What is the recommended dose of magnesium sulphate (MgSO4) for various medical conditions?

Magnesium Sulfate Dosing by Clinical Indication

For acute severe asthma in adults, administer 2 g IV magnesium sulfate over 20 minutes as adjunctive therapy when patients remain severe after 1 hour of intensive standard treatment; for preeclampsia/eclampsia, use a loading dose of 4-5 g IV over 20-30 minutes followed by 1-2 g/hour maintenance infusion. 1, 2

Acute Severe Asthma

Adult Dosing

• Administer 2 g IV magnesium sulfate diluted to 20% or less concentration over 20 minutes for patients with severe exacerbations (FEV1 <30% predicted) who have not responded adequately to standard therapy (oxygen, nebulized short-acting beta-agonists, and IV corticosteroids) 1, 3, 2
• The greatest benefit occurs in patients with FEV1 <20% predicted, where magnesium significantly improves lung function and reduces pulse rate at 4 hours 4
• A Cochrane meta-analysis demonstrated that IV magnesium reduces hospital admissions by approximately 7 per 100 patients treated (OR 0.75,95% CI 0.60-0.92) and improves spirometric indices 5

Pediatric Dosing

• Administer 25-75 mg/kg IV (maximum 2 g) over 20 minutes for children with life-threatening asthma exacerbations or those remaining severe after 1 hour of intensive conventional treatment 6
• Monitor for hypotension during administration, as rapid infusion may cause hypotension and bradycardia 6
• Have calcium chloride immediately available to counteract potential magnesium toxicity 1, 6

Important Considerations for Asthma

• Magnesium sulfate should be used as an adjunct to standard therapy, not as a replacement for inhaled beta-agonists, anticholinergics, and systemic corticosteroids 6, 3
• The mechanism involves bronchial smooth muscle relaxation independent of serum magnesium level, providing complementary bronchodilation 6, 3
• Common side effects include flushing, light-headedness, and hypotension, but the safety profile is generally favorable 3, 5

Preeclampsia/Eclampsia

Loading Dose

• Administer 4-5 g IV magnesium sulfate in 250 mL of 5% dextrose or 0.9% normal saline infused over 20-30 minutes 1, 2
• Alternatively, give 4 g IV by diluting the 50% solution to 10-20% concentration and inject over 3-4 minutes 2
• Simultaneously, IM doses of up to 10 g (5 g in each buttock using undiluted 50% solution) may be given for total initial dose of 10-14 g 2

Maintenance Infusion

• Continue with 1-2 g/hour by constant IV infusion after the loading dose 1, 2
• Alternatively, inject 4-5 g IM into alternate buttocks every 4 hours as needed, depending on presence of patellar reflex and adequate respiratory function 2
• Target serum magnesium level of 6 mg/100 mL is considered optimal for seizure control 2

Duration and Safety

• Continue magnesium sulfate for 24 hours postpartum as the standard recommendation 1
• Alternative approach: May discontinue after administering at least 8 g predelivery in select populations, though this requires consideration of local postpartum eclampsia incidence 1
• Do not exceed total daily dose of 30-40 g in 24 hours 2
• In severe renal insufficiency, maximum dosage is 20 g/48 hours with frequent serum magnesium monitoring 2
• Do not continue beyond 5-7 days in pregnancy as continuous maternal administration can cause fetal abnormalities 2

Other Indications

Torsades de Pointes

• Administer 1-2 g IV over 15 minutes for polymorphic VT associated with QT prolongation, regardless of serum magnesium level 1
• Have calcium immediately available to counteract magnesium toxicity 1

Magnesium Deficiency

• Mild deficiency: 1 g (8.12 mEq) IM every 6 hours for 4 doses (total 32.5 mEq/24 hours) 2
• Severe hypomagnesemia: Up to 250 mg/kg (approximately 2 mEq/kg) IM within 4 hours, or 5 g (40 mEq) added to 1 L IV fluid infused over 3 hours 2
• For acute hypomagnesemia: 1-2 g IV over 15 minutes, followed by maintenance infusion of 1 g/hour for 24 hours if needed 1

Other Uses

• Barium poisoning: 1-2 g IV 2
• Seizures (epilepsy, glomerulonephritis, hypothyroidism): 1 g IM or IV 2
• Paroxysmal atrial tachycardia: 3-4 g IV over 30 seconds with extreme caution, only if simpler measures have failed and no myocardial damage 2
• Cerebral edema: 2.5 g IV 2

Critical Administration Guidelines

Dilution Requirements

• Solutions for IV infusion must be diluted to 20% or less concentration prior to administration 2
• Common diluents are 5% dextrose or 0.9% normal saline 2
• For IM injection in children, dilute to 20% or less concentration (undiluted 50% solution appropriate only for adults) 2

Rate of Administration

• Do not exceed 150 mg/minute IV (1.5 mL of 10% concentration) except in severe eclampsia with seizures 2
• IM administration of undiluted 50% solution produces therapeutic levels in 60 minutes, whereas IV provides therapeutic levels almost immediately 2

Monitoring Requirements

• Monitor patellar reflex, respiratory function, and urine output during therapy 2
• Obtain frequent serum magnesium concentrations in renal insufficiency 2
• Observe for hypotension, bradycardia, flushing, and respiratory depression 1, 6, 5

Common Pitfalls

• Do not use magnesium as monotherapy for asthma—it must be combined with standard bronchodilators and corticosteroids 6, 3
• Avoid rapid IV infusion as this increases risk of hypotension and bradycardia 6
• Do not exceed renal excretory capacity in deficiency states—adjust dose in renal insufficiency 2
• Be aware of drug interactions: Magnesium may reduce antibiotic activity of streptomycin, tetracycline, and tobramycin when given together 2
• Nebulized magnesium is less effective than IV for acute asthma, though it may provide modest additional benefit as adjunctive therapy 6, 3, 7, 8