What is the recommended magnesium sulfate infusion rate for a pregnant woman with severe preeclampsia or eclampsia?

Magnesium Sulfate Infusion Rate for Severe Preeclampsia/Eclampsia

Administer a loading dose of 4-6 grams IV over 20-30 minutes, followed by a maintenance infusion of 2 grams per hour for most patients, particularly those with BMI ≥25 kg/m², and continue for 24 hours postpartum. 1, 2

Loading Dose Protocol

• Give 4-6 grams IV over 20-30 minutes as the initial loading dose 1, 2
• The FDA-approved alternative (Pritchard protocol) combines 4 grams IV plus 10 grams IM (5 grams in each buttock) for settings with limited IV access 1, 2
• IV administration provides therapeutic levels almost immediately, while IM administration achieves therapeutic levels within 60 minutes 2

Maintenance Infusion Rate

• Start at 2 grams per hour rather than 1 gram per hour for most patients, especially those with BMI ≥25 kg/m² 1
• The evidence shows 2 grams per hour is more effective at achieving therapeutic levels (4-6 mg/dL or 1.8-3.0 mmol/L) 1, 3
• The FDA label states 1-2 g/hour by constant IV infusion is acceptable, but recent guidelines favor the higher rate 2, 1
• One research study found 1 gram per hour was as effective as 2 grams per hour with fewer side effects, but this conflicts with guideline recommendations prioritizing therapeutic level achievement 4

Duration of Therapy

• Continue magnesium sulfate for 24 hours postpartum in most cases 1
• For eclampsia, continue until 24 hours after the last seizure 2
• Do not exceed 5-7 days of continuous administration in pregnancy, as this can cause fetal abnormalities 2
• Total daily dose should not exceed 30-40 grams 2

Critical Safety Monitoring

• Check deep tendon reflexes (patellar reflex must be present) 2, 3
• Monitor respiratory rate (must be ≥12 breaths/minute) 1
• Ensure urine output ≥30 mL/hour, as oliguria dramatically increases toxicity risk since magnesium is renally excreted 1, 5
• Loss of patellar reflex occurs at 3.5-5 mmol/L, respiratory paralysis at 5-6.5 mmol/L, and cardiac arrest at >12.5 mmol/L 3, 5
• Serum magnesium levels should NOT be routinely drawn; clinical monitoring (reflexes, respiratory rate, urine output) is sufficient 1

Fluid Management

• Limit total fluid intake to 60-80 mL/hour to prevent pulmonary edema in preeclamptic patients with capillary leak 1
• This is a critical pitfall—excessive fluid administration can cause life-threatening pulmonary edema 1

Drug Interactions to Avoid

• Never combine magnesium sulfate with calcium channel blockers (especially nifedipine), as this causes severe hypotension and myocardial depression 6, 1, 7
• If acute blood pressure control is needed, use hydralazine (5 mg IV bolus, then 10 mg every 20-30 minutes to maximum 25 mg) or labetalol (20 mg IV bolus, then escalating doses) 6

Renal Impairment Adjustments

• In severe renal insufficiency, maximum dose is 20 grams per 48 hours with frequent serum magnesium monitoring 2
• Patients with elevated creatinine require dose adjustment or discontinuation 1

Antidote for Toxicity

• Keep calcium gluconate 10% (15-30 mL IV over 2-5 minutes) or calcium chloride 10% (5-10 mL IV over 2-5 minutes) immediately available as a physiological antagonist 5
• Administer calcium immediately if respiratory depression, cardiac arrhythmias, or loss of reflexes occur 5

Alternative IM Regimen (Resource-Limited Settings)

• Loading: 4 grams IV + 10 grams IM (5 grams each buttock) 1, 2
• Maintenance: 5 grams IM every 4 hours in alternate buttocks for 24 hours 1, 2
• This regimen can be administered by trained midwives or nursing staff 7

Common Pitfalls

• Continuing magnesium when oliguria develops—this is the most dangerous error leading to toxicity 5
• Using NSAIDs for postpartum pain in preeclamptic patients, which worsens hypertension and increases acute kidney injury risk 1
• Waiting for laboratory confirmation before treating obvious clinical toxicity—treat based on clinical signs 5
• Administering oral antihypertensives during labor when gastrointestinal motility is reduced—use IV medications instead 1