Shock-induced figure-of-eight reentry in the isolated rabbit heart

The patterns of transmembrane potential on the whale heart during and immed iately after fibrillation-inducing shocks are unknown. To study arrhythmia induction, we recorded transmembrane activity from the anterior and posteri or epicardial surface of the isolated rabbit heart simultaneously using 2 c harge-coupled device cameras (32,512 pixels, 480 frames/second). Isolated h earts were paced from the apex at a cycle length of 250 ms. Two shock coils positioned inside the right ventricle (-) and atop the left atrium (+) del ivered shocks at 3 strengths (0.75, 1.5, and 2.25 A) and 6 coupling interva ls (130 to 230 ms). The patterns of depolarization and repolarization a ere similar, as is evident in the uniformity of action potential duration at 7 5% repolarization (131.4+/-8.3 ms). At short coupling intervals (<180 ms), shocks hyperpolarized a large portion of the ventricles and produced a pair of counterrotating waves, one on each side of the heart. The first beat af ter the shock wets reentrant in 90% of short coupling interval episodes. At long coupling intervals (>180 ms), increasingly stronger shocks depolarize d an increasingly Larger portion of the heart. The first bent after the sho ck was reentrant in 18% of long coupling interval episodes. Arrhythmias wer e most often induced at short coupling intervals (98%) than at long couplin g intervals (35%). The effect and outcome of the shock were related to the refractory state of the heart at the time of the shock. Hyperpolarization o ccurred at short coupling intervals, whereas depolarization occurred at lon g coupling intervals. Consistent with the "critical point" hypothesis, incr easing shock strength and coupling interval moved the location where reentr y formed (away from the shock electrode and pacing electrode, respectively) .