These are classified according to their electrophysiological effects observed in isolated cardiac tissues in vitro (Vaughan Williams, 1989). Fast sodium channel blockers (class I) which reduce the upstroke velocity of the action potential are usually subclassified into three groups, class I A-C, according to their effect on the action potential duration. Beta-adrenergic antagonists (class II) exert their effects by antagonizing the electrophysiological effects of beta-adrenergic catecholamines. Class III antiarrhythmic agents (eg amiodarone) prolong the action potential and slow calcium channel blockers (class IV) suppress the calcium inward current and calcium-dependent action potentials.