The Atrioventricular Node

AND BUNDLE OF HIS: SPECIFIC FEATURES

As mentioned in Section 3, the atrioventricular node and the bundle of His play critical roles in the maintenance and control of ventricular rhythms. In addition, both structures are frequently accessed during cardiac catheterization procedures: (1) as anatomical landmarks, (2) to allow insight into atrial-ventricular conduction behaviors, or (3) to ablate these structures or the surrounding tissues to terminate aberrant behaviors (e.g., reentrant tachycardias) or to prevent atrioventricular conduction in patients with chronic atrial fibrillation. Today, medical device designers have a strong interest to understand the details of the structural and functional properties of the atrio-ventricular node and the bundle of His for development of new therapies or to avoid inducing complications; hence, in the following section, such details are provided.

The myocytes located within the region of the atrioven-tricular node and the bundle of His have many unique characteristics. Specifically, both the atrioventricular node and His bundle are comprised primarily of "spiraled" myofibers that are then combined to form many collagen-encased fascicles. These fascicles are generally arranged in a parallel fashion in the proximal atrioventricular bundle (PAVB; the region of the atrioventricular node transitioning from the atrium into the body of the nodal tissues) and the distal atrioventricular bundle (DAVB; the penetrating portion of the bundle of His) and are interwoven within the atrioventricular node itself (the tortuosity of the cellular pathway within the atrioventricular node likely is a major contributor to the conduction delay in this region).

In general, the myocytes of the bundle of His are larger than those of the PAVB and the atrioventricular node, and the peri-nuclear regions of these myocytes are filled with glycogen. These cells uniquely utilize anaerobic metabolism instead of the normal aerobic metabolism used by the more abundant con tractile myocardium. His myocytes have longer intercalated disks, and although all of the nodal tissues have thin end processes, they are less numerous in the His myocytes. 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. Table 2 is a summary of the histological characteristics of the bundle of His in comparison to the other nodal tissues.

It should be noted that the bundle of His can receive inputs from both the atrioventricular node and from transitional cells in the atrial septum In general, the His bundle is located adjacent to the annulus of the tricuspid valve, distal to the atrio-ventricular node, and slightly proximal to the right and left bundle branches. The functional origin may be ill defined, but as described above, it is typically considered to begin anatomically at the point 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 at which the atrioventricular nodal tissues end and the bundle begins is difficult to define because it occurs over a transitional region. The penetrating bundle has been described as oval and was 1-1.5 mm long in young canines and 0.25-0.75 mm long in neonates (1).

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. The 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. The bundle is also at its closest to both the right and left ventricular chambers at this point. After leaving the central fibrous body, the bundle then bifurcates into the bundle branches; the right bundle branch passes into the myocardium of the interventricular septum, and the left bundle branch travels subendocardially along the septum in the left ventricle (as noted in Section 2). Figures 13 and 14 show canine histological sections of the bundle of His as they exit the central fibrous body (the branching bundle).

Electrophysiological studies of the bundle of His have most commonly been performed using catheters with polished electrodes and a short interelectrode spacing (i.e., those with 2-mm diameters). Because of the small amplitude of the His potential, special high-pass filtering must be used (>30 Hz). This highpass setting must be used to separate the His signal from the low-frequency shift in the isopotential line between the atrial depolarization and the atrial repolarization/ventricular depolarization. His potentials can commonly be mapped by deploying an electrode in one of three ways: (1) endocardially in the right atrium at a point on the tricuspid annulus near the membranous septum; (2) epicardially at the base of the aorta near the right

Table 2

Summary of the Histological Characteristics of the Nodal and Perinodal Tissues in Canines

Table 2

Summary of the Histological Characteristics of the Nodal and Perinodal Tissues in Canines

Feature

Atrioventricular bundle (DAVB, His bundle)

Atrioventricular node (AVN)

Proximal atrioventricular bundle (PAVB)

Nucleus

Clear perinuclear zone filled

Clear perinuclear zone filled

Clear perinuclear zone filled

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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