A young adult receives a large dose of prilocaine for a regional block. Two hours later he is centrally cyanotic with an oxygen saturation reading fixed around 85% despite a 100% oxygen facemask. Venous blood is described as chocolate brown. Which treatment will reverse this picture?

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

Explanation

Prilocaine is metabolised to o-toluidine, which oxidises haemoglobin iron from the ferrous to the ferric state, producing methaemoglobinaemia that cannot carry oxygen. Pulse oximetry reads falsely low and saturation does not correct with supplemental oxygen. Intravenous methylene blue, 1 to 2 mg/kg, reduces methaemoglobin back to functional haemoglobin through the NADPH-dependent pathway. Hydroxocobalamin treats carbon monoxide and cyanide poisoning, not this condition.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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