What is the recommended dose of magnesium sulphate (MgSO4) for severe asthma?

Magnesium Sulphate Dosing in Severe Asthma

For adults with severe asthma exacerbations, administer 2g of intravenous magnesium sulphate over 20 minutes as an adjunct to standard therapy (inhaled β2-agonists, anticholinergics, and systemic corticosteroids). 1

Indications for Use

Magnesium sulphate should be reserved for patients with life-threatening exacerbations or those whose exacerbations remain severe after 1 hour of intensive conventional treatment. 1

• The greatest benefit occurs in patients with FEV1 <25% predicted or those presenting with severe baseline impairment 1
• Multiple major guidelines including the American Academy of Allergy, Asthma, and Immunology, American Heart Association, and British Thoracic Society recommend IV magnesium sulphate specifically for severe asthma exacerbations 1
• Nine out of 16 international pediatric guidelines support the use of IV magnesium sulfate for severe exacerbations 2

Dosing Recommendations

Adults

• Standard dose: 2g IV administered over 20 minutes 1
• This represents unanimous consensus across major guidelines for adult dosing 2, 1

Pediatric Patients

• Dose: 25-75 mg/kg IV (maximum 2g) administered over 20 minutes 3
• The National Asthma Education and Prevention Program Expert Panel endorses this pediatric dosing range 3

Administration Protocol

Magnesium sulphate must be used as an adjunct to standard therapy, never as a replacement. 1

• Administer after ensuring the patient has received at least 1 hour of intensive conventional treatment including inhaled short-acting β2-agonists (albuterol 2.5-5 mg nebulized every 20 minutes for 3 doses), anticholinergics (ipratropium), and systemic corticosteroids 1
• Infuse over the full 20-minute period to avoid hypotension and bradycardia 3
• Have calcium chloride available to reverse potential magnesium toxicity if needed 3

Evidence Supporting Efficacy

The evidence strongly favors IV magnesium sulphate in severe cases:

• A Cochrane meta-analysis demonstrated that IV magnesium sulphate improves pulmonary function and reduces hospital admissions, particularly in patients with the most severe exacerbations 1
• In patients with severe asthma (FEV1 <25% predicted), admission rates dropped from 78.6% with placebo to 33.3% with magnesium sulphate (p=0.009) 4
• IV magnesium increases FEV1 % predicted at 4 hours and lowers pulse rate at 240 minutes compared to placebo 1

Critical Caveat: Severity Matters

Magnesium sulphate is ineffective in moderate asthma exacerbations. 4

• In patients with moderate asthma (baseline FEV1 25-75% predicted), there was no significant difference in admission rates (22.4% placebo vs 22.2% magnesium, p=0.98) or FEV1 improvement 4
• One study even showed less improvement in peak flow with magnesium in moderately severe asthmatics (174 L/min vs 212 L/min placebo, p=0.04) 5
• This underscores the importance of restricting use to truly severe exacerbations 6

Nebulized Magnesium Sulphate

Nebulized magnesium sulphate is less effective than IV administration and should not be routinely used. 1, 6

• While one guideline proposed inhaled magnesium for mild and moderate exacerbations, the evidence is less convincing 2, 1
• If used, the dose is 3 mL of 260 mmol/L solution every 20-60 minutes via nebulizer 1, 7
• Inhaled magnesium performs no better than salbutamol alone and shows no apparent synergy when combined 6

Safety Profile

• Minor side effects include flushing and light-headedness 1
• Rapid infusion may cause hypotension and bradycardia, emphasizing the importance of the 20-minute infusion time 3
• Overall safety profile is favorable with high-quality evidence 1

Mechanism of Action

• Magnesium causes bronchial smooth muscle relaxation independent of serum magnesium level, providing a complementary bronchodilator effect to β2-agonists 1, 3
• This mechanism explains why it works as an adjunct rather than a replacement for standard therapy 1