What are the guidelines for administering magnesium sulfate (MgSO4) at a dose of 2 grams continuously during labor, particularly for patients at risk of eclampsia or requiring fetal neuroprotection?

Magnesium Sulfate 2 Grams Continuous Infusion During Labor

For patients with severe preeclampsia or those requiring fetal neuroprotection before 32 weeks gestation, administer magnesium sulfate as a 4-6 gram IV loading dose over 20-30 minutes, followed by a continuous maintenance infusion of 2 grams per hour, particularly in patients with BMI ≥25 kg/m² 1.

Standard Dosing Protocol

Loading Dose

• Administer 4-6 grams IV over 20-30 minutes as the initial loading dose 1, 2
• This achieves rapid therapeutic levels for seizure prevention in severe preeclampsia 1

Maintenance Infusion: The 2 Gram Per Hour Regimen

• For patients with BMI ≥25 kg/m², start maintenance at 2 grams per hour rather than the traditional 1 gram per hour 1
• Evidence demonstrates that overweight patients achieve therapeutic magnesium levels more reliably with 2 grams per hour (52.6% vs 15.8% before delivery, and 84.2% vs 42.1% after delivery) 3
• The standard 1 gram per hour maintenance dose is inadequate in overweight patients, with 85.7% of eclamptic seizures during therapy occurring in patients with BMI ≥25 kg/m² who had subtherapeutic magnesium levels 3

Primary Clinical Indications

Seizure Prevention in Severe Preeclampsia

• Magnesium sulfate is the most effective agent for preventing eclamptic seizures, superior to phenytoin and diazepam 1, 4
• Indicated for severe preeclampsia with blood pressure ≥160/110 mmHg with significant proteinuria, or moderate hypertension with proteinuria and neurological symptoms (headache, visual disturbances, clonus) 1, 5
• Approximately halves the seizure rate in patients with preeclampsia 5

Fetal Neuroprotection

• Administer when delivery is anticipated before 32 weeks gestation 6
• Reduces cerebral palsy risk (relative risk 0.68,95% CI 0.54-0.87) without increasing mortality 4

Duration of Therapy

• Continue for 24 hours postpartum in most cases 5
• Some evidence suggests that if a woman has received at least 8 grams before delivery, continuing for a further 24 hours postpartum may not provide additional benefit 5
• Never exceed 5-7 days of continuous administration, as prolonged use can cause fetal abnormalities 5, 7, 8

Critical Safety Monitoring

Clinical Parameters (No Routine Lab Monitoring Required)

• Check patellar reflexes before each dose—loss of reflexes indicates impending toxicity at 3.5-5 mmol/L 1, 2
• Maintain respiratory rate >12 breaths per minute (respiratory paralysis occurs at 5-6.5 mmol/L) 1, 2
• Ensure urine output ≥30 mL/hour, as magnesium is renally excreted and oliguria increases toxicity risk 5, 4, 2
• Maintain oxygen saturation >90% 1, 5

When to Check Serum Magnesium Levels

• Only check serum levels in renal impairment (elevated creatinine), not routinely 4
• Therapeutic range is 1.8-3.0 mmol/L (4.8-8.4 mg/dL) 2, 3
• Cardiac conduction alterations occur at >7.5 mmol/L, with cardiac arrest risk at >12.5 mmol/L 2

Absolute Contraindications and Critical Precautions

Never Combine with Calcium Channel Blockers

• Do not use magnesium sulfate with calcium channel blockers (especially nifedipine), as this causes severe myocardial depression and precipitous hypotension 5, 4

Fluid Management

• Limit total fluid intake to 60-80 mL/hour to prevent pulmonary edema 1, 5
• Aim for euvolemia—avoid "running dry" as preeclamptic patients are at risk for acute kidney injury, but also avoid fluid overload 5

Neonatal Considerations

• Neonatal hypotonia occurs in approximately 6% of infants exposed to maternal magnesium sulfate 9
• Lower Apgar scores, increased intubation rates, and special care nursery admissions correlate with higher maternal serum magnesium concentrations before birth 9
• These effects are generally transient and resolve as magnesium is cleared from the neonatal circulation 9

Common Clinical Pitfalls

• Underdosing overweight patients: Using 1 gram per hour maintenance in patients with BMI ≥25 kg/m² results in subtherapeutic levels and breakthrough seizures 3
• Inadequate monitoring: Clinical examination (reflexes, respiratory rate, urine output) is sufficiently sensitive to detect toxicity before serious complications—routine serum levels are unnecessary and may delay appropriate clinical assessment 4
• Combining with calcium channel blockers: This is the most dangerous drug interaction and must be avoided 5, 4
• Excessive fluid administration: Preeclamptic patients are at high risk for pulmonary edema; strict fluid restriction is essential 1, 5