Magnesium Sulfate IV Preparation and Administration
Magnesium sulfate IV solutions must be diluted to a concentration of 20% or less prior to administration, with the most common diluents being 5% Dextrose or 0.9% Sodium Chloride. 1
Preparation Guidelines
Concentration Requirements
- All IV infusions require dilution to ≤20% concentration before administration 1
- Standard diluents: 5% Dextrose Injection or 0.9% Sodium Chloride Injection 1
- The undiluted 50% solution should never be given IV directly 1
Rate of Administration
- General IV injection rate should not exceed 150 mg/minute (1.5 mL of 10% concentration) 1
- Exception: severe eclampsia with active seizures may require faster administration 1
Dosing Regimens by Clinical Indication
Preeclampsia/Eclampsia
Loading dose: 4-6g IV over 20-30 minutes, followed by maintenance infusion of 1-2g/hour 2
FDA-approved regimen:
- Initial: 4-5g in 250 mL of 5% Dextrose or 0.9% Sodium Chloride, infused IV 1
- Alternative: 4g IV by diluting 50% solution to 10-20% concentration (40 mL of 10% or 20 mL of 20% solution) given over 3-4 minutes 1
- Maintenance: 1-2g/hour by continuous IV infusion 1, 2
- Duration: Continue for 24 hours postpartum as standard recommendation 2
Important dosing considerations:
- Higher maintenance doses (2g/hour) achieve therapeutic levels more reliably in overweight patients (BMI ≥25 kg/m²) compared to 1g/hour 3, 4
- The 2g/hour regimen resulted in therapeutic magnesium levels in 84.2% vs 42.1% of patients postpartum 3
- Despite higher side effects with 2g/hour, all were mild and no overdoses occurred 4
Torsades de Pointes (Polymorphic VT with Long QT)
Dose: 1-2g IV over 15 minutes 5, 2
Preparation:
- Dilute in 10 mL D5W for bolus administration 6
- This is the ONLY cardiac arrest indication for magnesium sulfate 6
- Do not use magnesium for any other ACLS rhythm (VF, pVT, asystole, PEA) 6
Severe Refractory Asthma
Dose: 2g IV diluted to ≤20% concentration, administered over 20 minutes 2
Hypomagnesemia
Mild deficiency:
- 1g (8.12 mEq) IM every 6 hours for 4 doses 1
Severe hypomagnesemia:
- 5g (approximately 40 mEq) added to 1 liter of 5% Dextrose or 0.9% Sodium Chloride for slow IV infusion over 3 hours 1
- Alternative: Up to 250 mg/kg (approximately 2 mEq/kg) IM within 4 hours if necessary 1
Critical Safety Monitoring
Therapeutic Range
- Target serum magnesium: 1.8-3.0 mmol/L (4.8-8.4 mg/dL) for eclampsia treatment 7, 3
- Maintenance therapy should be titrated to maintain levels of 1.3-2.2 mEq/L 2
Toxicity Warning Signs (in ascending order of severity)
- Loss of patellar reflex: 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
Essential Monitoring Parameters
- Deep tendon reflexes (patellar reflex) 7
- Respiratory rate (must remain adequate) 1
- Urine output 7
- Serum magnesium concentrations 7
- Have calcium chloride immediately available to counteract magnesium toxicity 5, 2
Common Pitfalls and Precautions
Duration Warnings
- Do not continue magnesium sulfate beyond 5-7 days in pregnancy—can cause fetal abnormalities 1
- For preeclampsia/eclampsia, standard duration is 24 hours postpartum 2
Adverse Effects
- Hypotension, bradycardia, CNS toxicity, respiratory depression 5
- Flushing is common with IV administration 2
- Follow magnesium levels closely if frequent or prolonged dosing required, particularly with impaired renal function 5