How to replete magnesium in patients with magnesium deficiency?

Magnesium Repletion in Patients with Magnesium Deficiency

For patients with magnesium deficiency, oral magnesium oxide at a dose of 12-24 mmol daily is recommended as first-line therapy, with administration preferably at night when intestinal transit is slowest to maximize absorption. 1

Oral Magnesium Supplementation

First-Line Therapy

• Magnesium oxide is the preferred oral supplement as it contains more elemental magnesium than other salts and is converted to magnesium chloride in the stomach 1
• Initial dose should be 12 mmol given at night, with total daily dose ranging from 12-24 mmol depending on severity and response 1
• Administering magnesium at night when intestinal transit is slowest helps maximize absorption 1, 2
• Dividing supplementation into multiple doses throughout the day is recommended for continuous repletion 3

Alternative Oral Options

• Organic magnesium salts (aspartate, citrate, lactate) have higher bioavailability than magnesium oxide or hydroxide and can be considered as alternatives 3, 2
• For patients with gastrointestinal disorders, these organic salts may be better tolerated 2

Parenteral Magnesium Therapy

Indications for IV Therapy

• For severe hypomagnesemia, IV magnesium sulfate is indicated 4
• For mild deficiency, 1 g (8.12 mEq) of magnesium sulfate can be administered IM every six hours for four doses 4
• For severe hypomagnesemia, up to 250 mg/kg of body weight may be given IM within four hours if necessary 4
• Alternatively, 5 g (approximately 40 mEq) can be added to one liter of 5% Dextrose or 0.9% Sodium Chloride for slow IV infusion over three hours 4

Administration Considerations

• IV infusion solutions must be diluted to a concentration of 20% or less prior to administration 4
• The rate of IV injection should generally not exceed 150 mg/minute 4
• In emergencies such as convulsions or ventricular arrhythmias, a bolus injection of 1.0 g of MgSO4 is indicated 5

Treatment Algorithm

1. Correct underlying factors first:

   • Address water and sodium depletion to correct secondary hyperaldosteronism, which can worsen magnesium deficiency 1, 2

2. For mild to moderate deficiency:

   • Start with oral magnesium oxide 12 mmol at night, increasing to 24 mmol daily if needed 1
   • Target serum magnesium level should be within normal range (1.8-2.2 mEq/L) 1
   • A reasonable minimum target level is >0.6 mmol/L 3, 1

3. For severe deficiency or when oral therapy fails:

   • Administer IV magnesium sulfate 1-2 g for symptomatic patients with severe hypomagnesemia (<1.2 mEq/L) 1
   • For ongoing repletion, 5 g (approximately 40 mEq) can be added to one liter of IV fluid for slow infusion over three hours 4

4. For refractory cases:

   • Consider oral 1-alpha hydroxy-cholecalciferol in gradually increasing doses to improve magnesium balance 1, 2
   • Monitor serum calcium regularly to avoid hypercalcemia 1, 2
   • Subcutaneous administration with saline is an option for patients requiring supplementation 1-3 times weekly 1

Special Considerations

• Renal impairment: In patients with severe renal insufficiency, reduce dosage and monitor serum magnesium levels frequently 4
• Cardiac arrhythmias: For arrhythmias associated with hypomagnesemia, administer IV magnesium 1-2 g bolus regardless of measured serum levels 1
• Malabsorption: Patients with short bowel syndrome or malabsorption may require higher doses of oral magnesium or parenteral supplementation 1, 2
• Potassium relationship: Hypokalemia is often refractory to treatment until magnesium deficiency is corrected 2, 6

Common Pitfalls and Monitoring

• Most magnesium salts can worsen diarrhea or stomal output in patients with gastrointestinal disorders 1, 2
• Serum magnesium levels do not accurately reflect total body magnesium status, as less than 1% of magnesium is found in the blood 2
• Monitor for signs of magnesium toxicity including hypotension, drowsiness, and muscle weakness 1
• Long-term oral magnesium repletion may be accomplished with 300-600 mg of elemental magnesium daily 7
• Complete repletion occurs slowly, so continued supplementation is often necessary 5