Learning Center/Pharmacology/Magnesium Sulfate

Magnesium Sulfate

Various

bronchodilator sabaIV · IM (for eclampsia when no IV access)Onset: 20–30 minutes (IV for bronchodilation)Duration: 30–60 minutes

Mechanism of Action

Divalent cation. Physiological calcium antagonist in smooth muscle → inhibits calcium-mediated bronchoconstriction → bronchodilation. Also inhibits mast cell and T-lymphocyte activation. Reduces neuromuscular transmission at high doses.

Dosing

Adult

Severe asthma: 2 g IV over 20 min (single dose). May repeat. IV infusion: 1–2 g/hr for eclampsia

Pediatric

Severe asthma: 25–75 mg/kg IV over 20 min (max 2 g)

Indications

Severe/refractory acute asthma (adjunct to inhaled bronchodilators and steroids)
Pre-eclampsia and eclampsia (seizure prophylaxis/treatment)
Torsades de pointes (cardiac arrhythmia)
Hypomagnesemia

Contraindications

Renal failure (impaired excretion → toxicity)
Heart block
Myasthenia gravis
Hypermagnesemia

Side Effects

Flushing / warmth
Hypotension
Loss of deep tendon reflexes (first sign of toxicity)
Respiratory depression (toxicity — levels >5 mg/dL)
Cardiac arrest (severe toxicity — levels >10–15 mg/dL)
Nausea

Monitoring Parameters

Deep tendon reflexes (disappear first — DTR loss = early toxicity)
Respiratory rate (depression = dangerous toxicity)
Serum magnesium levels (therapeutic 4–7 mg/dL for eclampsia; 2–4 mg/dL bronchodilation)
Urine output (required for excretion — minimum 25–30 mL/hr)
Blood pressure

Drug Interactions

Calcium gluconate (antidote for toxicity)
Neuromuscular blockers (potentiated — reduced dose needed)
Digoxin (hypermagnesemia may cause toxicity)

Clinical Pearls

For severe asthma (NAEPP Step 5 or ED refractory): 2 g IV over 20 min as adjunct
Monitor DTRs — loss of patellar reflex signals early toxicity
Antidote for toxicity: calcium gluconate 1 g IV
Renal function must be adequate for safe use
Mechanism in asthma: calcium antagonism → bronchodilation (smooth muscle relaxation)
Evidence strongest for severe/near-fatal asthma; routine use not recommended

Mnemonic

💡 MgSO4: Must Give when asthma Stays Obstinate for 4th-line therapy

NBRC High-Yield

Adjunct bronchodilator for severe/refractory asthma: 2 g IV over 20 min
Monitor DTRs — loss = early toxicity sign
Antidote: calcium gluconate 1 g IV
Requires adequate renal function for excretion
Mechanism: calcium antagonism → smooth muscle relaxation

← All Drugs

Clinical Pearls

For severe asthma (NAEPP Step 5 or ED refractory): 2 g IV over 20 min as adjunct

Monitor DTRs — loss of patellar reflex signals early toxicity

Antidote for toxicity: calcium gluconate 1 g IV

Renal function must be adequate for safe use

Mechanism in asthma: calcium antagonism → bronchodilation (smooth muscle relaxation)

Evidence strongest for severe/near-fatal asthma; routine use not recommended