Dispersion of Repolarization Induced by a Nonuniform Shock Field

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Abstract
Dispersion of repolarization may contribute to arrhythmias. To determine whether an electrical field stimulus (S2) with a nonuniform potential gradient can induce a dispersion of repolarization, we applied 5 ms rectangular S2 that had a nonuniform or uniform potential gradient during the action potential (AP) of bathed frog ventricular strips. One group had a partitioned bath to produce a nonuniform S2 of 39 +/- 11 V/cm (mean +/- SD) in one half of the 1 x 6 mm strip (H) and 0.3 +/- 0.2 V/cm in the other half (L), and simultaneous intracellular AP recordings in H and L with glass microelectrodes positioned 1.4 +/- 0.4 mm apart. Another group had uniform S2 and an AP recorded near the center of the strip. S1 pacing at 0.5 Hz was performed at one end of the strip and conduction along the strip was monitored. In each experiment, the S2 trials had an S1-S2 interval of 300 ms so that S2 was given during an AP (shocked AP). In both H and L, nonuniform S2 produced cumulative shortening of paced APs and lengthening of each shock AP compared with the paced AP preceding it. Uniform S2 of 1 V/cm did not shorten the paced APs or lengthen the shocked APs indicating that the AP changes in L were not due to the small potential gradient in L. Before beginning nonuniform S2 trials, the AP duration determined at the maximum repolarization rate was 601 +/- 72 ms in H and 602 +/- 71 ms in L (P = ns). During 13-20 nonuniform S2 trials over a 60-80 minute period, paced APs were shortened to 490 +/- 51 ms in H and 515 +/- 39 ms in L while each shocked AP was lengthened, compared with the paced AP preceding it, to 636 +/- 40 ms in H and 561 +/- 21 ms in L (P less than 0.05). Therefore, paced APs after shocks repolarized 25 ms earlier in H than in L and shocked APs repolarized 75 ms later in H than in L. The results show that during the shortened AP in H, the AP in L is shortened, which is consistent with intracellular current from L to H during repolarization. During the prolonged AP in H, the AP in L is prolonged compared with the paced AP preceding it, consistent with intracellular current from H to L during repolarization. Thus, nonuniform shocks can induce a dispersion of repolarization and may induce cell-to-cell interactions during repolarization.