Cardiac Fibrous Skeleton #5: The Atrioventricular Conduction Axis

1. Overview

The atrioventricular conduction axis is the specialized myocardial pathway providing the normal electrical connection between the atria and ventricles across the otherwise insulating cardiac fibrous skeleton. In the normally arranged heart, the compact atrioventricular node continues as the nonbranching penetrating bundle, which traverses the central fibrous body. The axis then reaches the crest of the muscular ventricular septum, becomes the branching bundle, gives rise to the left bundle branch, and continues as the right bundle branch. The branching portion is located immediately beneath the membranous septal region, classically beneath the interventricular component of the membranous septum [1].

The actual position, depth, and length of each component depend on the alignment of the atrial septum, ventricular septum, atrioventricular junctions, tricuspid valve attachment, and aortic root. These relationships are particularly important in congenital heart surgery because the operative field often lies directly adjacent to the penetrating or branching axis. Injury may produce transient conduction delay, bundle-branch block, or complete atrioventricular block.

2. The Fibrous Skeleton as an Electrical and Structural Framework

The cardiac fibrous skeleton supports the atrioventricular and arterial valves while electrically separating atrial working myocardium from ventricular working myocardium. Normal activation is therefore directed through the atrioventricular node and conduction axis rather than passing diffusely across the atrioventricular junction.

The septal atrioventricular junction is the central region in which the atrial septum, ventricular septum, aortic root, mitral valve, and tricuspid valve converge. Its important structures include the triangle of Koch, inferior pyramidal space, central fibrous body, membranous septum, and muscular ventricular septal crest [2]. These structures form a three-dimensional complex rather than isolated two-dimensional landmarks.

The central fibrous body consists principally of the right fibrous trigone and adjacent fibrous continuities among the aortic, mitral, and tricuspid valves. The penetrating bundle crosses this fibrous barrier at a restricted site, creating a small region in which the normal atrioventricular electrical connection is particularly vulnerable.

3. Atrioventricular Node and Triangle of Koch

In the normally structured heart, the compact atrioventricular node lies within the triangle of Koch. The triangle is bounded by the tendon of Todaro, the hinge of the septal leaflet of the tricuspid valve, and the coronary sinus orifice. Its apex points toward the central fibrous body, where the node transitions into the penetrating bundle [1].

The node lies within the inferior pyramidal space, where the atrial septal structures, atrioventricular junction, ventricular septum, and central fibrous body meet. Visible right atrial landmarks therefore guide localization but do not define its precise depth.

Substantial normal variation exists. Histologic and imaging studies demonstrate variation in the transition from the atrioventricular node to the penetrating bundle relative to the apex of the triangle of Koch and septal tricuspid hinge. In slightly more than one-half of examined hearts and patients, penetration occurred on the atrial side of the tricuspid hinge, but the dimensions of the axis and its proximity to the right-sided endocardium varied markedly [3]. A suture that is safe in one heart may therefore be hazardous in another despite similar surface anatomy.

4. Penetrating Bundle and Central Fibrous Body

The compact node becomes the penetrating bundle as the axis enters the central fibrous body. The term identifies the nonbranching segment crossing the fibrous atrioventricular plane. It should not imply that the bundle invariably passes through the center of a visible membranous septum. More precisely, the axis penetrates the central fibrous body and reaches the muscular septal crest beneath the membranous septal region [1].

The penetrating segment varies in length, orientation, and depth. Its route is influenced by the relationship between the inferior atrial septum and muscular ventricular septum. With normal septal alignment, the triangle of Koch and fibrous continuity provide useful landmarks. With abnormal alignment or ventricular topology, the nodal position and route of penetration may be displaced.

This region is vulnerable during procedures involving the septal tricuspid annulus, perimembranous ventricular septum, aortic annulus, subaortic septum, or central fibrous body. Injury may result from direct needle penetration, patch or prosthetic compression, excessive tension, hematoma, edema, traction, or thermal spread.

5. Membranous Septum and Tricuspid Valve Attachment

The membranous septum is the fibrous component of the septal complex. The hinge of the septal tricuspid leaflet may divide it into two components. The atrioventricular component separates the left ventricle from the right atrium, whereas the interventricular component separates the ventricles.

