The mechanism of complete and incomplete bundle branch block

Abstract

The propagation of the wave of activation in the interventricular septal mass of the dog's heart was studied under control conditions and with different degrees of bundle branch block. Unipolar leads, differential leads with contiguous electrodes, and distant bipolar leads were recorded at different levels of the septum. The following are the main findings: 1. 1. Under control conditions the process of activation in the interventricular septum develops from left to right and below upward. The velocity of propagation in most of the septal mass, which is formed mainly by the left ventricular muscle, is very rapid, approximately 1,000 to 1,200 mm. per second; this suggests that the ramifications of the Purkinje penetrate deeply into the interventricular septum muscle. 2. 2. Varying degrees of right bundle branch block do not modify the sense nor the sequence of the process of activation in the left ventricular mass so that the same velocity exists in the left ventricle as was found in control experiments. 3. 3. Minor degrees of left bundle branch block cause a delay in the time of arrival of the wave of activation to all the left ventricular septal mass, but fail to modify the sense or the velocity of propagation within the same mass. When the degree of the block is of great magnitude, the sense of the activation is inverted in the whole left septal mass. This process is therefore carried out from right to left. Under these conditions, when the impulse propagates in a direction counterwise to the normal, the velocity of propagation within the left septal mass diminishes and in cases of complete block falls around 360 mm. per second. 4. 4. In right as well in left bundle branch block there is a delay (approximately 0.03 to 0.04 second) which occurs in a relatively small portion of the septum (1.5 mm. to 2 mm). very near the right septal surface. This delay occurs precisely when the impulse passes from the regions activated by the normally functioning branch to regions which were previously activated by the blocked branch. It is probable that the delay represents a latency in the propagation of the impulse in this zone. This suggests that the right and the left systems are functionally independent and thus eliminates the possible existence of communications between both branches. We believe that this is the first experimental evidence of the site where the delay takes place in bundle branch blocks. 5. 5. The duration and importance of the septal phenomenon in the determination of ventricular curves is pointed out in normal conditions as well as in defects of intraventricular conduction. 6. 6. The principal implications which have a bearing on clinical electrocardiography are discussed.