magnesium sulfate

electrolyte supplement· Brands: Epsom saltsIMIVPOTOP

Mechanism of action

Magnesium sulfate provides magnesium, an essential mineral that acts as a cofactor for numerous enzymes and stabilises excitable membranes. It blocks voltage-gated calcium channels, reducing calcium influx and thereby decreasing neurotransmitter release, including acetylcholine at the neuromuscular junction. This results in skeletal muscle relaxation and anticonvulsant activity. At higher doses, it also causes vasodilation, which can lower blood pressure, and it depresses central nervous system excitability. The therapeutic effects are dose-dependent and require careful monitoring.

Pharmacokinetics

Oral absorption of magnesium sulfate is incomplete, with approximately 30% bioavailability; the route is used mainly for laxative effects. Intravenously administered magnesium sulfate is rapidly distributed throughout body water, crosses the placenta, and is excreted in breast milk. The drug is not metabolised and is eliminated unchanged primarily by the kidneys. Its half-life is prolonged in renal impairment, increasing the risk of toxicity. Excretion correlates with renal function and dose adjustment is needed in patients with reduced glomerular filtration.

Indications & dosing

Hypomagnesemia

For moderate to severe deficiency. Infuse over 2 hours. Repeated doses may be required.

adult · IV: 2–4 g single infusion · For moderate to severe deficiency. Infuse over 2 hours. Repeated doses may be required.

Preeclampsia and eclampsia

Loading dose over 15-20 minutes. Follow with maintenance infusion of 1-2 g/hour for 24 hours. Monitor respiratory rate and reflexes.

adult · IV: 4 g single dose · Loading dose over 15-20 minutes. Follow with maintenance infusion of 1-2 g/hour for 24 hours. Monitor respiratory rate and reflexes.

Severe acute asthmaoff-label

Adjunct to standard therapy. Infuse over 20 minutes. Often used when initial nebulised treatment is unsatisfactory.

adult · IV: 2 g single dose · Adjunct to standard therapy. Infuse over 20 minutes. Often used when initial nebulised treatment is unsatisfactory.

Torsades de pointes (ventricular tachycardia)off-label

Give as a slow bolus over 2-5 minutes. May be repeated if torsades persists.

adult · IV: 1–2 g single dose · Give as a slow bolus over 2-5 minutes. May be repeated if torsades persists.

Contraindications & precautions

Heart block (second or third degree) · Magnesium can worsen conduction disturbances.
Severe renal impairment · Magnesium is renally excreted; accumulation can occur. Use with caution and reduced dose.
Myasthenia gravis · Magnesium can exacerbate neuromuscular weakness.
Monitor blood pressure, respiratory rate, and deep tendon reflexes during intravenous administration.
Monitor serum magnesium levels, especially in patients with renal impairment or on magnesium therapy for prolonged periods.
Have calcium gluconate available as an antidote for magnesium toxicity.
Use with caution in patients with hypocalcemia, as magnesium can worsen calcium deficiency.
Avoid rapid intravenous administration to prevent severe hypotension and cardiac arrhythmias.

Adverse effects

Flushing
Hypotensionserious
Respiratory depressionserious
Cardiac arrestserious
Loss of deep tendon reflexesserious

Renal & hepatic adjustment

eGFR 30–59: Use with caution, reduce dose; monitor serum magnesium levels.
eGFR 0–29: Avoid or use with extreme caution; consider alternative if possible. (avoid)

Monitoring, pregnancy & toxicity

Serum magnesium · After each dose or as clinically indicated
Blood pressure · During infusion
Respiratory rate · During infusion
Deep tendon reflexes · Before each dose
Urine output · Continuously during infusion
Renal function (serum creatinine) · Baseline and periodically
Pregnancy: Use in pregnancy for pre-eclampsia, eclampsia, and tocolysis; considered safe when used as indicated.
Toxicity: Loss of deep tendon reflexes · Stop infusion; monitor serum magnesium; administer calcium gluconate 10% IV
Toxicity: Respiratory depression · Stop infusion; provide ventilation; administer calcium gluconate
Toxicity: Hypotension · Stop infusion; give IV fluids; administer calcium gluconate
Toxicity: Cardiac arrhythmias · Stop infusion; give calcium gluconate; treat arrhythmias as per ALS protocols

Antidotes & reversal

Calcium gluconate (IV) · 10 mL 10% slowly IV for clinically significant hypermagnesaemia (loss of reflexes, respiratory depression).

Clinical notes

Magnesium sulfate is used parenterally for the management of eclampsia and pre-eclampsia, and for torsades de pointes. It is also used as a bronchodilator in severe acute asthma, and orally as a laxative. When used for eclampsia, a loading dose is given intravenously followed by a continuous infusion, with monitoring of blood pressure, respiratory rate, deep tendon reflexes, and serum magnesium concentration. Calcium gluconate should be available as an antidote for magnesium toxicity. Use with caution in patients with renal impairment or heart failure, and avoid in myasthenia gravis. Practice varies by region and formulary.

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BNF; local formulary