What is the recommended dosage and administration of Magnesium Sulfate (MgSO4) in the treatment of preeclampsia?

Magnesium Sulfate Dosage and Administration in Preeclampsia

The recommended dosage of magnesium sulfate for preeclampsia is a loading dose of 4-5g IV over 15-20 minutes, followed by a maintenance dose of 1-2g/hour by continuous IV infusion for 24 hours postpartum. 1

Intravenous Administration Protocol

Loading Dose

• 4-5g IV in 250mL of 5% Dextrose or 0.9% Sodium Chloride over 15-20 minutes 1, 2

Maintenance Dose

• 1-2g/hour by continuous IV infusion 1
• Higher maintenance doses (2g/hour) may be required for overweight patients (BMI ≥25 kg/m²) 1
• Continue for 24 hours postpartum 1

Alternative Intramuscular Protocol

When IV administration is not feasible:

• Loading dose: 4g IV followed immediately by 5g IM in each buttock (total 14g loading) 3
• Maintenance dose: 5g IM every 4 hours in alternate buttocks 2

Monitoring Requirements

Clinical Monitoring

• Deep tendon reflexes (first sign of toxicity is loss of patellar reflex at 3.5-5 mmol/L) 1, 4
• Respiratory rate (respiratory depression occurs at 5-6.5 mmol/L) 4
• Urine output (should be >30mL/hour) 1
• Level of consciousness 1

Laboratory Monitoring

• Serum magnesium levels (target: 1.8-3.0 mmol/L for therapeutic effect) 4
• Renal function (creatinine) 1
• Liver function tests 1
• Complete blood count (platelets, hemoglobin) 1

Special Considerations

Fluid Management

• Restrict total fluid intake to 60-80 mL/hour during labor 1
• Aim for euvolemia by replacing insensible losses (30 mL/hour) plus urinary losses (0.5-1 mL/kg/hour) 1
• Avoid fluid overload as preeclamptic women are at risk of pulmonary edema due to capillary leak 1

Dose Adjustments

• In severe renal insufficiency, maximum dosage should not exceed 20g/48 hours with frequent serum magnesium monitoring 2
• Total daily dose should not exceed 30-40g in 24 hours 2

Duration Limitations

• Continuous use beyond 5-7 days can cause fetal abnormalities 1, 2

Efficacy and Safety Considerations

• Recent research suggests that shorter maintenance regimens (12-hour) may be as effective as the standard 24-hour regimen with fewer side effects 5
• Some studies indicate that a single loading dose without maintenance may be sufficient in certain cases of severe preeclampsia 6
• Clinical monitoring is generally sufficient; serum monitoring is not always necessary if clinical parameters are closely followed 7

Common Pitfalls and Caveats

1. Inadequate duration: Discontinuing MgSO₄ too early can increase risk of eclampsia, as seizures can still occur after delivery 1

2. Overdosage: Signs of magnesium toxicity progress from:

   • Loss of patellar reflex (3.5-5 mmol/L)
   • Respiratory depression (5-6.5 mmol/L)
   • Altered cardiac conduction (>7.5 mmol/L)
   • Cardiac arrest (>12.5 mmol/L) 4

3. Underdosage: Particularly in overweight patients who may require higher maintenance doses 1

4. Failure to adjust for renal impairment: Magnesium is primarily excreted by the kidneys, so dosage must be reduced in renal insufficiency 2

5. Inadequate monitoring: Clinical monitoring of reflexes, respiration, and urine output is essential even when serum levels aren't measured 7

The evidence strongly supports the use of magnesium sulfate as the drug of choice for seizure prophylaxis in preeclampsia, with careful attention to dosing, administration, and monitoring to ensure both efficacy and safety.