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Normal Magnesium 0.7-1.0 mmol/L — FRCA Primary MCQ

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HardPhysiology - Endocrine & MetabolicNormal Magnesium 0.7-1.0 mmol/LFRCA Primary

A patient receiving a QT-prolonging antiarrhythmic develops recurrent self-terminating polymorphic ventricular tachycardia with twisting QRS axes. Potassium is 3.2 mmol L−1 and magnesium 0.82 mmol L−1. What is the most appropriate immediate drug treatment?

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Correct answer: C — Give intravenous magnesium and correct potassium simultaneously

The best answer is “Give intravenous magnesium and correct potassium simultaneously”. The ECG description is torsades de pointes. Intravenous magnesium is indicated even when the measured magnesium concentration is within the laboratory range, while hypokalaemia and offending QT-prolonging drugs must be corrected. Amiodarone can further prolong QT, calcium is not definitive therapy for torsades, and spontaneous termination does not remove the risk of degeneration to ventricular fibrillation.

Reference: Resuscitation Council UK 2025 Special circumstances guidelines: electrolyte disorders: https://www.resus.org.uk/professional-library/2025-resuscitation-guidelines/special-circumstances-guidelines; Royal College of Anaesthetists, Primary FRCA syllabus v2.2: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf