Right Atrium Anatomy #4:

Membranous Septum

The membranous septum is a small but strategically important fibrous structure located at the central atrioventricular junction. It lies where the atrial septum, muscular ventricular septum, atrioventricular valves, and central fibrous body converge. Despite its limited size, it is one of the most important landmarks in congenital heart surgery because the atrioventricular conduction axis penetrates this region before entering the ventricular septum [1].

From the right atrial surgical view, the membranous septum should not be regarded merely as a passive septal structure. Rather, it represents a three-dimensional warning zone where septal anatomy, tricuspid valve attachment, the fibrous skeleton, and the conduction system are closely integrated [2].

1. Anatomical Concept

The membranous septum is composed primarily of fibrous tissue, in contrast to the surrounding muscular ventricular septum. It is positioned near the apex of the Triangle of Koch, adjacent to the septal leaflet of the tricuspid valve and the central fibrous body.

Anatomically, it forms a compact junction between:

1. The atrial septum
2. The muscular ventricular septum
3. The septal attachment of the tricuspid valve
4. The central fibrous body
5. The atrioventricular conduction axis

This explains why the membranous septum is simultaneously an anatomic landmark, a surgical reference point, and a conduction-risk zone.

2. Division by the Septal Tricuspid Leaflet

A key feature of the membranous septum is that the septal leaflet of the tricuspid valve divides it into two distinct components.

2.1 Atrioventricular Membranous Septum

AVMS

The atrioventricular membranous septum lies superior to the attachment of the septal tricuspid leaflet.

It separates the:

• Right atrium
• Left ventricle

This is anatomically unique because it creates a fibrous partition between an atrial chamber and a ventricular chamber. Therefore, the AVMS is not an interventricular structure; it belongs to the deeper architecture of the atrioventricular junction.

2.2 Interventricular Membranous Septum

IVMS

The interventricular membranous septum lies inferior to the septal tricuspid leaflet.

It separates the:

• Right ventricle
• Left ventricle

This component is especially relevant to perimembranous ventricular septal defects, because these defects are located adjacent to the membranous septum and its surrounding muscular rims.

3. Relationship to the Conduction System

The clinical importance of the membranous septum is defined by its relationship to the AV node–His bundle axis.

The conduction pathway can be understood in the following sequence:

1. The AV node lies within the Triangle of Koch.
2. The penetrating His bundle passes through the central fibrous body.
3. The conduction axis then runs close to the membranous septum.
4. The bundle courses along the inferior margin of the interventricular membranous septum before dividing into the right and left bundle branches [1, 2].

This relationship explains why operations around the membranous septum may produce conduction disturbances, including:

• Right bundle branch block
• Left bundle branch block
• Complete atrioventricular block
• Need for permanent pacing, in severe cases

The conduction system is usually invisible in the operative field. Therefore, safe surgery depends on recognizing the anatomic landmarks that predict its course, rather than attempting to identify the conduction tissue directly.

4. Perimembranous VSD and the Conduction Axis

A perimembranous VSD is not simply a hole in the membranous septum itself. More accurately, it is usually a defect in the muscular ventricular septum that is directly adjacent to the membranous septum.

Classic anatomic studies demonstrated that, in membranous and perimembranous defects, the common atrioventricular bundle typically courses along the posteroinferior margin of the defect and bifurcates near its inferior border [3].

However, the relationship is not identical in all VSDs. The location of the conduction tissue varies according to the extension of the defect:

• Perimembranous inlet VSD: the nonbranching bundle may lie very close to the septal rim and can be embedded in membranous remnants.
• Perimembranous outlet VSD: the conduction tissue is often displaced away from the crest and may be protected by surrounding muscular structures.
• High perimembranous defects: the penetrating bundle may run immediately adjacent to the defect margin, increasing the risk of injury during patch closure [4].

Thus, the surgical risk is not determined only by the diagnosis of “perimembranous VSD,” but by the precise spatial relationship between the defect rim, membranous septum, tricuspid leaflet, and conduction axis.

5. Surgical Implications During VSD Closure

During closure of a perimembranous VSD, the key danger zone is the region near the posteroinferior rim of the defect. Sutures placed too deeply in this area may injure the AV conduction axis.

Practical surgical principles

• Avoid deep sutures at the posteroinferior margin of the VSD.
• Use shallow, controlled bites when approaching the suspected conduction zone.
• Respect the hinge line of the septal tricuspid leaflet.
• Use tricuspid leaflet tissue for suture placement when appropriate.
• Maintain a three-dimensional mental image of the AV node, penetrating bundle, branching bundle, and right bundle branch.
• Recognize that the safest suture line may differ according to VSD subtype.

In tetralogy of Fallot, detailed anatomic work showed that the membranous flap can serve as a useful and relatively safe structure for suturing in selected perimembranous outlet defects. In one clinical series, a membranous flap-based technique was applied in 79 patients, with sinus rhythm preserved and no complete heart block reported [5].

6. Right Bundle Branch: Beyond the His Bundle

While complete AV block is usually associated with injury to the AV node or His bundle, postoperative right bundle branch block reflects injury to the right-sided distal conduction system. This is particularly relevant in VSD repair and tetralogy of Fallot repair.

Recent systematic review data suggest that the course of the right bundle branch differs according to VSD subtype. In normal hearts, the right bundle branch typically runs posterior to the muscle of Lancisi. In perimembranous outlet VSDs, especially tetralogy of Fallot, it may course approximately 2 mm anterior to the medial papillary muscle in a substantial proportion of cases [6].

