Continuous fractionated electrical activity after stimulation of the ventricles during the vulnerable period: evidence for local reentry.

Ventricular multiple extrasystoles and fibrillation were induced in open chest dogs by the delivery of either a single electrical pulse or a train of pulses to the right ventricle during the vulnerable period of the cardiac cycle. Bipolar epicardial electrodes delivered current while bipolar plunge electrodes recorded the activation complexes at various distances from the stimulation site. At stimulus intensities below the threshold for multiple ventricular responses, the electrograms recorded from muscle adjacent to the stimulation site showed fragmented activation complexes that lasted considerably longer than the control complexes. At stimulus intensities that evoked multiple ventricular extrasystoles or fibrillation, the fragmented electrical activity became continuous and bridged the diastolic interval between successive ectopic complexes. The continuous fractionated electrical activity that preceded the appearance of multiple extrasystoles and fibrillation was recorded only from electrodes placed within a few millimeters of the stimulating electrodes and could be evoked only by stimulation during the vulnerable period. Disappearance of the fractionated local electrical activity during a run of extrasystoles was followed immediately by the resumption of a normal rhythm. In addition, the fractionated continuous electrical activity that appeared after delivery of a single stimulus during the vulnerable period could be abolished by the delivery of a second stimulus during the protective zone. The presence of continuous local electrical activity before the development of multiple extrasystoles and fibrillation provides strong evidence that the ventricular fibrillation threshold technique operates by producing local reentrant activity.