Magnesium Sulfate
MgSO4
Divalent cation / NMDA antagonist / calcium-channel modulator
Multiple mechanisms: (1) NMDA receptor antagonism (anticonvulsant, analgesic); (2) Voltage-gated calcium channel blockade in vascular + uterine smooth muscle (vasodilation, tocolysis); (3) Decreased ACh release at neuromuscular junction (NMB potentiation); (4) Membrane stabilization (antiarrhythmic, especially torsades).
Indications
- •Eclampsia / severe preeclampsia seizure prophylaxis
- •Tocolysis for preterm labor (24–32 weeks)
- •Torsades de pointes (drug of choice)
- •Asthma exacerbation refractory to bronchodilators
- •Hypomagnesemia replacement
- •Adjunct analgesic / opioid-sparing (intraop infusion)
- •Pheochromocytoma (vasodilation + arrhythmia prophylaxis)
Dosing
| Context | Adult | Pediatric |
|---|---|---|
| Eclampsia/preeclampsia | 4 g IV bolus over 15–20 min, then 1–2 g/h infusion | — |
| Torsades de pointes | 1–2 g IV over 5–15 min; repeat | — |
| Asthma adjunct | 2 g IV over 20 min | 25–50 mg/kg over 20 min |
| Opioid-sparing analgesia (intraop) | 30–50 mg/kg IV bolus + 8–15 mg/kg/h infusion | — |
| Pediatric tocolysis (rarely) | (see eclampsia dosing) | 20–40 mg/kg load + 0.5–1 g/h |
| Hypomagnesemia repletion | 1–2 g IV over 1 h × 2–4 g total | — |
Pharmacokinetics
Onset 1 min IV. Distribution to bone (50%), ECF (~50%). T½ ~30 min initial, longer with repeated dosing. RENAL excretion (>90%) — accumulates in renal failure.
Hemodynamic effects
↓SVR (vasodilation) → ↓BP. Bradycardia. Negative inotropy at high doses.
Side effects
- !Therapeutic 4–7 mEq/L; toxicity above 10 (loss DTRs), 12 (resp depression), 15 (cardiac arrest)
- !Hypotension (bolus too fast)
- !Flushing, warmth (vasodilation)
- !POTENTIATES NEUROMUSCULAR BLOCKADE — dramatically reduces NMB requirement
- !Bradycardia + AV block at high levels
- !Pulmonary edema (rare, with prolonged tocolytic infusions)
Contraindications
- ×Heart block (without pacing)
- ×Myasthenia gravis (worsens weakness)
- ×Severe renal impairment (toxicity risk)
Reversal / antidote
CALCIUM GLUCONATE 1 g IV slowly is the antidote — competes with Mg at NMJ + cardiac membrane. Hold infusion. Supportive ventilation if respiratory depression.
Clinical pearls
- ★Preeclampsia/eclampsia: standard maintenance 1–2 g/h × 24 h post-delivery (most seizures occur postpartum). Check DTRs hourly + clinical exam.
- ★RENAL FAILURE: massively reduce dose (e.g., 1 g/h instead of 2) and monitor levels closely.
- ★INTRAOP MAGNESIUM (analgesia adjunct): 30–50 mg/kg bolus + infusion reduces opioid requirements ~30%. Watch for prolonged paralysis at end of case — reverse with sugammadex if rocuronium used.
- ★TORSADES: 2 g IV bolus stabilizes regardless of magnesium level (membrane stabilization).
- ★MgSO₄ + nondepolarizer: reduce NMB dose 25–50%, monitor TOF closely. Even at 'normal' doses, residual paralysis common.