Anisotropic conduction and reentry in perfused epicardium of rabbit left ventricle.

An experimental model of anisotropy was developed, consisting of a thin layer (1 mm) of perfused ventricular epicardium created by cryodestruction of the interventricular septum and the inner four-fifths of the free wall of the left ventricle. The electrophysiological properties of the 1-mm-thick surviving layer were the same as before freezing. Epicardial conduction was mapped with a 192-fold mapping electrode (spatial resolution 1 mm). In the intact ventricle transverse conduction was interrupted near the site of stimulation by epicardial breakthrough of faster intramural wavefronts, whereas in the thin epicardial layer transverse conduction proceeded over a much longer distance, resulting in enlargement of the area of effective anisotropy. Induction of premature beats or incremental pacing resulted in the development of lines of transverse conduction block. The occurrence of transverse conduction block, however, only rarely led to reentry, because fast longitudinal activation of the tissue distal to the line of block provided insufficient delay for the fibers proximal to the line of block to restore their excitability. Initiation of reentry was associated with the development of longitudinal arcs of conduction block during which the area distal to the line of block was activated by a slowly propagating transverse wavefront providing a longer delay for the proximal fibers to restore their excitability. In conclusion, thin layers of myocardium may provide a substrate for reentry because of enlargement of the effective area of anisotropy.