Magnesium Sulphate Dosing for Eclampsia
For eclampsia treatment, administer a loading dose of 4-6 grams IV over 20-30 minutes, followed by a maintenance infusion of 2 grams per hour IV for 24 hours postpartum. 1, 2
Loading Dose Regimen
Standard IV Protocol
- Administer 4-6 grams IV over 20-30 minutes as the initial loading dose 1, 3
- This can be given as 4 grams in 250 mL of 5% dextrose or 0.9% sodium chloride infused over 20-30 minutes 3
- Alternatively, dilute the 50% solution to 10% or 20% concentration and inject 40 mL of 10% solution or 20 mL of 20% solution IV over 3-4 minutes 3
Alternative Pritchard Protocol (Resource-Limited Settings)
- When continuous IV access is limited, use 4 grams IV plus 10 grams IM (5 grams in each buttock) as the combined loading dose 4, 1, 3
- This regimen is particularly useful in low and middle-income countries where IV infusion pumps may not be available 4
Maintenance Dose Regimen
Standard IV Maintenance
- Administer 2 grams per hour by continuous IV infusion for 24 hours postpartum 1, 2
- The 2 grams/hour rate is more effective than 1 gram/hour in achieving therapeutic magnesium levels, particularly in patients with BMI ≥25 kg/m² 1, 5, 6
- Therapeutic serum magnesium concentration for seizure control is 1.8-3.0 mmol/L (approximately 4.8-8.4 mg/dL) 7, 6
Alternative IM Maintenance (Pritchard Protocol)
- If continuous IV infusion is not feasible, give 5 grams IM every 4 hours in alternate buttocks for 24 hours 4, 1
- This regimen requires checking patellar reflexes and respiratory function before each dose 3
Duration of Therapy
- Continue magnesium sulfate for 24 hours postpartum in most cases 1
- Therapy should continue until seizures cease and the patient remains stable 3
- Do not exceed 30-40 grams total in 24 hours 3
- In severe renal insufficiency, maximum dosage is 20 grams per 48 hours with frequent serum magnesium monitoring 3
Critical Safety Monitoring
Clinical Parameters to Monitor
- Patellar (deep tendon) reflexes must be present before each dose - loss occurs at 3.5-5 mmol/L 7
- Respiratory rate must be ≥12 breaths per minute 1, 7
- Urine output must be ≥30 mL/hour 1, 7
- Respiratory paralysis occurs at 5-6.5 mmol/L, and cardiac arrest at >12.5 mmol/L 7
Fluid Management
- Limit total fluid intake to 60-80 mL/hour to prevent pulmonary edema 1, 2
- Preeclamptic patients have capillary leak and reduced plasma volume, making them vulnerable to fluid overload 2
Critical Drug Interactions
Never combine magnesium sulfate with calcium channel blockers (especially nifedipine) without extremely careful monitoring due to risk of severe hypotension and myocardial depression 1, 2, 8
Toxicity Management
Signs of Magnesium Toxicity
- Loss of patellar reflexes: 3.5-5 mmol/L 7
- Respiratory depression: 5-6.5 mmol/L 7
- Cardiac conduction abnormalities: >7.5 mmol/L 7
Antidote
- Immediately administer calcium chloride 10% 5-10 mL IV or calcium gluconate 10% 15-30 mL IV over 2-5 minutes if toxicity develops 8
- Calcium acts as a physiological antagonist to magnesium 8
Special Populations
Overweight Patients (BMI ≥25 kg/m²)
- Start maintenance at 2 grams/hour rather than 1 gram/hour 1, 6
- Studies show 1 gram/hour maintenance achieves therapeutic levels in only 15.8% of overweight patients before delivery versus 52.6% with 2 grams/hour 6
Renal Impairment
- Reduce dosing significantly as magnesium is 90% renally excreted 7
- Maximum 20 grams per 48 hours with frequent serum level monitoring 3
- Patients can develop toxicity at relatively lower doses 8
Common Pitfalls to Avoid
- Do not continue magnesium administration if oliguria develops - this dramatically increases toxicity risk 8
- Do not use NSAIDs for postpartum pain in preeclamptic patients - they worsen hypertension and increase acute kidney injury risk 1
- Do not use magnesium sulfate continuously in pregnancy beyond 5-7 days - this can cause fetal abnormalities 3
- Do not use loading dose-only regimens - limited data suggest they are less effective than loading plus maintenance regimens 9