What is the anatomy of the AV junction relevant to pediatric right bundle branch block?

Updated: Jul 26, 2018
  • Author: Glenn T Wetzel, MD, PhD; Chief Editor: Syamasundar Rao Patnana, MD  more...
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The specialized conduction system of the heart is composed of cells that conduct electrical impulses faster than the surrounding myocardium. The conduction system can be divided into distinct anatomic segments, and each segment is described in sequence beginning at the AV junction and ending with the Purkinje fibers.

The AV junction can be divided into three regions as follows: transitional cell zone, AV node, and penetrating portion of the AV bundle (His bundle, common bundle).

The transitional cell zone is where the right atrium merges with the compact AV node by means of discrete atrial pathways termed the slow and fast pathways.

The next segment is the AV node, which lies anterior and superior to the ostium of the coronary sinus, directly above the insertion of the septal leaflet of the tricuspid valve. This area is located at the apex of the triangle of Koch, which is formed by the tricuspid annulus, the tendon of Todaro, and the ostium of the coronary sinus. Blood supply to the AV node is derived from the AV nodal artery, which is a branch of the right coronary artery in 85%-90% of individuals and a branch of the left circumflex coronary artery in 10%-15% of individuals.

At the apex of the triangle of Koch, the compact AV node becomes the penetrating bundle of His. It penetrates the central fibrous body at the attachment of the tendon of Todaro, runs between the membranous septum and the muscular septum, and bifurcates at the crest of the muscular ventricular septum. The His bundle is divided into three anatomic segments. The proximal or nonpenetrating segment lies distal to the AV node and proximal to the central fibrous body. The middle or penetrating segment penetrates the central fibrous body and runs posterior to the membranous septum. The distal or branching segment bifurcates at the crest of the muscular septum into the right and left bundle branches (see the image below).

Anatomy of the penetrating portion of the atrioven Anatomy of the penetrating portion of the atrioventricular (AV) bundle is shown.

The right bundle branch, a direct continuation of the penetrating bundle of His, originates distal to the attachment of the septal leaflet of the tricuspid valve with the membranous septum and surfaces on the right ventricular septum just below the papillary muscle of the conus. It is unbranched and proceeds toward the apex of the right ventricle along the posterior margin of the septal band, courses through the moderator band to the base of the anterior papillary muscle, and proceeds to the right ventricular free wall.

The left bundle branch originates at the crest of the muscular ventricular septum just distal to the membranous septum. It arises in a fanlike fashion that descends inferiorly along the left ventricular septal surface beneath the noncoronary cusp of the aortic valve. The left bundle branch usually branches into three major fascicles. The anterior fascicle is directed to the base of the anterolateral papillary muscle, the posterior fascicle is directed to the base of the posteromedial papillary muscle, and, in 60% of hearts, a central fascicle proceeds to the midseptal region. When no central fascicle is present, as in 40% of hearts, the midseptal region is supplied by radiations from the anterior fascicle or the anterior and posterior fascicles.

At the terminal aspect of each bundle branch, Purkinje fibers are interlaced on the endocardial surface of both ventricles and tend to be concentrated at the tips of the papillary muscles.

For a discussion on the anatomy of subtypes, see below.

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