The branching bundle is traditionally described as lying beneath the interventricular membranous septum. This description is useful but not universally constant. In almost three-fifths of examined hearts, a distinct interventricular component of the fibrous membranous septum could not be identified [3]. In such hearts, the axis remains related to the fibrous–muscular junction and muscular septal crest, although the expected membranous landmark is absent or minimal.

Absence of an obvious interventricular membranous component does not indicate absence of conduction tissue. The surgeon must identify the true junction between the central fibrous body and muscular septum rather than assuming a constant membranous plate. The septal tricuspid leaflet attachment may further obscure the relationship of the penetrating and branching axis.

6. Branching Bundle and Bundle Branches

After reaching the muscular ventricular septal crest, the penetrating bundle becomes the branching bundle. This segment gives rise to the left bundle branch and then continues as the right bundle branch.

The left bundle branch spreads as a broad sheet over the left ventricular septal surface rather than remaining a single narrow cord. Its proximal fibers may approach the left ventricular outflow tract and hinge of the right coronary aortic leaflet. In an anatomic study, the superior edge of the left bundle was an average of 3.3 mm from that hinge, with a range of 0.4–10.2 mm [4]. Its position was influenced by the depth of the inferoseptal recess and angulation of the muscular ventricular septum.

The right bundle branch is comparatively narrow. It continues along or within the right ventricular septum and becomes subendocardial near the base of the medial papillary muscle, traditionally termed the muscle of Lancisi. It then extends toward the moderator band and anterior papillary muscle. This discrete course makes it vulnerable during right ventricular septal incisions, aggressive muscle resection, or outlet septal procedures.

7. Ventricular Septal Defects

The relationship between the conduction axis and a ventricular septal defect is determined by the defect’s borders and extension, not merely its visible location. In a perimembranous defect, the central fibrous body forms part of the margin, and the axis typically courses along the posteroinferior rim. It is especially vulnerable where the fibrous margin transitions to the muscular ventricular septal crest.

Phase-contrast computed tomography of eight hearts with ventricular septal defects localized the penetrating bundle at a median distance of 1.43 mm from the septal crest, with a range of 0.99–1.54 mm [5]. The study suggested that longitudinally oriented sutures along the posteroinferior rim may reduce the likelihood of crossing the axis and warned against sutures within the valley between the limbs of the septomarginal trabeculation.

These measurements are not a universal “safe distance.” They demonstrate how close the axis may lie to the surgical margin and reinforce the need to control suture depth, direction, and tension. A shallow bite may cause residual shunting or patch instability, whereas a deep or transseptal bite may capture the penetrating or branching bundle.

A muscular inlet defect has a different relationship: the conduction axis may course along the anterosuperior rim rather than the posteroinferior border [6]. Two defects encountered through the right atrium may therefore require opposite assumptions regarding the danger zone. Precise phenotypic classification must precede patch placement.

8. Conduction Anatomy in Other Congenital Malformations

In atrioventricular septal defects, the normal atrioventricular septal structures are deficient and the triangle of Koch is distorted. The atrioventricular node is displaced posteriorly and inferiorly, and the nonbranching bundle follows an elongated route before reaching the ventricular septum [6]. The axis is particularly vulnerable near the inferior margin of the ventricular component and beneath the inferior bridging leaflet. Normal-heart landmarks should not be directly transferred to this morphology.

Tetralogy of Fallot generally retains a conduction axis related to the area of aortic–mitral–tricuspid fibrous continuity, where the bundle penetrates near the posteroinferior margin of the ventricular septal defect [6]. Outlet septal malalignment changes the geometry of the defect but does not eliminate the importance of the fibrous margin.

More profound displacement is expected with atrioventricular discordance, ambiguous atrioventricular connection with left-handed ventricular topology, selected univentricular atrioventricular connections, and straddling tricuspid valves [6]. In these settings, the atrioventricular node may be anterior, posterior, or duplicated, and the penetrating bundle may follow a long and unexpected route.

