A consideration of antiarrhythmic therapy.

Antiarrhythmic drugs Types. A variety of antiarrhythmic drugs are available, both conventional and investigational. They can be divided into groups: those that exert blocking actions predominantly on sodium, potassium, or calcium channels, and those that block /3-adrenergic receptors. Exensive subclassification' of these drugs does not seem to be useful, primarily because it does not help in choosing effective drug therapy for a given patient. The classifications are also limited because they are based on the electrophysiologic effects exerted by an arbitrary concentration of the drug, generally on normal Purkinje fibers, often not even in arrhythmic preparations. The effects of these drugs depend on tissue type, species, the degree of acute or chronic damage, heart rate, membrane potential, the ionic composition of the extracellular milieu, and other factors. Thus, for example, lidocaine exerts little effect on a normal Purkinje fiber action potential upstroke at a pacing cycle length of 1000 msec but can profoundly depress it when the cell becomes depolarized and discharges at a rapid rate, such as might occur during ischemia-induced tachycardia. Many drugs exhibit actions that belong in multiple categories or operate indirectly, such as by altering hemodynamics, myocardial metabolism, or autonomic transmission. Some drugs have active metabolites that exert effects different from the parent compound. Not all drugs in the same class have identical effects, while some drugs in different classes have similar actions.