Membranous Septum #2: Conduction Axis Relationship

Relationship Between the Membranous Septum, AV Node, and His Bundle

The membranous septum is a small but surgically critical structure located at the central fibrous body, between the atrioventricular junction and the ventricular septum. Although it is anatomically small, it has major clinical importance because the atrioventricular conduction axis passes in intimate relationship to this region. Injury to this area during septal surgery can result in transient or permanent atrioventricular block.

The key concept is that the membranous septum is not simply a passive fibrous partition. It is the anatomical gateway through which the atrioventricular conduction axis passes from the AV node into the ventricular septum. In the normal heart, the AV node gives rise to the penetrating bundle near the atrioventricular component of the membranous septum. The bundle then courses along the inferior margin of the interventricular component before bifurcating into the right and left bundle branches [1, 2].

1. Anatomical Components of the Membranous Septum

The membranous septum is traditionally divided into two components: the atrioventricular component and the interventricular component. This distinction is clinically important because the septal leaflet of the tricuspid valve divides the membranous septum into atrial-facing and ventricular-facing portions.

1.1 Atrioventricular Component

The atrioventricular component separates the right atrium from the left ventricle. It is located superior to the septal leaflet of the tricuspid valve and immediately adjacent to the central fibrous body.

This region is surgically important because the compact AV node and penetrating bundle are located at or near this fibrous junction. The conduction axis transitions from the atrial septal region into the ventricular septal region by passing through the fibrous skeleton of the heart.

1.2 Interventricular Component

The interventricular component separates the left ventricle from the right ventricle. It is located inferior to the atrioventricular component and is bordered on the right ventricular side by the septal leaflet of the tricuspid valve.

After penetrating the central fibrous body, the His bundle courses along the inferior margin of this interventricular component. This relationship explains why sutures placed deeply along the posteroinferior rim of a perimembranous VSD may endanger the conduction axis.

2. Course of the Conduction Axis

The normal atrioventricular conduction axis can be understood in four sequential segments: the AV node, penetrating bundle, non-branching bundle, and bifurcating bundle.

2.1 AV Node

The AV node is located within the triangle of Koch. The practical surgical boundaries of this triangle are:

1. The tendon of Todaro
2. The septal leaflet of the tricuspid valve
3. The coronary sinus orifice

From a surgical perspective, the AV node lies on the atrial side of the central fibrous body. It is not usually visible directly during surgery, but its location can be inferred from anatomical landmarks. In hearts with normally aligned atrial and ventricular septal structures, the triangle of Koch remains a useful guide to the location of the AV node and penetrating bundle [2].

2.2 Penetrating Bundle

The AV node continues as the penetrating bundle. This bundle passes through the fibrous skeleton of the heart near the atrioventricular component of the membranous septum.

This is one of the most vulnerable portions of the conduction axis because the tissue transitions from the atrial septal region into the ventricular septal region. Surgical trauma, deep sutures, aggressive traction, excessive cautery, or resection near the central fibrous body may injure the penetrating bundle.

2.3 Non-branching Bundle

After penetrating the fibrous body, the conduction axis becomes the non-branching His bundle. It courses along the inferior margin of the interventricular component of the membranous septum.

This segment is particularly relevant during closure of perimembranous ventricular septal defects. In many perimembranous defects, the conduction tissue runs along the posteroinferior margin of the defect, close to the hinge line of the septal tricuspid leaflet [2, 3].

2.4 Bifurcating Bundle and Bundle Branches

The His bundle subsequently bifurcates into the right and left bundle branches. The right bundle branch courses toward the moderator band and right ventricular free wall, while the left bundle branch fans out beneath the left ventricular septal endocardium.

Once the conduction axis has bifurcated, the right and left bundle branches become more widely distributed along the ventricular septum. However, injury proximal to bifurcation may produce complete atrioventricular block, whereas more distal injury may produce bundle branch block.

3. Surgical Significance

The membranous septum is one of the most important conduction landmarks in congenital heart surgery. Its clinical importance is greatest in operations involving the ventricular septum, atrioventricular valves, and left ventricular outflow tract.

Relevant procedures include:

1. Perimembranous VSD closure
2. Tetralogy of Fallot repair
3. Complete AVSD repair
4. Double-outlet right ventricle repair
5. Subaortic membrane resection
6. LVOT enlargement procedures
7. Tricuspid valve repair near the septal leaflet
8. Aortic valve or aortic root surgery involving the right-non-coronary commissural region

The surgeon must mentally map the conduction axis even when it is not directly visible. The membranous septum should therefore be treated not only as an anatomical structure, but also as a high-risk conduction zone.

4. Relationship to Perimembranous VSD

In a perimembranous VSD, the defect is adjacent to the membranous septum and central fibrous body. The conduction axis typically runs along the posteroinferior margin of the defect, where atrioventricular fibrous continuity is present [2, 3].

This has several operative implications:

1. Sutures should avoid deep bites along the posteroinferior rim.
2. The septal leaflet of the tricuspid valve may be used as a protective landmark.
3. Patch sutures should be placed with awareness of the expected course of the non-branching bundle.
4. Excessive traction on the septal leaflet or membranous septal remnant should be avoided.
5. Temporary postoperative conduction disturbance may occur from edema even without direct transection of conduction tissue.

The goal is complete VSD closure without compromising the penetrating bundle, non-branching bundle, or proximal bundle branches.

5. Conduction Axis Variation by VSD Subtype

The relationship between the conduction axis and the VSD margin is not identical across all VSD subtypes. This is one of the most important practical points for congenital heart surgeons.

