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# Magnesium Sulphate
## Overview
Magnesium sulphate is an essential mineral that plays a role in numerous biochemical functions, including neuromuscular transmission and muscle contraction. It is available in intravenous (IV) and intramuscular (IM) formulations.
## Primary Indications
* Hypomagnesemia
* Eclampsia and pre-eclampsia (eclampsia prevention and treatment)
* Torsades de pointes (polymorphic ventricular tachycardia)
* Bronchospasm (adjunctive therapy in severe cases)
* Severe asthma exacerbations (adjunctive therapy)
* Preeclampsia/Eclampsia
* Hypomagnesemia
## Adult Dosing
**Hypomagnesemia:**
* **IV:** 4 grams IV initially, followed by 1-2 grams IV every 4-6 hours as needed. Alternatively, a continuous infusion of 1 gram/hour (or 0.5-1 mEq/kg/hour) may be used.
* **IM:** 1 gram IM every 4-6 hours for 4 doses (total 4 grams). IM injections are painful and can cause tissue damage.
**Eclampsia/Pre-eclampsia:**
* **IV loading dose:** 4-6 grams IV over 5-20 minutes.
* **IV maintenance infusion:** 1-2 grams/hour.
* **IM (alternative):** 5 grams IM into each buttock (total 10 grams) as a loading dose, followed by 5 grams IM into alternating buttocks every 4 hours. (IM route is less preferred due to pain and absorption variability).
**Torsades de Pointes:**
* **IV:** 1-2 grams IV in 10-50 mL D5W or NS over 5-10 minutes. Followed by an infusion of 0.5-1 gram/hour if needed.
**Bronchospasm/Severe Asthma:**
* **IV:** 1-2 grams IV in 50-100 mL NS over 15-30 minutes. Dosing may vary based on institutional protocols.
## Pediatric Dosing
**Hypomagnesemia:**
* **IV:** 20-50 mg/kg/dose (as anhydrous magnesium sulphate) IV over 10-60 minutes. Maximum dose typically 2 grams. May repeat every 4-6 hours.
* **IM:** 25-50 mg/kg/dose (as anhydrous magnesium sulphate) IM every 4-6 hours. Maximum dose typically 2 grams. IM route is painful.
**Eclampsia/Pre-eclampsia:**
* Dosing is often based on adult protocols or institutional guidelines. A common pediatric regimen for status epilepticus (which may be adapted for eclampsia) includes a loading dose of 25-50 mg/kg IV, followed by an infusion of 10-30 mg/kg/hour.
## Dose Adjustments
* **Renal Impairment:** Use with caution. Reduce dose and monitor serum magnesium levels closely. Patients with severe renal impairment may require significantly reduced doses or may not tolerate magnesium sulphate therapy.
## Contraindications
* Hypermagnesemia
* Heart block
* Myocardial infarction (relative contraindication, especially with IV administration)
* Hypotension (relative contraindication, especially with rapid IV administration)
* Myasthenia gravis (relative contraindication)
* Severe renal impairment
## Adverse Effects
* **Common:** Flushing, sweating, hypotension, nausea, vomiting, decreased deep tendon reflexes, lethargy, respiratory depression, cardiac arrhythmias, and cardiac arrest.
* **Rare:** Hypermagnesemia, especially with impaired renal function or excessive doses.
## Key Drug Interactions
* **Neuromuscular Blocking Agents (e.g., succinylcholine, rocuronium):** Magnesium can potentiate neuromuscular blockade, leading to prolonged paralysis and respiratory depression.
* **Calcium Channel Blockers:** Increased risk of hypotension and bradycardia.
* **Digoxin:** IV magnesium can potentially worsen digoxin toxicity.
* **Antibiotics (Tetracyclines, Quinolones):** Magnesium can decrease the absorption of oral tetracyclines and fluoroquinolones; administer at least 2 hours before or 4-6 hours after these agents.
## Monitoring
* **Serum Magnesium Levels:** Essential, especially with IV infusions, renal impairment, or prolonged therapy. Therapeutic range for hypomagnesemia is typically 1.7-2.2 mEq/L (0.85-1.1 mmol/L). Aim for >1.7 mEq/L for hypomagnesemia treatment. For eclampsia, target levels are often 4-7 mEq/L (2-3.5 mmol/L).
* **Deep Tendon Reflexes:** Diminished reflexes are an early sign of toxicity.
* **Respiratory Rate and Depth:** Monitor for respiratory depression.
* **Blood Pressure and Heart Rate:** Especially with IV administration.
* **Urine Output:** Important indicator of renal function.
## Clinical Pearls
* Magnesium sulphate IV infusions should be administered slowly to avoid hypotension and other adverse effects.
* IM injections are painful and can cause tissue necrosis; a 2% lidocaine solution may be mixed with the magnesium sulphate to reduce pain.
* Discontinue infusion immediately if signs of magnesium toxicity appear (e.g., absent deep tendon reflexes, respiratory depression, hypotension).
* Calcium gluconate or calcium chloride is the antidote for magnesium toxicity.
* The dose of magnesium sulphate is often expressed in grams, but it is important to remember the relationship to elemental magnesium (1 gram of magnesium sulphate heptahydrate is approximately 0.12 grams or 1 mEq of elemental magnesium). However, most clinical dosing guidelines use grams of magnesium sulphate.
* In patients with normal renal function, it is difficult to achieve hypermagnesemia with oral magnesium products.
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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 and institutional protocols before administering any medication.
