Detailed activation sequence in the region of electrically induced ventricular fibrillation in dogs

We determined the details of ventricular activation sequence during the onset of ventricular fibrillation in 6 dogs. Unipolar electrograms were simultaneously recorded from 192 ventricular epicardial sites evenly spaced in a 3 by 3 cm square area surrounding 2 central sites to which stimuli for inducing fibrillation were delivered. In 5 dogs, 2 to 10 msec duration pulses with amplitudes of 150 to 220 v were used to induce fibrillation, and in 1 dog, 3 successive permature stimuli of 3 v intensity were used to induce fibrillation. In 2 dogs the pattern of activation at the onset of fibrillation was compatible with local reentrant paths near the stimulus site. Fibrillation was induced with a single high intensity pulse in both of these dogs. In the other 4 dogs, the pattern of activation at the onset of fibrillation was highly suggestive of reentrant paths distant from the stimulating electrodes. In 3 of these dogs fibrillation was induced by a single high intensity pulse and in the other it was induced by 3 repetitive stimuli. Activation patterns resembling supraventricular activation occurred prior to fibrillation in the latter animal. The data demonstrate 2 distinct modes of onset of electrically induced ventricular fibrillation. The findings support local reentry in the region of the stimulus site as the mode of onset of fibrillation in some animals and possible reentry at sites distant from the stimulus site with participation of the specialized conduction system in other animals. The differences in mode of onset of electrically induced fibrillation may be responsible for instability of fibrillation threshold measurements in some animals. In addition the role of the specialized conduction system in the onset of fibrillation may be responsible for the effects of sympathetic and vagal stimulation on fibrillation threshold.