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# Magnesium Sulfate
## Overview
Magnesium sulfate is an electrolyte that plays a crucial role in numerous physiological processes, including neuromuscular transmission and muscle contraction. It is available in injectable and oral formulations.
## Primary Indications
* Treatment of hypomagnesemia.
* Management of pre-eclampsia and eclampsia.
* Treatment of severe asthma exacerbations.
* Management of torsades de pointes.
* Prevention and treatment of seizures associated with hyperactive deep tendon reflexes in patients with pre-eclampsia/eclampsia.
## Adult Dosing
**Hypomagnesemia:**
* **Severe deficiency (serum magnesium < 1.3 mg/dL):** Initial dose of 4-6 grams (32-48 mEq) IV infused over 15-60 minutes, followed by a maintenance infusion of 1-2 grams (8-16 mEq) per hour, or intermittent IM injections of 1 gram (8 mEq) every 4 hours as needed. The total daily dose should generally not exceed 30 grams (240 mEq) in 24 hours.
* **Less severe deficiency:** 1-2 grams (8-16 mEq) IV or IM every 6-12 hours.
**Pre-eclampsia/Eclampsia:**
* **Loading dose:** 4-6 grams (32-48 mEq) IV infused over 5-10 minutes.
* **Maintenance dose:** 1-2 grams (8-16 mEq) per hour as a continuous IV infusion. Dosing may vary based on local protocol and patient response.
**Torsades de Pointes:**
* **Acute management:** 1-2 grams (8-16 mEq) IV diluted in 10-20 mL of D5W infused over 5-10 minutes. Additional doses may be administered.
* **Recurrent or sustained:** 5 grams (40 mEq) diluted in 250 mL D5W infused over 1 hour.
**Severe Asthma:**
* 2 grams (16 mEq) IV diluted in 50 mL of saline infused over 15-30 minutes. This is an adjunct therapy.
## Pediatric Dosing
Dosing is highly variable and dependent on indication and magnesium level. Specific protocols are often followed.
**Hypomagnesemia:**
* **Severe:** 25-50 mg/kg/dose (elemental magnesium) IV infused over 3-6 hours. Maximum single dose 2 grams.
* **Maintenance:** 10-20 mg/kg/dose (elemental magnesium) IV every 4-6 hours.
**Eclampsia:**
* Dosing varies. Refer to specific pediatric emergency protocols or obstetric guidelines.
**Asthma:**
* 25-75 mg/kg (elemental magnesium) IV over 15-30 minutes. Maximum dose 2 grams.
## Dose Adjustments
* **Renal impairment:** Magnesium is renally excreted. Use with caution in patients with impaired renal function. Monitor serum magnesium levels closely. Lower doses or extended intervals may be necessary.
## Contraindications
* Hypermagnesemia.
* Heart block.
* Myocardial infarction with digitalis intoxication.
* Hypersensitivity to magnesium sulfate.
## Adverse Effects
Common adverse effects include:
* Flushing
* Nausea, vomiting
* Drowsiness
* Hypotension
* Warm sensation
* Decreased deep tendon reflexes
* Respiratory depression (at high doses)
* Cardiac arrhythmias
* Hyperthermia
## Key Drug Interactions
* **Neuromuscular blocking agents (e.g., succinylcholine, rocuronium):** May potentiate neuromuscular blockade.
* **Calcium channel blockers:** Increased risk of hypotension and bradycardia.
* **Digitalis glycosides:** Increased risk of cardiac arrhythmias, especially with hypermagnesemia.
* **Nifedipine:** May cause profound hypotension when used concurrently for pre-eclampsia.
## Monitoring
* **Serum magnesium levels:** Crucial, especially with IV infusion or in renal impairment. Target levels vary by indication.
* **Renal function:** Monitor BUN and creatinine.
* **Respiratory rate and depth:** Monitor for signs of respiratory depression.
* **Deep tendon reflexes:** Loss of reflexes indicates magnesium toxicity.
* **Blood pressure and heart rate:** Monitor for hypotension and bradycardia.
* **Urine output:** Monitor for adequate renal function.
* **ECG:** Monitor for arrhythmias, particularly in patients with cardiac conditions.
## Clinical Pearls
* Magnesium sulfate is administered as anhydrous magnesium sulfate; doses are typically expressed in grams (g) or milliequivalents (mEq) of elemental magnesium. 1 gram of magnesium sulfate is approximately 8.12 mEq or 4.06 mmol of elemental magnesium.
* Rapid IV infusion can cause hypotension and cardiac arrhythmias. Always infuse as directed.
* Monitor for signs of magnesium toxicity, which include loss of deep tendon reflexes, respiratory depression, altered mental status, and cardiac arrest. Calcium gluconate is the antidote for magnesium toxicity.
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