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Additional file 1: Video S1. Transmembrane voltage mapping of rat heart epicardium. An adult rat heart was imaged during ventricular pacing at cycle length 250 msec. The voltage wavefront can be seen originating from the center of the posterior ventricle and propagating across the surface, followed by a slight delay and retrograde atrial conduction. Calcium imaging was performed simultaneously (see Additional file 2: Video S2). The images were box blurred using a 15 × 15 uniform kernel and the length of the heart is approximately 2.5 cm from base to apex. (MP4 7953 kb)