A 52-year-old man with hypertension and asthma is prescribed a beta blocker. He develops severe bronchospasm within 48 hours. Which feature of the beta blocker used best explains this adverse effect compared to an alternative agent?

Explanation

## Beta Blockers and Bronchospasm: The Cardioselectivity Principle ### Mechanism of Beta-2 Blockade in Airways **Key Point:** Non-selective β-blockers (propranolol, nadolol, timolol) block both β1 (cardiac) and β2 (bronchial) receptors. β2 blockade in airway smooth muscle removes the bronchodilatory effect of endogenous catecholamines, leading to unopposed parasympathetic (cholinergic) tone and bronchoconstriction. **High-Yield:** Cardioselective β-blockers (metoprolol, atenolol, bisoprolol) preferentially block β1 receptors at therapeutic doses, sparing β2 receptors in the lungs. This makes them safer in asthma and COPD. ### Pathophysiology of Bronchospasm ```mermaid flowchart TD A[Non-selective β-blocker administered]:::action --> B[β1 blockade in heart]:::outcome A --> C[β2 blockade in airways]:::outcome C --> D[Loss of catecholamine-mediated bronchodilation]:::outcome D --> E[Unopposed parasympathetic tone]:::outcome E --> F[Airway smooth muscle contraction]:::urgent F --> G[Bronchospasm & wheezing]:::urgent B --> H[Therapeutic benefit: ↓ HR, ↓ BP]:::outcome style A fill:#fff4e6 style G fill:#fee2e2 ``` ### Comparison: Non-Selective vs. Cardioselective Beta Blockers | Property | Non-Selective | Cardioselective | |----------|---------------|------------------| | **β1 Blockade** | Yes | Yes | | **β2 Blockade** | Yes (significant) | Minimal at therapeutic doses | | **Airway Safety** | Contraindicated in asthma | Relatively safe | | **Bronchospasm Risk** | High | Low | | **Examples** | Propranolol, nadolol, timolol | Metoprolol, atenolol, bisoprolol | **Clinical Pearl:** In the scenario above, the patient should have been prescribed a cardioselective agent (metoprolol or atenolol) from the start. If a β-blocker is absolutely necessary in asthma, cardioselectivity is non-negotiable. **Mnemonic:** **CAB** = **C**ardioselective **A**gents are **B**ronchus-safe. ### Why Non-Selective Agents Are Contraindicated Non-selective β-blockers are relatively contraindicated (not absolutely, but high-risk) in: - Asthma - COPD - Reactive airway disease - Severe allergic rhinitis Cardioselective agents are preferred because they maintain β2-mediated bronchodilation at therapeutic doses. [cite:KD Tripathi 8e Ch 12]