Cardiac Fibrous Skeleton #3: Tricuspid Valve, Membranous Septum, and the Atrioventricular Conduction Axis

1. Overview

The tricuspid valve is frequently described as having an annulus, but the right atrioventricular junction does not possess a complete, uniformly dense fibrous ring. Its strongest fibrous support is concentrated along the septal hinge, where the tricuspid valve is integrated with the central fibrous body, membranous septum, and atrioventricular conduction axis. The anterior and posterior portions of the hinge are supported predominantly by ventricular myocardium, fibro-fatty tissue, and discontinuous fibrous strands rather than by a continuous annular structure [1].

This regional anatomy makes the septal tricuspid junction simultaneously a structural anchor, an electrophysiologic landmark, and a major surgical danger zone. The septal leaflet overlies the membranous septum, the compact atrioventricular node is located within the inferior pyramidal space represented on the right atrial surface by Koch’s triangle, and the penetrating atrioventricular bundle traverses the central fibrous body immediately adjacent to the septal hinge [2,3].

Consequently, operations involving the septal tricuspid annulus, perimembranous ventricular septal defects, atrioventricular septal defects, or the membranous septum require a three-dimensional understanding of the conduction axis rather than reliance on a single visible landmark.

2. Fibrous Architecture of the Tricuspid Annulus

The right atrioventricular junction is nonplanar, dynamic, and structurally heterogeneous. Its free-wall components enlarge and contract during the cardiac cycle because they are supported mainly by myocardium and fibro-fatty tissue. This differs from the more substantial fibrous continuity present in portions of the mitral annulus.

The anterior and posterior tricuspid leaflet hinge lines are attached primarily to the right ventricular free wall and atrioventricular groove. Discontinuous fibrous strands, sometimes termed the fila coronaria, contribute support but do not form a complete ring. The true fibrous component is concentrated along the septal hinge, particularly near the central fibrous body and membranous septum [1].

This architecture explains several clinically important features:

• The tricuspid annulus changes considerably in size and shape with right ventricular loading.
• Functional tricuspid regurgitation commonly results from annular dilation and leaflet tethering rather than primary leaflet pathology.
• Free-wall annuloplasty sutures engage relatively compliant tissue.
• Septal annuloplasty sutures engage more fixed structures but carry greater risk of conduction injury.

The term annulus is therefore useful as a geometric description of the leaflet hinge line, but it should not be interpreted as indicating a continuous fibrous ring.

3. Central Fibrous Body and Right Fibrous Trigone

The central fibrous body is the dense fibrous intersection connecting the aortic root, mitral valve, tricuspid valve, and membranous septum. It incorporates the region of the right fibrous trigone and forms part of the insulating plane between atrial and ventricular myocardium [1,2].

From the right atrial perspective, the central fibrous body lies near the apex of Koch’s triangle and the anterosuperior portion of the septal tricuspid hinge. From the left ventricular outflow tract, it is related to the noncoronary and right coronary aortic sinuses. From the ventricular side, it is located above the crest of the muscular ventricular septum.

The central fibrous body has two essential functions:

1. Mechanical integration: It anchors the adjacent valve hinges and maintains continuity between the atrioventricular valves and aortic root.
2. Electrical insulation: It separates atrial from ventricular myocardium except where the specialized atrioventricular conduction axis penetrates the fibrous plane.

The penetrating atrioventricular bundle passes through this fibrous structure, making it the only normal electrical bridge across the insulating atrioventricular junction [3]. The same tissue that provides mechanical stability therefore contains one of the most vulnerable components of the cardiac conduction system.

4. Septal Tricuspid Leaflet and the Membranous Septum

The membranous septum is a small fibrous plate situated between the aortic root, central fibrous body, right atrium, and muscular ventricular septum. Although small, it is among the most surgically important structures in congenital cardiac anatomy.

The hinge of the septal tricuspid leaflet divides the membranous septum into two potential components:

• The atrioventricular membranous septum, located above the septal leaflet hinge, separates the left ventricle from the right atrium.
• The interventricular membranous septum, located below the hinge, separates the left and right ventricles [1,2].

