Functional Transverse Interconnections within the His Bundle and the Bundle Branches

The existence of transverse interconnections within the His bundle and the bundle branches was investigated in excised preparations from dogs and rabbits. The preparations included the His bundle or one of the bundle branches along with portions of the peripheral conduction system. They were superfused with Tyrode's solution, and conventional electrophysiological techniques for extra- and intracellular recordings were employed. The effects of incisions and of intracellular stimulation within the proximal conduction system on distal activation were observed. Transection of 90% of a right bundle branch or intracellular stimulation within that bundle branch did not alter the distal activation pattern observed during surface stimulation of fully recovered fibers. Transection of 90% of the left bundle branch or intracellular stimulation produced a temporal shift of no more than 4 msec in the activation of widely scattered distal sites. Intracellular stimulation in the left bundle branch never produced dispersion of more than 3 msec in distal activation compared with the effects of surface stimulation. Intracellular stimulation at various points within the His bundle always activated the right bundle branch in a manner identical with activation during physiological atrioventricular conduction. Transverse velocity in the conduction system was less than longitudinal velocity. It is concluded that functional transverse interconnections are relatively numerous in the His bundle and the bundle branches.