THE SPREAD OF THE IMPULSE IN THE MAMMALIAN VENTRICLE

Abstract

The subject of impulse spread in the ventricles was investigated in a series of 12 dogs. The surface of the heart was prevented from drying, and its temp. was maintained at body level. Under these conditions, the normal sinus impulse reached the epi-cardial surface of both ventricles almost simultaneously. No correlation could be made between the rate of propagation of an artificially induced impulse on the one hand and the relationship of the points of stimulation and pick-up with respect to the direction of the superficial muscle bundles on the other. In other words, the impulse did not spread any more rapidly when the points were situated along the muscle bands than when they lay in a line across them. When the stimulating electrode was on one ventricle and the recording electrode on the other, a marked delay in the spread of the artificially induced impulse was noted, even when the 2 electrodes were placed in a line parallel to the superficial sino-spiral fibers which cross the anterior interventricular sulcus. By noting the effect of epicardial cuts on the rate of propagation of the artificially induced impulse, further evidence was collected against the hypothesis that the excitation wave spreads parallel to the superficial muscle bundles in a direction from apex to base. The average time consumed in the spread of an artificially induced impulse over a constant epicardial distance on the right ventricle was only slightly less than that on the left. In view of the marked difference in thickness of the outer walls of the 2 ventricles, this finding is opposed to the concept that an artificially induced impulse arising on the epicardial surface of either ventricle penetrates the muscle wall at a slow rate of speed, enters and spreads rapidly through the subendocardial Purkinje network, and then traverses muscle again to reach the epi-cardium. By obtaining the times of arrival of artificially induced ectopic impulses at various spots before and after cutting the right branch of the bundle of His, evidence was found which indicated that the impulse originating on one ventricle might reach the opposite one through the myo-cardial Purkinje fibers in the septum, rather than over the bundle branch pathway via the bifurcation of the bundle of His. Since the data could not for the most part be reconciled with the current concepts of impulse spread in the ventricles, an attempt was made to correlate the results with the anatomic findings of a wide-spread myocardial Purkinje network which up to the present has not received physiological significance. On the basis of the results herein presented, it seems justifiable to assume that the impulse does not leave the subendocardial Purkinje network to spread out over muscle itself, but rather that it remains in the continuation of the network, i.e., in the above-mentioned myocardial Purkinje fibers, and, spreading out over this extensive plexus, excites the muscle in numerous places ulmost simultaneously.