MYOCARDIAL DISCONTINUITIES - A SUBSTRATE FOR PRODUCING VIRTUAL ELECTRODES THAT DIRECTLY EXCITE THE MYOCARDIUM BY SHOCKS

Background-Theoretical models suggest that an electrical stimulus caus es regions of depolarization and hyperpolarization on either side of a myocardial discontinuity. This study determined experimentally whethe r an artificial discontinuity gives rise to an activation front in res ponse to an electrical stimulus, consistent with the creation of such polarized regions. Methods and Results-After a thoracotomy in six dogs , a 504-unipolar-electrode plaque was sutured to the right ventricular epicardium to map activations. From a line electrode parallel to one side of the plaque, 10 S-1 stimuli were delivered, followed by S-2 and S-3 Stimuli (S1S1, S1S2, S2S3 interval=300 ms). S-1 and S-3 stimuli w ere 25 mA; 5-ms S-2 stimuli of both polarities were initially 25 mA an d increased in 25 mA increments. The plaque was removed, and a transmu ral incision was made through the ventricular wall in the middle of th e mapped region and sutured closed. The plaque was replaced and the st imulation protocol repeated. Before the incision, S-2 stimuli directly activated tissue only near the stimulation site. An activation front arose at the border of the directly activated region and propagated ac ross the plaque. As the S-2 stimulus strength was increased, the size of the directly activated region increased. After the incision, suffic iently large S-2 Stimuli caused direct activation of tissue adjacent t o the transmural incision as well as at the stimulation site. Activati on fronts that arose adjacent to the transmural incision either propag ated proximally toward the stimulation site and collided with the acti vation front originating from the stimulation wire or propagated dista lly away from the incision. Minimum S-2 stimulus strengths activating areas adjacent to the incision were only 45+/-14% (cathode) and 39+/-1 8% (anode) of the strengths required to directly activate the same are a before the incision was formed (P<.05). Conclusions-Myocardial disco ntinuities can give rise to activation fronts after a stimulus, sugges ting the presence of polarized regions adjacent to the discontinuity.