The most widely used classification system for antiarrhythmic drugs is a modification of the system proposed by Vaughan Williams. It classifies drugs according to their effects on action potentials in individual cells. Class I drugs block sodium channels responsible for the fast response in atrial, ventricular, and Purkinje tissues, thus depressing conduction velocity. Class II drugs are beta-adrenergic receptor antagonists (or beta blockers). Class III drugs prolong cardiac repolarization, predominantly by blocking potassium channels during phases 2 and 3 of the action potential, thereby increasing tissue refractoriness. Class IV drugs block calcium channels (calcium channel blockers), depressing the slow response in SA nodal and AV nodal cells, and perhaps in other cells as well.
This classification system is an oversimplification, however, and does not account for many other effects. Additionally, it does not account for the multiple effects a drug may have on cardiac cells. For example, sotalol has beta-blocking activity (Class II), but it also significantly prolongs the action potential duration (Class III). Another drug, amiodarone, has been shown to have Class I, II, III, and IV effects and perhaps others as well. Furthermore, many drugs undergo metabolism to electrophysiologically active metabolites, which may have electrophysiologic effects that differ from those of the parent compound.