Anti-arrhythmic drugs

Anti-arrhythmic drugs are often classified by the Vaughan-Williams (VW) method. This system of classification is based not only on the fundamental sites or mechanisms of action of these drugs but also on the electrophysiological changes that they evoke. However, it does not take into account the fact that some anti-arrhythmic drugs have multiple mechanisms of action. It does not allow for the fact that drug action on diseased tissue may differ from that in healthy tissue. Furthermore the VW system excludes some potential sites of anti-arrhythmic drug action. An alternative method of classifying anti-arrhythmic drugs is based on their clinical efficacy against arrhythmias originating from a supraventricular site, arrhythmias originating from within the ventricles or arrhythmias originating from above or from within the ventricles. Adenosine, atropine and digoxin all find use against supraventricular arrhythmias but are not classified under the VW system. Verapamil (VW class IV) is another agent used to control supraventricular arrhythmias. Amiodarone (VW class III/IV), disopyramide (VW class Ia), flecainide (VW class Ic), procainamide (VW class Ia), propafenone (VW class Ic), esmolol (VW class II), and sotalol (VW class II/III) are used in the control of arrhythmias arising either above or within the ventricles. Lidocaine (VW class Ib) and mexiletine (VW class Ib) are used in the control of arrhythmias of ventricular origin. Drugs in the VW classes Ia, Ib and Ic all cause use-dependent blockade of voltage-sensitive Na+ channels. Those in VW classes II, III and IV are antagonists at β-adrenoceptors, K+-channel blockers or Ca2+-channel blockers, respectively.