Rate related block in the canine His-Purkinje system after ligation of the anterior septal artery--observation by His bundle electrocardiography.

In order to investigate the pathophysiology of tachycardia and bradycardia dependent block in the His-Purkinje system, anterior septal artery was ligated and the effects of vagal stimulation and atrial pacing were serially checked in 26 dogs. Tachycardia dependent intra-His bundle block or H-V blocks developed in all cases following ligation of the anterior septal artery and bradycardia dependent block ensued in 3 cases. The latter appeared only in the form of split His bundle or lengthening of the H-V intervals. Spontaneous second or third degree AV block appeared thereafter. Bundle branch block appeared in 19 cases after ligation of the artery. Rate dependent bundle branch block was observed before the development of stable bundle branch block. Frequently, a very long pause was necessary to produce bradycardia dependent block. On the other hand, bradycardia dependent H-V blocks by vagal stimulation were not necessarily dependent upon cardiac cycle length alone. Tachycardia dependent paroxysmal A-V blocks were induced in 7 cases by atrial pacing, soon before or after the onset of spontaneous second degree A-V block. In addition, this block was induced in the very early phase after ligation of the artery in one case. This repetitive intra-His bundle block or H-V block was induced not only by progressive increase in pacing rate but also by gradual decrease in pacing rate after a Wenckebach type of second degree A-H block.