The Atrioventricular Node and Bundle of His: Specific Features

The atrioventricular node and bundle of His play critical roles in the maintenance and control of ventricular rhythms. As mentioned above, the atrioventricular node is composed of heterogeneous gap junctions with electrical communication via the protein connexin. More specifically, within the human myocardium there are four connexin proteins identified to date: Cx43, Cx40, Cx45, and Cx30.2/31.9 [25].

Additionally, during clinical cardiac catheterization procedures, frequent attempts are made to electrically identify both the atrioventricular node and the bundle of His in order to: 1) provide anatomic landmarks; 2) gain insights on atrial-ventricular conduction behaviors; and/or 3) determine locations to ablate these structures or the surrounding tissues. Such ablations are commonly performed to terminate aberrant behaviors (e.g., reentrant tachycardias) or to prevent atrioventricular conduction in patients with chronic atrial fibrillation. There remains strong interest by medical device designers to understand details of the structural and functional properties of the atrioventricular node and the bundle of His in order to develop new therapies and/or avoid inducing complications. For example, in a recent study in our laboratory, we attempted to provide direct correlations between means to record and activate the bundle of His in an in vitro swine heart study [26]. In addition, several recent studies have explored 3D reconstruction of the human atrioventricular node [27], but much more work on this topic needs to be performed [1].

In general, the His bundle is located adjacent to the annulus of the tricuspid valve, distal to the atrioventricular node, and slightly proximal to the right bundle branch and left bundle branch. The functional origin may be ill defined, but as described above, it is typically considered to anatomically begin at the point where the atrioventricular nodal tissue enters the central fibrous body. The bundle of His is described as having three regions; the penetrating bundle, nonbranching bundle, and branching bundle. The penetrating bundle is the region that enters the central fibrous body. At this point, the His fascicles are insulated but are surrounded by atrial tissue (superiorly and anteriorly), the ventricular septum (inferiorly), and the central fibrous body (posteriorly). Thus, the exact point where the atrioventricular nodal tissues end and the bundle begins is difficult to define, since it occurs over a transitional region. The nonbranching bundle passes through the central fibrous body and is surrounded on all sides by the central fibrous body. In this cardiac region, the His bundle still has atrial tissue superior and anterior to it, the ventricular septum inferior to it, and now the aortic and mitral valves posterior to it. It should be noted that His myocytes are innervated, but to a lesser extent than those in the atrioventricular node. Unlike the sinoatrial and atrioventricular nodes, the His bundle has no large blood vessels that supply it specifically.

The common branching bundle is described to begin as the His exits the central fibrous body. At this point, it is inferior to the membranous septum and superior to the ventricular septum. After leaving the central fibrous body, it then bifurcates into the right and left bundle branches. The right bundle branch passes within the myocardium of the interventricular septum and the left bundle branch primarily travels subendocardially along the septum in the left ventricle (as noted above).