The relative size of these components depends on the level at which the septal leaflet attaches. A more apical tricuspid hinge creates a larger atrioventricular component, whereas attachment closer to the aortic root reduces or eliminates the interventricular component.

The septal leaflet overlies the membranous septum and may obscure it during a right atrial or trans-tricuspid surgical approach. The surgeon therefore sees the leaflet and its hinge rather than the entire fibrous plate. This distinction is important during ventricular septal defect closure because the visible leaflet margin does not directly delineate the underlying course of the penetrating and branching atrioventricular bundle.

Adhesion of tricuspid valvar tissue to the margins of a perimembranous ventricular septal defect may produce partial or complete functional closure. The resulting tissue may appear aneurysmal and can contain components of the septal leaflet, accessory tricuspid tissue, and fibrous tissue rather than representing a true aneurysm of the original membranous septum [2].

5. Koch’s Triangle and the Inferior Pyramidal Space

Koch’s triangle is the principal right atrial surface landmark used to identify the compact atrioventricular node. Its conventional boundaries are:

1. The tendon of Todaro
2. The septal tricuspid hinge
3. The coronary sinus orifice

The triangle is best understood as the endocardial surface of a deeper three-dimensional region known as the inferior pyramidal space. The compact atrioventricular node lies within this space near the anterosuperior apex of the triangle, adjacent to the central fibrous body [2,4].

The tendon of Todaro is a fibrous continuation extending from the region of the Eustachian valve toward the central fibrous body. Its direction helps identify the apex of Koch’s triangle, but the tendon may be indistinct, broad, interrupted, or difficult to recognize. It should therefore be used in combination with the coronary sinus orifice and septal tricuspid hinge.

The atrioventricular node is not necessarily located at the geometric center of the triangle. Its position is influenced by the dimensions of the triangle, the depth of the inferior pyramidal space, the arrangement of the atrial septal buttress, and the relationship between the atrial and ventricular septal structures.

The coronary sinus orifice marks the posteroinferior region of the triangle, while the septal tricuspid hinge forms its anterior-inferior boundary. The compact node is generally closer to the apex, where the tendon of Todaro approaches the central fibrous body.

6. Course of the Atrioventricular Conduction Axis

The atrioventricular conduction axis consists of several anatomically distinct components:

• Compact atrioventricular node
• Penetrating atrioventricular bundle
• Nonbranching atrioventricular bundle
• Branching atrioventricular bundle
• Right and left bundle branches

The compact node is connected to atrial myocardium within the inferior pyramidal space. It then becomes the penetrating bundle as it enters the insulating tissues of the central fibrous body. After penetrating the fibrous plane, the axis courses along the crest of the muscular ventricular septum beneath or adjacent to the membranous septum [3].

The nonbranching bundle remains insulated from surrounding ventricular myocardium before reaching the branching portion. The branching bundle gives rise to the left bundle branch and continues as the right bundle branch. The left bundle spreads beneath the left ventricular septal endocardium, whereas the right bundle courses along the right ventricular aspect of the septum toward the moderator band and anterior papillary muscle.

The relationship between the conduction axis and membranous septum is not uniform. The bundle may lie directly on the septal crest, slightly leftward, or within fibrous tissue adjacent to the membranous septum. Its branching point may occur proximal to, beneath, or distal to the visible membranous component.

This arrangement explains why injury can occur from either side of the septum. A suture placed through the tricuspid side, left ventricular outflow tract, aortic annulus, or ventricular septal crest may compromise the same conduction axis.

7. Anatomical Variability

Although Koch’s triangle, the central fibrous body, and the membranous septum provide reproducible conceptual landmarks, the precise location of the atrioventricular conduction axis varies considerably among individuals.

In a combined histological and clinical imaging study, Cabrera and colleagues demonstrated substantial variation in the point at which the conduction axis penetrated the central fibrous body within the region of Koch’s triangle. The axis did not consistently occupy a single predictable location relative to the septal tricuspid hinge or membranous septum [4].

