Magnesium Sulfate Dosing for Severe Asthma
For adults with severe asthma exacerbations, administer 2g of intravenous magnesium sulfate over 20 minutes; for children, use 25-75 mg/kg IV (maximum 2g) over 20 minutes. 1, 2
When to Administer Magnesium Sulfate
Magnesium sulfate should be given to patients with life-threatening asthma exacerbations or those whose exacerbations remain severe after 1 hour of intensive conventional treatment with inhaled β2-agonists, anticholinergics, and systemic corticosteroids. 1, 2
The evidence strongly supports this timing:
- Multiple major guidelines (American Academy of Allergy, Asthma, and Immunology, American Heart Association, British Thoracic Society) recommend IV magnesium sulfate specifically for severe exacerbations 1
- Nine out of 16 international guidelines suggest IV magnesium sulfate for severe asthma exacerbations 3
- The greatest benefit occurs in patients with FEV1 <20% predicted or initial FEV1 <25% predicted 1, 4
Specific Dosing Protocols
Adults
- Standard dose: 2g IV over 20 minutes 1
- Administer as a single bolus; continuous infusion has not shown additional benefit 5
- Dilute to 20% or less concentration before administration 6
Pediatric Patients
- Dose: 25-75 mg/kg IV (maximum 2g) over 20 minutes 2
- The wide dosing range allows for severity-based adjustment, though 50 mg/kg is commonly used 2
Critical Implementation Points
Magnesium sulfate must be used as an adjunct to standard therapy, never as a replacement. 1, 2 Standard therapy includes:
- Inhaled short-acting β2-agonists (albuterol 2.5-5 mg nebulized every 20 minutes for 3 doses) 1
- Anticholinergics (ipratropium bromide) 1
- Systemic corticosteroids (methylprednisolone 1-2 mg/kg IV or prednisolone 1-2 mg/kg orally) 3, 1
- Oxygen supplementation targeting saturation 92-95% 3, 1
Evidence Supporting This Approach
The recommendation is based on high-quality evidence:
- A Cochrane meta-analysis demonstrated that IV magnesium sulfate improves pulmonary function and reduces hospital admissions in severe exacerbations 1, 7
- In patients with severe asthma (baseline FEV1 <25% predicted), admission rates dropped from 78.6% to 33.3% with magnesium sulfate 4
- Peak expiratory flow rate improved by 52.3 L/min in severe cases 7
However, magnesium sulfate does NOT benefit patients with moderate asthma exacerbations - admission rates and pulmonary function improvements were not significant in this population 4. This is why the 1-hour reassessment after intensive conventional treatment is critical for appropriate patient selection.
Safety Considerations
Monitor for hypotension and bradycardia during administration, particularly if infused too rapidly. 2, 6
Common side effects include:
- Flushing and light-headedness (minor and well-tolerated) 1, 6
- Hypotension if administered too rapidly 2, 6
Have calcium chloride immediately available to counteract potential magnesium toxicity. 2, 6
Alternative Route: Nebulized Magnesium Sulfate
Inhaled magnesium sulfate (3 ml of 260 mmol/L solution every 20-60 minutes) has been proposed for mild to moderate exacerbations 3, 2, 8, but nebulized magnesium is less effective than IV administration and should not be used as a substitute in severe cases 1, 2. One study showed reduced hospitalization rates (44% vs 72%) with nebulized magnesium in moderate to severe asthma 8, but this remains a secondary option compared to IV administration for severe exacerbations.
Common Pitfalls to Avoid
- Do not use magnesium sulfate as first-line therapy - it is only indicated after 1 hour of intensive conventional treatment fails 1, 2
- Do not use continuous infusion protocols - a single 2g bolus over 20 minutes is the evidence-based approach; one study showed no benefit from adding a 2g/hour infusion after the initial bolus 5
- Do not administer to patients with only moderate exacerbations - these patients do not benefit and should continue with standard therapy alone 4
- Do not delay standard therapy to give magnesium - it is an adjunct only 1, 2