Standing Order for Magnesium Replacement
For adult patients with hypomagnesemia, a standard magnesium replacement protocol should include 1g of magnesium sulfate (equivalent to 8.12 mEq of magnesium) administered intramuscularly every six hours for four doses for mild deficiency, or 5g (approximately 40 mEq) added to one liter of IV fluid for slow infusion over three hours for severe deficiency. 1
Magnesium Deficiency Assessment
- Serum magnesium level < 1.8 mg/dL indicates deficiency
- Clinical symptoms may include:
- Abdominal cramps
- Impaired healing
- Fatigue
- Bone pain
- Neurological symptoms (confusion, irritability, seizures) 2
Replacement Protocol Based on Severity
Mild Magnesium Deficiency (1.5-1.8 mg/dL)
- Dose: 1g magnesium sulfate (2mL of 50% solution)
- Route: Intramuscular injection
- Frequency: Every 6 hours for 4 doses (total 32.5 mEq/24 hours) 1
- Alternative: Oral magnesium oxide 12-24 mmol daily (preferably at night) 2
Severe Magnesium Deficiency (<1.5 mg/dL)
- Dose: Up to 250 mg/kg body weight (0.5 mL of 50% solution)
- Route: Intramuscular injection
- Timeframe: Within a 4-hour period 1
IV Alternative for Severe Deficiency
- Dose: 5g magnesium sulfate (approximately 40 mEq)
- Dilution: Add to 1L of 5% Dextrose or 0.9% Sodium Chloride
- Rate: Slow infusion over 3 hours 1
Monitoring Parameters
- Check serum magnesium level within 24 hours after replacement
- Target serum level: ≥2 mEq/L 3
- Monitor for signs of hypermagnesemia:
- Loss of patellar reflex (occurs at 3.5-5 mmol/L)
- Respiratory depression (occurs at 5-6.5 mmol/L)
- Cardiac conduction changes (occurs at >7.5 mmol/L) 4
Special Considerations
Renal Impairment
- In severe renal insufficiency: Maximum dosage of 20g/48 hours
- Require more frequent serum magnesium monitoring 1
- Avoid in patients with creatinine clearance <20 mg/dL 2
Electrolyte Correction Sequence
- Correct hypomagnesemia before addressing hypokalemia
- Potassium replacement may be ineffective until magnesium is repleted 2
Pregnancy Considerations
- For severe pre-eclampsia or eclampsia: Initial dose of 4-5g IV in 250mL of fluid
- Maintenance: 1-2g/hour by constant IV infusion
- Target serum level: 6 mg/100mL for seizure control
- Maximum daily dose: 30-40g should not be exceeded
- Continuous use beyond 5-7 days can cause fetal abnormalities 1
Important Precautions
- Solutions for IV infusion must be diluted to ≤20% concentration prior to administration
- IV injection rate should not exceed 150 mg/minute except in severe eclampsia with seizures
- For children, dilute solution to ≤20% concentration for IM injection 1
- Monitor for diarrhea which can worsen with magnesium supplementation 2
Common Pitfalls
- "Rule of thumb" estimations (1g IV magnesium raises serum level by 0.15 mEq/L) often fail to achieve target levels in critically ill patients 3
- Medications such as diuretics, proton pump inhibitors, and certain antibiotics can cause hypomagnesemia 2
- Failure to monitor renal function can lead to magnesium toxicity
- Inadequate follow-up measurements may result in under-replacement or over-replacement
By following this structured approach to magnesium replacement, clinicians can effectively address magnesium deficiency while minimizing risks of adverse effects.