1. Clinical Overview
Magnesium Sulphate (50% w/v) is a concentrated, sterile, hypertonic solution of magnesium sulfate heptahydrate, providing 500 mg of magnesium sulfate per mL (equivalent to 2 mmol/mL of magnesium ions and 4 mmol/mL of sulfate ions). It is a critical care medication with diverse therapeutic roles, primarily as an anticonvulsant in severe pre-eclampsia and eclampsia, a tocolytic in preterm labor, an electrolyte replenisher for severe hypomagnesemia, and an antiarrhythmic for specific life-threatening arrhythmias. Its actions stem from magnesium's physiological role as a cofactor for over 300 enzymatic reactions and its direct pharmacological effects on neuromuscular and cardiovascular systems.
| Onset | Duration | Bioavailability |
|---|---|---|
| Intravenous (IV): Immediate to within minutes. Intramuscular (IM): Approximately 60 minutes. | IV Bolus: 30 minutes. IV Infusion: Duration of infusion plus 30-60 minutes post-infusion. IM: 3-4 hours. | Intravenous: 100%. Intramuscular: High but variable, approximately 90-95%. |
2. Mechanism of Action
Magnesium acts as a physiological calcium antagonist. It competitively blocks calcium channels, inhibiting calcium influx into cells. This leads to: 1) Reduced acetylcholine release at the neuromuscular junction, causing peripheral neuromuscular blockade and vasodilation. 2) Cerebral vasodilation, reversing vasospasm in eclampsia. 3) Uterine smooth muscle relaxation (tocolytic effect). 4) Membrane stabilization in cardiac myocytes. It also acts as a non-competitive antagonist at N-methyl-D-aspartate (NMDA) glutamate receptors in the CNS, contributing to its anticonvulsant and potential neuroprotective effects.
3. Indications & Uses
- Management and prevention of seizures in severe pre-eclampsia and eclampsia
- Treatment of severe, symptomatic hypomagnesemia
- Tocolysis for short-term (24-48 hours) management of acute preterm labor (to allow for corticosteroid administration)
- Management of life-threatening ventricular arrhythmias (e.g., Torsades de Pointes) refractory to conventional therapy
4. Dosage & Administration
Adult Dosage: Dose varies drastically by indication. **Eclampsia:** Loading dose: 4-6 g IV over 15-20 minutes, followed by maintenance infusion of 1-2 g/hour. **Hypomagnesemia:** Up to 8 g (16 mL of 50% solution) in divided doses over 12-24 hours, diluted and given IV slowly. **Tocolysis:** Loading dose: 4-6 g IV over 20-30 min, then maintenance infusion of 1-4 g/hour. **Torsades de Pointes:** 1-2 g IV over 5-60 minutes, diluted.
Administration: **MUST BE DILUTED.** 50% w/v solution is hypertonic and vesicant. For IV use: Dilute in 100-250 mL of compatible IV fluid (e.g., 0.9% NaCl, 5% Dextrose). For IV bolus (loading dose): Infuse over 15-30 minutes. For IV infusion: Use an infusion pump. For IM use: Use the undiluted 50% solution, administer deep IM (gluteal) using Z-track technique, not exceeding 2 g (4 mL) per injection site. Monitor patellar reflex, respiratory rate (>16/min), and urine output (>25-30 mL/hr) throughout therapy.
5. Side Effects
Common side effects may include:
- Flushing, feeling of warmth
- Sweating
- Hypotension
- Injection site pain/reaction (especially IM)
- Lethargy, drowsiness
- Nausea
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Neuromuscular Blocking Agents (e.g., Succinylcholine, Rocuronium) | Potentiates neuromuscular blockade, leading to prolonged apnea and paralysis. | Major |
| Calcium Channel Blockers (e.g., Verapamil, Diltiazem) | Additive effects on cardiac conduction and contractility; risk of hypotension, bradycardia, and heart block. | Major |
| Cardiac Glycosides (Digoxin) | Magnesium may exacerbate digoxin-induced heart block. Concurrent use requires extreme ECG monitoring. | Major |
| CNS Depressants (e.g., Opioids, Benzodiazepines, Barbiturates) | Additive CNS and respiratory depression. | Moderate |
| Antibiotics (Aminoglycosides, Polymyxins) | May have additive neuromuscular blocking effects. | Moderate |
| Diuretics (Loop diuretics like Furosemide) | Increased renal excretion of magnesium, potentially reducing efficacy. | Moderate |
7. Patient Counselling
- Inform all healthcare providers you are receiving magnesium sulfate.
- Report immediately any feeling of extreme weakness, difficulty breathing, or extreme drowsiness.
- For IM injection, expect significant pain at the injection site; warm compresses may help.
8. Toxicology & Storage
Overdose: Progressive sequence: Loss of patellar reflex → Respiratory depression (RR < 12/min) → Somnolence → Complete heart block → Cardiac arrest. Other signs: Flaccid paralysis, hypothermia, coma.
Storage: Store at room temperature (15-25°C). Protect from freezing. The 50% w/v solution may crystallize at low temperatures; if crystals form, warm to 37°C in a water bath and shake to redissolve. Do not use if solution is discolored or contains particulate matter. Keep out of reach of children. Single-use vial; discard any unused portion.