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# Magnesium sulphate
## Overview
Magnesium sulphate is a concentrated electrolyte used for parenteral replacement and as a pharmacologic agent for eclampsia, severe asthma, and arrhythmias. Intravenous administration requires caution due to the risk of toxicity and rapid hemodynamic changes.
## Primary Indications
* **Obstetrics:** Prevention/control of seizures in pre-eclampsia/eclampsia.
* **Cardiology:** Torsades de pointes (arrhythmia) and digitalis-induced arrhythmias.
* **Respiratory:** Severe, acute asthma exacerbations (adjunctive therapy).
* **Metabolic:** Symptomatic hypomagnesemia.
## Adult Dosing
* **Eclampsia:** Loading dose of 4–6 g IV over 15–20 minutes, followed by a continuous infusion of 1–2 g/hour.
* **Torsades de pointes:** 1–2 g IV push (over 5–20 minutes); may be repeated once if rhythm persists.
* **Severe Asthma:** 2 g IV over 20 minutes as a single dose.
* **Hypomagnesemia:** 1–4 g IV/IM. Rates should not exceed 1 g per hour for asymptomatic deficiency; symptomatic (e.g., seizures/arrhythmia) may be faster at clinician discretion under cardiac monitoring.
## Pediatric Dosing
* **Asthma:** 25–75 mg/kg (max 2 g) IV as a single dose over 20–30 minutes.
* **Hypomagnesemia/Arrhythmia:** 25–50 mg/kg/dose IV; may repeat every 6 hours as needed.
*(Note: Strict weight-based dosing required. Confirm local pediatric protocol as guidelines vary widely.)*
## Dose Adjustments
* **Renal Impairment:** Reduce dosage by 50% in patients with severe renal failure (CrCl <30 mL/min). Monitor magnesium levels and clinical signs of toxicity frequently. Avoid or use with extreme caution in end-stage renal disease (ESRD).
## Contraindications
* Heart block (unless pacemaker present).
* Myocardial damage.
* Chronic renal failure (except in life-threatening scenarios with rigorous monitoring).
## Adverse Effects
* **Early signs of toxicity:** Loss of deep tendon reflexes (patellar reflex), hypotension, flushing, diaphoresis.
* **Severe toxicity:** Respiratory depression, cardiac conduction abnormalities (prolonged PR/QRS), respiratory arrest, cardiac arrest.
* **Antidote:** Calcium gluconate 1g IV for magnesium toxicity/respiratory depression.
## Key Drug Interactions
* **Neuromuscular Blocking Agents:** Potentiates neuromuscular blockade.
* **CNS Depressants:** May increase sedative effects.
* **Digoxin:** May increase risk of digitalis toxicity; monitor carefully.
* **Aminoglycosides:** Potential for additive neuromuscular blockade.
## Monitoring
* **Clinical:** Check patellar reflexes (absence is a harbinger of respiratory depression), respiratory rate (must be ≥12/min), and urine output (≥30 mL/hr).
* **Laboratory:** Serum magnesium concentrations. Target for eclampsia is typically 4.8–8.4 mg/dL (2.0–3.5 mmol/L).
* **Cardiac:** ECG monitoring mandatory for rapid administration (Torsades) or large loading doses.
## Clinical Pearls
* **Administration:** Concentrated IV magnesium (e.g., 50%) must be diluted to 10–20% concentration for peripheral infusion to avoid vein irritation. Use an infusion pump for all continuous rates.
* **Eclampsia:** Always keep Calcium Gluconate (1g ampule) at the bedside when infusing magnesium.
* **Correction:** Magnesium levels can be deceptive as they do not reflect intracellular stores. Repletion should continue for 24–48 hours after symptoms resolve.
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**Educational Disclaimer:** This information is intended for educational purposes for healthcare professionals. Dosing, specific protocols, and safety requirements may vary significantly by institution. Always verify prescribing information against current institutional policy and authoritative drug references (e.g., Lexicomp, Micromedex) before administration.
