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Magnesium potentiation of neuromuscular block — FRCA Primary MCQ

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HardPharmacologyMagnesium potentiation of neuromuscular blockFRCA Primary

A woman with severe pre-eclampsia has received magnesium sulphate and then requires general anaesthesia for postpartum haemorrhage. After rocuronium, neuromuscular recovery is slower than expected. What is the most likely mechanism?

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Correct answer: D — Reduced presynaptic acetylcholine release

Magnesium reduces calcium-dependent acetylcholine release at the neuromuscular junction and potentiates non-depolarising blockade. Increased plasma cholinesterase activity would shorten suxamethonium or mivacurium effect, not prolong rocuronium. Opioid receptor blockade and cyclo-oxygenase inhibition are unrelated. Magnesium-treated patients require careful dosing and quantitative neuromuscular monitoring.

Reference: Peck, Hill & Williams Pharmacology for Anaesthesia and Intensive Care; Obstetric anaesthesia textbooks