Tricuspid atresia also demonstrates phenotype-specific variation. In two histologically examined hearts with type IB tricuspid atresia, the atrioventricular node was adjacent to the central fibrous body, the left bundle branches arose unusually close to the nodal–bundle junction, and the right bundle branch had a markedly elongated course to the right septal endocardium [7]. Although based on a small series, these findings illustrate how altered chamber connections and septal geometry reshape distal conduction pathways.

9. Mechanisms and Patterns of Operative Conduction Injury

Conduction injury may be immediate or delayed. Direct transection, needle penetration, or suture entrapment may cause abrupt block during cardiopulmonary bypass or after reperfusion. Edema, hematoma, ischemia, tissue traction, patch compression, or thermal injury may produce delayed or potentially reversible dysfunction.

The postoperative rhythm may help localize the injured segment. Injury to the atrioventricular node or penetrating bundle may cause high-grade or complete atrioventricular block. Injury to the proximal branching axis may produce bifascicular disease or complete infranodal block. Damage to the broad proximal left bundle may result in left bundle branch block or fascicular block, whereas injury along the discrete right ventricular septal pathway may produce right bundle branch block.

Transient block does not prove that the injury is benign. Serial electrocardiography should assess the PR interval, QRS duration, frontal-plane axis, bundle-branch morphology, and intermittent atrioventricular block. Temporary pacing capability is essential after operations involving the perimembranous septum, atrioventricular junction, or subaortic region.

10. Operative Principles for Conduction Preservation

Conduction preservation begins with morphological analysis rather than a fixed suture rule.

1. Define the atrioventricular connection, ventricular topology, septal alignment, and exact defect borders.
2. Identify the central fibrous body, tricuspid valve hinge, muscular septal crest, and fibrous–muscular junction.
3. Predict the expected conduction route for the specific phenotype.
4. Control needle direction, bite depth, knot position, and patch tension along the predicted axis.
5. Avoid unnecessary traction, deep septal annular sutures, aggressive muscle resection, and thermal energy near the penetrating or branching bundle.
6. Compare preoperative and postoperative conduction, with temporary pacing available when the operative field approaches the axis.

Anatomic imaging, surgical inspection, and histology-derived rules remain the principal basis for predicting the conduction axis. Intraoperative electrical mapping may offer patient-specific localization in selected complex lesions, but the available literature does not establish standardized protocols, comparative effectiveness, or clearly defined indications. Mapping should be considered an adjunct to, rather than a substitute for, detailed morphological understanding.

References

[1] Anderson RH, Ho SY, Becker AE. The surgical anatomy of the conduction tissues. Thorax. 1983;38(6):408-420. doi:10.1136/thx.38.6.408.

[2] Saremi F, Hassani C, Sánchez-Quintana D. Septal atrioventricular junction region: comprehensive imaging in adults. Radiographics. 2016. doi:10.1148/RG.2016160010.

[3] Cabrera JA, Anderson RH, Macías Y, Nevado-Medina J, Porta-Sánchez A, Rubio J, Sánchez-Quintana D. Variable arrangement of the atrioventricular conduction axis within the triangle of Koch: implications for permanent His bundle pacing. JACC Clin Electrophysiol. 2020. doi:10.1016/j.jacep.2019.12.004.

[4] Macías Y, Tretter JT, Sánchez-Quintana D, Cabrera JA, Spicer DE, de Almeida MC, Anderson RH. The atrioventricular conduction axis and the aortic root: inferences for transcatheter replacement of the aortic valve. Clin Anat. 2021. doi:10.1002/ca.23793.

[5] Yoshitake S, Kaneko Y, Morita K, Hoshino M, Oshima Y, Takahashi M, Anderson RH. Visualization and quantification of the atrioventricular conduction axis in hearts with ventricular septal defect using phase-contrast computed tomography. J Thorac Cardiovasc Surg. 2020. doi:10.1016/j.jtcvs.2020.02.036.

[6] Ho SY, Anderson RH. Conduction tissue in congenital heart surgery. World J Surg. 1985. doi:10.1007/BF01656057.

[7] Guller B, DuShane JW, Titus JL. The atrioventricular conduction system in two cases of tricuspid atresia. Circulation. 1969;40(2):217. doi:10.1161/01.CIR.40.2.217.