What are the preparation and administration instructions for a magnesium sulfate loading and maintenance infusion for fetal neuroprotection in a pregnant patient at 24–34 weeks gestation with imminent delivery?

Magnesium Sulfate Preparation and Administration for Fetal Neuroprotection

For a pregnant patient at 24-34 weeks with imminent delivery, prepare magnesium sulfate as a 4-gram IV loading dose diluted in 100 mL of normal saline or 5% dextrose, infused over 20-30 minutes, followed by a maintenance infusion of 1 gram per hour until delivery or for a maximum of 12-24 hours. 1, 2, 3

Detailed Nursing Instructions for Solution Preparation

Loading Dose Preparation

• Draw up 8 mL of 50% magnesium sulfate solution (this equals 4 grams)
• Dilute in 100 mL of either 0.9% sodium chloride or 5% dextrose - the 50% solution MUST be diluted to 20% concentration or less before IV administration 2
• Label the bag clearly: "MgSO4 4g in 100 mL - Loading Dose"
• Infuse over 20-30 minutes using an IV pump 1, 2

Maintenance Infusion Preparation

• Add 20 grams (40 mL of 50% solution) to 1000 mL of normal saline or 5% dextrose
• This creates a concentration of 20 mg/mL (2% solution)
• Set pump to run at 50 mL/hour (delivers 1 gram/hour) 1, 2, 3
• Continue until delivery occurs or for maximum 12 hours 3
• Do not exceed cumulative dose of 50 grams 1

Critical Monitoring Requirements

Before each dose and hourly during infusion, the nurse MUST assess:

Essential Safety Checks (Stop infusion immediately if any absent)

• Patellar reflex (knee jerk) present - reflexes disappear at toxic levels 2
• Respiratory rate ≥16 breaths per minute - respiratory depression indicates toxicity 2
• Urine output ≥100 mL in preceding 4 hours - magnesium is renally excreted 2

Therapeutic Monitoring

• Target serum magnesium level: 3-6 mg/dL (2.5-5 mEq/L) 2
• Optimal cord blood level for neuroprotection: ≥3.8 mg/dL 4
• Deep tendon reflexes diminish when levels exceed 4 mEq/L 2
• Reflexes absent and respiratory paralysis risk at 10 mEq/L 2

Alternative Regimen: Pritchard (IM) Protocol

If IV access is unavailable or in resource-limited settings:

Loading Dose

• 4 grams IV (if any IV access available) PLUS
• 5 grams IM in each buttock (10 grams total IM)
• Total loading dose = 14 grams 1

Maintenance Dose

• 5 grams IM every 4 hours alternating buttocks
• Continue for 24 hours 1
• Dilute 50% solution to 20% before IM injection 2

Note: Even if only IM administration is possible, give at minimum 5 grams IM in each buttock (10 grams total) as loading dose before transfer - this is better than no treatment 1

Duration of Therapy

Continue magnesium sulfate:

• Until delivery occurs, OR
• For 24 hours postpartum 1
• Maximum infusion duration: 12 hours if delivery has not occurred and is no longer imminent 2, 3

Critical warning: Do NOT continue magnesium sulfate beyond 5-7 days total as this causes fetal skeletal abnormalities (hypocalcemia, demineralization, osteopenia) 2

Timing Considerations for Maximum Benefit

Optimal administration window:

• Give within 12 hours before delivery 5
• Infusion duration >6 hours before delivery achieves better cord blood levels 4
• Duration >18 hours may maximize neuroprotection 5
• IV route achieves therapeutic levels immediately; IM route takes 60 minutes 2

Common Maternal Side Effects to Anticipate

Expected effects (not reasons to stop):

• Flushing and warmth 2
• Sweating 2
• Nausea
• Headache
• Muscle weakness

Severe effects requiring immediate discontinuation:

• Loss of patellar reflexes 2
• Respiratory rate <16/minute 2
• Respiratory depression
• Decreased level of consciousness

Have calcium gluconate 1 gram IV immediately available as antidote for magnesium toxicity 2

Drug Interactions to Avoid

• Reduce doses of CNS depressants (narcotics, barbiturates, anesthetics) - additive effects 2
• Use extreme caution with neuromuscular blocking agents - excessive blockade 2
• Avoid in digitalized patients - risk of heart block if calcium needed for toxicity 2
• Caution with nifedipine - can cause precipitous blood pressure drop 6

Contraindications

Do not administer if:

• Myasthenia gravis
• Heart block
• Severe renal impairment (adjust dose, maximum 20g/48 hours) 2

Evidence for Neuroprotection

The MAGPIE trial and subsequent meta-analyses demonstrate magnesium sulfate reduces cerebral palsy by 29% (RR 0.71) and death or cerebral palsy by 13% (RR 0.87) in preterm infants, with a number needed to treat of 60 to prevent one case of cerebral palsy 7, 8. It also reduces severe intraventricular hemorrhage (grade 3-4) by 24% 7, 8.

This protocol applies to all women with imminent delivery <32-34 weeks gestation, regardless of single/multiple pregnancy or cause of prematurity 1, 3.