A 38-year-old woman with asthma and exercise-induced bronchoconstriction requires a selective β2-adrenergic agonist for acute symptom relief. Which of the following agents is the drug of choice for rapid bronchodilation?

Explanation

## β2-Selective Agonists in Asthma: Receptor Selectivity and Clinical Use **Key Point:** Salbutamol (albuterol) is a selective β2-adrenergic agonist that produces rapid bronchodilation with minimal cardiac (β1) stimulation, making it the gold-standard rescue therapy for acute asthma exacerbations and exercise-induced bronchoconstriction. ### β-Agonist Selectivity and Cardiac Effects | Drug | β1 Selectivity | β2 Selectivity | Cardiac Effects | Onset | Duration | Use in Asthma | |------|----------------|----------------|-----------------|-------|----------|---------------| | **Salbutamol** | Minimal | High | Minimal tachycardia | 5–15 min (inhaled) | 4–6 hours | **First-line rescue** | | Isoproterenol | Non-selective | Non-selective | Severe tachycardia, arrhythmias | Rapid | Short | Obsolete in asthma | | Dobutamine | β1 > β2 | Minimal | Marked tachycardia, ↑ BP | Rapid | Short | Cardiac support only | | Epinephrine | α, β1, β2 | Non-selective | Severe hypertension, tachycardia | Very rapid | Very short | Anaphylaxis only | **High-Yield:** Salbutamol's β2-selectivity (especially at therapeutic doses) means it relaxes bronchial smooth muscle without significant β1-mediated cardiac stimulation, making it safe and effective for repeated use in asthma. ### Pharmacological Basis 1. **Mechanism:** Activation of β2-adrenergic receptors on airway smooth muscle → ↑ cAMP → smooth muscle relaxation and bronchodilation 2. **β2-selectivity:** At clinical doses, salbutamol preferentially binds β2 receptors; β1 effects (tachycardia, increased contractility) are minimal 3. **Rapid onset:** Inhaled route achieves peak effect in 5–15 minutes; oral form takes 30–60 minutes 4. **Duration:** 4–6 hours (allows 4–6 hourly dosing) **Clinical Pearl:** Salbutamol is used as a rescue inhaler (metered-dose inhaler, MDI) for acute symptoms and can be used before exercise to prevent exercise-induced bronchoconstriction. Overuse (>2 days/week) signals need for controller therapy (inhaled corticosteroids). **Mnemonic:** **SABA** = **S**elective **A**drenergic **B**ronchodilator **A**gent (salbutamol, terbutaline) — emphasizes selectivity for β2. **Tip:** When a question asks for "rapid" bronchodilation in asthma with minimal cardiac side effects, think salbutamol — it is the only option that combines β2-selectivity with fast onset.