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# Magnesium sulfate ## Overview - An inorganic magnesium salt used IV, IM, and orally depending on formulation and indication. - Doses may be expressed as **magnesium sulfate**, **magnesium**, or **mmol of elemental magnesium**; verify the product concentration. - Approximate conversion for **magnesium sulfate heptahydrate**: - **1 g magnesium sulfate ≈ 4 mmol elemental magnesium** - **1 g ≈ 98.6 mg elemental magnesium** - A **50% injection (500 mg/mL)** contains approximately **2 mmol/mL**. - IV administration should use the prescribed dilution and infusion rate. Rapid IV administration increases toxicity risk. ## Primary Indications - Prevention and treatment of seizures in: - Severe preeclampsia - Eclampsia - Magnesium deficiency or symptomatic hypomagnesemia - Torsades de pointes or suspected magnesium-related ventricular arrhythmia - Severe acute asthma refractory to initial bronchodilator and corticosteroid therapy - Selected obstetric, neonatal, and critical-care protocols ## Adult Dosing ### Severe preeclampsia/eclampsia - **IV loading:** 4–6 g IV over 15–30 minutes. - **Maintenance:** 1–2 g/hour by continuous IV infusion. - **Recurrent seizure:** 2–4 g IV over 5–15 minutes; follow local obstetric protocol. - If IV access is unavailable, some protocols use **10 g IM** initially, divided as 5 g into each buttock, followed by **5 g IM every 4 hours** if reflexes, respirations, and urine output remain adequate. - Continue commonly for **24 hours after delivery or the last seizure**, according to local protocol. - There is no universal maximum total dose; dosing must be guided by clinical monitoring and renal function. ### Hypomagnesemia Dose depends on severity, symptoms, renal function, and serum magnesium: - **Mild/asymptomatic:** oral magnesium may be used if gastrointestinal tolerance permits. - **Symptomatic, severe, or unable to take oral therapy:** commonly **1–2 g IV over 15–60 minutes**. - Severe depletion may require **4–8 g IV over 12–24 hours**, with repeat dosing based on magnesium levels and clinical response. - For unstable or life-threatening manifestations, use a monitored IV regimen according to institutional protocol. ### Torsades de pointes - **1–2 g IV** diluted and administered over approximately **5–20 minutes**. - For cardiac arrest or pulseless torsades, follow advanced life-support protocol; rapid IV/IO administration may be used. - Continuous ECG monitoring is required. ### Severe acute asthma - **2 g IV over 20 minutes** as a single dose, generally after standard emergency therapy has been initiated. - Routine repeated doses are not established and should follow specialist or local protocol. ## Pediatric Dosing ### Hypomagnesemia - **25–50 mg/kg IV magnesium sulfate** ≈ **0.1–0.2 mmol/kg elemental magnesium**. - Maximum commonly used single dose: **2 g magnesium sulfate**. - Administer over at least **2 hours** when clinically appropriate; faster administration may be used for severe instability under monitored conditions. ### Torsades de pointes or life-threatening ventricular arrhythmia - **25–50 mg/kg IV/IO**, maximum **2 g**. - Administer according to pediatric resuscitation protocol and cardiac rhythm. ### Severe acute asthma - **25–75 mg/kg IV over 20 minutes**, maximum **2 g**. - Use as an adjunct for severe or refractory exacerbation; routine use in mild/moderate asthma is not recommended. ### Neonates - Dosing varies substantially by gestational age, indication, serum concentration, and neonatal protocol. Use a neonatal formulary or specialist-guided regimen rather than extrapolating adult doses. ## Dose Adjustments - **Renal impairment:** reduce dose and/or extend dosing interval; avoid routine maintenance infusions in severe renal failure unless specialist-directed. - In severe renal impairment, consider substantially reducing the dose and monitor serum magnesium frequently. - Avoid or use extreme caution in **oliguria/anuria**, because magnesium accumulation can cause respiratory depression, hypotension, and cardiac arrest. - **Elderly patients:** use conservative dosing because renal