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Electrolyte Pregnancy: A — first-line for eclampsia and seizure prophylaxis

Magnesium Sulphate

Brand names: Magnesium Sulphate 50%

Adult dose

Dose: Eclampsia: 4g IV over 5–15 min. Asthma / VF: 2g IV over 10–20 min
Route: IV
Frequency: Loading dose then infusion (eclampsia: 1g/h for 24h)
Eclampsia: 4g in 100ml 5% glucose over 5–15 min, then 1g/h infusion. Refractory VF (ALS): 2g IV bolus. Severe acute asthma (NICE): 1.2–2g IV over 20 min. Torsades de Pointes: 2g IV over 5–10 min.

Paediatric dose

Dose: 25 mg/kg
Route: IV slow over 20 min
Frequency: Single dose
Max: 2000mg
Concentration: 500 mg/ml

Dose adjustments

Renal

Use with extreme caution in renal impairment — magnesium is renally excreted. Monitor serum levels closely. Avoid if GFR <30 unless critical.

Paediatric weight-based calculator

Clinical pearls

  • Magpie Trial: magnesium sulphate reduces eclampsia by 58% and recurrence by 67%. Drug of choice for eclampsia.
  • Toxicity monitoring in eclampsia: check patellar reflexes before each maintenance dose. Absent reflexes = stop infusion.
  • Toxicity antidote: calcium gluconate 10ml of 10% IV over 10 min — immediately reverses respiratory arrest from Mg toxicity.
  • For VF/Torsades: dilute in 5% glucose (not saline), give over 10–15 min.
  • Neonatal use: neuroprotection for preterm <30 weeks — 4g IV bolus to mother.

Contraindications

  • Heart block
  • Myasthenia gravis
  • Hypermagnasaemia
  • Severe renal impairment (relative — monitor levels)

Side effects

  • Flushing, sweating, nausea — common during infusion
  • Hypotension if given too rapidly
  • Loss of deep tendon reflexes (first toxicity sign — serum Mg >3.5 mmol/L)
  • Respiratory paralysis (serum Mg >5 mmol/L)
  • Cardiac arrest (serum Mg >7.5 mmol/L)

Interactions

  • Calcium channel blockers: enhanced hypotension and neuromuscular effects
  • Non-depolarising NMBAs (rocuronium, vecuronium): enhanced and prolonged block
  • Digoxin: enhanced toxicity at high magnesium levels

Monitoring

  • Respiratory rate (must be >12/min)
  • patellar reflexes (must be present before each dose)
  • urine output (>25ml/h)
  • serum Mg (therapeutic 2–3.5 mmol/L)

Reference: BNFc; Magpie Trial, Lancet 2002; NICE NG133; UK ALS Guidelines 2021; NICE BNF 84. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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