How to administer magnesium (Mg) intravenously in a patient with hypokalemia, normal renal function, and hypernatremia, considering the choice of diluent, such as 5% dextrose?

Intravenous Magnesium Administration in Hypernatremia: Use 5% Dextrose as Diluent

Yes, 5% dextrose is the appropriate diluent for intravenous magnesium sulfate in patients with hypernatremia, as it avoids additional sodium load while providing safe dilution for administration. 1

Diluent Selection for IV Magnesium

• The FDA-approved diluents for magnesium sulfate IV infusion are 5% Dextrose Injection, USP and 0.9% Sodium Chloride Injection, USP 1
• In hypernatremia, 5% dextrose is strongly preferred over normal saline to avoid exacerbating sodium elevation 1
• Magnesium sulfate injection (50%) must be diluted to a concentration of 20% or less prior to IV infusion 1
• The rate of IV injection should generally not exceed 150 mg/minute (1.5 mL of a 10% concentration or equivalent) 1

Specific Dosing Protocol for Hypomagnesemia with Normal Renal Function

• For severe hypomagnesemia, 5 g (approximately 40 mEq) can be added to one liter of 5% Dextrose Injection for slow IV infusion over a three-hour period 1
• For mild magnesium deficiency, the usual adult dose is 1 g (equivalent to 8.12 mEq) every six hours for four doses 1
• The American College of Cardiology recommends maintaining serum magnesium ≥0.70 mmol/L (approximately 1.7 mg/dL) 2
• Target serum magnesium levels of 3 to 6 mg/100 mL (2.5 to 5 mEq/L) are generally sufficient for therapeutic effect 1

Critical Monitoring Requirements

• Monitor serum magnesium levels and clinical status continuously to avoid overdosage 1
• Test patellar reflexes (knee jerk) before each dose—if absent, hold additional magnesium until reflexes return 1
• Ensure respiratory rate ≥16 breaths/minute before each dose 1
• Deep tendon reflexes begin to diminish when magnesium levels exceed 4 mEq/L, and reflexes may be absent at 10 mEq/L where respiratory paralysis becomes a hazard 1
• Urine output should be maintained at ≥100 mL during the four hours preceding each dose 1

Addressing Concurrent Hypokalemia

• Correct hypomagnesemia before or simultaneously with potassium replacement, as magnesium deficiency causes refractory hypokalemia 2, 3
• The target potassium level is >4 mmol/L 2
• Magnesium replacement significantly improves potassium retention—in critically ill patients, magnesium-treated groups showed positive potassium balance (+72 mmol) versus negative balance (-74 mmol) in controls over 48 hours 3
• Despite similar serum potassium values, patients receiving magnesium required 30% less potassium replacement by 30 hours 3

Duration of Effect and Redosing

• After a 2 g IV dose, serum magnesium drops below 2.0 mg/dL within 12 hours in most patients 4
• To maintain total serum magnesium above 2.0 mg/dL, expect to administer 2 g IV magnesium sulfate at least twice daily 4
• The renal response to magnesium deficiency is to lower fractional excretion to <2%, but this compensatory mechanism is often insufficient in critically ill patients 5, 6

Critical Pitfalls to Avoid

• Never use normal saline as diluent in hypernatremia—this will worsen sodium elevation despite being FDA-approved for general use 1
• Do not administer magnesium if renal impairment is present (CrCl <20 mL/min) without careful monitoring, as hypermagnesemia can cause cardiac complications 2, 1
• In patients with severe renal impairment, maximum dosage is 20 grams/48 hours with frequent serum magnesium monitoring 1
• Have injectable calcium salt immediately available to counteract potential magnesium toxicity 1
• Avoid administering magnesium in digitalized patients without extreme caution, as serious cardiac conduction changes and heart block may occur if calcium is required to treat magnesium toxicity 1
• Magnesium should be used with caution when barbiturates, narcotics, or other CNS depressants are being administered due to additive effects 1

Practical Administration Example

For a patient with hypernatremia, hypokalemia, and normal renal function requiring magnesium replacement:

• Dilute 5 g magnesium sulfate (10 mL of 50% solution) in 1 liter of 5% dextrose 1
• Infuse over 3 hours (approximately 333 mL/hour) 1
• Check magnesium, potassium, patellar reflexes, and respiratory rate before each dose 1
• Repeat dosing every 12-24 hours as needed based on serum levels 4
• Simultaneously correct potassium with target >4 mmol/L 2