A 52-year-old man with hypertension is started on a non-selective beta-blocker. He develops severe bronchoconstriction within hours. Which autonomic feature best explains why a non-selective beta-blocker causes this adverse effect, in contrast to a selective β₁-blocker?

Explanation

## Non-Selective Beta-Blocker-Induced Bronchoconstriction ### Mechanism of Adverse Effect **Key Point:** β₂-adrenergic receptors on bronchial smooth muscle mediate bronchodilation. Non-selective beta-blockers block both β₁ (cardiac) and β₂ (bronchial) receptors, removing the tonic bronchodilatory effect of sympathetic noradrenaline. ### Autonomic Control of Airway Caliber | Receptor | Location | Agonist Effect | Antagonist Effect | |----------|----------|----------------|-------------------| | **β₂** | Bronchial smooth muscle | Relaxation → Bronchodilation | Contraction → Bronchoconstriction | | **M3** | Bronchial smooth muscle | Contraction → Bronchoconstriction | Relaxation → Bronchodilation | | **β₁** | Heart (SA node, ventricle) | Increased HR, contractility | Decreased HR, contractility | ### Why Non-Selective Beta-Blockers Cause Bronchoconstriction **High-Yield:** The lungs have **minimal sympathetic innervation** but **robust parasympathetic (vagal) innervation**. Normally, tonic β₂-mediated bronchodilation from circulating catecholamines balances parasympathetic bronchoconstriction. Non-selective beta-blockers remove this balance. **Clinical Pearl:** In asthma or COPD patients, the loss of β₂-mediated bronchodilation tips the scale toward unopposed parasympathetic bronchoconstriction, triggering severe airway obstruction. ### Pathophysiology Flowchart ```mermaid flowchart TD A[Normal airway tone]:::outcome --> B[β₂ agonism from noradrenaline]:::action A --> C[M3 antagonism from lack of ACh]:::action B --> D[Bronchial smooth muscle relaxation]:::action C --> D D --> E[Patent airways]:::outcome F[Non-selective beta-blocker given]:::action --> G[β₂ blockade]:::urgent G --> H[Loss of bronchodilation]:::urgent H --> I[Unopposed M3 parasympathetic tone]:::urgent I --> J[Bronchial smooth muscle contraction]:::urgent J --> K[Bronchoconstriction & airway obstruction]:::urgent ``` **Mnemonic:** **β₂ = Bronchi** (and also **B**lood vessels, **B**ladder). Blocking β₂ causes bronchoconstriction and vasoconstriction. ### Selective β₁-Blockers Are Safe **Key Point:** Selective β₁-blockers (metoprolol, atenolol, bisoprolol) at therapeutic doses preferentially block cardiac β₁ receptors and spare β₂ receptors on airways. Bronchial β₂-mediated dilation is preserved, preventing bronchoconstriction. **Clinical Pearl:** In patients with asthma, COPD, or reactive airway disease, always use a **cardioselective (β₁-selective) beta-blocker** or avoid beta-blockers entirely.