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# Magnesium Sulphate
## Overview
Magnesium sulphate is an electrolyte that plays a vital role in numerous biochemical functions. It is available for intravenous (IV) and intramuscular (IM) administration.
## Primary Indications
* **Hypomagnesemia:** Treatment and prevention of low magnesium levels.
* **Eclampsia/Preeclampsia:** Prophylaxis and treatment of seizures associated with severe preeclampsia and eclampsia.
* **Torsades de Pointes:** Treatment of polymorphic ventricular tachycardia associated with a prolonged QT interval.
* **Bronchodilator:** Adjunctive therapy in severe acute asthma exacerbations unresponsive to standard treatment.
## Adult Dosing
* **Hypomagnesemia (Symptomatic):**
* **IV:** 4-5 grams (32-40 mEq) diluted in 1000 mL of normal saline or dextrose 5% in water infused over 15-30 minutes. May be followed by a continuous infusion of 1-2 grams (8-16 mEq) per hour, not to exceed 3 grams (24 mEq) in 24 hours, depending on serum magnesium levels and clinical response.
* **IM:** 4-5 grams (32-40 mEq) divided into two doses and injected into alternate buttocks, typically given as 1 gram (8 mEq) every 4 hours as needed. This route is painful and can cause induration.
* **Hypomagnesemia (Asymptomatic):** Dosing varies based on severity and local protocol, often lower continuous infusions or oral magnesium replacement.
* **Eclampsia/Preeclampsia (Loading Dose):** 4-6 grams (32-48 mEq) IV infused over 5-10 minutes.
* **Eclampsia/Preeclampsia (Maintenance Dose):** 1-2 grams (8-16 mEq) per hour IV infusion. Some protocols utilize IM dosing: 5 grams (40 mEq) IM into alternate buttocks every 4 hours.
* **Torsades de Pointes:** 1-2 grams (8-16 mEq) IV in 50-100 mL of D5W over 5-10 minutes. May be followed by infusion of 0.5-1 gram (4-8 mEq) per hour.
* **Asthma (Adjunctive):** 1-2 grams (8-16 mEq) IV in 50-100 mL normal saline over 15-30 minutes.
## Pediatric Dosing
Dosing is highly variable and dependent on indication and patient weight. Reference to pediatric-specific guidelines or local protocols is essential.
* **Hypomagnesemia:** Typically dosed based on serum magnesium deficit. Common IV infusion rates range from 25-50 mg/kg/dose (0.2-0.4 mEq/kg/dose), not to exceed 1000 mg (8 mEq) per dose, given over several hours. Maintenance infusions vary.
* **Eclampsia:** Extrapolation from adult data, with careful titration and monitoring.
* **Torsades de Pointes:** 25-50 mg/kg (0.2-0.4 mEq/kg) IV over 10-20 minutes, not to exceed adult doses.
## Dose Adjustments
* **Renal Impairment:** Magnesium is renally excreted. Use with caution in patients with impaired renal function. Reduce dose and monitor serum magnesium levels closely. Specific dose adjustments depend on the degree of renal impairment and local guidelines.
## Contraindications
* Hypermagnesemia
* Heart block
* Myocardial infarction
* Shock
* Hypersensitivity to magnesium sulphate
## Adverse Effects
* **Common:** Flushing, diaphoresis, hypotension, nausea, vomiting, sedation, decreased deep tendon reflexes.
* **Serious:** Respiratory depression, cardiac arrhythmias, cardiac arrest, hypermagnesemia (especially with renal impairment).
## Key Drug Interactions
* **Neuromuscular Blocking Agents:** Magnesium sulphate may potentiate the effects of neuromuscular blockers.
* **Calcium Channel Blockers:** Increased risk of hypotension and bradycardia.
* **Tetracyclines and Fluoroquinolones:** Magnesium can reduce the absorption of these antibiotics; separate administration by at least 2 hours.
## Monitoring
* **Serum Magnesium Levels:** Monitor closely, especially during continuous infusions or in patients with renal impairment. Target levels vary by indication (e.g., 4-7 mEq/L for eclampsia).
* **Renal Function:** Assess baseline and monitor periodically.
* **Deep Tendon Reflexes:** Assess before and during administration, as loss of reflexes is an early sign of hypermagnesemia.
* **Respiratory Rate:** Monitor for signs of respiratory depression.
* **Blood Pressure and Heart Rate:** Monitor for hypotension and bradycardia.
* **Urine Output:** Should be adequate (at least 100 mL in 4 hours) before administering repeat doses in eclampsia prophylaxis.
## Clinical Pearls
* Always dilute magnesium sulphate for IV administration to prevent local irritation and reduce risk of rapid hypermagnesemia.
* IM administration is painful and should be administered deep into large muscle masses.
* In patients with impaired renal function, the risk of magnesium accumulation and toxicity is significantly increased.
* Magnesium sulphate is the drug of choice for treating torsades de pointes, even in the absence of confirmed hypomagnesemia.
* Be aware of the potential for rebound hypomagnesemia after cessation of prolonged infusions.
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*Disclaimer: This information is intended for healthcare professionals and is not a substitute for professional medical advice. Always consult the most current prescribing information, official guidelines, and drug reference resources before making any clinical decisions.*
