Magnesium Sulfate Dosing
For preeclampsia and eclampsia, administer a loading dose of 4-6 grams IV over 20-30 minutes, followed by a maintenance infusion of 1-2 grams per hour for 24 hours postpartum. 1, 2
Standard Dosing Regimen for Preeclampsia/Eclampsia
Loading Dose
- Administer 4-6 grams IV over 20-30 minutes as the initial loading dose 1, 2
- The FDA-approved regimen specifies 4-5 grams in 250 mL of 5% dextrose or 0.9% sodium chloride infused IV 3
- Alternatively, use the Pritchard protocol: 4 grams IV plus 10 grams IM (5 grams in each buttock) for combined loading, particularly useful in resource-limited settings with limited IV access 2, 3
Maintenance Dose
- Continue with 1-2 grams per hour by continuous IV infusion for 24 hours postpartum 1, 2
- Evidence supports 2 grams per hour as more effective than 1 gram per hour in achieving therapeutic levels, particularly in patients with BMI ≥25 kg/m² 2
- For the IM-based Pritchard regimen, give 5 grams IM every 4 hours in alternate buttocks for 24 hours 2, 3
Duration of Therapy
- Continue magnesium sulfate for 24 hours postpartum in most cases 2
- The 24-hour protocol remains the safer standard despite some evidence suggesting women who received ≥8 grams before delivery may not benefit from continuing the full 24 hours 2
- Do not exceed 30-40 grams total daily dose 3
- Never continue magnesium sulfate beyond 5-7 days in pregnancy, as this can cause fetal abnormalities 1, 3
Special Population Considerations
Overweight Patients
- Start at 2 grams per hour maintenance rather than 1 gram per hour for patients with BMI ≥25 kg/m² 2
Renal Impairment
- Maximum dosage is 20 grams per 48 hours in severe renal insufficiency 3
- Frequent serum magnesium monitoring is required in renal impairment 2, 3
- Oliguria (<30 mL/hour urine output) increases toxicity risk as magnesium is renally excreted 2, 4
Critical Safety Monitoring
Clinical Monitoring (Preferred Over Laboratory)
- Do not routinely draw magnesium levels; clinical monitoring should guide therapy 2
- Check deep tendon reflexes (must be present) 2, 5
- Monitor respiratory rate (must be ≥12 breaths/minute) 2
- Assess urine output (must be ≥30 mL/hour) 2, 4
- Maintain oxygen saturation >90% 2
When to Check Serum Magnesium Levels
- Only check in high-risk situations: renal impairment with elevated creatinine, oliguria developing during therapy, or signs of toxicity 2, 4
- Therapeutic range is 4-7 mEq/L (4.8-8.4 mg/dL); seizure control is optimal at 6 mg/100 mL 3, 5
Critical Drug Interactions and Contraindications
Absolute Contraindication
- Never combine magnesium sulfate with calcium channel blockers (especially nifedipine), as this causes severe hypotension and myocardial depression 1, 2, 6
Fluid Management
- Limit total fluid intake to 60-80 mL/hour to prevent pulmonary edema in preeclamptic patients with capillary leak 2
Pain Management
- Avoid NSAIDs for postpartum pain when possible, as they worsen hypertension and increase acute kidney injury risk 2
Magnesium Toxicity Recognition and Treatment
Signs of Toxicity by Serum Level
- Loss of tendon reflexes, sedation, severe muscular weakness, and respiratory depression occur at 4-5 mmol/L 4
- ECG changes (prolonged PR, QRS, QT intervals) occur at 2.5-5 mmol/L 4
- AV nodal block, bradycardia, and hypotension occur at 6-10 mmol/L 4
Immediate Treatment of Toxicity
- Administer calcium chloride 10% 5-10 mL or calcium gluconate 10% 15-30 mL IV over 2-5 minutes immediately 4
- Stop magnesium infusion 2, 4
- Provide respiratory support if needed 4
- Continuous cardiac monitoring for arrhythmias 4
- Blood pressure support with vasopressors if hypotension persists 4
Alternative Indications and Dosing
Fetal Neuroprotection
- Administer magnesium sulfate when delivery is anticipated before 32 weeks gestation to reduce cerebral palsy risk 6
- Use standard preeclampsia dosing regimen 6
Magnesium Deficiency
- For mild deficiency: 1 gram (8.12 mEq) IM every 6 hours for 4 doses 3
- For severe hypomagnesemia: up to 250 mg/kg IM over 4 hours, or 5 grams in 1 liter IV over 3 hours 3
Other Acute Conditions
- Barium poisoning: 1-2 grams IV 3
- Seizures from epilepsy, glomerulonephritis, or hypothyroidism: 1 gram IM or IV 3
- Paroxysmal atrial tachycardia: 3-4 grams IV over 30 seconds (only if simpler measures failed and no myocardial damage) 3
Common Pitfalls to Avoid
- Do not delay calcium administration while waiting for laboratory confirmation when clinical signs strongly suggest toxicity 4
- Do not continue magnesium when oliguria develops, especially in pregnant women 4
- Do not use loading dose-only regimens without maintenance therapy, as limited data support this approach 7
- Do not rely solely on total magnesium levels, as they may not correlate with ionized (active) magnesium, particularly during therapy 8