This finding is surgically important because the medial papillary muscle and septomarginal trabeculation are often used as operative landmarks. The surgeon must understand that these landmarks do not always indicate the same conduction pathway in every VSD morphology.

7. Other Congenital Lesions Around the Membranous Septum

The membranous septum and conduction axis are also important in more complex congenital lesions.

7.1 Atrioventricular Septal Defect

In AVSD, the atrioventricular junction is abnormal, and the conduction axis may be displaced from its usual position. The risk of conduction injury is therefore related not only to the VSD component, but also to the abnormal arrangement of the AV node and bridging leaflets.

7.2 Double Outlet Right Ventricle

In DORV, surgical risk depends heavily on the relationship between the VSD, outflow tracts, and conduction axis. When the VSD is perimembranous or remote, intracardiac baffling may bring sutures close to conduction tissue.

7.3 Truncus Arteriosus

In truncus arteriosus, the VSD is often anterior and related to the truncal root. However, when the VSD is close to the membranous septum, the atrioventricular bundle and early bundle branches may become vulnerable during closure [7].

These examples emphasize that the membranous septum should be interpreted as part of a broader atrioventricular septal junction complex, rather than as an isolated structure.

8. Modern Imaging and Functional Anatomy

Modern CT and MRI have improved visualization of the septal atrioventricular junction, including the membranous septum, central fibrous body, inferior pyramidal space, and surrounding valve attachments [2]. Although the conduction tissue itself is generally not directly visible on routine clinical imaging, imaging helps define the structures that predict its location.

In electrophysiology, the same region has become increasingly important for His bundle pacing and conduction system pacing. These techniques reinforce the concept that the membranous septum, tricuspid valve tissue, and adjacent ventricular myocardium form a compact functional zone where small differences in position can determine whether pacing captures atrial myocardium, ventricular myocardium, or the conduction axis itself [8].

For the congenital surgeon, this modern perspective supports a simple principle:

the membranous septum is not just anatomy; it is functional anatomy.

9. Surgeon’s View

From the right atrium, the membranous septum is interpreted through surrounding landmarks:

• Septal leaflet of the tricuspid valve
• Triangle of Koch
• Coronary sinus
• Tendon of Todaro
• Central fibrous body
• VSD rim, when present
• Muscle of Lancisi and septomarginal trabeculation
• Medial papillary muscle, especially in outlet-type defects

The surgeon rarely sees the conduction tissue directly. Instead, safe repair depends on translating visible landmarks into an invisible conduction map.

In this sense, the membranous septum functions as a surgical compass. It helps the surgeon estimate where the penetrating bundle enters the ventricular septum, where the branching bundle lies, and where sutures must be placed with particular caution.

10. Clinical Summary

The membranous septum is a small fibrous structure with major surgical importance. It forms the central landmark of the atrioventricular septal junction and is divided by the septal tricuspid leaflet into the atrioventricular membranous septum and interventricular membranous septum.

Its importance comes from its intimate relationship with the AV node–His bundle axis. The penetrating bundle passes through this region and courses near the inferior margin of the interventricular membranous septum before bifurcating into the bundle branches. Therefore, operations around the perimembranous septum—especially VSD closure—require precise three-dimensional understanding of the conduction system.

Essential points

• The membranous septum is a fibrous landmark at the central atrioventricular junction.
• The septal tricuspid leaflet divides it into the AVMS and IVMS.
• The AVMS separates the right atrium from the left ventricle.
• The IVMS separates the right ventricle from the left ventricle.
• The His bundle penetrates the central fibrous body and runs close to the IVMS.
• In perimembranous VSDs, the conduction axis is usually at risk near the posteroinferior margin.
• The exact risk zone varies according to VSD subtype and associated congenital anatomy.
• Safe repair requires landmark-based prediction of the invisible conduction pathway.

Concise Version for Notion

The membranous septum is a compact fibrous landmark at the central atrioventricular junction, where the atrial septum, ventricular septum, atrioventricular valves, and central fibrous body converge. The septal leaflet of the tricuspid valve divides it into the atrioventricular membranous septum, which separates the right atrium from the left ventricle, and the interventricular membranous septum, which separates the two ventricles.

Its surgical importance is defined by the conduction axis. The AV node gives rise to the penetrating His bundle, which passes through the central fibrous body and courses near the inferior margin of the interventricular membranous septum before bifurcating into the bundle branches. Therefore, repair around the perimembranous septum—especially VSD closure—requires meticulous suture placement and a three-dimensional understanding of the invisible conduction pathway.

References

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

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

[3] Latham RA, Anderson RH. Anatomical variations in atrioventricular conduction system with reference to ventricular septal defects. Br Heart J. 1972;34(2):185-190.

[4] 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.

[5] Kurosawa H, Imai Y, Becker AE. Surgical anatomy of the atrioventricular conduction bundle in tetralogy of Fallot. New findings relevant to the position of the sutures. J Thorac Cardiovasc Surg. 1988;95(4):586-591.

[6] 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.

[7] Bharati S, Karp RB, Lev M. The conduction system in truncus arteriosus and its surgical significance. A study of five cases. J Thorac Cardiovasc Surg. 1992;104(4):954-960.

[8] Mulpuru SK, Cha YM, Asirvatham SJ. Synchronous ventricular pacing with direct capture of the atrioventricular conduction system: Functional anatomy, terminology, and challenges. Heart Rhythm. 2016;13(11):2237-2246.