## α₁-Selective Adrenergic Agonists **Key Point:** Phenylephrine is a selective α₁-adrenergic agonist with minimal β-receptor activity, making it ideal for topical nasal decongestant use. ### Mechanism of Action Phenylephrine binds preferentially to α₁-adrenergic receptors on vascular smooth muscle, causing vasoconstriction without significant cardiac stimulation (β₁ effects). ### Clinical Applications - Nasal decongestant (topical spray, oral formulations) - Mydriasis in ophthalmology - Hypotension management in perioperative settings - Parenteral use for acute hypotension ### Selectivity Comparison Table | Drug | α₁ | α₂ | β₁ | β₂ | Clinical Use | | --- | --- | --- | --- | --- | --- | | **Phenylephrine** | +++++ | − | − | − | Nasal decongestant, mydriasis | | Epinephrine | ++++ | ++++ | +++++ | +++++ | Anaphylaxis, cardiac arrest | | Isoproterenol | − | − | +++++ | +++++ | Bronchospasm, bradycardia | | Dobutamine | + | − | +++++ | + | Cardiogenic shock | **High-Yield:** Phenylephrine's pure α₁ selectivity means it causes **reflex bradycardia** (baroreceptor-mediated) without direct β₁ stimulation of the heart — a key distinguishing feature from epinephrine. **Clinical Pearl:** Phenylephrine is preferred over ephedrine for nasal congestion because it lacks the CNS stimulation and systemic effects of ephedrine, making it safer for prolonged use.
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.