Electrolyte · Antiarrhythmic · Antiepileptic · Bronchodilator
Magnesium sulfate (MgSO4)
Also called Mag, MgSO4
Indications
- Seizure prevention and treatment in severe preeclampsia and eclampsia
- Hypomagnesemia, including with arrhythmias or refractory hypokalemia or hypocalcemia
- Torsades de pointes (off-label)
- Severe asthma not responding to initial treatment (off-label)
- Fetal neuroprotection before 32 weeks (off-label)
- AF rate-control adjunct and postoperative AF prevention (off-label)
Dosing
Magnesium sulfate drip card and mL/h converterStart: Severe preeclampsia or eclampsia: 4–6 g over 15–20 minutes, then 1–2 g/h
| Where | Dose |
|---|---|
| Cardiac arrest | Magnesium: 1–2 g IV/IO over 5–20 min for torsades |
| Rhythm control | Magnesium: 2 g over 15 min for polymorphic VT |
| Metabolic and electrolyte boluses | Magnesium sulfate: 1–2 g over 15–60 min (4–6 g load for preeclampsia, then 1–2 g/h); avoid in myasthenia gravis (can trigger crisis) |
| Unit conversions | Magnesium: 1 mEq/L = 0.5 mmol/L = 1.2 mg/dL |
| Unit conversions | Magnesium sulfate: 1 g = 8.1 mEq |
| Other ICU drugs | Magnesium, potassium, phosphate replacement: Smaller doses and closer levels |
Mechanism of action
Magnesium acts as a physiologic calcium antagonist: it competes with Ca²⁺ at voltage-gated L-type channels in smooth and cardiac muscle and at presynaptic nerve terminals, so less Ca²⁺ enters. At the neuromuscular junction this cuts acetylcholine release; in the brain Mg²⁺ blocks the NMDA receptor channel and relaxes cerebral vessels, the likely basis of its effect in eclampsia. In the heart it suppresses the Ca²⁺-dependent early afterdepolarizations that trigger torsades without shortening the QT; as a Na⁺/K⁺-ATPase cofactor it helps cells hold K⁺.
Pharmacology
- CNS: prevents eclamptic seizures better than diazepam or phenytoin (Collaborative Eclampsia Trial 1995); it does not treat severe hypertension
- Neuromuscular: weakness and loss of deep tendon reflexes as levels rise; potentiates all neuromuscular blockers
- Cardiovascular: vasodilation and flushing with fast boluses; slows SA and AV conduction; outside the Vaughan Williams classes
- Respiratory: bronchodilation; respiratory depression at toxic levels
- Obstetric: relaxes the uterus; given before 32 weeks it lowered cerebral palsy (BEAM, Rouse 2008)
- Metabolic: needed for PTH release and to stop renal K⁺ wasting
Pharmacokinetics
| Onset | IV immediate; IM about 1 h (label) |
|---|---|
| Duration | About 30 min after IV and 3–4 h after IM (label), hence the infusion |
| Half-life | About 4–5 h in preeclampsia with normal kidneys; much longer in kidney failure |
| Distribution | Extracellular, then into cells and bone; Vd 14–49 L in pregnancy (Okusanya 2016); crosses the placenta |
| Protein binding | About 30%, mainly to albumin |
| Metabolism | None |
| Elimination | Kidneys only, in proportion to plasma level and GFR (label) |
| Target levels | Eclampsia prophylaxis about 4.8–8.4 mg/dL (4–7 mEq/L) in common protocols; the label gives 2.5–7.5 mEq/L |
| Dialysis | Removed by hemodialysis |
Chemistry
- Magnesium sulfate heptahydrate (MgSO₄·7H₂O, Epsom salt, MW 246.5): a fully dissociated inorganic salt, so pKa, lipid solubility and stereochemistry do not apply
- 1 g supplies about 98 mg (8.1 mEq, 4.06 mmol) of elemental magnesium
- 50% vials (4 mEq/mL, about 4,060 mOsm/L, pH 5.5–7.0) must be diluted to 20% or less for IV use (label)
- Premixed 4% bags (20 g/500 mL, 40 g/1000 mL) reduce mixing errors; a high-alert drug with an independent double-check
Adverse effects and cautions
In the one-pagers
- Obstetric EmergenciesPreeclampsia and eclampsia: Eclampsia: new seizures with no other cause. Call for help, protect from injury, side-lying, prevent aspiration, oxygen. Most stop on their own; magnesium prevents the next.
- ACLS Algorithms in BriefCPR quality, shocks and drugs: Not routine in arrest: calcium, bicarbonate, magnesium, vasopressin, high-dose epinephrine, pacing. Hyperkalemic arrest: calcium and bicarbonate may be considered (2025: benefit unproven).
- Myasthenic Crisis and Guillain-Barré SyndromeMyasthenic crisis: Worsen MG: fluoroquinolones (boxed warning), aminoglycosides, macrolides, IV magnesium, β-blockers, procainamide, neuromuscular blockers, volatile anesthetics, checkpoint inhibitors
- Asthma and COPD ExacerbationsSevere asthma: IV magnesium 2 g over 20 min for severe attacks
- Rhythm GalleryVentricular: Torsades: polymorphic VT twisting around the baseline, usually with a long QT. Pulseless: defibrillate. With a pulse: magnesium 1–2 g IV over about 15 min; avoid QT-prolonging drugs such as amiodarone.
- Subarachnoid HemorrhageNimodipine and delayed cerebral ischemia: No benefit: Prophylactic triple-H (harmful), statins, IV magnesium, clazosentan
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In the algorithms
Other electrolyte drugs
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Sources
- FDA label: Magnesium sulfate 50% (2025)
- ACOG Practice Bulletin 222 (2020)
- Okusanya 2016 (pharmacokinetics review)
- Iseri 1984 (magnesium as a calcium blocker)
For education and exam preparation only. Follow your institution’s protocols. Typical adult values for study. Follow the order, the label and your institution's protocols.