What is the appropriate magnesium dosage for an adult patient with severe hypomagnesemia (magnesium level of 1.1) and no significant renal impairment?

Management of Severe Hypomagnesemia (Magnesium 1.1 mg/dL)

For a magnesium level of 1.1 mg/dL, administer 4-5 g IV magnesium sulfate (32-40 mEq) diluted in 250 mL of normal saline or 5% dextrose, infused over 3-4 hours, followed by 1-2 g/hour continuous infusion or 4-5 g IM every 4 hours until symptoms resolve and levels normalize. 1

Immediate Assessment Required

Before administering magnesium, you must:

• Check renal function immediately - if creatinine clearance is <20 mL/min, magnesium supplementation is absolutely contraindicated due to life-threatening hypermagnesemia risk 2, 3
• Assess for volume depletion and correct with IV normal saline first, as secondary hyperaldosteronism increases renal magnesium wasting 2, 3
• Check potassium and calcium levels, as hypomagnesemia causes refractory hypokalemia and hypocalcemia that will not respond to supplementation until magnesium is corrected 2, 4
• Obtain ECG to assess for QTc prolongation or arrhythmias 3

Treatment Protocol for Severe Hypomagnesemia

Parenteral Magnesium (First-Line for Severe Deficiency)

For severe hypomagnesemia (<1.2 mg/dL), parenteral therapy is mandatory: 1, 5

• Initial IV dose: 4-5 g magnesium sulfate (32-40 mEq) in 250 mL normal saline or 5% dextrose infused over 3-4 hours 1
• Alternative rapid dosing: Dilute 50% magnesium sulfate solution to 10-20% concentration and inject 40 mL of 10% solution (or 20 mL of 20% solution) IV over 3-4 minutes 1
• Maintenance: After initial dose, continue with 1-2 g/hour continuous IV infusion OR 4-5 g (8-10 mL of 50% solution) IM into alternate buttocks every 4 hours 1
• Maximum rate: Do not exceed 150 mg/minute (1.5 mL of 10% solution) except in life-threatening emergencies 1

For life-threatening presentations (torsades de pointes, severe arrhythmias):

• Give 1-2 g magnesium sulfate IV bolus over 5 minutes regardless of baseline magnesium level 3, 6

IM Administration Alternative

• Deep IM injection: 5 g (10 mL of undiluted 50% solution) in each buttock simultaneously with IV dose, then 4-5 g IM every 4 hours as needed 1
• IM administration provides therapeutic levels within 60 minutes 1

Monitoring During Treatment

• Monitor for magnesium toxicity: Loss of patellar reflexes, respiratory depression (rate <16/min), hypotension, bradycardia 1, 6
• Have calcium chloride immediately available to reverse magnesium toxicity if needed 2, 3
• Recheck magnesium level within 24-48 hours after IV administration 2
• Monitor potassium and calcium as these will normalize 24-72 hours after magnesium repletion begins 2

Transition to Oral Therapy

Once magnesium level reaches >1.2 mg/dL and patient is asymptomatic:

• Magnesium oxide 12-24 mmol daily (approximately 480-960 mg elemental magnesium) 2, 7
• Administer at night when intestinal transit is slowest to maximize absorption 2, 7
• Target serum magnesium >0.6 mmol/L (>1.46 mg/dL) 7, 3

Critical Pitfalls to Avoid

• Never give magnesium without checking renal function first - creatinine clearance <20 mL/min is an absolute contraindication 2, 3
• Do not attempt to correct hypokalemia or hypocalcemia before correcting magnesium - they will be refractory to treatment 2, 4
• Do not mix magnesium sulfate with calcium or vasopressors in the same IV line 3
• In severe renal insufficiency, maximum dose is 20 g/48 hours with frequent serum monitoring 1
• Rapid infusion causes hypotension and bradycardia - respect maximum infusion rates 3, 1

Renal Dosing Adjustments

• CrCl 30-60 mL/min: Use reduced doses with close monitoring 2
• CrCl 20-30 mL/min: Avoid unless life-threatening emergency, use extreme caution 2
• CrCl <20 mL/min: Absolute contraindication 2, 3