Why is magnesium given rapidly in asthma exacerbation?

Magnesium Administration in Asthma Exacerbations

Intravenous magnesium sulfate should be administered over 20 minutes (not faster) in severe asthma exacerbations to ensure both efficacy and safety. 1

Recommended Dosing and Administration

• For adults: 2g IV over 20 minutes 1
• For pediatric patients: 25-50 mg/kg (maximum 2g) over 10-20 minutes 1
• For COPD exacerbations: 1.2-2g over 20 minutes 1

Mechanism and Efficacy

Magnesium sulfate works as a bronchodilator in acute severe asthma through several mechanisms:

• Relaxes smooth muscle in the airways
• Decreases neuromuscular excitability
• Modulates calcium concentration in smooth muscle cells 2

The timing of administration (over 20 minutes) is critical because:

1. It allows for optimal therapeutic serum levels to be achieved
2. It minimizes the risk of adverse effects associated with rapid infusion
3. It provides sufficient time to monitor for potential toxicity 1

Evidence for Effectiveness

Intravenous magnesium sulfate has demonstrated significant benefits in severe asthma exacerbations:

• Reduces hospitalization rates 1, 3
• Improves pulmonary function 3, 4
• Provides rapid bronchodilation comparable to albuterol in some studies 4

It's important to note that while IV magnesium is effective for acute exacerbations, it does not appear beneficial in chronic stable asthma 5.

Monitoring During Administration

During the 20-minute infusion period, careful monitoring is essential:

• Vital signs (blood pressure, heart rate, oxygen saturation)
• Deep tendon reflexes
• Respiratory function
• Signs of magnesium toxicity (flushing, sweating, hypotension, respiratory depression) 1

Important Considerations and Precautions

• Calcium salts should be available to counteract potential magnesium toxicity 1
• Use with caution in patients with renal insufficiency (maximum 20g/48 hours with frequent monitoring) 1
• Exercise caution in patients on digitalis due to potential cardiac conduction changes 1
• Monitor for excessive neuromuscular blockade in patients receiving neuromuscular blocking agents 1

Clinical Application

The American Academy of Allergy, Asthma, and Immunology recommends magnesium sulfate as part of a comprehensive treatment protocol for severe asthma exacerbations that includes:

• High-flow oxygen
• Short-acting beta-agonists
• Systemic corticosteroids
• Ipratropium bromide 1

While nebulized magnesium has been studied, intravenous administration has more consistent evidence supporting its efficacy 3, 6.