Magnesium Sulfate Dosage for Pre-eclampsia
For pre-eclampsia, the recommended magnesium sulfate dosage is a 4-5g intravenous loading dose over 15-30 minutes, followed by a maintenance infusion of 1-2g/hour for 24 hours or until 24 hours postpartum. 1, 2
Initial Loading Dose
- 4-5g magnesium sulfate diluted in 250mL of 5% Dextrose or 0.9% Sodium Chloride
- Administer IV over 15-30 minutes
Alternative Loading Dose Options
- IV loading dose of 4g over 15-30 minutes PLUS
- Simultaneous IM injection of 10g (5g in each buttock) when using the Pritchard regimen
Maintenance Dose
Two evidence-based options:
- IV Infusion (Preferred): 1-2g/hour via controlled infusion pump for 24 hours
- IM Regimen: 5g IM every 4 hours in alternating buttocks
Duration of Treatment
- Continue for 24 hours after delivery
- Some recent evidence suggests that if at least 8g has been given before delivery, continuing postpartum may not be necessary, but the ISSHP still recommends continuing for 24 hours postpartum 1
Therapeutic Monitoring
- Target serum magnesium level: 1.8-3.0 mmol/L (therapeutic range) 3
- Monitor for toxicity:
- Loss of patellar reflexes occurs at 3.5-5 mmol/L
- Respiratory depression at 5-6.5 mmol/L
- Cardiac conduction abnormalities >7.5 mmol/L
- Cardiac arrest >12.5 mmol/L 3
Clinical Considerations
Dosage Adjustments
- In renal insufficiency: Maximum 20g/48 hours with frequent serum magnesium monitoring 2
- Limit continuous use in pregnancy beyond 5-7 days due to risk of fetal abnormalities 2
Efficacy Considerations
- Recent research suggests that a 1g/hour maintenance dose may be as effective as 2g/hour with fewer side effects 4
- However, older studies found that 1g/hour produced inadequate serum levels compared to the standard regimen 5
Administration Pitfalls
- Ensure proper dilution - IV solutions must be diluted to ≤20% concentration prior to administration 2
- Use controlled infusion pump for IV maintenance to prevent accidental overdose
- Avoid combining with calcium channel blockers (particularly nifedipine) as this may cause excessive hypotension 1
- Do not administer magnesium sulfate sublingually
Monitoring Requirements
- Deep tendon reflexes every 1-2 hours
- Respiratory rate (should be >12/min)
- Urine output (should be >30 mL/hour)
- Consider serum magnesium levels in cases of:
- Renal dysfunction
- Signs of toxicity
- Inadequate clinical response
The MAGPIE trial demonstrated that magnesium sulfate approximately halves the rate of eclampsia, with about 100 women needing treatment to prevent one seizure 1. While selective use may be reasonable in high-income settings with specialized centers, all pre-eclamptic women in low and middle-income countries should receive magnesium sulfate due to the favorable cost-benefit ratio 1.