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# Magnesium Sulfate ## Overview Magnesium sulfate is an essential mineral that plays a vital role in numerous biochemical functions, including neuromuscular transmission, cardiac excitability, and enzyme activity. It is available for intravenous (IV) and intramuscular (IM) administration, as well as oral formulations. ## Primary Indications * **Hypomagnesemia:** Treatment and prevention of magnesium deficiency. * **Eclampsia/Preeclampsia:** Prevention and treatment of seizures associated with severe preeclampsia and eclampsia. * **Torsades de Pointes:** Treatment of polymorphic ventricular tachycardia with a prolonged QT interval. * **Bronchospasm:** Adjunctive therapy in severe, acute asthma exacerbations unresponsive to standard treatment. * **Constipation (Oral):** Osmotic laxative. ## Adult Dosing * **Hypomagnesemia:** * **Severe (serum Mg < 1.5 mEq/L or < 0.75 mmol/L):** 4-6 grams IV infused over 5-60 minutes, followed by a continuous infusion of 1-2 grams per hour. Alternatively, 1-2 grams IM every 4 hours for 4 doses. * **Moderate (serum Mg 1.5-2.5 mEq/L or 0.75-1.25 mmol/L):** 2-4 grams IV infused over 5-60 minutes. * **Maintenance:** Dosing depends on serum magnesium levels and renal function, typically 2-4 grams IV every 12-24 hours. * **Eclampsia/Preeclampsia:** Loading dose: 4-6 grams IV infused over 5-10 minutes. Maintenance infusion: 1-2 grams per hour. IM dosing: 5 grams IM in each buttock (total 10g) for loading, followed by 5 grams IM every 4 hours. * **Torsades de Pointes:** 1-2 grams IV diluted in 10-20 mL of D5W infused over 5-10 minutes. Additional doses may be given, followed by an infusion of 0.5-1 gram per hour. * **Bronchospasm:** 1-2 grams IV infused over 15-30 minutes. * **Constipation (Oral):** 25-30 mL of a 50% solution (12.5-15 g magnesium) taken with a full glass of water. ## Pediatric Dosing Dosing is highly variable and depends on indication, age, weight, and magnesium level. Consult specific pediatric guidelines. * **Hypomagnesemia:** Dosing often ranges from 20-50 mg/kg/dose IV over 5-15 minutes for acute repletion, with maintenance infusions adjusted based on levels. * **Eclampsia:** Dosing typically follows adult regimens, adjusted for weight. * **Torsades de Pointes:** 25-50 mg/kg IV over 10-20 minutes, with a maximum single dose of 2 grams. ## Dose Adjustments * **Renal Impairment:** Magnesium is renally excreted. Reduce dose and monitor serum magnesium levels closely in patients with impaired renal function. * **CrCl 10-50 mL/min:** Reduce maintenance dose by 50%. * **CrCl < 10 mL/min:** Reduce maintenance dose by 75% or avoid if possible; requires close monitoring. ## Contraindications * Hypermagnesemia. * Heart block. * Myocardial infarction (caution). * Anuria. * Severe renal impairment. * Hypersensitivity to magnesium sulfate. ## Adverse Effects * **Common:** Flushing, sweating, hypotension, nausea, vomiting, drowsiness, decreased deep tendon reflexes. * **Serious:** Respiratory depression, cardiac arrhythmias, cardiac arrest, hypermagnesemia (especially with renal impairment), neuromuscular blockade. * **IM injection:** Pain, swelling, sterile abscess. ## Key Drug Interactions * **Neuromuscular Blocking Agents (e.g., rocuronium, vecuronium):** Magnesium can potentiate neuromuscular blockade, leading to prolonged paralysis. * **Calcium Channel Blockers (e.g., nifedipine, verapamil):** Additive hypotensive and cardiotoxic effects may occur. * **Antibiotics (Tetracyclines, Quinolones):** Magnesium can decrease the absorption of oral tetracyclines and fluoroquinolones by forming chelates. Separate administration by at least 2-4 hours. * **Bisphosphonates:** Magnesium can decrease absorption. Separate administration by at least 2 hours. ## Monitoring * **Serum Magnesium Levels:** Essential, especially with IV infusions, renal impairment, or prolonged therapy. Target levels depend on indication (e.g., 4-7 mEq/L for eclampsia). * **Renal Function:** Assess baseline and monitor periodically. * **Deep Tendon Reflexes:** Monitor for loss of reflexes, an early sign of hypermagnesemia. * **Respiratory Rate:** Monitor for depression, especially with high doses or rapid infusion. * **Blood Pressure:** Monitor for hypotension. * **Urine Output:** Ensure adequate renal function. ## Clinical Pearls * Magnesium sulfate is a potent medication; accurate dose calculation and slow infusion rates are crucial to prevent toxicity. * Always dilute IV magnesium sulfate before administration. * In patients with severe renal impairment, monitor magnesium levels very closely; toxicity can be life-threatening. * Intramuscular injections can be painful; consider lidocaine in the injection site if indicated and permitted by protocol. * When treating hypomagnesemia, aim for a serum magnesium level of 2-3 mEq/L (1-1.5 mmol/L) for symptomatic patients and potentially higher (4-7 mEq/L or 2-3.5 mmol/L) for patients with life-threatening arrhythmias or eclampsia. *** **Disclaimer:** This information is intended for healthcare professionals and is not a substitute for professional medical advice. Always consult the most current prescribing information and relevant clinical guidelines before making any decisions about drug therapy. Dosing and management may vary based on individual patient factors and local protocols.