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Magnesium Neuromuscular Effects — FRCA Primary MCQ

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HardPharmacology - CardiovascularMagnesium Neuromuscular EffectsFRCA Primary

A patient treated with intravenous magnesium sulfate for severe asthma receives rocuronium. At the end of surgery the train-of-four ratio remains markedly depressed despite a dose that is usually short acting. Which interaction best explains this?

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Correct answer: D — Magnesium prolongs non-depolarising neuromuscular blocker action

The best answer is “Magnesium prolongs non-depolarising neuromuscular blocker action”. Parenteral magnesium reduces neuromuscular transmission and potentiates the action of non-depolarising blockers, so rocuronium can have greater depth and duration than expected. Quantitative monitoring, adjusted blocker dosing and confirmed recovery are therefore important. Magnesium neither increases acetylcholine release nor converts rocuronium into a depolarising drug, and the observed effect is not simply sedation.

Reference: Magnesium sulfate 50% w/v solution for injection, UK SmPC: https://www.medicines.org.uk/emc/product/3539/smpc; Royal College of Anaesthetists, Primary FRCA syllabus v2.2: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf