How should intravenous magnesium sulfate be administered (loading dose, dilution, infusion rate, duration, monitoring parameters, and contraindications) to a pregnant woman for neuroprotection (e.g., preeclampsia)?

Intravenous Magnesium Sulfate Administration for Preeclampsia/Eclampsia

For severe preeclampsia or eclampsia, 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 for 24 hours postpartum. 1, 2, 3, 4

Loading Dose Protocol

Administer 4-6 grams of magnesium sulfate intravenously over 20-30 minutes as the initial loading dose. 3, 4

• Dilute the magnesium sulfate to a 10-20% concentration before IV administration (never give the 50% solution undiluted intravenously). 4
• For a 4 gram loading dose: dilute to 40 mL of 10% solution or 20 mL of 20% solution, then infuse over 3-4 minutes for rapid effect in eclampsia with active seizures. 4
• For standard severe preeclampsia without active seizures, infuse the loading dose over 20-30 minutes. 3
• Common diluents include 5% dextrose or 0.9% sodium chloride. 4

Alternative Pritchard Protocol (Resource-Limited Settings)

• Give 4 grams IV plus 10 grams IM (5 grams in each buttock) as the combined loading dose when continuous IV infusion is unavailable. 3, 4
• This provides therapeutic levels within 60 minutes via the IM route. 4

Maintenance Infusion

Continue with 2 grams per hour by continuous IV infusion, as this dose is more effective than 1 gram per hour in achieving therapeutic magnesium levels (1.8-3.0 mmol/L). 2, 3, 5

• Evidence demonstrates that 2 grams per hour achieves therapeutic levels in 70-80% of patients versus only 17-37% with 1 gram per hour. 5
• This is particularly important for patients with BMI ≥25 kg/m², who may require the higher maintenance dose. 3
• The maximum infusion rate should generally not exceed 150 mg/minute (1.5 mL of 10% solution), except during active eclamptic seizures. 4

Alternative IM Maintenance (Pritchard Protocol)

• Give 5 grams IM every 4 hours in alternating buttocks for 24 hours when continuous IV infusion is not feasible. 3, 4

Duration of Therapy

Continue magnesium sulfate for 24 hours postpartum in most cases, as eclamptic seizures can occur for the first time in the early postpartum period. 2, 3, 6

• The 24-hour postpartum protocol is the safer standard despite some evidence suggesting women who received ≥8 grams before delivery may not require the full 24 hours. 3
• Maximum total dose should not exceed 30-40 grams per 24 hours. 4
• Never continue magnesium sulfate beyond 5-7 days, as prolonged maternal administration can cause fetal abnormalities. 4

Clinical Monitoring Parameters

Monitor respiratory rate (must be ≥12 breaths/minute), patellar reflexes (must be present), and urine output (must be ≥30 mL/hour) before each dose or continuously during infusion. 2, 7, 8

Essential Clinical Checks

• Respiratory rate: Respiratory paralysis occurs at magnesium levels of 5-6.5 mmol/L. 2, 8
• Patellar reflexes: Loss of deep tendon reflexes indicates impending toxicity at levels of 3.5-5 mmol/L. 8
• Urine output: Maintain ≥30 mL/hour, as magnesium is renally excreted and oliguria increases toxicity risk. 2, 3
• Oxygen saturation: Maintain >90%. 2

Laboratory Monitoring

Serum magnesium levels are NOT routinely required; clinical monitoring is sufficient. 2, 3

• Check serum magnesium only in high-risk situations: renal impairment (elevated creatinine), urine output <30 mL/hour, loss of patellar reflexes, or respiratory rate <12 breaths/minute. 2, 3
• Therapeutic range is 1.8-3.0 mmol/L (4.8-7.2 mg/dL). 8

Critical Fluid Management

Limit total IV fluid intake to 60-80 mL/hour to prevent pulmonary edema, as preeclamptic patients have capillary leak and reduced plasma volume. 2, 6

• Avoid diuretics, as plasma volume is already reduced in preeclampsia. 1, 6
• Preeclamptic women are at high risk for both pulmonary edema and acute kidney injury. 2

Contraindications and Critical Safety Warnings

Absolute Contraindications

Never combine magnesium sulfate with calcium channel blockers (especially IV or sublingual nifedipine), as this causes severe myocardial depression and precipitous hypotension. 2, 6

• If concurrent blood pressure control is needed, use IV labetalol or oral nifedipine separately with extremely careful monitoring. 1, 6
• The combination of magnesium and calcium channel blockers can cause cardiovascular collapse. 2, 6

Relative Contraindications

• Severe renal insufficiency: Maximum dose is 20 grams per 48 hours with frequent serum magnesium monitoring. 4
• Myasthenia gravis: Magnesium can worsen neuromuscular blockade. 8

Concurrent Blood Pressure Management

Magnesium sulfate does NOT control blood pressure; separate antihypertensive therapy is required to achieve target BP <160/105-110 mmHg. 2, 6

First-Line Antihypertensive Options

• IV labetalol: Preferred agent, can be used cautiously with magnesium. 1, 6
• Oral nifedipine (immediate-release): Use with extreme caution if patient is on magnesium; avoid IV nifedipine completely. 6
• IV hydralazine: Second-line option only, associated with adverse perinatal outcomes. 6

Special Scenario: Pulmonary Edema

• Use IV nitroglycerin 5 mcg/min (increased every 3-5 minutes to maximum 100 mcg/min) instead of calcium channel blockers when pulmonary edema is present. 1, 6

Indications for Magnesium Sulfate

Administer magnesium sulfate for:

• Eclampsia treatment: All cases with seizures. 2
• Severe preeclampsia: BP ≥160/110 mmHg with proteinuria. 2, 6
• Moderate preeclampsia with warning signs: BP ≥150/100 mmHg with proteinuria plus neurological symptoms (severe headache, visual scotomata, hyperreflexia, clonus, or epigastric pain). 2, 6

Magnesium Toxicity Management

Signs of Toxicity (in order of severity)

• Loss of patellar reflexes: 3.5-5 mmol/L 8
• Respiratory depression: 5-6.5 mmol/L 8
• Altered cardiac conduction: >7.5 mmol/L 8
• Cardiac arrest: >12.5 mmol/L 8

Antidote

Administer 1 gram (10 mL of 10% solution) calcium gluconate IV over 3 minutes if magnesium toxicity occurs. 8

Common Pitfalls to Avoid

• Do not use magnesium sulfate for blood pressure control—it is for seizure prophylaxis only. 2, 6
• Do not combine with calcium channel blockers without intensive monitoring. 2, 6
• Do not use NSAIDs for postpartum pain in preeclamptic patients, as they worsen hypertension and increase acute kidney injury risk. 3
• Do not rely on proteinuria level or serum uric acid as indications for delivery timing. 2
• Do not exceed 5-7 days of continuous therapy due to risk of fetal abnormalities. 4

Postpartum Monitoring

Continue monitoring BP and clinical condition at least every 4 hours while awake for at least 3 days postpartum, as eclampsia can develop for the first time in the postpartum period. 2