Hypomagnesemia Correction
Immediate Treatment Decision
For severe symptomatic hypomagnesemia or life-threatening presentations (seizures, torsades de pointes, cardiac arrhythmias), give 1-2 g magnesium sulfate IV bolus over 5-15 minutes immediately, regardless of baseline magnesium level. 1, 2 This is the single most critical intervention that takes priority over all other considerations.
Treatment Algorithm Based on Severity and Renal Function
Step 1: Assess Renal Function FIRST
- Check creatinine clearance before any magnesium administration 1, 2, 3
- Absolute contraindication: CrCl <20 mL/min - risk of life-threatening hypermagnesemia 1, 4
- Maximum dose in severe renal insufficiency: 20 grams/48 hours with frequent serum monitoring 1, 2
- CrCl 20-30 mL/min: Avoid unless life-threatening emergency (torsades), then use extreme caution 4
- CrCl 30-60 mL/min: Use reduced doses with close monitoring 4
Step 2: Correct Volume Depletion BEFORE Magnesium Supplementation
This is the most commonly missed step that causes treatment failure. 1, 4
- Administer IV normal saline (2-4 L/day initially) to correct sodium and water depletion 1, 4
- Rationale: Secondary hyperaldosteronism from volume depletion drives renal magnesium wasting at rates exceeding any supplementation you provide 1, 4
- Hyperaldosteronism increases renal retention of sodium at the expense of magnesium and potassium 1, 4
- Failure to correct volume status first will result in continued magnesium losses despite aggressive supplementation 1, 4
Step 3: Determine Severity and Route
Severe Symptomatic Hypomagnesemia (<0.50 mmol/L or <1.2 mg/dL)
Parenteral magnesium is mandatory. 1, 2, 3
Life-threatening presentations (torsades de pointes, seizures, cardiac arrest):
- Give 1-2 g magnesium sulfate IV bolus over 5 minutes 1, 2
- For torsades specifically: 2 g IV over 5 minutes regardless of serum level 1, 4
- Follow with continuous infusion of 1-2 g/hour 1
Severe hypomagnesemia without immediate life threat:
- 1-2 g IV magnesium sulfate over 15 minutes 1, 2
- Alternative: 5 g (approximately 40 mEq) added to 1 L D5W or NS for slow IV infusion over 3 hours 2
- Maximum infusion rate: 150 mg/minute (1.5 mL of 10% solution) 2
IM administration option:
- For mild deficiency: 1 g (8.12 mEq) IM every 6 hours for 4 doses (total 32.5 mEq/24 hours) 2
- For severe: Up to 250 mg/kg (approximately 2 mEq/kg) IM within 4 hours if necessary 2
- Therapeutic levels achieved in 60 minutes with IM vs immediately with IV 2
Mild to Moderate Hypomagnesemia (0.50-0.70 mmol/L)
Oral magnesium is first-line. 1, 4
- Magnesium oxide 12-24 mmol daily (approximately 480-960 mg elemental magnesium) 1, 4
- Divide doses throughout the day, with larger dose at night when intestinal transit is slowest 1, 4
- For constipation indication: Start 400-500 mg daily, titrate to 1.5 g/day based on response 4
Critical Concurrent Electrolyte Management
Replace Magnesium FIRST Before Attempting Other Corrections
This is non-negotiable and the most common cause of treatment failure. 1
Hypokalemia will be refractory to potassium supplementation until magnesium is normalized 1, 4, 5
Hypocalcemia will be refractory to calcium supplementation until magnesium is normalized 1
Special Clinical Scenarios
Short Bowel Syndrome / High GI Losses
Oral supplementation frequently fails in these patients. 1, 4
- Each liter of jejunostomy fluid contains ~100 mmol/L sodium plus substantial magnesium 1, 4
- Most magnesium salts are poorly absorbed and may worsen diarrhea 1, 4
- First correct volume depletion with IV saline 1, 4
- If oral fails: IV or subcutaneous magnesium sulfate (4-12 mmol added to saline bags) 1-3 times weekly 1, 4
- Consider oral 1-alpha hydroxy-cholecalciferol (0.25-9.00 μg daily) in gradually increasing doses 1, 4
- Monitor serum calcium regularly to avoid hypercalcemia 1, 4
Continuous Renal Replacement Therapy (CRRT)
- Hypomagnesemia occurs in 60-65% of critically ill patients on CRRT 1, 4
- Use dialysis solutions containing magnesium to prevent ongoing losses 1, 4
- Regional citrate anticoagulation increases magnesium losses as magnesium-citrate complexes 1, 4
Cardiac Patients with QTc Prolongation
- Maintain magnesium >2 mg/dL in patients with QTc >500 ms or on QT-prolonging medications 1, 4
- Obtain ECG immediately if patient has arrhythmias, heart failure, or digoxin therapy 1
Monitoring for Magnesium Toxicity During IV Replacement
This is critical to prevent life-threatening complications. 1, 2
Signs of Toxicity (in order of appearance):
- Loss of deep tendon reflexes (begins >4 mEq/L, absent at 10 mEq/L) 2
- Hypotension and bradycardia 1, 2
- Respiratory depression progressing to paralysis 1, 2
- Complete cardiovascular collapse (at 6-10 mmol/L) 1, 2
Antidote:
- Have calcium chloride 10% (5-10 mL) or calcium gluconate 10% (15-30 mL) immediately available 1, 2
- Administer IV over 2-5 minutes to reverse magnesium toxicity 1
Monitoring Timeline
- Baseline: Serum magnesium, potassium, calcium, renal function 1, 4
- During IV replacement: Continuous monitoring for toxicity signs 1, 2
- After IV bolus for cardiac emergency: Recheck within 24-48 hours 4
- After starting oral supplementation: Recheck at 2-3 weeks 4
- Stable maintenance: Every 3 months 4
- High GI losses or CRRT: Every 2 weeks initially, then monthly 4
Common Pitfalls to Avoid
Never supplement magnesium in volume-depleted patients without first correcting with IV saline - secondary hyperaldosteronism will cause continued renal wasting 1, 4
Never attempt to correct hypokalemia or hypocalcemia before normalizing magnesium - these will be refractory to treatment 1, 4
Never give magnesium if CrCl <20 mL/min except in life-threatening torsades - risk of fatal hypermagnesemia 1, 4, 2
Never assume normal serum magnesium excludes deficiency - less than 1% of total body magnesium is in blood 4, 5
Never mix magnesium sulfate with calcium in the same IV solution - will precipitate 1, 2
Never exceed 150 mg/minute IV infusion rate except in severe eclampsia with seizures 2
Never use continuous magnesium sulfate in pregnancy beyond 5-7 days - causes fetal abnormalities 2