What is the appropriate dosing regimen for intravenous (IV) magnesium in a patient with severe hypomagnesemia (serum magnesium level of 0.32 mmol/L) if 200mmol of IV magnesium is considered?

Severe Hypomagnesemia (0.32 mmol/L): IV Magnesium Dosing Protocol

For severe hypomagnesemia at 0.32 mmol/L, administer 5 g (approximately 40 mEq) magnesium sulfate IV over 3 hours, followed by continuous infusion of 1-2 g/hour for 24 hours, with total replacement typically requiring 32-48 mEq over the first 24 hours. 1, 2

Immediate Assessment Required

Before administering any magnesium:

• Check renal function immediately - if creatinine clearance <20 mL/min, IV magnesium is absolutely contraindicated due to life-threatening hypermagnesemia risk 3, 2
• Obtain ECG urgently - a magnesium level of 0.32 mmol/L places the patient at high risk for torsades de pointes and ventricular arrhythmias 2, 4
• Assess volume status - correct sodium and water depletion with IV normal saline (2-4 L/day initially) BEFORE magnesium replacement, as secondary hyperaldosteronism from volume depletion causes renal magnesium wasting that will prevent effective repletion 3, 2
• Check potassium and calcium levels - hypomagnesemia causes refractory hypokalemia and hypocalcemia that will not respond to supplementation until magnesium is corrected 3, 2, 5

IV Magnesium Dosing Protocol for Severe Hypomagnesemia

Initial Loading Dose

If life-threatening arrhythmia present (torsades de pointes, ventricular arrhythmia):

• Give 1-2 g magnesium sulfate IV bolus over 5 minutes regardless of baseline level 2, 1
• Have calcium chloride immediately available to reverse toxicity if needed 3

If severe but asymptomatic (0.32 mmol/L without arrhythmia):

• Administer 5 g (approximately 40 mEq) magnesium sulfate added to 1 liter of 5% dextrose or 0.9% sodium chloride, infused IV over 3 hours 1, 2
• Alternative: Give 1-2 g IV over 15 minutes, followed by continuous infusion 2

Maintenance Infusion

• After initial loading, continue 1-2 g/hour by constant IV infusion for 24 hours 1
• Total daily dose should not exceed 30-40 g in 24 hours 1
• In severe renal insufficiency, maximum dose is 20 g/48 hours with frequent serum monitoring 1, 2

Rate Limitations

• Do not exceed 150 mg/minute (1.5 mL of 10% solution) except in eclamptic seizures 1
• Rapid infusion causes hypotension and bradycardia 2

Critical Monitoring During IV Replacement

• Monitor patellar reflexes hourly - loss indicates impending magnesium toxicity 1
• Monitor respiratory rate - respiratory depression occurs with toxicity 2, 1
• Continuous cardiac monitoring - watch for bradycardia, hypotension, heart block 2, 4
• Recheck magnesium level within 24-48 hours after IV administration 3
• Target serum magnesium ≥0.70 mmol/L (1.7 mg/dL) 2, 6

Concurrent Electrolyte Management

Replace magnesium FIRST before attempting to correct other electrolytes:

• Hypokalemia will be refractory until magnesium is normalized - potassium supplementation will fail due to magnesium-induced dysfunction of potassium transport systems 3, 2, 5
• Hypocalcemia will be refractory until magnesium is normalized - magnesium deficiency impairs parathyroid hormone release 3, 2
• Expect calcium normalization within 24-72 hours after magnesium repletion begins 2

Transition to Oral Maintenance

After initial IV correction:

• Start oral magnesium oxide 12-24 mmol daily (approximately 480-960 mg elemental magnesium) 3, 2
• Administer at night when intestinal transit is slowest for better absorption 3
• Continue oral supplementation for weeks to months to replete total body stores 3

Common Pitfalls to Avoid

• Never give magnesium without checking renal function first - this is the most dangerous error 3, 2, 7
• Never attempt to correct potassium or calcium before normalizing magnesium - these efforts will fail and waste time 3, 2, 5
• Never assume volume status is adequate - failure to correct volume depletion first results in continued renal magnesium wasting despite supplementation 3, 2
• Never infuse faster than 150 mg/minute except in life-threatening seizures - rapid infusion causes cardiovascular collapse 1
• Never mix magnesium sulfate with calcium in the same IV solution - precipitate formation occurs 1

Special Considerations

• If patient is on dialysis: Use magnesium-enriched dialysate rather than IV supplementation 6
• If regional citrate anticoagulation during CRRT: Expect dramatically increased magnesium losses requiring higher replacement 6
• If pregnant beyond 5-7 days: Continuous magnesium can cause fetal abnormalities - use shortest duration possible 1