beta blocker
Summary
Beta blockers are drugs that antagonize β-adrenergic receptors, decreasing heart rate, contractility, and conduction velocity, thereby reducing myocardial oxygen demand and blood pressure. They are used in hypertension, angina, heart failure (specific agents), arrhythmias, and post-MI management. Cardioselective agents (β1-selective) minimize bronchospasm risk compared to nonselective agents.
Detail
Beta blockers competitively inhibit catecholamine (epinephrine/norepinephrine) binding at β-adrenergic receptors. β1 receptors (primarily cardiac) mediate increased heart rate, contractility, and AV conduction via Gs-protein/cAMP pathway; blocking them decreases heart rate, contractility, and myocardial oxygen demand—useful in angina, hypertension, and arrhythmias. β2 receptors (bronchial, vascular smooth muscle) mediate bronchodilation and vasodilation; nonselective blockers (e.g., propranolol) can cause bronchospasm and are relatively contraindicated in asthma/COPD. Cardioselective β1 blockers (metoprolol, atenolol, bisoprolol, esmolol) are preferred in patients with pulmonary disease, though selectivity is lost at high doses. Some agents have additional properties: carvedilol and labetalol also block α1 receptors (used in heart failure and hypertensive emergencies); nebivolol has vasodilating (nitric oxide-mediated) effects; some have intrinsic sympathomimetic activity (pindolol, acebutolol), causing partial agonism. Clinical uses: hypertension (second-line in many guidelines), angina pectoris (reduce oxygen demand), post-MI (reduce mortality, prevent remodeling), heart failure with reduced ejection fraction (carvedilol, metoprolol succinate, bisoprolol—improve survival by blunting sympathetic overactivation), arrhythmias (rate control in atrial fibrillation, suppression of ventricular arrhythmias), migraine prophylaxis, essential tremor, performance anxiety, and thyrotoxicosis (symptom control). Esmolol, ultra-short-acting, is used in acute settings for rate control. Side effects include bradycardia, AV block, fatigue, erectile dysfunction, masking of hypoglycemia symptoms in diabetics (except sweating), and exacerbation of Raynaud's phenomenon or peripheral vascular disease. Abrupt discontinuation can cause rebound tachycardia/hypertension due to receptor upregulation. Contraindications include severe bradycardia, high-grade AV block, decompensated heart failure (acute), and caution in asthma/COPD (nonselective agents) and cocaine intoxication (unopposed alpha stimulation risk). Pharmacologically, beta blockers decrease renin release from juxtaglomerular cells (β1-mediated), contributing to antihypertensive effect. Overdose treatment includes glucagon, which bypasses the beta receptor to increase cAMP directly.
Sources
- Katzung's Basic and Clinical Pharmacology
- First Aid for the USMLE Step 1
- Goodman & Gilman's The Pharmacological Basis of Therapeutics
- Harrison's Principles of Internal Medicine
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