Magnesium Sulfate Dosing for Asthma Exacerbations
For severe asthma exacerbations not responding to conventional therapy, intravenous magnesium sulfate should be administered at a dose of 2 grams over 20 minutes in adults. 1
Indications for IV Magnesium Sulfate
Magnesium sulfate is not indicated for all asthma exacerbations but should be considered in specific scenarios:
- Life-threatening asthma exacerbations
- Severe exacerbations that remain severe after 1 hour of intensive conventional treatment
- Patients with FEV1 < 25-30% predicted despite standard therapy
- Impending respiratory failure
IV magnesium sulfate has no apparent value in patients with exacerbations of lower severity 1.
Dosing Recommendations
Adults:
- Standard dose: 2 grams IV administered over 20 minutes 1
- This is a one-time dose, not to be repeated routinely
Children:
- 25-50 mg/kg (maximum 2 grams) IV over 20 minutes 2
Administration Protocol
Ensure patient is receiving standard asthma therapy first:
- Oxygen
- Inhaled short-acting β2-agonists (albuterol)
- Systemic corticosteroids
- Consider ipratropium bromide
If severe symptoms persist after 1 hour of intensive treatment, administer magnesium sulfate:
- Dilute in 50-100 mL normal saline
- Infuse over 20 minutes
- Monitor vital signs and oxygen saturation during infusion
Common side effects to monitor for:
- Flushing
- Light-headedness
- Hypotension
- Fatigue
- Nausea
- Headache
Clinical Evidence and Efficacy
The evidence strongly supports the use of IV magnesium sulfate in severe asthma:
A Cochrane meta-analysis concluded that IV magnesium sulfate improves pulmonary function and reduces hospital admissions, particularly for patients with the most severe exacerbations of asthma 1.
In patients with severe asthma (FEV1 < 25% predicted), IV magnesium significantly reduced admission rates from 78.6% to 33.3% compared to placebo 3.
IV magnesium has been shown to reduce hospital admissions by approximately 7 admissions for every 100 adults treated 4.
Important Considerations
Magnesium causes relaxation of bronchial smooth muscle independent of serum magnesium level 1.
While IV magnesium is effective, inhaled magnesium sulfate has shown less convincing evidence of benefit 5.
Calcium chloride should be available to reverse potential magnesium toxicity if needed 2.
The selective use of intravenous magnesium sulfate has been adopted by many academic emergency departments for severe asthma 1.
In cases of impending respiratory failure, rapid infusion of magnesium (2g over 2 minutes) has been reported to dramatically reverse severe bronchospasm and prevent intubation, though this is not the standard recommended administration rate 6.
IV magnesium sulfate represents an important adjunctive therapy for severe asthma exacerbations that don't respond to conventional treatments, with a well-established safety profile and demonstrated efficacy in reducing hospitalization rates and improving lung function.