The same study found that an identifiable interventricular component of the membranous septum was absent in almost three-fifths of examined hearts [4]. In such hearts, the tricuspid leaflet hinge attached near the border between the central fibrous body and muscular septum, leaving little or no fibrous tissue separating the conduction axis from the ventricular septal crest.

This variability has several implications:

• A large membranous septum does not guarantee a large margin of safety.
• A small or absent interventricular component may place the branching bundle close to the tricuspid hinge.
• The apex of Koch’s triangle is a regional guide rather than a precise point.
• Visible anatomy cannot reliably define the exact depth of the conduction tissue.
• Congenital malalignment may further alter all expected relationships.

Surgical planning should therefore be based on an anticipated danger zone rather than a presumed linear course of the bundle.

8. Perimembranous Ventricular Septal Defects

A perimembranous ventricular septal defect is defined by the presence of fibrous continuity forming part of its margin, usually involving the central fibrous body and adjacent atrioventricular or arterial valve tissue. The atrioventricular conduction axis typically runs along the posteroinferior margin of the defect [5].

From a transatrial approach, the defect is viewed through the tricuspid valve. The central fibrous body and tricuspid–mitral fibrous continuity form part of the surgeon’s right-hand margin. The penetrating and nonbranching components of the conduction axis are generally related to this fibrous border.

The exact relationship depends on the extension of the defect:

• In central or trabecular perimembranous defects, the bundle usually passes along the posteroinferior rim.
• In perimembranous inlet defects, the penetrating bundle may detour deeply into the central fibrous body before reaching the ventricular septal crest.
• In defects with outlet extension, the bundle may be farther from portions of the visible margin but remains vulnerable near the central fibrous body.
• In muscular inlet defects, the conduction axis follows a different course and may be related to the opposite margin from that expected in a perimembranous defect [5,6].

Chiu and colleagues emphasized that, in perimembranous inlet defects, deep sutures placed into the central fibrous body or tricuspid annular tissue may injure the penetrating bundle [6]. The danger cannot be avoided merely by staying away from the visible ventricular septal crest.

Patch sutures should therefore be placed with controlled depth along the posteroinferior margin. The objective is secure tissue purchase without penetrating deeply into the central fibrous body. Sutures may be placed slightly away from the apparent defect edge when necessary to avoid the predicted conduction pathway.

Temporary detachment of the septal tricuspid leaflet can improve exposure, but reattachment sutures must also remain superficial. After closure, the septal leaflet should be inspected for restricted motion, distortion, or residual tricuspid regurgitation.

9. Implications for Tricuspid Valve Surgery

The septal annulus requires a different surgical strategy from the anterior and posterior annulus. Aggressive annular reduction near the central fibrous body may damage the atrioventricular node or penetrating bundle. Septal annuloplasty sutures should therefore be shallow and should avoid excessive incorporation of the fibrous septal tissues.

The greatest conduction risk is concentrated near the anterosuperior septal hinge, where the conduction axis penetrates the central fibrous body. More posterior sutures are farther from the penetrating bundle but remain close to Koch’s triangle and the compact atrioventricular node.

During tricuspid leaflet detachment, cleft closure, commissural reconstruction, or septal annuloplasty, the surgeon should preserve the original level of the septal hinge. Sutures placed too ventricularly may capture conduction tissue or restrict leaflet motion. Sutures placed too atrially may produce inadequate leaflet support and residual regurgitation.

In congenital tricuspid valve disease, normal landmarks may be distorted. Ebstein malformation, straddling tricuspid valve, atrioventricular discordance, ventricular topology abnormalities, and prior septal repair may alter the expected position of both the annulus and conduction axis [5].

10. Atrioventricular Septal Defects

In atrioventricular septal defects, the normal atrioventricular septal junction is deficient and the expected landmarks of Koch’s triangle are displaced. The compact atrioventricular node is usually located more posteriorly and inferiorly than in the normal heart, and the nonbranching bundle is elongated before reaching the ventricular septum [5].

