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# Magnesium Sulfate ## Overview Magnesium sulfate is an inorganic salt that plays a role in numerous biochemical processes, including neuromuscular transmission and cardiac excitability. It is available for intravenous (IV) and intramuscular (IM) administration, as well as oral formulations. ## Primary Indications * Treatment of hypomagnesemia. * Prevention and treatment of seizures in preeclampsia and eclampsia. * Management of torsades de pointes. * Bronchodilation in severe asthma exacerbations. * Laxative (oral use). ## Adult Dosing * **Hypomagnesemia:** * **Severe (serum Mg < 1.3 mEq/L):** 4-6 grams IV infused over 1 hour, followed by 2-4 grams IV every 4-12 hours as needed. * **Less severe (serum Mg 1.3-1.7 mEq/L):** 2-4 grams IV or IM. * Maintenance: 4-6 grams total daily dose divided into 4 doses IM or continuously infused IV. * **Preeclampsia/Eclampsia:** * **Loading dose:** 4-6 grams IV infused over 5-20 minutes. * **Maintenance infusion:** 1-2 grams per hour IV. Alternatively, 5 grams IM into each buttock every 4 hours. Continue for 24 hours postpartum or 1 hour after last seizure. * **Torsades de Pointes:** 1-2 grams IV in 50-100 mL D5W over 5-10 minutes. Repeat infusion if necessary, followed by a continuous infusion of 0.5-1 gram per hour. * **Asthma Exacerbation (adjunctive):** 2 grams IV infused over 15-30 minutes. * **Laxative (oral):** 15-30 mL of a 50% solution, or 2-4 grams of oral granules, taken with a full glass of water. ## Pediatric Dosing * **Hypomagnesemia:** Dosing is highly variable and dependent on serum magnesium levels. A common empiric regimen is 25-50 mg/kg/dose IV every 4-6 hours for 3-4 doses. Maximum single dose generally not to exceed 2 grams. * **Eclampsia:** Loading dose: 40-50 mg/kg IV (max 4 grams). Maintenance: 20-30 mg/kg/hour IV (max 2 grams/hour). * **Asthma Exacerbation:** 25-40 mg/kg/dose IV infused over 15-30 minutes (max 2 grams). ## Dose Adjustments * **Renal Impairment:** Magnesium is renally excreted. Reduce dose and monitor serum magnesium levels closely in patients with impaired renal function. Dose reduction may be necessary in moderate to severe renal impairment. ## Contraindications * Hypermagnesemia. * Heart block or myocardial damage. * Use with caution in patients with myasthenia gravis. ## Adverse Effects * **Common:** Flushing, warmth, hypotension, sweating, nausea, vomiting, drowsiness, decreased reflexes. * **Serious:** Respiratory depression, cardiac arrest, hypermagnesemia (muscle weakness, absent deep tendon reflexes, bradycardia, hypotension, cardiac arrhythmias, respiratory arrest). ## Key Drug Interactions * **Neuromuscular Blocking Agents (e.g., succinylcholine, vecuronium):** Potentiates neuromuscular blockade. * **Calcium Channel Blockers:** Additive hypotensive effects and potential for neuromuscular blockade. * **Tetracyclines and Bisphosphonates:** Magnesium can decrease absorption; separate administration by at least 2-3 hours. ## Monitoring * **Serum magnesium levels:** Essential, especially with repeated doses or in renal impairment. Target levels vary by indication (e.g., therapeutic range for hypomagnesemia is typically 2-3.5 mEq/L). * **Renal function:** Creatinine and BUN. * **Deep tendon reflexes:** Diminished reflexes can indicate rising magnesium levels. * **Respiratory rate and depth:** Monitor for respiratory depression. * **Blood pressure and heart rate:** Monitor for hypotension and bradycardia. * **Urine output:** Ensure adequate renal function. ## Clinical Pearls * Administer IV magnesium sulfate slowly to avoid hypotension and flushing. Maximum infusion rate for standard maintenance is typically 1 gram/hour, although higher rates may be used in specific emergent situations under close monitoring. * IM injections can be painful and should be administered deep into the gluteal muscle, ideally divided between two sites if doses exceed 5 grams. * Oral magnesium sulfate has a laxative effect and is often used for constipation. * Magnesium sulfate is the treatment of choice for torsades de pointes, even in the absence of documented hypomagnesemia. * Monitor for signs and symptoms of hypermagnesemia, especially in patients with renal impairment or receiving high doses. Calcium gluconate or calcium chloride can be used as an antidote for severe hypermagnesemia. *** *Please verify this information with the current official prescribing information and relevant institutional protocols before use.*