Magnesium Sulphate (MgSO₄)

Overview

Magnesium sulphate is an inorganic salt used extensively in clinical medicine. It acts as a CNS depressant, anticonvulsant, smooth muscle relaxant, and antiarrhythmic agent. Its mechanism centers on antagonism of calcium (blocking calcium channels and NMDA receptors) and membrane stabilization.

Pharmacology

Property	Detail
Class	Electrolyte / CNS depressant / Anticonvulsant
Mechanism	Blocks NMDA receptors; antagonizes Ca²⁺ at cell membranes; slows neuromuscular transmission
Route	IV, IM, oral, topical
Elimination	Renal (dose reduction in renal impairment)
Monitoring	Serum Mg²⁺, deep tendon reflexes (DTR), respiratory rate, urine output

Key Clinical Indications

1. Eclampsia & Pre-eclampsia (First-line)

Per Management of Elevated Blood Pressure and Hypertension (p. 74):

Loading dose: 4 g IV over 5 min, then 1 g/h IV (maintenance)
- OR: 5 g IM into each buttock, then 5 g IM every 4 h
Indicated for:
- Eclampsia (convulsions)
- Severe pre-eclampsia with severe hypertension + proteinuria
- Pre-eclampsia with hypertension + neurological symptoms/signs
Caution: Concomitant nifedipine increases risk of hypotension

2. Torsades de Pointes (TdP) / Polymorphic VT

Per Harrison's Principles of Internal Medicine, 21st Ed. (p. 7104):

Dose: 1–2 g IV — usually suppresses recurrent episodes
If magnesium alone is insufficient: increase heart rate with isoproterenol or pacing (target 100–120 bpm) to suppress PVCs
Address underlying causes: hypokalemia, hypocalcemia, bradycardia, offending drugs

3. Other Established Indications

Indication	Dose (typical)
Severe acute asthma (refractory)	1.2–2 g IV over 20 min
Hypomagnesaemia	1–2 g IV replacement
Preterm labour tocolysis	4–6 g IV load, then 1–2 g/h
Neuroprotection in preterm birth (<32 weeks)	4 g IV load
Constipation / bowel prep	Oral (saline laxative effect)

Toxicity & Antidote

Magnesium toxicity is dose-dependent and correlates with serum levels:

Serum Mg²⁺ (mmol/L)	Effect
2–3.5	Therapeutic range (eclampsia)
3.5–5	Loss of deep tendon reflexes
5–7.5	Respiratory paralysis
>7.5	Cardiac arrest

Antidote: Calcium gluconate 1 g IV (10 mL of 10% solution) — reverses magnesium toxicity promptly.

Monitoring parameters: Check DTRs before each IM dose; urine output ≥25 mL/h; respiratory rate ≥12/min.

Contraindications / Precautions

Renal failure — impaired excretion; reduce dose or avoid
Myasthenia gravis — can precipitate crisis
Heart block — magnesium slows conduction
Hypocalcaemia — may worsen

Key Drug Interactions

Nifedipine — enhanced hypotension
Neuromuscular blocking agents — potentiated
Aminoglycosides — additive neuromuscular blockade

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