Magnesium sulphate
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Searching for comprehensive information on magnesium sulphate across its clinical uses, pharmacology, and dosing.
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magnesium sulphate pharmacology mechanism indications
Harrison's Principles of Internal Medicine, Twenty First Edition
Page 7104
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magnesium sulphate eclampsia preeclampsia dosing protocol
Management of Elevated Blood Pressure and Hypertension
Page 74
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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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