Magnesium Sulfate Maintenance Dose for Preeclampsia
The recommended maintenance dose of magnesium sulfate for preeclampsia is 1-2 grams/hour by continuous IV infusion after an initial loading dose of 4-5 grams IV over 15-20 minutes. 1, 2
Dosing Regimen
Loading Dose
- 4-5 grams IV in 250 mL of 5% Dextrose or 0.9% Sodium Chloride over 15-20 minutes 1, 2
- Alternative approach: Initial IV dose of 4 grams may be given by diluting the 50% solution to a 10% or 20% concentration and injecting over 3-4 minutes 2
Maintenance Dose
Clinical Considerations for Dosing
Factors Affecting Dose Selection
- Body weight: Research suggests overweight patients (BMI ≥25 kg/m²) may require higher maintenance doses (2 grams/hour) to achieve therapeutic levels 4
- Severity of preeclampsia: More severe cases may benefit from higher doses within the recommended range
- Renal function: Maximum dosage should not exceed 20 grams/48 hours in severe renal insufficiency 2
Therapeutic Targets
- Target serum magnesium level: 4.8-8.4 mg/dL (2-3.5 mmol/L) 5
- A 1 gram/hour maintenance dose may be insufficient in some patients, particularly those who are overweight 6
Monitoring During Therapy
Clinical assessment for signs of magnesium toxicity:
- Deep tendon reflexes (patellar reflex)
- Respiratory rate (should be >12/min)
- Urine output (should be >30 mL/hour)
- Level of consciousness
Laboratory monitoring:
- Serum magnesium levels if clinically indicated or in cases of renal impairment
- Creatinine, liver function tests, platelets, and hemoglobin at least twice weekly 3
Duration of Treatment
- Continue magnesium sulfate for 24 hours postpartum 3
- Recent research suggests that a 12-hour regimen may be as effective as a 24-hour regimen with fewer side effects 7, but until further studies confirm these findings in diverse populations, continuing for 24 hours postpartum remains the standard recommendation 3
Important Precautions
- Total daily dose should not exceed 30-40 grams/24 hours 2
- Fluid intake should be limited to 60-80 mL/hour during labor to avoid pulmonary edema 3
- Reduced gastrointestinal motility during labor may decrease absorption of oral antihypertensives, necessitating IV medications 1
- Continuous maternal administration beyond 5-7 days can cause fetal abnormalities 2
Common Side Effects and Management
- Pain, inflammation, or bruising at injection site (with IM administration)
- Flushing, sweating, sensation of warmth
- Nausea, vomiting
- Muscle weakness
- Drowsiness
Magnesium sulfate therapy should be discontinued immediately if signs of toxicity develop, including loss of patellar reflexes, respiratory depression, or altered consciousness.
Remember that magnesium sulfate is the drug of choice for prevention of eclampsia and has been shown to reduce the risk of eclampsia by approximately half 1.