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Figure 5 Top: Sections 1 and 3 of the activation maps of beats V1 and V2 from the short–long sequence shown in Figures 3 and 4. Bottom: Unipolar electrograms recorded from two plunge needle electrodes, A and B, from sections 1 and 3, respectively. The numbers represent local activation–recovery intervals (ARI) in ms. The numbers in parentheses are the cardiac cycle length in ms. Note the development of conduction block between electrode sites #5 and #4 during V2 in needle B. The changes in a preceding cardiac cycle were associated with significant changes in local ARIs that were more marked in midmyocardial (Mid) and endocardial (End) zones compared with epicardial (Epi) zones. See text for details. E = electrotonic potential.