Magnesium Sulphate (MgSO4) Injection - Administration

Preparation & Available Formulation

• Available as: 500 mg/mL (50% solution) for injection
• Before administration, always dilute the 50% solution - concentrated MgSO4 must NOT be given as a direct rapid IV push undiluted (except in cardiac arrest situations where speed is critical)

Indications

Standard Dosing Protocols

1. Eclampsia / Severe Preeclampsia (IV Route - Preferred)

• 4-6 g IV diluted in 100 mL normal saline or 5% dextrose
• Infused over 15-30 minutes

• 1-2 g/hour as a continuous IV infusion
• Continue for at least 24 hours after the last seizure or delivery

In renal insufficiency (creatinine >1 mg/dL): reduce bolus to 2 g IV, then obtain serum magnesium level before increasing infusion rate - Tintinalli's Emergency Medicine

2. Intramuscular Route (Pritchard Regimen - resource-limited settings)

• Loading: 4 g IV slow push (over 3-5 min) + 10 g IM (5 g into each buttock as 50% solution with 1 mL of 2% lignocaine to reduce pain)
• Maintenance: 5 g IM every 4 hours into alternating buttocks
• IM injections are painful and can cause tissue injury; IV preferred when available

3. Torsades de Pointes / Cardiac Arrhythmia (IV Route)

• 1-2 g IV diluted in normal saline or 5% dextrose
• Given as a rapid bolus in patients with a pulse
• Note: Rapid IV administration can cause vasodilation, flushing, and hypotension - Tintinalli's Emergency Medicine

4. Fetal Neuroprotection (Preterm <32 weeks)

• 4 g IV loading dose followed by a maintenance infusion
• Used when delivery is imminent <32 weeks to reduce risk of cerebral palsy - Creasy & Resnik's Maternal-Fetal Medicine

Serum Magnesium Levels and Clinical Effects

Presence of deep tendon reflexes = serum magnesium is NOT dangerously high. Loss of DTR = first clinical warning sign of toxicity. - Creasy & Resnik's Maternal-Fetal Medicine

Monitoring During Infusion

1. Deep tendon reflexes (patellar reflex) - loss = stop infusion
2. Respiratory rate - must be ≥12-16/min; respiratory depression occurs at ~12 mg/dL
3. Urine output - must be ≥25-30 mL/hour (MgSO4 is renally excreted; half-life ~4 hours in normal renal function, prolonged in renal impairment)
4. Serum magnesium levels (where available)

• Loss of patellar reflexes
• Respiratory rate <12/min
• Urine output <25 mL/hour

Management of Toxicity / Hypermagnesaemia

Special Populations

• Renal impairment: Reduce maintenance infusion rate; monitor serum levels closely (excretion depends on GFR and tubular transport maximum)
• Oliguric patients: Use with extreme caution; consider reducing dose to 0.5 g/hour with frequent monitoring
• Neonate: Neonatal hypermagnesaemia possible if mother is on MgSO4 near delivery - monitor baby for respiratory depression, hypotonia, and poor suck

Drug Interaction Note