Left Posterior Fascicular Block in Canine and Primate Hearts

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Abstract
To determine the electrocardiographic consequences of experimentally produced left posterior fascicular block, electrocardiograms of three baboons and three dogs were recorded from extremity and chest leads, from an esophageal lead, and from atraumatic exploring epicardial leads both before and after left posterior fascicular block and again after right bundle-branch block. The following electrocardiographic changes were noted: In direct epicardial leads from sites overlying zones of delayed myocardial excitation (1) increased ratio of R to S; (2) later intrinsic deflection time; and (3) appearance or augmentation of the Q wave. These altered potentials also appeared as rotations of the mean spatial QRS electrical axis. The QRS duration increased slightly. The addition of RBBB produced a further delay in deflection time in epicardial leads from the RV and a further increase in QRS interval. The epicardial envelopment by the excitation process is portrayed in diagrams. These observations emphasize the need for caution in the diagnosis of left posterior fascicular block alone or in combination with RBBB.