FIG. 15.

Frequency dependence of propagation and propagation block at a so-called “isthmus,” a site of source-to-sink mismatch. Panel (a): Scheme of a small cleft (interruption of dark vertical line) produced in a sheet of subepicardial tissue form sheep ventricle. Source-to-sink mismatch is characterized by curved spread of propagation beyond the obstacle. Panel (b): Decrease of propagation velocity with decreasing cleft width (corresponding to increasing source-to-sink mismatch). Panel (c): Frequency dependence of propagation block at the isthmus. At a basic cycle length (BCL) = 500 ms (corresponding to a stimulation rate of 2 Hz), 1:1 propagation is preserved down to an isthmus width of 0.5 mm. At BCL of 150 ms (corresponding to a stimulation rate of 6.66 Hz), propagation block starts to develop at isthmus widths < 2 mm. Reproduced with permission from C. Cabo et al., Circ. Res. 75, 6 (1994). Copyright 1994 Wolters Kluwer Health, Inc.⁸³