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Electrolyte / Anticonvulsant / Tocolytic Pregnancy: Used in pre-eclampsia/eclampsia — well-established safety for mother and fetus at therapeutic doses; neonatal hypotonia if given close to delivery

Magnesium Sulphate (IV — ICU/Anaesthesia)

Brand names: MgSO4, Magnesium Sulphate 50%

Adult dose

Dose: Hypomagnesaemia: 8 mmol (2 g) IV over 10–15 min then 65 mmol/24h; Eclampsia: 4 g (16 mmol) IV over 5–15 min then 1 g/hour; Torsades de pointes: 2 g IV over 1–2 min; Severe asthma: 1.2–2 g IV over 20 min
Route: IV
Frequency: Loading dose then maintenance
Max: Titrated to clinical response and serum Mg levels
50% solution = 2 mmol/mL (500 mg/mL) — must be diluted before use. 10% or 20% for infusions. Target serum Mg in eclampsia: 2–3.5 mmol/L.

Paediatric dose

Dose: 0.1–0.2 mmol/kg
Route: IV
Frequency: Over 10–20 min
Max: 8 mmol
Severe hypomagnesaemia or torsades: 0.1–0.2 mmol/kg IV over 10–20 min. Severe acute asthma: 40 mg/kg (max 2 g) IV over 20 min.

Dose adjustments

Renal

Significant risk of accumulation in renal impairment — reduce dose and monitor levels closely.

Hepatic

No specific adjustment required.

Paediatric weight-based calculator

Severe hypomagnesaemia or torsades: 0.1–0.2 mmol/kg IV over 10–20 min. Severe acute asthma: 40 mg/kg (max 2 g) IV over 20 min.

Clinical pearls

  • Antidote for magnesium toxicity: calcium gluconate 10 mL of 10% IV — competes at the calcium channel; have at bedside whenever Mg infusion is running
  • Loss of patellar reflex = serum Mg ~3.5 mmol/L → reduce or stop infusion. Respiratory arrest = ~5 mmol/L
  • In eclampsia: superior to diazepam and phenytoin for seizure prevention — Magpie trial (2002, Lancet)

Contraindications

  • Hypermagnesaemia
  • Heart block
  • Myasthenia gravis (potentiates NMB)

Side effects

  • Flushing and warmth (infusion)
  • Hypotension
  • Respiratory depression (Mg >3.5 mmol/L)
  • Loss of deep tendon reflexes (>3.5 mmol/L — warning sign)
  • Cardiac arrest (>6 mmol/L)
  • Potentiation of NMB agents

Interactions

  • Neuromuscular blocking agents (profound potentiation — reduce NMB doses in Mg-treated patients)
  • Calcium channel blockers (additive hypotension)
  • Digoxin (additive cardiac depression)

Monitoring

  • Serum magnesium levels (every 4–6h)
  • Patellar reflex check before each bolus (eclampsia protocol)
  • Respiratory rate and SpO2
  • Urine output (must be >25 mL/hour — renal excretion)

Reference: BNFc; BNF 90; Magpie Trial (Lancet 2002); NICE NG133 (Hypertension in Pregnancy); BNFc. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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