clearance may be reduced. - Adjust dosing based on: - Serum magnesium - Urine output - Respiratory rate - Deep-tendon reflexes - Blood pressure and clinical status ## Contraindications - Hypersensitivity to magnesium sulfate - Significant hypermagnesemia - Severe renal failure or anuria unless specifically directed by a specialist - Heart block or clinically significant myocardial conduction disease, unless managed in a monitored specialist setting - **Myasthenia gravis:** generally avoid or use only with specialist oversight because neuromuscular blockade may worsen - Caution with respiratory depression, hypotension, or concomitant neuromuscular-blocking therapy ## Adverse Effects ### Common or dose-related - Flushing, warmth, sweating - Nausea and vomiting - Headache - Drowsiness - Muscle weakness - Hypotension - Bradycardia - Injection-site pain, particularly with IM administration ### Toxicity - Reduced or absent deep-tendon reflexes - Respiratory depression or apnea - Severe hypotension - Bradyarrhythmias, conduction delay, or cardiac arrest - Paralysis - Coma ### Management of significant toxicity - Stop magnesium administration. - Provide airway and ventilatory support. - Administer **calcium gluconate 10%: 10 mL IV (1 g) over 5–10 minutes in adults**, or **100 mg/kg IV, maximum 1 g, in children**, with repeat dosing guided by clinical response and local protocol. - Calcium chloride may be used in resuscitation settings via an appropriate IV route. ## Key Drug Interactions - **Neuromuscular blockers:** additive or prolonged neuromuscular blockade. - **CNS depressants, opioids, and sedatives:** increased respiratory and CNS depression. - **Calcium-channel blockers and other antihypertensives:** additive hypotension or bradycardia; clinically important interaction is particularly relevant in obstetric patients. - **Digoxin and other cardiac glycosides:** potential conduction and rhythm effects; monitor closely. - **Oral tetracyclines, fluoroquinolones, levothyroxine, and bisphosphonates:** reduced absorption. Separate oral administration by at least **2–4 hours**, according to the interacting drug’s labeling. - Other magnesium-, calcium-, or aluminum-containing products may increase total electrolyte exposure. ## Monitoring - Serum magnesium, especially with repeated doses, infusion therapy, renal impairment, or toxicity risk. - Respiratory rate and oxygenation; hold therapy if respiratory depression develops. - Deep-tendon reflexes during obstetric infusions; absent reflexes suggest toxicity. - Urine output; in obstetric use, commonly require at least **25–30 mL/hour** or approximately **100 mL over 4 hours**, depending on protocol. - Blood pressure, heart rate, ECG, and level of consciousness. - Serum calcium, potassium, and renal function when clinically indicated. - Keep injectable calcium immediately available during IV magnesium therapy for eclampsia or high-dose treatment. ## Clinical Pearls - Confirm whether the prescribed dose refers to **grams of magnesium sulfate** or **grams/mmol of elemental magnesium**. - Do not give concentrated magnesium sulfate IV undiluted unless the product labeling and emergency protocol specifically permit it. - In eclampsia, magnesium sulfate is preferred for seizure prevention and treatment; benzodiazepines or phenytoin are generally alternatives when magnesium is contraindicated or ineffective. - A therapeutic serum magnesium range is protocol-dependent; clinical assessment is essential because toxicity can occur with rising levels, particularly in renal impairment. - Magnesium sulfate does not replace urgent correction of other causes of torsades, such as hypokalemia or QT-prolonging drugs. - For asthma, magnesium is an adjunct—not a substitute for oxygen, inhaled short-acting bronchodilators, anticholinergics, corticosteroids, and escalation of respiratory support when needed. - Verify the vial concentration, dilution, infusion rate, and local maximum dose before administration. *Educational information only; verify current prescribing information, product concentration, institutional protocols, and patient-specific dosing before use.*