When should magnesium sulphate (MgSO4) be administered to a pregnant patient with severe hypertension or preeclampsia?

When to Administer Magnesium Sulfate in Pregnancy

Magnesium sulfate should be administered to pregnant women with preeclampsia who have severe hypertension (≥160/110 mmHg) with proteinuria, or those with moderate hypertension plus neurological symptoms (severe headache, visual disturbances, hyperreflexia, or clonus) for seizure prophylaxis. 1, 2

Primary Indications for Magnesium Sulfate

Severe Preeclampsia with High-Risk Features

• Administer MgSO4 when blood pressure is ≥160/110 mmHg with proteinuria to prevent eclamptic seizures 1, 2
• Give MgSO4 for moderate hypertension (≥150/100 mmHg) with proteinuria ≥2+ plus signs of imminent eclampsia including:
  • Severe intractable headache 1
  • Visual scotomata or blurred vision 1, 3
  • Hyperreflexia or clonus 1, 4
  • Epigastric tenderness 3

Additional Clinical Scenarios Requiring MgSO4

• HELLP syndrome with severe hypertension warrants magnesium sulfate administration 5
• Active eclamptic seizures require immediate MgSO4 as first-line treatment, superior to phenytoin and diazepam 5
• Fetal neuroprotection: Give MgSO4 when delivery is anticipated before 32 weeks' gestation to reduce cerebral palsy risk 5

Standard Dosing Protocol

Loading and Maintenance Regimen

• Loading dose: 4-6 grams IV over 20-30 minutes 6, 2, 7
• Maintenance infusion: 1-2 grams per hour by continuous IV infusion 6, 7
• Consider 2 grams/hour for patients with BMI ≥25 kg/m² as this achieves therapeutic levels more effectively than 1 gram/hour 6, 2
• Continue for 24 hours postpartum in most cases 6, 5

Alternative Regimen (Resource-Limited Settings)

• Pritchard protocol: 4 grams IV plus 10 grams IM (5 grams each buttock) as loading dose, followed by 5 grams IM every 4 hours in alternate buttocks 6, 7

When NOT to Give Magnesium Sulfate

Late Postpartum Severe Hypertension (>48 Hours)

• Reserve MgSO4 for patients with neurological symptoms only in late postpartum presentations 8
• Eclampsia occurring >48 hours postpartum is rare (16% of all eclampsia cases) and almost always preceded by cerebral symptoms 8
• Focus on antihypertensive management rather than routine MgSO4 for asymptomatic late postpartum severe hypertension 8

Mild Preeclampsia Without Risk Features

• Routine use in all preeclampsia cases is not justified as eclampsia incidence is lower in mild disease 3
• MgSO4 should be reserved for cases with concern about eclampsia risk 3

Critical Safety Considerations

Absolute Contraindications and Precautions

• Never combine MgSO4 with calcium channel blockers (especially nifedipine) due to risk of severe myocardial depression and precipitous hypotension 6, 5, 2
• Limit total fluid intake to 60-80 mL/hour to prevent pulmonary edema in preeclamptic patients with capillary leak 6, 2
• Do not exceed 5-7 days of continuous therapy as prolonged use causes fetal abnormalities 5, 7
• Maximum dose: 30-40 grams per 24 hours in normal renal function; reduce to 20 grams/48 hours in severe renal insufficiency 7

Clinical Monitoring (No Routine Lab Levels Needed)

• Monitor respiratory rate ≥12 breaths/minute 6, 5
• Check patellar reflexes - discontinue if absent 5, 7
• Maintain urine output ≥30 mL/hour 6, 5
• Serum magnesium levels only needed for: renal impairment, oliguria <30 mL/hour, loss of reflexes, or respiratory rate <12 5

Common Pitfalls to Avoid

• Do not use NSAIDs for postpartum pain in preeclamptic patients as they worsen hypertension and increase acute kidney injury risk 6
• Do not delay MgSO4 for laboratory confirmation - clinical criteria are sufficient to initiate therapy 1, 2
• Do not withhold MgSO4 in the presence of placental abruption - abruption is not a contraindication and MgSO4 does not increase this risk 5
• Avoid sublingual or IV nifedipine when MgSO4 is running due to dangerous drug interaction 5