beta blockers
Summary
Beta blockers are antagonists of beta-adrenergic receptors used to treat hypertension, angina, heart failure, arrhythmias, and post-MI to reduce mortality. They decrease heart rate, contractility, and myocardial oxygen demand, and can be cardioselective (β1) or nonselective (β1/β2).
Detail
Mechanism: Beta blockers competitively inhibit catecholamine binding at beta-adrenergic receptors, decreasing cAMP production and downstream sympathetic effects. β1 receptors (heart, kidney JGA) mediate increased heart rate, contractility, and renin release; β2 receptors (lungs, vasculature, liver) mediate bronchodilation, vasodilation, and glycogenolysis.
Classification: - Nonselective (β1=β2): propranolol, timolol, nadolol, pindolol (partial agonist/ISA), carvedilol & labetalol (also block α1, used in HTN and HF) - Cardioselective (β1>β2): metoprolol, atenolol, esmolol (short-acting, IV, used in acute settings), bisoprolol - Third-gen with vasodilating properties: carvedilol (α1 block), nebivolol (NO-mediated vasodilation)
Clinical uses: Hypertension, angina (reduce O2 demand), post-MI (mortality benefit via reduced arrhythmia/remodeling), heart failure with reduced EF (bisoprolol, carvedilol, metoprolol succinate - improve survival), arrhythmias (rate control in AFib, SVT), migraine prophylaxis (propranolol), essential tremor, hyperthyroidism symptom control, performance anxiety, glaucoma (timolol - decreases aqueous humor production).
Adverse effects: Bradycardia, AV block, bronchospasm (avoid nonselective in asthma/COPD), fatigue, erectile dysfunction, mask hypoglycemia symptoms in diabetics (except sweating), metabolic effects (may worsen dyslipidemia), cold extremities (peripheral vasoconstriction).
Contraindications/cautions: Decompensated heart failure, severe bradycardia, high-grade AV block, cardiogenic shock, severe reactive airway disease (relative for cardioselective).
Toxicity/overdose: Bradycardia, hypotension, hypoglycemia (masked); treat with IV glucagon (bypasses beta receptor, activates cAMP directly), atropine, or pacing.
Board pearls: Esmolol - ultra short half-life, good for acute rate control/surgery. Nonselective beta blockers can cause unopposed alpha stimulation if given during pheochromocytoma or cocaine toxicity without prior alpha blockade - can precipitate hypertensive crisis. Abrupt discontinuation can cause rebound tachycardia/hypertension due to receptor upregulation.
Sources
- First Aid for the USMLE Step 1
- Katzung's Basic and Clinical Pharmacology
- Goodman & Gilman's The Pharmacological Basis of Therapeutics
- UpToDate
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