What is the best way to replace magnesium intravenously (IV) in a patient with a magnesium deficiency?

Intravenous Magnesium Replacement Protocol

For severe symptomatic hypomagnesemia or life-threatening presentations (torsades de pointes, seizures, severe arrhythmias), give 1-2 g magnesium sulfate IV bolus over 5-15 minutes, followed by continuous infusion; for mild-to-moderate deficiency, administer 1 g IM every 6 hours for 4 doses or 5 g (40 mEq) in 1 liter of fluid over 3 hours. 1, 2, 3

Critical First Step: Assess Renal Function

Magnesium supplementation is absolutely contraindicated when creatinine clearance is <20 mL/min due to life-threatening hypermagnesemia risk. 4, 1 Between 20-30 mL/min, use extreme caution and only in emergencies. 4

Correct Volume Depletion BEFORE Magnesium Replacement

In patients with high-output stomas, diarrhea, or gastrointestinal losses, rehydration with IV normal saline (2-4 L/day initially) is the crucial first step to eliminate secondary hyperaldosteronism, which drives renal magnesium wasting and prevents effective repletion. 4, 1, 2 Hyperaldosteronism increases renal retention of sodium at the expense of magnesium and potassium, causing continued urinary losses despite total body depletion. 4

IV Magnesium Dosing by Clinical Scenario

Life-Threatening Emergencies

• Torsades de pointes with prolonged QT: Give 1-2 g magnesium sulfate IV bolus over 5 minutes regardless of baseline magnesium level. 1, 2, 3
• Severe symptomatic hypomagnesemia (<0.50 mmol/L or <1.2 mg/dL): Administer 1-2 g IV over 5-15 minutes, followed by continuous infusion. 1, 2
• Eclampsia with seizures: Initial dose of 4-5 g in 250 mL fluid infused IV, with simultaneous IM doses of up to 10 g (5 g in each buttock), then 4-5 g IM every 4 hours as needed. 3
• Severe hypomagnesemia: Up to 250 mg/kg (approximately 2 mEq/kg) may be given IM within 4 hours if necessary. 3

Moderate Deficiency

• Standard replacement: 1 g (8.12 mEq) IM every 6 hours for 4 doses (total 32.5 mEq/24 hours). 3
• Alternative IV approach: 5 g (40 mEq) added to 1 liter of D5W or normal saline for slow IV infusion over 3 hours. 3

Maintenance Therapy

• Continuous infusion: After initial bolus, administer 1-2 g/hour by constant IV infusion. 3
• Target serum level: Maintain magnesium at 6 mg/100 mL (2.5 mmol/L) for seizure control in eclampsia. 3
• Maximum daily dose: Do not exceed 30-40 g per 24 hours. 3

Administration Guidelines

The rate of IV injection should generally not exceed 150 mg/minute (1.5 mL of 10% solution), except in severe eclampsia with seizures. 3 Solutions for IV infusion must be diluted to 20% concentration or less prior to administration. 3

For rapid bolus dosing in emergencies: 10-15 mL of 20% magnesium sulfate solution given IV over 1 minute, followed by slow 4-6 hour infusion of 500 mL of 2% magnesium sulfate in D5W. 5, 6

Critical Monitoring During IV Administration

• Monitor for magnesium toxicity: Loss of patellar reflexes (first sign), respiratory depression, hypotension, bradycardia, and complete cardiovascular collapse. 1, 2, 3
• Have calcium chloride immediately available to reverse magnesium toxicity if needed. 1, 2
• Check magnesium levels: Recheck within 24-48 hours after IV administration in cardiac emergencies or QTc prolongation. 4
• In severe renal insufficiency: Maximum dose is 20 g/48 hours with frequent serum magnesium monitoring. 3

Address Concurrent Electrolyte Abnormalities

Always replace magnesium BEFORE attempting to correct hypocalcemia or hypokalemia—these will be refractory to treatment until magnesium is normalized. 4, 2 Hypomagnesemia causes dysfunction of multiple potassium transport systems and increases renal potassium excretion, making hypokalemia resistant to potassium treatment alone. 4, 1, 2 Hypomagnesemia also impairs parathyroid hormone release, causing calcium deficiency. 4

Special Clinical Scenarios

Continuous Renal Replacement Therapy (CRRT)

Use dialysis solutions containing magnesium to prevent hypomagnesemia, which occurs in 60-65% of critically ill patients on CRRT. 4, 1, 2 Regional citrate anticoagulation increases magnesium losses through chelation. 4

Short Bowel Syndrome/High-Output Stomas

After volume repletion with IV saline, if oral magnesium fails (which is common due to poor absorption and worsening diarrhea), use IV or subcutaneous magnesium sulfate 4-12 mmol added to saline bags, administered 1-3 times weekly. 4, 2, 7

Post-Transplant Patients on Calcineurin Inhibitors

Increased dietary intake alone is insufficient; magnesium supplements are typically required, with monitoring following transplant protocols. 2

Common Pitfalls to Avoid

• Never assume normal serum magnesium excludes deficiency—less than 1% of total body magnesium is in blood, so normal levels can coexist with significant intracellular depletion. 4
• Avoid rapid infusion outside of life-threatening emergencies—rapid administration causes hypotension and bradycardia. 2
• Do not use in pregnancy beyond 5-7 days—continuous maternal administration can cause fetal abnormalities. 3
• Avoid mixing with calcium or vasoactive amines in the same solution—precipitation will occur. 2, 3
• Never give magnesium before correcting volume depletion in patients with GI losses—ongoing aldosterone-driven renal wasting will exceed supplementation. 4, 1

When to Transition to Oral Therapy

Once acute symptoms resolve and serum magnesium normalizes, transition to oral magnesium oxide 12-24 mmol daily (480-960 mg elemental magnesium), preferably given at night when intestinal transit is slowest. 4, 2 For refractory cases despite oral therapy, consider adding oral 1-alpha hydroxy-cholecalciferol (0.25-9.00 μg daily) to improve magnesium balance, while monitoring serum calcium regularly to avoid hypercalcemia. 4, 2