A 65-year-old with heart failure on carvedilol develops worsening bronchospasm. Switching to metoprolol succinate reduces bronchospasm. The property of metoprolol that accounts for this differential effect is:
- A Intrinsic sympathomimetic activity at beta-2 receptors
- B Additional alpha-1 blockade that bronchodilates via vascular unloading
- C Selective blockade of cardiac beta-1 receptors with relative sparing of beta-2 receptors ✓
- D Membrane-stabilizing activity that prevents reflex bronchospasm
Correct answer: C. Selective blockade of cardiac beta-1 receptors with relative sparing of beta-2 receptors
Explanation
Metoprolol is a cardioselective (beta-1 selective) blocker with approximately 75-fold greater affinity for beta-1 than beta-2 receptors. Carvedilol is a non-selective beta blocker (plus alpha-1 blockade) with equal beta-1 and beta-2 blockade, precipitating bronchoconstriction via beta-2 receptor antagonism in airways. Cardioselectivity is relative and dose-dependent but clinically meaningful at standard therapeutic doses.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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