What are the parameters for initiating magnesium (Mg) therapy in a pregnant woman?

Parameters for Initiating Magnesium Sulfate in Pregnant Women

Primary Indications for Starting Magnesium Sulfate

Magnesium sulfate should be initiated in pregnant women with severe preeclampsia who have at least one clinical sign of seriousness to prevent eclamptic seizures, and in all cases of eclampsia for seizure control. 1, 2

Specific Clinical Scenarios Requiring Magnesium Initiation

• Severe preeclampsia with clinical signs of seriousness - This includes blood pressure ≥160/110 mmHg with significant proteinuria (≥3+) or moderate hypertension (≥150/100 mmHg) with proteinuria ≥2+ plus neurological symptoms such as severe headache, visual disturbances, or hyperreflexia 3

• Active eclamptic seizures - Magnesium sulfate is the most effective agent for controlling eclamptic seizures, superior to phenytoin and diazepam 1, 2

• Fetal neuroprotection - When delivery of a potentially viable infant is anticipated before 32 weeks gestation, magnesium sulfate should be administered for fetal neuroprotection 2, 4

Standard Dosing Regimen

Intravenous Protocol (Preferred)

• Loading dose: 4-6 grams IV over 20-30 minutes 5, 3, 6

• Maintenance infusion: 1-2 grams per hour by continuous IV infusion 5, 6

• Higher maintenance dose consideration: 2 grams per hour is more effective than 1 gram per hour in achieving therapeutic levels, particularly in patients with BMI ≥25 kg/m² 5

• Duration: Continue for 24 hours postpartum in most cases 5

Alternative Intramuscular Protocol (Pritchard Regimen)

• Loading dose: 4 grams IV plus 10 grams IM (5 grams in each buttock) 5, 6

• Maintenance: 5 grams IM every 4 hours in alternate buttocks 5, 6

• This regimen is particularly useful in resource-limited settings with limited IV access 5, 2

Critical Safety Parameters Before and During Administration

Pre-Administration Requirements

• Patellar reflex must be present - Absence of knee jerk reflexes is a contraindication to administration 6, 7

• Respiratory rate ≥12 breaths per minute - Respiratory depression is a contraindication 5, 6

• Urine output ≥30 mL/hour - Oliguria increases toxicity risk as magnesium is renally excreted 5, 6

• Renal function assessment - In severe renal impairment, maximum dosage should not exceed 20 grams in 48 hours 6

Ongoing Monitoring Requirements

• Check patellar reflexes before each dose - Reflexes begin to diminish when magnesium levels exceed 4 mEq/L and may be absent at 10 mEq/L 6, 7

• Monitor respiratory rate continuously - Respiratory paralysis occurs at serum levels of 5-6.5 mmol/L 7

• Maintain urine output ≥30 mL/hour - Critical for preventing toxicity 5

• Oxygen saturation >90% 5

• Serum magnesium levels - Routine monitoring is NOT recommended; clinical monitoring (reflexes, respiratory rate, urine output) should guide therapy 5. Check serum levels only in high-risk situations including renal impairment, prolonged therapy, or signs of toxicity 5, 6

Therapeutic Target Range

• Optimal serum magnesium concentration: 3-6 mg/100 mL (2.5-5 mEq/L or 1.8-3.0 mmol/L) for seizure control 6, 7

Critical Safety Considerations and Contraindications

Absolute Contraindications

• Never combine magnesium sulfate with calcium channel blockers (especially nifedipine) - This combination can cause severe hypotension and myocardial depression 5, 2, 3

• Myasthenia gravis - Magnesium can worsen neuromuscular blockade 6

Fluid Management

• Limit total fluid intake to 60-80 mL/hour - Preeclamptic patients have capillary leak and reduced plasma volume, making them susceptible to pulmonary edema 5, 3

• Avoid diuretics - Plasma volume is already reduced in preeclampsia 3

Duration Limitations

• Do not exceed 5-7 days of continuous administration - Prolonged use beyond this period can cause fetal abnormalities including hypocalcemia, skeletal demineralization, and osteopenia 6, 4

• Maximum 24-hour dose: 30-40 grams should not be exceeded 6

Antidote and Toxicity Management

• Calcium gluconate must be immediately available - 1 gram (10 mL of 10% solution) IV over 3 minutes to counteract magnesium toxicity 6

• Signs of toxicity in order of increasing severity:

  • Loss of patellar reflexes: 3.5-5 mmol/L 7
  • Respiratory paralysis: 5-6.5 mmol/L 7
  • Altered cardiac conduction: >7.5 mmol/L 7
  • Cardiac arrest: >12.5 mmol/L 7

Common Pitfalls to Avoid

• Do not use NSAIDs for postpartum pain in preeclamptic patients when possible, as they worsen hypertension and increase acute kidney injury risk 5

• Do not use for prolonged tocolysis - The FDA warns against using magnesium sulfate for more than 5-7 days to stop preterm labor; this is an unindicated use 6, 4

• Do not combine with neuromuscular blocking agents without extreme caution - Excessive neuromuscular blockade can occur 6

• Adjust dosing in digitalized patients - Serious cardiac conduction changes and heart block may occur if calcium administration is required to treat magnesium toxicity 6

Special Population Considerations

• Overweight patients (BMI ≥25 kg/m²): Start maintenance at 2 grams per hour rather than 1 gram per hour 5

• Geriatric patients: Require reduced dosage due to impaired renal function 6

• Neonatal effects: Newborns may show signs of magnesium toxicity including neuromuscular or respiratory depression, especially when administered for more than 24 hours preceding delivery 6, 8