Magnesium Replacement

Magnesium replacement treats hypomagnesaemia (serum magnesium under 0.7 mmol/L) and several non-deficiency conditions: torsades de pointes, severe pre-eclampsia and eclampsia, and acute severe asthma. Low magnesium often coexists with low potassium and calcium and causes arrhythmia, tremor, tetany and seizures.

Mechanism. Magnesium is a cofactor for the sodium-potassium pump, calcium handling and many enzymes. Replacement restores these functions and stabilises cardiac membranes. At higher doses it blocks calcium entry into muscle and nerve cells, which relaxes smooth muscle and raises seizure threshold.

Pharmacokinetics. Oral absorption is incomplete (about 30%) and limited by diarrhoea. Intravenous magnesium reaches the kidney quickly; up to half of an intravenous dose is lost in the urine in 24 hours, so replacement takes several days. Elimination is renal.

Antidote. Calcium gluconate 10% 10 mL (1 g) intravenously over 10 minutes for magnesium toxicity.

Toxicity ladder. Loss of patellar reflexes (4 to 5 mmol/L) comes first, then respiratory depression (over 6 mmol/L) and heart block (over 7.5 mmol/L).

• First-line. Oral magnesium for mild deficiency; intravenous magnesium for symptoms or arrhythmia.
• Alternative. Reduce loss by stopping offending drugs (proton pump inhibitors, loop diuretics, amphotericin, cisplatin).
• Specialist. Obstetric seizure prevention and treatment, hypomagnesaemia from tubular disorders, long-term replacement in short bowel disease.
• Emergency. Torsades de pointes, eclampsia, life-threatening asthma.

Selection. Choose oral replacement for slow correction. Use intravenous for urgent need. Reduce the dose in kidney impairment.

Contraindications. Heart block, myasthenia gravis, severe kidney failure with oliguria (reduce dose), hypermagnesaemia.

Serious adverse effects.

• Flushing, hypotension and warmth during infusion.
• Loss of tendon reflexes, drowsiness, respiratory depression.
• Heart block and cardiac arrest in overdose.
• Diarrhoea with oral salts.

Major interactions. Neuromuscular blockers (prolonged paralysis); calcium channel blockers (hypotension); aminoglycosides; oral magnesium lowers absorption of tetracyclines, quinolones, levothyroxine and bisphosphonates.

Common prescribing errors.

• Giving magnesium in severe kidney failure without reducing the dose.
• Failing to monitor reflexes, respiratory rate and urine output during obstetric infusions.
• Confusing grams, millimoles and millilitres of magnesium sulfate.
• Not replacing magnesium in resistant hypokalaemia.

Special groups.

• Pregnancy: first choice for eclampsia; monitor mother and neonate.
• Breastfeeding: compatible.
• Children: weight-based dosing; confirm in a paediatric formulary.
• Older adults: lower doses and closer monitoring.
• Kidney: reduce dose by half or more and monitor levels.
• Liver: no adjustment.
• East Africa: magnesium sulfate for eclampsia is widely available and often the only anticonvulsant stocked; ensure calcium gluconate is on hand and that nurses check reflexes, respiratory rate and urine output; check which strength of ampoule is in stock (commonly 50%, 10 mL, 5 g) before drawing doses.

Monitoring. Serum magnesium, potassium and calcium; ECG in arrhythmia; reflexes, respiratory rate (at least 16 a minute) and urine output (at least 30 mL/hour) during obstetric infusions.

Referral and escalation. Respiratory depression, loss of reflexes, heart block, anuria, or persistent arrhythmia.

Comparison with alternative classes. Magnesium sulfate is more effective than phenytoin or diazepam for eclampsia and is first-line for torsades. Oral salts are cheaper but cause diarrhoea and absorb poorly.