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Magnesium Toxicity Antidote — FRCA Final MCQ

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ModerateObstetric AnaesthesiaMagnesium Toxicity AntidoteFRCA Final

A 28-year-old woman with severe pre-eclampsia is receiving a magnesium sulphate loading dose (4 g IV over 20 minutes). She develops flushing, nausea, and feels warm. These are followed by loss of deep tendon reflexes. Her respiratory rate drops to 8/min. What is the antidote?

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Correct answer: D — Flumazenil 0.2 mg IV

Magnesium toxicity causes progressive loss of deep tendon reflexes (first sign at ~4 mmol/L), followed by respiratory depression, cardiac conduction abnormalities, and cardiac arrest (>6 mmol/L). The antidote is IV calcium gluconate 10 mL of 10% (or calcium chloride), which directly antagonises the cardiac and neuromuscular effects of magnesium. The magnesium infusion should be stopped immediately. Monitoring during magnesium administration must include: patellar reflexes (checked hourly), respiratory rate, urine output (magnesium is renally excreted), and SpO2.

Reference: NICE – 2023 – NG133 Hypertension in pregnancy; BNF – 2025 – Magnesium sulphate toxicity; OAA – 2024 – QRH