyngeal mucosa can cause a serious complication. Which finding on pulse oximetry would alert the anaesthetist to this complication?
- A SpO2 98% with normal waveform; spuriously reassuring
- B SpO2 85% despite patient breathing normally and appearing comfortable ✓
- C SpO2 70% with flat plethysmographic waveform indicating poor perfusion
- D SpO2 100% with tachycardia indicating sympathetic stimulation
Explanation
Benzocaine is a local anaesthetic associated with oxidising the haemoglobin iron from Fe2+ to Fe3+, producing methaemoglobinaemia. Methaemoglobin cannot carry oxygen and absorbs light at both 660 nm and 940 nm wavelengths used by standard pulse oximetry; as MetHb rises, the SpO2 reading tends to plateau at approximately 85% regardless of actual arterial oxygenation. A patient with MetHb 20–30% may appear comfortable (central cyanosis present) while SpO2 reads 85% — this combination should prompt co-oximetry. Option C would reflect peripheral arterial disease or shock, not MetHb specifically.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.