What a clinician needs to know about the mechanisms of action of antiarrhythmic drugs.

Vaughan Williams' classifications of antiarrhythmic drugs are briefly discussed, including the fact that these drugs are classified based upon their effects on normal cardiac cells; that more and more subclassifications need to be introduced; that many drugs have actions belonging to different classes. A plea is made not to consider just one effect of an antiarrhythmic drug--that is, its effect on conduction velocity only or on refractory period only, but to evaluate its effect on wavelength, the product of refractory period and conduction velocity. Experimental studies on atrial arrhythmias induced by electrical stimulation in chronically instrumented dogs have shown a very good correlation between wavelength and type of arrhythmia induced. The predictive value of wavelength was much greater than that of refractory period alone or conduction velocity alone. It is emphasized that drugs that shorten wavelength especially at rapid rates may exhibit dangerous proarrhythmic effects.