Two-Dimensional Perfused Rabbit Epicardium

Background. Prolonged membrane depolarization induced by an electric shock in the heart may produce propagation block leading to repetitive beats. We studied prolonged depolarization and its role in repetitive beats in a thin epicardial layer of endocardially prefrozen arterially perfused rabbit heart. Methods and Results. A laser scanner recorded optical action potentials at 63 sites within a 1-cm2 area on the left ventricle of hearts stained with potentiometric fluorescent dye. Pacing (S1) produced propagation across the myofibers; then, a 3-millisecond shock (S2) given in the S1 refractory period produced an electric field that decreased in strength with distance along the fibers. The S2 strengths at the center of the scanned region (C) were 2.1±0.2 or 5.6±03 V/cm (mean±SD, n=4). Repetitive beats occurred in 50% of hearts when C was 2.1 V/cm and in 100% of hearts when C was 5.6 V/cm. With each occurrence of repetitive beats, prolonged depolarization of the shocked action potential occurred within 1