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# Magnesium sulfate
## Overview
Magnesium sulfate is an essential mineral used for replacement, seizure prophylaxis, and as a smooth muscle relaxant. Pharmacokinetics depend heavily on route (IV vs. IM) and renal function.
## Primary Indications
* **Obstetrics:** Eclampsia and preeclampsia (seizure prevention).
* **Neurology/Emergency:** Torsades de pointes (arrhythmia), severe acute asthma (refractory), and hypomagnesemia.
## Adult Dosing
* **Hypomagnesemia:** 1–4 g IV/IM. Common protocol: 1–2 g IV over 15–60 minutes. Severe cases: up to 8 g over 24 hours.
* **Torsades de Pointes:** 1–2 g IV bolus over 5–20 minutes, may repeat once.
* **Eclampsia/Preeclampsia:** 4–6 g IV loading dose over 15–20 minutes, followed by 1–2 g/hr continuous infusion.
* **Acute Severe Asthma:** 2 g IV over 20 minutes (adjunctive).
## Pediatric Dosing
* **Hypomagnesemia:** 25–50 mg/kg/dose IV/IM (max 2 g/dose).
* **Acute Asthma:** 25–75 mg/kg IV (max 2 g) as a single dose over 20 minutes.
* *Note: Always verify pediatric dosing against weight-based institutional formularies.*
## Dose Adjustments
* **Renal Impairment:** Must reduce dose/rate and monitor levels closely. Patients with CrCl <30 mL/min are at high risk for toxicity. Adjust frequency or discontinue if hypermagnesemia occurs.
## Contraindications
* Myasthenia gravis (may exacerbate muscle weakness).
* Heart block (caution/avoid).
* Anuria/severe renal impairment (relative).
## Adverse Effects
* **Common:** Flushing, sweating, injection site reactions.
* **Serious (Toxicity):** Loss of deep tendon reflexes (early sign), hypotension, respiratory depression, cardiac arrest, AV block.
* **Antidote:** Calcium gluconate (1 g IV) for respiratory or cardiac depression.
## Key Drug Interactions
* **Neuromuscular Blockers:** Potentiates effects (increased risk of paralysis).
* **Aminoglycosides:** May exacerbate neuromuscular blockade.
* **CNS Depressants:** Enhances sedative effects.
* **Digoxin:** Potential for increased digitalis toxicity risk (hypomagnesemia correction).
## Monitoring
* **Clinical:** Respiratory rate (>12-16 breaths/min), deep tendon reflexes (patellar), urine output (>0.5–1 mL/kg/hr).
* **Laboratory:** Serum magnesium levels (Target range: 2.0–4.0 mEq/L for seizure prophylaxis). Monitor calcium and potassium concurrently.
## Clinical Pearls
* **IV Compatibility:** Rapid infusions can cause hypotension and cardiac arrhythmias.
* **Pregnancy/Lactation:** Category A for hypomagnesemia; cross-links placenta. Neonatal respiratory depression is a risk if mother is treated shortly before birth.
* **Administration:** Ensure calcium gluconate is at the bedside for rapid reversal in the event of toxicity.
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**EDUCATIONAL DISCLAIMER:** This information is for educational purposes only. Drug dosing, local protocols, and clinical guidelines vary significantly between institutions. Always consult your facility’s specific formulary, clinical pharmacists, and current prescribing information (package inserts or resources like Lexicomp/UpToDate) before administering medication.
