How do I prepare a magnesium sulfate infusion that delivers 1 gram per hour and what step‑by‑step instructions should I give to the nurse?

Magnesium Sulfate 1 Gram Per Hour Infusion: Nursing Instructions

For a standard 1 gram per hour maintenance infusion of magnesium sulfate, instruct the nurse to dilute 20 grams of magnesium sulfate in 500 mL of normal saline or D5W (creating a 40 mg/mL concentration) and run at 25 mL/hour on an IV pump. This is the most straightforward preparation method that delivers exactly 1 gram per hour.

Step-by-Step Preparation Instructions

Standard Dilution Method (Recommended)

• Take: 40 mL of 50% magnesium sulfate solution (contains 20 grams total)
• Add to: 500 mL IV bag of normal saline or D5W
• Final concentration: 40 mg/mL (20 grams in 500 mL)
• Infusion rate: 25 mL/hour = 1 gram/hour
• Duration: This bag will last 20 hours

Alternative Concentrated Method

• Take: 20 mL of 50% magnesium sulfate solution (contains 10 grams total)
• Add to: 250 mL IV bag of normal saline or D5W
• Final concentration: 40 mg/mL (10 grams in 250 mL)
• Infusion rate: 25 mL/hour = 1 gram/hour
• Duration: This bag will last 10 hours

Critical Safety Monitoring Requirements

The FDA label mandates specific safety monitoring that must be performed before each dose 1:

Before Starting and During Infusion:

• Patellar reflex (knee jerk): Must be present before each dose
• Respiratory rate: Must be ≥16 breaths/minute
• Urine output: Must maintain ≥100 mL over 4 hours preceding each dose
• Serum magnesium levels: Therapeutic range is 4.8-8.4 mg/dL (or 4-7 mEq/L per some sources)

Hold Infusion If:

• Absent patellar reflexes
• Respiratory rate <16 breaths/minute
• Urine output <25 mL/hour
• Signs of magnesium toxicity (severe flushing, muscle weakness, hypotension)

Emergency Preparation:

• Keep calcium gluconate 1 gram IV at bedside as antidote for magnesium toxicity 1

Context: When 1 Gram/Hour May Be Insufficient

Important caveat: While 1 gram/hour is a commonly used maintenance dose, recent evidence suggests this may be subtherapeutic for many patients, particularly those with higher body weight. The 2018 ISSHP guidelines reference the MAGPIE trial protocol which used 1 gram/hour maintenance 2, but newer research challenges this dosing.

Evidence for Higher Dosing:

• Obese patients (BMI ≥35): A 2020 RCT found that 100% of obese women receiving 1 gram/hour had subtherapeutic magnesium levels at 4 hours, compared to 63% receiving 2 grams/hour 3
• Overweight patients (BMI ≥25): Studies show only 15.8-23% achieve therapeutic levels with 1 gram/hour versus 52.6-70% with 2 grams/hour 4, 5
• Standard weight patients: Even in unselected populations, 2 grams/hour achieves therapeutic levels in 70-80% versus only 17-23% with 1 gram/hour 5

Clinical Implications:

If your patient has BMI ≥25 or if serum magnesium levels are subtherapeutic on 1 gram/hour, consider increasing to 1.5-2 grams/hour. For 2 grams/hour, simply double the infusion rate to 50 mL/hour using the same dilution (40 mg/mL concentration).

Duration of Therapy

Per ISSHP guidelines, continue magnesium sulfate for 24 hours postpartum for preeclampsia/eclampsia prophylaxis 2. Some protocols allow discontinuation after 8 grams if given before delivery, but the 24-hour postpartum standard remains most widely recommended.

Common Pitfalls to Avoid

• Concentration error: The 50% magnesium sulfate solution MUST be diluted to ≤20% before IV infusion per FDA requirements 1
• Rate confusion: Ensure pump is set to mL/hour, not grams/hour
• Inadequate monitoring: Reflexes and respiratory rate must be checked regularly, not just at start
• Renal impairment: Magnesium is renally cleared; patients with renal dysfunction require dose reduction and closer monitoring 1
• Drug interactions: Use extreme caution with neuromuscular blockers, CNS depressants, and in digitalized patients 1