A key point to know about antiarrhythmics in Vaughn-Williams Class I-C is that they
|A||Are only given for arrhythmias during acute myocardial infarction|
|B||Are particularly suited for patients with low ejection fractions|
|C||Are drugs of choice for relatively innocuous ventricular arrhythmias|
|D||Have a significant pro-arrhythmic effect|
a. Flecainide, propafenone, and to a degree moricizine, the class I-C antiarrhythmics, are associated with a higher incidence of severe proarrhythmic events than virtually any other antiarrhythmics in other classes.
b. This risk partially explains why and when these drugs were first approved, they were indicated only for life- threatening ventricular arrhythmias that failed to respond to all other reasonable (and safer) alternatives. (This risk also contributed to why another I-C agent, encainide, was withdrawn from the market.)
c. Nowadays, these I-C agents are still used for serious (life-threatening) and refractory ventricular arrhythmias, their efficacy arising from significant sodium channel blockade. However, they also block some potassium channels, which accounts for modestly growing interest in and use of these drugs for some atrial arrhythmias. Regardless of whether the use is for an atrial or ventricular arrhythmia, the proarrhythmic effects should be of concern.
d. Finally it is probably worth opining that memorizing which antiarrhythmic agents are in which Vaughn-Williams class may not be profitable.
e. Among the reasons why:
i. this classification is based largely on electrophysiologic effects of the drugs in largely normal, isolated cardiac cells, not in diseased intact hearts;
ii. some antiarrhythmic drugs have electrophysiologic/ionic mechanisms of action that would place them in more than one class;
iii. belonging to a particular Vaughn-Williams class does not necessarily predict clinical use; and
iv. side effect profile of drugs in the same class—both cardiac and extracardiac side effects and toxicities—can differ substantially