A patient develops cyanosis, chocolate-coloured blood, and SpO2 reading fixed around 85% despite high-flow oxygen, shortly after topical benzocaine spray was used before upper GI endoscopy. Pulse oximetry is unreliable because of abnormal light absorption. What is the definitive treatment?

• A Intravenous hydroxocobalamin
• B Hyperbaric oxygen therapy
• C Exchange transfusion
• D Intravenous methylene blue ✓

Explanation

Benzocaine and prilocaine are oxidising agents that convert haemoglobin iron from the ferrous to the ferric state, producing methaemoglobinaemia which cannot carry oxygen. Intravenous methylene blue 1 to 2 mg/kg acts through NADPH-dependent methaemoglobin reductase to reduce ferric iron back to ferrous form. Hydroxocobalamin treats cyanide poisoning, and hyperbaric oxygen is reserved for carbon monoxide poisoning. Standard pulse oximeters cannot distinguish methaemoglobin, giving falsely low readings near 85%.

Reference: Morgan and Mikhail's Clinical Anesthesiology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.