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Rapid IV magnesium can cause cardiac arrest, respiratory depression, and hypotension. Use infusion pump. Monitor deep tendon reflexes. Cardiac monitoring for severe cases.
Calculate magnesium replacement dosing for hypomagnesemia
Normal: 1.8-2.6 mg/dL (or 1.5-2.1 mEq/L)
| Serum Mg (mg/dL) | Severity | IV Dose (MgSO4) | PO Dose | Monitoring |
|---|---|---|---|---|
| <1.0 | Severe | 4-6 g over 8-24 hrs | 800 mg/day divided | ICU, cardiac monitor, check Mg q6-12h, reflexes |
| 1.0-1.1 | Moderate-Severe | 4 g over 4-8 hrs | 600 mg/day | Cardiac monitor, check Mg q12h |
| 1.2-1.4 | Moderate | 2 g over 2-4 hrs | 400 mg/day | Check Mg in 24 hours |
| 1.5-1.7 | Mild | 1 g over 1-2 hrs | 400 mg/day (preferred) | Check Mg in 24-48 hours |
| 1.8-2.0 | Low-Normal | Not needed | 200 mg/day if indicated | Routine |
| 2.1-2.6 | Normal | None | None | Routine |
| Formulation | Elemental Mg Content | Route | Notes |
|---|---|---|---|
| Magnesium Sulfate (MgSO4) | 1 g = 8.12 mEq = 98 mg elemental | IV/IM | Standard IV formulation |
| Mag Oxide | 400 mg tablet = 240 mg elemental | PO | Poorly absorbed, causes diarrhea |
| Mag Citrate | 400 mg tablet = 64 mg elemental | PO | Better absorbed, less diarrhea |
| Mag Chloride | 535 mg tablet = 64 mg elemental | PO | Good absorption |
| Mag Glycinate | 100 mg tablet = 14 mg elemental | PO | Best absorbed, least GI effects |
Magnesium is crucial for >300 enzymatic reactions, muscle/nerve function, and maintaining potassium and calcium levels. Hypomagnesemia causes arrhythmias, muscle cramps, seizures, and prevents potassium correction. It's often overlooked in ICU patients.
Monitor deep tendon reflexes (patellar reflex). Loss of reflexes occurs at Mg >4 mg/dL. Signs of toxicity: hypotension, bradycardia, respiratory depression, cardiac arrest. Levels: >4 mg/dL = loss of reflexes, >6 mg/dL = respiratory paralysis, >12 mg/dL = cardiac arrest.
Calcium gluconate 1-2 g IV over 5-10 minutes antagonizes the cardiac and neuromuscular effects of magnesium. Also stop Mg infusion, support respiration if needed, and consider hemodialysis for severe toxicity in renal failure.
Oral magnesium (especially mag oxide and citrate) acts as an osmotic laxative. To minimize: use divided doses (e.g., 200 mg TID instead of 600 mg once daily), take with food, or switch to better-absorbed forms like magnesium glycinate.
Use IV for: Mg <1.2 mg/dL, symptomatic hypomagnesemia (arrhythmias, seizures, tetany), NPO status, or severe GI losses. Use PO for: Mg 1.2-1.8 mg/dL in stable patients who can tolerate oral intake. IV is more predictable and faster.