MgSO4 (Magnesium Sulfate) in Obstetrics - 5 Mark Exam Answer

Introduction

1. Indications

• Drug of choice for prevention and treatment of eclamptic seizures
• Superior to phenytoin and benzodiazepines (diazepam) for both prophylaxis and treatment - confirmed by two landmark RCTs in 1995 and the Magpie trial
• Recommended in all women with preeclampsia with severe features
• In a systematic review, MgSO4 vs. placebo: halved the risk of eclampsia (RR 0.41, 95% CI 0.29-0.58), reduced placental abruption risk (RR 0.64), and reduced maternal mortality (RR 0.54)
• NOT routinely recommended in preeclampsia without severe features in resource-rich settings (ACOG)

• Recommended by ACOG for neuroprotection when preterm delivery is anticipated at <32 weeks gestation
• Reduces the risk of moderate to severe cerebral palsy (CP) in the premature neonate
• Mechanism: NMDA receptor antagonism reduces excitotoxic brain injury in the fetus
• Rescue dosing may be considered if >12 hours since last dose when delivery becomes imminent

• Used to arrest preterm labor (delay delivery by 24-48 hours) to allow administration of antenatal corticosteroids and facilitate maternal transport to a tertiary facility
• Less commonly used today as first-line tocolytic compared to nifedipine (calcium channel blocker) and indomethacin, but still used especially in fetal surgery settings where infusions are continued intraoperatively and ~24 hours postoperatively

2. Mechanism of Action

• Physiological calcium antagonist - competes with Ca²+ at voltage-gated channels and at the motor end-plate
• Blocks NMDA (N-methyl-D-aspartate) glutamate receptors - the proposed mechanism for both anticonvulsant activity and fetal neuroprotection
• Causes smooth muscle relaxation - explains both tocolytic effect and vasodilation
• Decreases neuromuscular transmission by inhibiting acetylcholine release at the motor nerve terminal

3. Dosing Regimens

• IM route: mix with 1 mL of 2% lignocaine to reduce injection pain
• Continue for 24 hours after delivery (or 24 hours after last seizure in eclampsia)
• For cesarean section: begin before surgery and continue throughout surgery and for 24 hours postpartum
• Therapeutic serum level: 4-8 mEq/L (4.8-9.6 mg/dL)

4. Monitoring for Toxicity

• Patellar/knee reflex - must be PRESENT; loss = toxicity warning
• Respiratory rate - must be >12/min
• Urine output - must be >25-30 mL/hr (drug is renally excreted; dose reduce in renal impairment)

5. Treatment of Toxicity

• Stop infusion immediately
• Antidote: Calcium gluconate 1 g IV (10 mL of 10% solution) given slowly - antagonizes magnesium toxicity at the neuromuscular junction
• Supportive: oxygen, assisted ventilation if respiratory arrest
• Forced diuresis with loop diuretic (furosemide) + IV fluids enhances renal magnesium excretion in patients with adequate renal function

6. Anesthetic Implications (Critical for Anesthesia Exam)

1. Potentiation of neuromuscular blocking agents (NMBAs): MgSO4 potentiates BOTH depolarizing (suxamethonium) and non-depolarizing (rocuronium, vecuronium, atracurium) muscle relaxants. Mechanism: decreased acetylcholine release + reduced muscle membrane excitability. Therefore - reduce NMB doses and use a nerve stimulator (TOF monitoring) in these patients.
2. Hypotension: MgSO4 causes vasodilation and myocardial depression. Combined with neuraxial anesthesia (spinal/epidural), this can exacerbate hypotension. Use vasopressors (phenylephrine preferred in obstetrics) judiciously.
3. Neonatal effects: Crosses the placenta freely - the neonate may be born with hypermagnesemia. Average neonatal serum Mg after maternal therapy: ~3.7 mEq/L. Effects include hypotonia, respiratory depression, poor feeding. Serum level does not correlate with Apgar scores.
4. Calcium gluconate at bedside: Must always be immediately available for the anesthesiologist managing these patients.
5. Reduced MAC: Magnesium reduces anesthetic requirements (reduced MAC of volatile agents) due to its CNS depressant properties.

7. Contraindications/Cautions

• Myasthenia gravis (absolute - can precipitate myasthenic crisis)
• Renal impairment (GFR <30 mL/min) - significantly reduces excretion; dose reduction mandatory
• Use phenytoin instead in cases with pulmonary edema, pulmonary concerns, or myasthenia gravis

Summary Table