Adrenergic Blockers

Adrenergic blockers reduce the physiologic response to norepinephrine and epinephrine. The receptor being blocked determines what changes first. Beta-adrenergic blockers competitively antagonize beta receptors. At clinically relevant doses, they reduce heart rate, myocardial contractility, and renin release. The result may be lower blood pressure, less myocardial oxygen demand, and slower conduction through the atrioventricular node. Those same effects can become harmful when the patient already has poor perfusion, conduction disease, or inadequate cardiac reserve. The class is not interchangeable. Propranolol and carvedilol block beta receptors broadly; carvedilol also blocks alpha-1 receptors, adding vasodilation. Other agents are more beta-1 selective, but selectivity is relative rather than absolute. A patient receiving a beta-1-selective drug can still develop bronchospasm, especially at higher exposure or with reactive airway disease. Alpha-1 blockers follow a different bedside pattern: they primarily reduce vascular tone and are more likely to produce orthostatic hypotension and first-dose syncope than the marked bradycardia expected from beta blockade. Identify the receptor target before predicting the adverse effect.