Magnesium Sulfate (MgSO4): Indications and Dosing Guidelines
Magnesium sulfate is indicated primarily for prevention and treatment of eclampsia in pre-eclampsia, treatment of magnesium deficiency, and fetal neuroprotection in preterm birth, with specific dosing regimens established for each indication. 1
Indications for Magnesium Sulfate
1. Pre-eclampsia and Eclampsia
- Prevention and treatment of seizures in pre-eclampsia and eclampsia 2, 1
- Management of hypertensive crisis in pregnancy 2
- HELLP syndrome with co-existing severe hypertension 3
2. Magnesium Deficiency
- Treatment of mild to severe hypomagnesemia 1
- Replacement therapy in magnesium deficiency with signs of tetany 1
3. Other Indications
- Total parenteral nutrition (TPN) supplementation 1
- Neuroprotection for preterm fetuses when delivery is required before 32 weeks' gestation 3, 4
- Counteracting barium poisoning 1
- Control of seizures associated with epilepsy, glomerulonephritis, or hypothyroidism 1
- Paroxysmal atrial tachycardia (when simpler measures have failed) 1
- Reduction of cerebral edema 1
Dosing Guidelines by Indication
1. Pre-eclampsia and Eclampsia
Intravenous Regimen (Preferred)
- Loading dose: 4-5 g IV in 250 mL of 5% Dextrose or 0.9% Sodium Chloride over 15-20 minutes 3, 1
- Maintenance dose: 1-2 g/hour by continuous IV infusion for 24 hours 3, 1
- Target serum magnesium level: 1.8-3.0 mmol/L 5
Intramuscular Regimen (Alternative)
- Loading dose: 4 g IV over 5 minutes, followed immediately by 5 g IM in each buttock (total 10 g) 2, 1
- Maintenance dose: 5 g IM every 4 hours in alternating buttocks 2, 1, 6
2. Magnesium Deficiency
Mild Deficiency
- 1 g (8.12 mEq) IM every 6 hours for 4 doses (total 32.5 mEq/24 hours) 1
Severe Deficiency
- Up to 250 mg/kg IM within 4 hours, or
- 5 g (40 mEq) added to 1 L of 5% Dextrose or 0.9% Sodium Chloride for slow IV infusion over 3 hours 3, 1
3. Total Parenteral Nutrition (TPN)
4. Fetal Neuroprotection in Preterm Birth
- Similar dosing to pre-eclampsia regimen 4
- Administered to women at risk of preterm birth <34 weeks' gestation 4
Monitoring and Safety Considerations
Clinical Monitoring
- Deep tendon reflexes: Loss indicates impending toxicity (serum levels 3.5-5 mmol/L) 3, 5
- Respiratory rate: Respiratory depression may occur at levels 5-6.5 mmol/L 5
- Urine output: Ensure adequate renal function (>30 mL/hour) 3
- Level of consciousness: Monitor for sedation 3
Laboratory Monitoring
- Serum magnesium levels: Target 1.8-3.0 mmol/L for eclampsia treatment 5
- Renal function: Creatinine and eGFR 3
- Other labs: Liver function tests, platelets, hemoglobin 3
Dosage Adjustments
- Renal impairment:
- Overweight patients (BMI ≥25 kg/m²): May require higher maintenance doses (2 g/hour) 3, 7
Toxicity Management
- Antidote: Calcium chloride (10%) 5-10 mL or calcium gluconate (10%) 15-30 mL IV 3
- Toxicity signs by serum level:
- 3.5-5 mmol/L: Loss of patellar reflex
- 5-6.5 mmol/L: Respiratory paralysis
7.5 mmol/L: Altered cardiac conduction
12.5 mmol/L: Cardiac arrest 5
Important Considerations and Precautions
- Continuous maternal administration beyond 5-7 days can cause fetal abnormalities 3, 1
- Solutions for IV infusion must be diluted to ≤20% concentration prior to administration 1
- Magnesium may reduce antibiotic activity of streptomycin, tetracycline, and tobramycin 1
- Risk of hypotension when given concomitantly with nifedipine 2
- Avoid diuretic therapy in pre-eclampsia as plasma volume is already reduced 2
- Recent evidence suggests 1 g/hour maintenance dose may be as effective as 2 g/hour with fewer side effects 7
By following these guidelines and carefully monitoring patients, magnesium sulfate can be safely and effectively administered for its various indications.