What is the recommended protocol for intravenous (IV) magnesium (Mg) replacement?

IV Magnesium Replacement Protocol

For hypomagnesemia, administer 1-2 g IV magnesium sulfate over 15 minutes for acute replacement, followed by maintenance infusion of 1 g/hour for 24 hours if needed, with the rate not exceeding 150 mg/minute except in life-threatening situations. 1, 2

Dosing by Clinical Indication

Hypomagnesemia (General)

• Mild deficiency: 1 g (8.12 mEq) IM every 6 hours for 4 doses, or 5 g added to 1 liter of fluid for slow IV infusion over 3 hours 2
• Severe hypomagnesemia: Up to 250 mg/kg (approximately 2 mEq/kg) may be given IM within 4 hours if necessary 2
• Acute replacement: 1-2 g IV over 15 minutes, followed by maintenance infusion of 1 g/hour for 24 hours if needed 1
• The FDA label specifies that caution must be observed to prevent exceeding renal excretory capacity during deficiency treatment 2

Life-Threatening Arrhythmias

• Torsades de pointes: 2 g IV as first-line therapy regardless of serum magnesium level 3
• For pediatric patients: 25-50 mg/kg (maximum 2 g) given as bolus for pulseless torsades, or over 10-20 minutes for torsades with pulses 4
• Repeat 2 g infusions may be necessary if episodes persist 3
• Polymorphic VT with QT prolongation: 1-2 g IV over 15 minutes 1

Severe Refractory Asthma

• Dose: 2 g IV magnesium sulfate over 20 minutes for patients with the most severe exacerbations who have failed initial bronchodilator therapy 3
• Must be diluted to 20% or less concentration before administration 4
• Do NOT use for mild or moderate asthma exacerbations as it shows no benefit 3

Preeclampsia/Eclampsia

• Loading dose: 4-6 g IV over 20-30 minutes 1
• Maintenance: 1-2 g/hour continuous infusion 1
• Alternative FDA-approved regimen: 4-5 g IV in 250 mL fluid, with simultaneous IM doses of up to 10 g (5 g in each buttock), followed by 4-5 g IM into alternate buttocks every 4 hours as needed 2
• Continue for 24 hours postpartum as standard recommendation 1
• Target serum magnesium level of 6 mg/100 mL for seizure control 2

Administration Guidelines

Rate and Concentration

• Maximum rate: Generally should not exceed 150 mg/minute (1.5 mL of 10% solution), except in severe eclampsia with seizures 2
• Dilution requirement: Solutions for IV infusion must be diluted to 20% or less concentration prior to administration 2
• Common diluents are 5% dextrose or 0.9% sodium chloride 2

Duration Considerations

• A 2024 study found that 2 g doses maintain total serum magnesium above 2.0 mg/dL for less than 12 hours on average, suggesting twice-daily dosing may be needed for sustained therapeutic levels 5
• Research shows that prolonging infusion rates (0.5 g/hour vs. 2 g/hour) does not improve magnesium retention or therapeutic outcomes 6

Safety Monitoring

Critical Precautions

• Have calcium chloride immediately available to counteract magnesium toxicity 4
• Monitor for hypotension, bradycardia, and flushing during administration 1
• Check patellar reflexes and respiratory function before each IM dose in eclampsia management 2

Maximum Dosing Limits

• Total daily dose: Should not exceed 30-40 g in 24 hours 2
• Severe renal insufficiency: Maximum 20 g/48 hours with frequent serum magnesium monitoring 2
• Pregnancy: Do not continue beyond 5-7 days as it can cause fetal abnormalities 2

Target Serum Levels

• Therapeutic range: 1.5-3 mmol/L (approximately 3.6-7.3 mg/dL) for most indications 7
• Maintenance therapy: Target 1.3-2.2 mEq/L 1
• Eclampsia control: 6 mg/100 mL (approximately 2.5 mEq/L) 2

Special Populations

Short Bowel Syndrome

• Rehydration to correct secondary hyperaldosteronism is the most important first step before magnesium replacement 4
• If oral supplementation fails, give IV or subcutaneous magnesium (4-12 mmol added to saline bag) 4

Cancer Patients

• Chemotherapy agents like cisplatin or cetuximab may cause significant hypomagnesemia requiring IV replacement 4
• IV magnesium sulfate may reverse neurological symptoms including confusion, hallucinations, irritability, and seizures 4

Pediatric Dosing

• For hypomagnesemia/torsades: 25-50 mg/kg (maximum 2 g) over 10-20 minutes 4
• Rapid infusion may cause hypotension and bradycardia 4

Common Pitfalls

• Inadequate maintenance dosing: An IV regimen with 1 g/hour maintenance is inadequate for preeclampsia management; 2 g/hour is required to achieve therapeutic levels comparable to IM regimens 8
• Forgetting dilution: Always dilute to ≤20% concentration for IV infusion to prevent adverse effects 2
• Ignoring renal function: Adjust dosing and increase monitoring frequency in renal insufficiency 2
• Assuming single doses suffice: For sustained effect, expect to dose at least twice daily when targeting serum levels above 2.0 mg/dL 5