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©2013 UpToDate ® Print Email Anisotropic reentry Panel A is an activation map of a single reentrant circuit at the epicardial border zone of a myocardial infarction. The large arrows represent the general activation pattern which occurs around a long line of block (blue arrow). Parallel isochrones 130 and 140 which are adjacent to the block suggest that activation is occurring across the block, resulting in a smaller circuit (shaded area) shown by the small black arrow. Panel B is an enlarged representation of this shaded area; the dark black rectangle represents an area of block around which there is a reentrant circuit (small arrows). Rapid activation occurs parallel to the long axis of the fiber orientation (isochrones 10-40 and 130-150), while in the transverse direction activation is slow (bunched isochrones 50-120).