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# Magnesium sulfate
## Overview
Magnesium sulfate is a parenteral or oral electrolyte used for magnesium replacement, seizure prophylaxis, and as a smooth muscle relaxant. It acts as a calcium antagonist and reduces acetylcholine release at the neuromuscular junction.
## Primary Indications
* **Hypomagnesemia:** Correction of deficiency.
* **Eclampsia/Preeclampsia:** Prophylaxis and treatment of seizures.
* **Torsades de Pointes:** Immediate management of refractory ventricular arrhythmias.
* **Asthma:** Adjunctive treatment for severe, refractory acute exacerbations.
## Adult Dosing
* **Hypomagnesemia:** 1–4 g IV/IM. Common protocol: 2 g IV over 15–60 minutes. Severe deficit may require up to 8–12 g/24 hours.
* **Eclampsia (Pritchard Protocol):** 4 g IV loading dose over 15–20 minutes, followed by 1–2 g/hour IV infusion. Continue for 24 hours post-delivery or post-seizure.
* **Torsades de Pointes:** 1–2 g IV push over 5–20 minutes; may repeat once or follow with an infusion (3–20 mg/min).
* **Severe Asthma:** 2 g IV administered over 20 minutes (single dose).
## Pediatric Dosing
* **Hypomagnesemia:** 25–50 mg/kg/dose IV/IM every 4–6 hours as needed; max 2 g per dose.
* **Asthma:** 25–75 mg/kg IV (max 2–2.5 g) given as a single dose over 20 minutes.
* *Note: Dosing is highly dependent on institutional protocols specific to pediatric critical care.*
## Dose Adjustments
* **Renal Impairment:** Reduce dose in patients with CrCl <30 mL/min. Magnesium accumulates rapidly in renal failure, increasing risk of toxicity. Monitor serum magnesium levels closely.
## Contraindications
* Heart block or myocardial damage.
* Severe renal impairment (CrCl <20 mL/min for prolonged use).
* Recent myocardial infarction (relative).
## Adverse Effects
* **Common:** Flushing, warm sensation, hypotension, nausea, injection site reactions.
* **Toxic (Hypermagnesemia):** Loss of deep tendon reflexes (early sign), muscle weakness, respiratory depression/paralysis, AV block, cardiac arrest.
## Key Drug Interactions
* **Neuromuscular Blockers:** Potentiates effect; risk of prolonged paralysis.
* **Calcium Channel Blockers:** Enhances risk of hypotension and neuromuscular blockade.
* **Digoxin:** Rapid changes in magnesium levels can precipitate digoxin toxicity.
## Monitoring
* **Serum Magnesium:** Maintain therapeutic range (typically 1.7–2.3 mg/dL for asymptomatic, up to 4–6 mg/dL for eclampsia).
* **Clinical Signs:** Assess deep tendon reflexes (must be present), respiratory rate (>12-16 breaths/min), and urine output (>0.5 mL/kg/hr) at least hourly during infusion for eclampsia.
* **Calcium Gluconate:** Keep 1 g IV at the bedside as an antidote for respiratory depression or cardiac toxicity.
## Clinical Pearls
* **Calcium Antidote:** Always have intravenous calcium gluconate or calcium chloride available when infusing magnesium for seizure prophylaxis or arrhythmia.
* **Route:** IV administration is preferred for acute/emergent indications. IM administration is painful and generally reserved for situations where IV access is impossible.
* **Preeclampsia:** Ensure fetal heart rate monitoring occurs during infusion, as magnesium crosses the placenta.
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**Disclaimer:** This information is for educational purposes only. Always verify drug doses, compatibility, and infusion protocols against current institutional guidelines and the manufacturer’s package insert before prescribing or administering medication.
