Magnesium Sulfate Regimens for Preeclampsia
The standard magnesium sulfate regimen for preeclampsia consists of 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, 2
Standard Regimens
Intravenous (IV) Regimen
- Loading dose: 4-5g 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, 2
- Duration: Continue for 24 hours postpartum 1
Intramuscular (IM) Regimen
- Loading dose: 4g IV over 15-20 minutes
- Initial IM dose: 10g (5g in each buttock)
- Maintenance dose: 5g IM every 4 hours in alternating buttocks 2, 3
Special Population Adjustments
Renal Impairment
- For impaired renal function, reduce maintenance dose to 0.5-0.75g/hour (50% reduction) 1
- With severe renal insufficiency, maximum dosage should not exceed 20g/48 hours 2
- Monitoring of serum magnesium levels is essential in these patients 1, 2
Overweight Patients
- Patients with BMI ≥25 kg/m² may require higher maintenance doses (2g/hour) to achieve therapeutic levels 1, 4
- Research shows that 2g/hour infusion in overweight mothers achieves therapeutic levels more consistently than 1g/hour (84.2% vs 42.1% after delivery) 4
Therapeutic Targets and Monitoring
Target Serum Levels
- Therapeutic serum magnesium level: 1.8-3.0 mmol/L (4.8-8.4 mg/dL) 3, 5
- This level is effective for prevention and treatment of eclamptic convulsions 3
Monitoring Parameters
- Clinical assessment for magnesium toxicity: deep tendon reflexes, respiratory rate, urine output, and level of consciousness 1
- First warning of toxicity: loss of patellar reflex at 3.5-5 mmol/L 3
- Respiratory paralysis occurs at 5-6.5 mmol/L 3
- Cardiac conduction is altered at >7.5 mmol/L 3
Important Considerations and Pitfalls
Risk Factors for Sub-therapeutic Levels
- High creatinine clearance (≥133 mL/min) 5
- Omission of loading dose 5
- Measurement of serum levels too early (before 2.375 hours of infusion) 5
Safety Precautions
- Do not exceed IV injection rate of 150 mg/minute except in severe eclampsia with seizures 2
- Total daily dose should not exceed 30-40g/24 hours 2
- Have calcium chloride (10%) 5-10 mL or calcium gluconate (10%) 15-30 mL immediately available to counteract magnesium toxicity 1
- Avoid continuous maternal administration beyond 5-7 days due to risk of fetal abnormalities 1, 2
Alternative Administration Methods
- Springfusor® pump has shown better acceptability and lower pain scores compared to standard IM regimen (95.3% vs 70.3% acceptability) 6
- IV administration is preferable where resources allow, as side effects and injection site problems are lower 7
Duration of Treatment
- Continue magnesium sulfate for 24 hours postpartum 1
- Duration should not normally exceed 24 hours if IV route is used 7
Remember that careful monitoring is essential regardless of the regimen used, and magnesium sulfate administration should be discontinued as soon as the desired effect is obtained.