The hydrochloride salt form of esmolol, a short, rapid-acting, selective beta-adrenergic receptor blocker, devoid of intrinsic sympathomimetic activity, and with anti-arrhythmic, antihypertensive and potential analgesic activities. Upon intravenous administration, esmolol binds to and blocks the beta-1 receptor in the myocardium thereby preventing the action of epinephrine and norepinephrine. This leads to a reduction in the force and rate of cardiac contractions and thereby preventing tachycardia, arrythmias and/or hypertension. At higher doses, esmolol also blocks beta-2 receptors located in bronchial and vascular smooth muscle, thereby leading to smooth muscle relaxation. In addition, esmolol exerts a peripheral analgesic effect and intraoperative use of this agent may decrease the amount of opioid administration postoperatively. As the ester moiety in esmolol is rapidly hydrolyzed byplasma esterases, this agent has a very short half-life.