714-1 Interaction Between Strong Electrical Stimulation and Reentrant Wavefronts in Canine Ventricular Fibrillation

Abstract

To test the hypothesis that the effects of a strong electrical stimulus on reentrant wavefronts (RW) in ventricular fibrillation (VF) is dependent upon the timing of the stimulus, we studied 6 open-chest dogs with computerized mapping techniques. A plaque electrode array with up to 509 bipolar electrodes was placed on the RV epicardium. The interelectrode distance was 1.6 mm and the interpolar distance was 0.5 mm. Following 8 baseline stimuli (S 1 ) at 300 ms cycle length, a strong premature stimulus (S 2 ) was given to the center of mapped tissue to induce VF. In subsequent episodes, a second premature stimulus (S 3 ) of the same strength as S 2 was given to the same site at progressively longer S 2 –S 3 intervals with 20 ms increments. At baseline, S 2 consistently induced figure 8 reentry and VF. The VF cycle length immediately after S 2 averaged 108 ± 17 ms. The S 3 resulted in one of the following responses: (I) termination of RW and VF, (II) induction of different RW, or a focal pattern of repetitive activation, and (III) persistence of the same figure 8 RW. The intervals between S 3 and the immediate preceding activation at the site near S 3 were 41 ± 18 ms, 60 ± 18 ms and 99 ± 14 ms (p < 0.001), for response patterns I, II and III, respectively. We conclude that the effects of strong electrical stimulation on the RW in VF is dependent upon the recovery interval since the previous local activation. A protective zone occurred first, during which a strong electrical stimulation can terminate RW and abort VF. This zone was followed by a vulnerable period during which new wavefronts could be induced. If the S 3 was given after the end of the vulnerable period, there was no change of the RW.