Mechanism of ventricular vulnerability to single premature stimuli in open-chest dogs.

To determine the mechanism of ventricular vulnerability to electrical stimulation, we simultaneously recorded from 120 transmural electrodes in a 35 X 20 X 5-mm portion of right ventricular infundibulum in seven dogs. Baseline pacing (S1) was performed from outside the mapped region followed by single premature stimulation (S2) of increasing strength at the center of the mapped region. In five of six episodes of ventricular fibrillation and 26 of 30 episodes of repetitive responses, complete reentrant pathways were observed. Earliest activation following S2 was not at the site of S2 stimulation but was at a point between the S1 and S2 sites of stimulation. Activation spread away from the early site toward the opposite side of the mapped region around the sides of an arc of block near the S2 site to form a "figure-of-eight." The activation fronts coalesced to activate the region around the S2 site last and, if the difference in times between activation at the early site and near the S2 site was large, reentered the tissue toward the S1 site. Ventricular refractory periods were determined in four dogs following S1 pacing; the regions with the greatest nonuniformity in the dispersion of refractoriness were not the regions of unidirectional block after S2 stimulation. Thus, 1) ventricular fibrillation and repetitive responses induced electrically with S1 and S2 stimuli at different ventricular sites arise by figure-of-eight reentry, 2) this reentry is caused by the ability of S2 stimulation both to prolong refractoriness near the S2 site and to initiate a propagated response in the region between the S1 and S2 sites, and 3) a nonuniform dispersion of refractoriness is not crucial for the electrical induction of reentry leading to ventricular fibrillation or repetitive responses when S1 and S2 stimuli are given at different locations on the right ventricular outflow tract.