Anaylsis of ventricular activation and repolarization from intramural and epicardial potential distributions for ectopic beats in the intact dog.

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Abstract
Ventricular activation and repolarization were examined by measuring intramural and epicardial potential distributions during ectopic sequences in intact dogs. Potential distributions were used because they provided a direct measure of all of repolarization. Ectopic sequences produced almost unidirectional excitation across the heart along with repolarization events that were different from normal. During ectopic repolarization, positive potentials occurred over a broad area surrounding the ectopic focus, and negative potentials occurred over a large area on the opposite side of the heart where excitation terminated. The potentials in the walls of both ventricles were more positive in the outer layers and more negative in the inner layers, a gradient similar to normal. A maximum initially was at the ectopic site with a magnitude greater than that of the minimum, but the maximum subsequently decreased in magnitude and shifted toward the minimum while the minimum increased in magnitude. The steepest gradients were initially around the maximum, and they then shifted toward the minimum. The results show that ventricular repolatization potential distributions during ectopic beats are predominantly influenced by gradients from one side of the heart to the other (transventricular gradients) in contrast to normal repolatization distributions which are predominantly influenced by gradients across the wall (transmural gradients).