Why is terbutaline preferred over epinephrine (adrenaline) intramuscularly (IM) for acute Chronic Obstructive Pulmonary Disease (COPD) exacerbation?

Terbutaline vs. Epinephrine IM in Acute COPD Exacerbation

Terbutaline is preferred over epinephrine IM for acute COPD exacerbations because it has greater β2-receptor selectivity, resulting in more effective bronchodilation with fewer cardiovascular side effects.

Pharmacological Rationale

• β2-receptor selectivity: Terbutaline has a preferential effect on β2-adrenergic receptors, which are predominant in bronchial smooth muscle, while having less effect on β1-receptors in the heart 1
• Duration of action: Terbutaline has a longer duration of action compared to epinephrine, providing more sustained bronchodilation 2
• Cardiovascular safety profile: While both medications can increase heart rate, terbutaline has fewer overall cardiovascular effects due to its greater β2-selectivity 1, 2

Efficacy in COPD Exacerbations

According to European Respiratory Society guidelines, β2-agonists like terbutaline (5-10 mg) are recommended first-line treatments for acute COPD exacerbations 3. The British Thoracic Society specifically recommends terbutaline 5-10 mg as an appropriate bronchodilator for moderate exacerbations 3.

Studies have demonstrated that terbutaline:

• Produces more pronounced increases in lung function parameters (FVC, FEV1, MEFR, MMEF) than epinephrine 2
• Has an earlier onset and longer duration of action than other sympathomimetics 4
• Improves both right and left ventricular function in COPD patients, which may contribute to clinical improvement 5

Administration Considerations

For acute COPD exacerbations:

• Nebulized terbutaline (5-10 mg) is the preferred delivery method 3
• Nebulization can be repeated every 20 minutes for the first hour and then every 4 hours as needed 6
• Nebulizers should ideally be driven by compressed air rather than oxygen in COPD patients with elevated PaCO2 or respiratory acidosis 3

Safety Considerations

Important safety advantages of terbutaline over epinephrine include:

1. Reduced risk of oxygen desaturation: β2-agonists may cause a fall in PaO2 due to pulmonary vascular effects, but this risk is greater with less selective agents like epinephrine 3

2. Cardiovascular safety: While both medications can increase heart rate, terbutaline's greater β2-selectivity results in proportionally greater changes in pulmonary function parameters than in heart rate or blood pressure 1

3. Fewer systemic side effects: The more selective β2-action of terbutaline results in fewer overall side effects compared to epinephrine 2

Clinical Approach

For acute COPD exacerbations:

1. Start with nebulized terbutaline 5-10 mg
2. Consider adding an anticholinergic agent (ipratropium bromide 0.25-0.5 mg) for severe exacerbations or if response to terbutaline alone is poor 3
3. Continue nebulized bronchodilators for 24-48 hours or until clinical improvement 3
4. Switch to metered-dose inhalers or dry powder inhalers once the patient stabilizes 3

Potential Pitfalls

• Monitor for tachycardia and other cardiovascular effects, especially in patients with pre-existing cardiac disease
• Be aware that concurrent use of methylxanthines (like theophylline) with β-agonists may increase the risk of cardiac arrhythmias 1
• Consider driving nebulizers with compressed air rather than oxygen in patients with elevated PaCO2 or respiratory acidosis to prevent worsening hypercapnia 3

In summary, terbutaline's greater β2-selectivity, longer duration of action, and better safety profile make it a preferred choice over epinephrine IM for acute COPD exacerbations.