Conduction Disorders in the Canine Proximal His-Purkinje System Following Acute Myocardial Ischemia

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Abstract
An intra-His bundle lesion developed in 26 out of 38 dogs (68%) that survived the initial period of arrhythmias that followed ligation of the anterior septal artery. The lesion was isolated in the His bundle in ten experiments and was associated with bundle branch block in 16. The evolution of intra-His bundle block (IHBB) was evaluated utilizing standard ECG leads, catheter electrode, and plunge wire recordings from the His bundle and bundle branches. In all experiments the His bundle potential recorded by the catheter electrode was split into at least two distinct deflections (H 1 and H 2 ). This was always associated with a progressive increase of the H 1 -V interval and represented "first degree IHBB." A "second degree IHBB," characterized by intermittent block of the atrial impulse between the two His deflections, consistently developed either spontaneously or in response to atrial pacing. The first "stage" of second degree IHBB had a Mobitz type II pattern with a constant P-R, H 1 -V, and H 1 -H 2 intervals of the beats preceding and following the block. In 20 of 26 observations, this stage merged into the second stage of second degree IHBB characterized by a progressive increment of the H 1 -H 2 intervals prior to failure of a ventricular response. The increment of conduction time, which was at first in the range of a few msec, would not be recognized in standard ECG recordings. This increment could increase up to 180 msec, giving rise to an obvious Wenckebach periodicity that would be recognized at the usual 25 mm/sec paper speed. Complete (third degree) IHBB was observed either in the form of paroxysmal or persistent block. Paroxysmal block occurred spontaneously or could be induced by a critical rate of rapid atrial pacing during early stages of second degree IHBB. Complete IHBB that developed later was persistent and usually associated with a slow idioventricular rhythm. The study suggests that Mobitz type II and the Wenckebach patterns of conduction disturbance in the His bundle are different stages of the same electrophysiological disturbance, with Mobitz type II block representing the initial stages of failure of impulse propagation.