The coronary sinus orifice and atrial septal structures may be displaced, making the normal apex of Koch’s triangle unreliable. Seo and colleagues demonstrated that the point at which the inferior bridging leaflet crosses the crest of the ventricular septum is a more consistent surgical landmark for predicting where the conduction axis penetrates in atrioventricular septal defects [7].

The bundle then courses along the inferior margin of the ventricular component. Sutures closing the ventricular component should be placed on the appropriate side of this anticipated pathway, particularly near the inferior bridging leaflet and ventricular septal crest.

The conduction axis in atrioventricular septal defects should therefore not be inferred from normal tricuspid annular anatomy. The morphology of the common atrioventricular junction, the bridging leaflets, and the relationship between the atrial and ventricular septa must determine the surgical danger zone.

11. Intraoperative Protection of the Conduction Axis

Before placing septal sutures, the surgeon should identify:

• The coronary sinus orifice
• The tendon of Todaro
• The septal tricuspid hinge
• The central fibrous body
• The membranous septum
• The crest of the muscular ventricular septum
• The morphology and extension of any ventricular septal defect

Deep cautery, traction, and dissection should be minimized near the apex of Koch’s triangle and central fibrous body. Sutures should be placed with controlled depth, particularly along the posteroinferior margin of perimembranous defects and the anterosuperior septal annulus.

New atrioventricular delay, junctional rhythm, bundle-branch block, or complete atrioventricular block during reconstruction should raise concern for conduction-axis compression or injury. When conduction changes occur immediately after placement of a specific suture, removal or repositioning may restore conduction if the mechanism is mechanical rather than destructive.

Temporary pacing wires and postoperative rhythm surveillance are appropriate after procedures involving the central fibrous body, membranous septum, or atrioventricular septal junction. Delayed atrioventricular block may result from edema, inflammation, hematoma, ischemia, or progressive fibrosis even when conduction is initially preserved.

12. Key Surgical Concept

The septal tricuspid valve is an integral component of the cardiac fibrous skeleton. Its hinge attaches to the central fibrous body, overlies the membranous septum, and forms a boundary of Koch’s triangle. The compact atrioventricular node lies within the deeper inferior pyramidal space, while the penetrating bundle traverses the central fibrous body and reaches the ventricular septal crest.

These structures form a continuous three-dimensional complex rather than separate anatomical landmarks. Safe surgery requires the surgeon to understand not only the visible leaflet and defect margins but also the probable depth and variability of the conduction axis.

The essential operative principle is to treat the septal tricuspid hinge, central fibrous body, and posteroinferior margin of a perimembranous defect as a conduction danger zone. Accurate morphological classification, shallow controlled suturing, preservation of leaflet mobility, and continuous attention to rhythm are fundamental to achieving durable reconstruction without atrioventricular block.

References

[1] Anderson RH, Ho SY, Becker AE. Anatomy of the human atrioventricular junctions revisited. Anat Rec. 2000;260(1):81-91. (Wiley Online Library)

[2] Saremi F, Hassani C, Sánchez-Quintana D. Septal atrioventricular junction region: comprehensive imaging in adults. RadioGraphics. 2016;36(7):1966-1986. (RSNA Publications Online)

[3] Anderson RH, Ho SY, Becker AE. The surgical anatomy of the conduction tissues. Thorax. 1983;38(6):408-420. (PubMed)

[4] Cabrera JA, Anderson RH, Macías Y, Nevado-Medina J, Porta-Sánchez A, Rubio JM, 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;6(4):362-377. (JAMA Network)

[5] Ho SY, Anderson RH. Conduction tissue in congenital heart surgery. World J Surg. 1985;9(4):550-567. (Springer)

[6] Chiu IS, Hung CR, Wang JK, Wu FF, How SW. The atrioventricular conduction axis of hearts with isolated ventricular septal defects. J Formos Med Assoc. 1990;89(11):997-1003. (PubMed)

[7] Seo JW, Zuberbuhler JR, Ho SY, Anderson RH. Surgical significance of morphological variations in the atrial septum in atrioventricular septal defect for determination of the site of penetration of the atrioventricular conduction axis. J Card Surg. 1992;7(4):324-332. (PubMed)