1971 Volume 12 Issue 3 Pages 281-289
Histological determination was made on the preparations from the rabbit heart presumed to be bypass tracts by the previous functional study. As the first bypass tract a muscle strip composed of the apparently ordinary cardiac muscle was found to leave the right atrium and reach the bundle of His at or near the bifurcation, bypassing the A-V node. As the second bypass tract a bundle of apparently ordinary cardiac muscle fibers leaves the lateral wall of the right atrium above the A-V groove, descending and reaching the lateral wall of the right ventricle. A part of the third bypass tract was found below the right atrium close to the interatrial septum and its terminal part was found to enter the left side of the A-V node anterior to the ordinary atrio-nodal connection.Since its middle part could not be differentiated clearly from the ordinary atrial muscle fibers, the histological confirmation is not considered to be complete as yet, as far as the third bypass tract is concerned.
These histological findings explain the short P-R interval and the almost normal ventricular complex of the first bypass conduction, the short P-R interval and the abnormal configuration of the ventricular complex of the second bypass conduction, and the long P-R interval and normal ventricular complex of the third bypass conduction. As additional functional findings the short P-R interval of the first bypass was found to show relatively constant values in spite of the change in the stimulus frequency and the longer than normal P-R interval of the third bypass to decrease at first and increase markedly later as the stimulus frequency was increased.
