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Prilocaine methaemoglobinaemia — FRCA Primary MCQ

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HardPharmacologyPrilocaine methaemoglobinaemiaFRCA Primary

A patient receives a large dose of prilocaine for intravenous regional anaesthesia and later appears cyanosed despite a PaO2 of 28 kPa. Pulse oximetry remains around 85%. What is the most likely explanation?

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Correct answer: A — Methaemoglobinaemia from o-toluidine

Prilocaine is metabolised to o-toluidine, which can oxidise haemoglobin to methaemoglobin and produce cyanosis with a high PaO2. Carbon monoxide poisoning gives a falsely reassuring pulse oximeter reading rather than a typical saturation near 85%. A shunt or embolism would usually lower PaO2. Co-oximetry confirms the diagnosis, and methylene blue is considered when clinically significant.

Reference: Peck, Hill & Williams Pharmacology for Anaesthesia and Intensive Care; Primary FRCA textbooks