Esmolol, commonly marketed under the trade name Brevibloc, is a cardioselective beta-1 receptor blocker. It has a rapid onset but short duration of action without causing significant intrinsic sympathomimetic or membrane stabilizing activities at recommended therapeutic doses. It works by blocking beta-adrenergic receptors in the heart, which leads to decreased force and rate of heart contractions. Esmolol prevents the action of two naturally occurring substances: epinephrine and norepinephrine.

For the rapid control of ventricular rate in patients with atrial fibrillation or atrial flutter in perioperative, postoperative, or other emergent circumstances where short term control of ventricular rate with a short-acting agent is desirable. Also used in noncompensatory sinus tachycardia where the rapid heart rate requires specific intervention.

Similar to other beta-blockers, esmolol blocks the agonistic effect of the sympathetic neurotransmitters by competing for receptor binding sites. Because it predominantly blocks the beta-1 receptors in cardiac tissue, it is said to be cardioselective. In general, so-called cardioselective beta-blockers are relatively cardioselective; at lower doses they block beta-1 receptors only but begin to block beta-2 receptors as the dose increases. At therapeutic dosages, esmolol does not have intrinsic sympathomimetic activity (ISA) or membrane-stabilizing (quinidine-like) activity. Antiarrhythmic activity is due to blockade of adrenergic stimulation of cardiac pacemaker potentials. In the Vaughan Williams classification of antiarrhythmics, beta-blockers are considered to be class II agents.

Esmolol undergoes rapid hydrolysis of ester linkage which is catalyzed by esterases found in the cytosol of red blood cells (RBCs). The plasma cholinersterases or RBC membrane acetylcholinesterases are not involved in this metabolic reaction. Metabolism of the drug occurs mainly in RBCs to form a free acid metabolite (with 1/1500 the activity of esmolol) and methanol.