Potential Changes Attending the Excitation Process in the Atrioventricular Conduction System of Bovine and Canine Hearts

Abstract

Potential changes attending excitation of the atrioventricular node, the common bundle, the bundle branches, and initial ramifications of the conducting tissue in the subendoeardium of the bovine heart beating in situ have been recorded from unipolar exploring electrodes. Correlations between the form of nodal deflection and the location in the node of the electrode tip have been established. Prom right bundle branch and subendocardial action potentials, data were derived indicating a rate of transmission through the right branch of 2.02 M/sec. Certain observations on the effects of acute injury and severe hypoxia are recorded. The character of nodal deflections attending retrograde excitation is reviewed. In studies on canine hearts beating in situ, eleetrograms have been made from the regions in which the atrioventricular node, the common bundle, and bundle brandies are believed to lie. Deflections comparable to those recorded in bovine experiments were encountered and, by analogy with those bovine studies, certain morphologic correlations postulated. In general, canine nodal deflections are smaller than their bovine counterparts and lack the variety of form ascribed to potential changes in the several portions of the bovine node. In canine studies, deflections of atrial, atrionodal, and nodal type coexisted in recordings from a single needle electrode suggesting a degree of proximity between the sources of these potential changes not encountered in bovine experiments.PART I Observations on Bovine Hearts I N human and canine hearts, component parts of the atrioventricular conduction system can be identified only by painstaking dissection. In the bovine heart, these structures are morphologically distinct and offer certain advantages to study of potential changes attending their excitation. During the past 3 years, a series of studies has been carried on in our laboratory in an attempt to exploit this morphologic advantage. These studies will be summarized here.Meanwhile, widespread interest in the problem of atrioventricular excitation has found expression in a number of excellent Prom the Mayo Clinic and the Mayo Foundation,

Morphology and electrophysiology of the mammalian atrioventricular node

“…There are only a few studies in which extracellular electrodes were used to record directly from the AV node (4,5,47,150,161,170,187). The interpretation of most extracellular waveforms recorded from the nodal area is very difficult.…”

Dissociation and delayed conduction in the canine right bundle branch.

“…The conduction delay caused by premature stimulation or manual pressure therefore may appear first in the transverse bridges (because of their higher resistance) and thereby favor dissociation of the RBB into separate longitudinal conduction tracts (figs. [4][5][6].…”

Slow potentials and catheter ablation for AVNRT.

“…'0 11 In humans such potentials were recorded for the first time in 1969 by Damato et al"2 and attributed to the compact AV node. '2 C In the course of studies involving radiofrequency ablation for AVNRT, we described slow potentials recordable during antegrade AV conduction along the tricuspid valve at the mid and posterior part of the septum-at a significant distance from the anatomic site of the AV node-in 95% of patients with AVNRT and their previously unreported rate-dependent behaviour.…”

“…[2][3][4][5] One approach to slow pathway ablation has therefore been anatomical, with radiofrequency energy sequentially applied to the posteroinferior interatrial septum and, later, if required, to the mid-septum. Published data show that the success rates usually achieved (> 90%) require a high number of radiofrequency energy applications (range of medians [4][5][6][7][8][9][10][11][12][13][14].34…”