Features of fascicular block imposed upon existing right bundle branch block in the dog and baboon

Abstract

To delineate in the presence of right bundle branch block the electrocardiographic features of either left anterior fascicular block or left posterior fascicular block, these lesions were produced experimentally in six dog and six baboon hearts using a small knife for right bundle branch block, then a ligature for either left anterior fascicular or left posterior fascicular block. After each resultant conduction defect was proved with epicardial mapping of respective excitation delays, the effects of such blocks on the electrocardiogram and the vectorcardiogram were observed. Corresponding anatomic lesions were confirmed at autopsy. Right bundle branch block alone caused a right-anterior-superior shift of the mean QRS axis that was more pronounced in the dog than in the baboon. Left anterior fascicular block then resulted in extreme left axis deviation in both species. Left posterior fascicular block did not cause further right axis deviation but shifted the mean frontal QRS axis counterclockwise, still superior in the dog but quite inferior in the baboon. QRS duration after right bundle branch block alone was prolonged more in the dog than in the baboon. After fascicular block, further prolongation was greatest in the baboon to quite comparable final values. In both species, the addition of left anterior fascicular block widened the QRS complex more than the addition of left posterior fascicular block. In the electrocardiogram and vectorcardiogram, left anterior fascicular block tended to enhance, and left posterior fascicular block to mask, the aberrancy associated with right bundle branch block alone, especially in the primate.