5.1 Perimembranous Inlet VSD

In perimembranous inlet VSDs, the conduction axis may lie extremely close to the septal rim. Because the defect extends toward the inlet septum and atrioventricular junction, the penetrating and non-branching bundles may be particularly vulnerable during patch placement.

The key surgical danger is assuming that the conduction axis has the same location as in a simple perimembranous defect. In inlet defects, the conduction axis can be displaced relative to the surgeon’s field, and the safe margin for suture placement may be very narrow [4].

5.2 Muscular Inlet VSD

Muscular inlet VSDs differ from perimembranous inlet defects because the defect has a muscular rim rather than fibrous continuity with the central fibrous body. This distinction changes the relationship between the conduction axis and the defect.

In surgical terms, the morphology of the rim—not merely the geographic location of the hole—determines where the conduction axis is likely to run. Therefore, inlet VSDs should not be treated as a single uniform surgical category [4].

5.3 Perimembranous Outlet VSD and Tetralogy of Fallot

In perimembranous outlet VSDs, including many defects encountered in tetralogy of Fallot, the proximal AV conduction axis is usually remote from the superior or outlet margin of the defect. However, the right bundle branch may be at risk during closure of the ventricular septal defect or during work around the septomarginal trabeculation and medial papillary muscle.

A systematic anatomical review showed that in normal hearts the right bundle branch runs posterior to Lancisi’s muscle, whereas in perimembranous outlet VSDs, especially tetralogy of Fallot, the right bundle branch typically courses anterior to the medial papillary muscle. This makes the medial papillary muscle and septomarginal trabeculation important landmarks during VSD closure [5].

6. Role of the Trabecula Septomarginalis

The trabecula septomarginalis modifies the relationship between the conduction bundle and the margins of ventricular septal defects. In some hearts, the trabecula septomarginalis provides a muscular landmark that helps separate the conduction tissue from the surgical margin. In other hearts, particularly when the trabecular structure is deficient or altered, the conduction bundle may become more exposed along the septal crest [6].

This is relevant in tetralogy of Fallot and other outlet-type VSDs, where the surgeon often works near the septomarginal trabeculation, medial papillary muscle, and right bundle branch. The absence, deficiency, or displacement of these structures may change the expected safe zone for sutures.

7. Relationship to the Tricuspid Valve

The septal leaflet of the tricuspid valve is closely related to the membranous septum. Its hinge line helps define the boundary between the atrioventricular and interventricular components.

This relationship is clinically useful because the tricuspid valve can obscure the true rim of a perimembranous VSD. In some operations, the surgeon may detach or retract the septal leaflet to improve exposure. When doing so, the surgeon must remember that the conduction axis lies deep and inferior to this region.

Injury may occur not only from sutures placed directly into the septum, but also from traction, cautery, patch distortion, or annular sutures placed too close to the central fibrous body.

8. Practical Surgical Concept

A useful operative rule is:

The AV node is atrial, the penetrating bundle is membranous, and the non-branching bundle is ventricular.

This concept helps translate three-dimensional anatomy into operative decision-making.

• The AV node lies near the atrial septal side of the central fibrous body.
• The penetrating bundle passes through the fibrous skeleton near the atrioventricular component of the membranous septum.
• The non-branching bundle runs along the inferior margin of the interventricular component.
• The bundle then bifurcates along the ventricular septum.
• In perimembranous defects, the posteroinferior rim is the principal danger zone.
• In outlet defects, the right bundle branch and its relationship to the medial papillary muscle become especially important.

Therefore, the membranous septum should always be interpreted as both an anatomical structure and a conduction-risk zone.

9. Clinical Consequences of Injury

Damage to the conduction axis can result in:

1. First-degree AV block
2. Bundle branch block
3. Junctional rhythm
4. Transient complete heart block
5. Permanent complete heart block requiring pacemaker implantation

Transient block may result from edema, inflammation, stretch, or local ischemia. Permanent block usually reflects direct structural injury to the AV node, penetrating bundle, non-branching bundle, or proximal His bundle.

In congenital heart surgery, complete anatomical repair must therefore be balanced against conduction preservation. This is particularly important in small infants, redo operations, complex VSD anatomy, AVSD, double-outlet right ventricle, tetralogy of Fallot, and lesions with abnormal atrioventricular alignment.

10. Key Takeaways

The membranous septum is small, but it defines one of the most important conduction landmarks in septal surgery.

The AV node gives rise to the penetrating bundle near the atrioventricular component of the membranous septum.

The His bundle then courses along the inferior margin of the interventricular component as a non-branching bundle.

The conduction axis subsequently bifurcates into the right and left bundle branches along the ventricular septum.

The location of the conduction axis varies by VSD subtype. Perimembranous inlet defects place the penetrating and non-branching bundles at particular risk, while outlet defects more commonly raise concern for the distal branching bundle and right bundle branch.

For the congenital heart surgeon, safe septal surgery requires more than identifying the hole. It requires understanding the fibrous rim, muscular rim, tricuspid valve relationship, trabecula septomarginalis, medial papillary muscle, and expected course of the conduction axis.

References

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

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

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

[4] Spicer DE, Anderson RH, Backer CL. Clarifying the surgical morphology of inlet ventricular septal defects. Ann Thorac Surg. 2013;95(1):236-241.

[5] Yoneyama F, Kato H, Mathis BJ, Suetsugu F, Hiramatsu Y. Right bundle branch in ventricular septal defects. Eur J Cardiothorac Surg. 2025;67(4):ezaf105.

[6] Kurosawa H, Becker AE. Modification of the precise relationship of the atrioventricular conduction bundle to the margins of the ventricular septal defects by the trabecula septomarginalis. J Thorac Cardiovasc Surg. 1984;87(4):605-615.