VENTRICULAR-FIBRILLATION IS NOT AN ANODALLY INDUCED PHENOMENON IN OPEN-CHEST DOGS

It is generally assumed that ventricular fibrillation evoked by electrical stimulation depends on anodal excitation. To test this hypothesis, six open-chest dogs were studied with computerized mapping techniques. A plaque electrode array containing 56 closely (2.5-5 mm) spaced bipolar electrodes was placed on the right ventricle. The patterns of activation after premature stimulation and at the onset of multiple responses or ventricular fibrillation were determined when the baseline driving stimuli (S1) were given to the center, and when a 5-ms bipolar single premature stimulus (S2) was given via two electrodes (one anodal and one cathodal) at the opposite edges of the plaque electrode array. The results showed that the classical anodal and cathodal strength-interval curves could be demonstrated by this method. A relatively supernormal period was observed only in the anodal strength-interval curve and coincided with the most vulnerable phase of the cardiac cycle. Although a supernormal period was found only at the anodal site, the origins of excitation at the onset of multiple responses or ventricular fibrillation could be anodal, cathodal, or both. When the S2 polarity was reversed, the origin of multiple responses or ventricular fibrillation stayed at the same site and did not change according to the polarity of the S2. These findings indicate that ventricular fibrillation is not an anodally induced phenomenon. The preexisting electrophysiological state at the site of stimulation determines the initiation and maintenance of multiple responses or ventricular fibrillation.