Heterogeneous APD60 maps. A: per-pixel APD60 mean maps with insets showing the last optical AP of the 250 ms PCL. In the insets, circles mark onset of the AP upstroke (red), AP activation (blue), and 60% of AP repolarization (green). Axes units are pixels, with 118 µm/pixel. B: per-pixel APD60 mean maps at 150 ms PCL. C: histograms of APD60 maps shown in A and B. Global mean and standard deviation are marked with dotted lines in each histogram. APD60, action potential duration at 60% repolarization; AP, action potential; PCL, pacing cycle length.