## Intrinsic Sympathomimetic Activity (ISA) in Beta Blockers **Key Point:** ISA is the ability of a beta blocker to produce partial agonist activity at the beta receptor, preventing excessive bradycardia and negative inotropic effects. ### Beta Blockers with ISA Pindolol is a non-selective beta blocker with significant ISA (approximately 50% of the maximal beta-agonist effect). Other beta blockers with ISA include: - Acebutolol (cardioselective) - Oxprenolol (non-selective) - Carteolol (non-selective) ### Clinical Significance of ISA **High-Yield:** Beta blockers with ISA cause: - Less reduction in resting heart rate - Minimal negative inotropic effects - Reduced peripheral vasoconstriction - Better tolerance in patients with baseline bradycardia or conduction abnormalities ### Comparison Table | Beta Blocker | Selectivity | ISA | Clinical Use | | --- | --- | --- | --- | | Propranolol | Non-selective | None | Hypertension, angina, arrhythmias | | Pindolol | Non-selective | Yes (50%) | Bradycardia-prone patients | | Atenolol | β₁-selective | None | Post-MI, hypertension | | Metoprolol | β₁-selective | None | Post-MI, heart failure | **Clinical Pearl:** ISA-containing beta blockers are contraindicated in post-MI patients and those with heart failure because the partial agonist activity may blunt the cardioprotective benefits of complete beta blockade. **Mnemonic:** **PAC** — **P**indolol, **A**cebutolol, **C**arteolol are the commonly tested beta blockers with ISA.
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