MgSO4 Dosing for Pregnancy-Induced Hypertension
For women with pregnancy-induced hypertension (PIH), administer magnesium sulfate as a 4-6 gram IV loading dose over 20-30 minutes, followed by a maintenance infusion of 1-2 grams per hour for 24 hours postpartum. 1, 2
Standard IV Regimen (Preferred)
Loading Dose:
- Administer 4-6 grams IV over 20-30 minutes 1, 2
- The FDA-approved regimen for severe pre-eclampsia/eclampsia specifies 4-5 grams in 250 mL of 5% dextrose or 0.9% sodium chloride 3
Maintenance Dose:
- Standard: 1-2 grams per hour by continuous IV infusion 1, 2
- For patients with BMI ≥25 kg/m²: Start at 2 grams per hour rather than 1 gram per hour, as 70-80% of overweight patients reach therapeutic levels within 2-4 hours at this dose 2
- Evidence from a 2019 randomized trial showed both 1 gram/hour and 2 grams/hour were equally effective in preventing eclampsia, though 2 grams/hour produced higher serum magnesium levels with more side effects 4
Duration:
- Continue for 24 hours postpartum in most cases 1, 2
- Maximum total duration should not exceed 5-7 days due to risk of fetal abnormalities 3
Alternative Pritchard Regimen (Resource-Limited Settings)
When continuous IV infusion is not feasible, use the combined IV/IM protocol validated in the MAGPIE trial: 2
Loading Dose:
Maintenance Dose:
- 5 grams IM every 4 hours in alternate buttocks for 24 hours 1, 2
- This regimen is particularly valuable when IV access is limited or continuous monitoring is unavailable 2
Critical Safety Monitoring
Clinical Parameters (Monitor Continuously):
- Patellar reflexes: Loss occurs at 3.5-5 mmol/L (first sign of toxicity) 5
- Respiratory rate: Must be ≥12 breaths/minute; respiratory paralysis occurs at 5-6.5 mmol/L 1, 5
- Urine output: Maintain ≥30 mL/hour (oliguria increases toxicity risk as magnesium is renally excreted) 1, 6
- Oxygen saturation: Keep >90% 6
Laboratory Monitoring:
- Serum magnesium levels are NOT routinely needed with standard dosing 6
- Check levels only in high-risk situations: renal impairment, oliguria (<30 mL/hour), loss of reflexes, or respiratory rate <12 breaths/minute 6
- Therapeutic range for seizure prevention: 1.8-3.0 mmol/L (4.8-7.2 mg/dL) 5
Absolute Contraindications and Critical Warnings
Never combine MgSO4 with calcium channel blockers (especially nifedipine) - this causes severe hypotension and myocardial depression 1, 2, 6
Fluid restriction:
- Limit total fluid intake to 60-80 mL/hour to prevent pulmonary edema in preeclamptic patients with capillary leak 1, 2
Renal impairment:
- Maximum dose is 20 grams/48 hours in severe renal insufficiency 3
- Requires frequent serum magnesium monitoring 3
Toxicity management:
- Antidote: 10 mL of 10% calcium gluconate IV 7
- Immediately stop infusion if reflexes are lost or respiratory rate drops below 12 1
Common Pitfalls to Avoid
- Do NOT use NSAIDs for postpartum pain in preeclamptic patients - they worsen hypertension and increase acute kidney injury risk 1, 2
- Do NOT rely on proteinuria levels to guide MgSO4 administration; base decision on blood pressure and clinical signs 6
- Do NOT continue beyond 5-7 days - prolonged use causes fetal abnormalities 3
- In community settings where full protocols cannot be administered, give at least 10 grams IM total (5 grams each buttock) as loading dose before referral 2
Indications for MgSO4 in PIH
- Blood pressure ≥160/110 mmHg with proteinuria 6
- Moderate hypertension (≥150/100 mmHg) with proteinuria PLUS signs of imminent eclampsia (severe headache, visual scotomata, clonus, or epigastric pain) 6
- Any woman with severe pre-eclampsia and at least one clinical sign of seriousness 6
Evidence Base
MgSO4 is superior to both phenytoin and diazepam for preventing eclamptic seizures - a 1995 randomized trial showed 10/1089 women on phenytoin developed eclampsia versus 0/1049 on MgSO4 (P=0.004) 8. This validates decades of clinical practice and is endorsed by ACOG, ISSHP, and European guidelines 1, 2, 6.