A young woman receives topical benzocaine spray before awake fibreoptic intubation. Twenty minutes later she is centrally cyanosed with SpO2 reading 88% that does not improve on 100% oxygen. Arterial blood is described as chocolate brown. What is the definitive treatment?
- A Intravenous hydroxocobalamin
- B Exchange transfusion
- C Intravenous methylene blue 1 to 2 mg/kg ✓
- D Hyperbaric oxygen therapy
Correct answer: C. Intravenous methylene blue 1 to 2 mg/kg
Explanation
This is acquired methaemoglobinaemia from benzocaine (also caused by prilocaine). The ferric iron cannot carry oxygen, pulse oximetry reads falsely around 85%, and arterial blood looks chocolate brown and does not brighten on exposure to air. Methylene blue provides the electron acceptor for NADPH dependent methaemoglobin reductase and is the definitive antidote. Hydroxocobalamin treats cyanide poisoning and exchange transfusion or hyperbaric oxygen are reserved for refractory cases.
Reference: Morgan and Mikhail's Clinical Anesthesiology, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.