| Summary: | Introduction Spatiotemporal distribution of myocardial action potential durations is heterogeneous but a role of these heterogeneities is largely unknown. Heart ventricular contraction needs muscle mechanical heterogeneity, namely activation spread from “slow” to “fast” muscle elements, which are expected to have longer and shorter action potentials, respectively. We hypothesized that the longer repolarization duration at the primary activation area provides the better pump function. The objective of the present study was to evaluate an association between the repolarization duration in the area of initial activation and aortic blood flow under electrical pacing of different ventricular regions. Having a single ventricle and primitive, if any, conduction system, a fish heart presents a useful model for assessing the functional significance of electrophysiological nonuniformities. Methods Experiments were performed in wild caught Atlantic cod (n = 18). Ventricular unipolar electrograms were led from an epicardial sock and intramural plunge electrodes (24–64 leads simultaneously) connected to a custom‐made mapping system. In each lead, activation times (ATs) and repolarization times (RTs) were determined as dV/dt min during QRS complex and dV/dt max during T wave, respectively. Activation‐repolarization intervals (ARIs, surrogate for action potential durations) were calculated as ARI = RT‐AT. Aortic blood flow was measured with transit time flow meter. Results In sinus rhythm, the posterior wall near the atrioventricular junction was earliest to be activated (median ATs 9 [IQR 3–15] ms) and had the longest median ARIs (516 [IQR 430–580] ms). The most remote from this area regions had expectedly longer ATs (heart apex: 33 [IQR 29–44] ms; anterior base 33 [IQR 25–43] ms; both p<0.05 vs posterior wall) and also demonstrated shorter ARIs (heart apex: 482 [IQR 427–544] ms; anterior base 416 [IQR 390–497]; both p<0.05 vs posterior wall). Posterior wall pacing yielded a highest median aortic flow 47.3 [IQR ...
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