Activation and repolarization patterns in the ventricular epicardium under sinus rhythm in frog and rabbit hearts

Our study compared the contributions of activation sequence and local repolarization durations distribution in the organization of epicardial repolarization in animals with fast (rabbit) and slow (frog) myocardial activation under sinus rhythm. Activation times, repolarization times and activation–recovery intervals (ARI) were obtained from ventricular epicardial unipolar electrograms recorded in 13 Chinchilla rabbits (Oryctolagus cuniculus) and 10 frogs (Rana temporaria). In frogs, depolarization travels from the atrioventricular ring radially. ARIs increased progressively from the apex to the middle portion and finally to the base (502 ± 75, 557 ± 73, 606 ± 79 ms, respectively; P < 0.01). In rabbits, depolarization spread from two epicardial breakthroughs with the duration of epicardial activation being lower than that in frogs (17 ± 3 vs. 44 ± 18 ms; P < 0.001). ARI durations were 120 ± 37, 143 ± 45, and 163 ± 40 ms in the left ventricular apex, left, and right ventricular bases, respectively (P < 0.05). In both species, repolarization sequence was directed from apex to base according to the ARI distribution with dispersion of repolarization being higher than that of activation (P < 0.001). Thus, excitation spread sequence and velocity per se do not play a crucial role in the formation of ventricular epicardial repolarization pattern, but the chief factor governing repolarization sequences is the distribution of local repolarization durations.