Membranous Septum: Small Structure, High-Stakes Anatomy

1. Core Concept

The membranous septum is a small but surgically critical fibrous component of the cardiac septal complex. It lies at the intersection of the muscular ventricular septum, atrial septum, atrioventricular junction, aortic root, tricuspid valve, and central fibrous body.

A perimembranous ventricular septal defect is defined by its relationship to this membranous septal region. The surgical importance is not only the size of the defect, but also the precise relationship of the defect to the conduction axis, septal leaflet of the tricuspid valve, aortic valve, and fibrous skeleton of the heart [1–3].

A key anatomic point is that a membranous flap may be present in some perimembranous VSDs, especially when there is outlet extension. This flap represents a remnant of the interventricular component of the membranous septum. It may be useful as part of the suture line, but it must be distinguished from the posteroinferior rim where the conduction axis is at risk.

2. Structural Anatomy of the Membranous Septum

The membranous septum is a fibrous septal structure located at the junction of:

• the muscular interventricular septum
• the atrial septum
• the atrioventricular junction
• the central fibrous body
• the aortic root, particularly beneath the interleaflet triangle between the right and non-coronary aortic cusps
• the septal hinge of the tricuspid valve

The hinge of the septal leaflet of the tricuspid valve divides the membranous septum into two components.

2.1 Atrioventricular Membranous Septum

The atrioventricular membranous septum lies above the hinge of the septal leaflet of the tricuspid valve.

It separates the right atrium from the left ventricle and is closely related to the central fibrous body and penetrating atrioventricular conduction bundle [2].

2.2 Interventricular Membranous Septum

The interventricular membranous septum lies below the hinge of the septal leaflet of the tricuspid valve.

It separates the right ventricle from the left ventricle. This component is directly relevant to perimembranous VSD anatomy. When a perimembranous defect is present, a portion of the interventricular membranous septum may remain as a fibrous flap along the margin of the defect.

3. Relationship to the Cardiac Conduction Axis

The atrioventricular conduction axis has a predictable but surgically hazardous relationship to the membranous septum.

The atrioventricular node gives rise to the penetrating bundle, which passes through the fibrous atrioventricular component of the membranous septum. The bundle then courses along the inferior margin of the interventricular membranous septum as a non-branching bundle before bifurcating into the right and left bundle branches [2].

In classic anatomic descriptions of ventricular septal defects, the conduction bundle passes along the posterior edge of membranous or perimembranous defects and bifurcates near the inferior border of the defect [4].

More recent three-dimensional phase-contrast computed tomography has quantified this relationship in hearts with VSD. In that study, the penetrating bundle was located very close to the septal crest, with a median distance of approximately 1.43 mm from the crest, ranging from 0.99 to 1.54 mm [5].

For the surgeon, the practical point is clear:

• the conduction axis is concentrated near the posteroinferior rim of the perimembranous VSD
• the highest-risk zone is near the septal leaflet hinge of the tricuspid valve
• the conduction tissue does not run randomly around the entire VSD margin
• the right bundle branch subsequently emerges onto the right ventricular septal surface and courses toward the septomarginal trabeculation

4. Perimembranous VSD: Anatomic Definition

A perimembranous VSD is a ventricular septal defect located adjacent to the membranous septum and central fibrous body. It usually opens into the right ventricle near the septal leaflet of the tricuspid valve and lies close to the aortic valve.

The defect may extend in different directions:

• Inlet extension toward the atrioventricular valve region
• Trabecular extension into the muscular septum
• Outlet extension toward the right ventricular outflow tract and subaortic region

This directional extension is surgically important. The same perimembranous defect may appear different depending on whether it is viewed from the right atrium, right ventricle, left ventricle, or aortic root. The surgeon must therefore define the true margins of the defect, not simply the visible opening between tricuspid valve tissue [1,3].

In outlet extension, the VSD may approach the subaortic region and the right and non-coronary aortic cusps. Patch closure must eliminate the full extent of the defect while avoiding distortion of the aortic valve [6].

5. The Membranous Flap

In some perimembranous VSDs, especially those with outlet extension, a membranous flap may be visible along the margin of the defect.

This flap represents a remnant of the interventricular membranous septum. It is anatomically distinct from the conduction axis.

The conduction axis courses along the posteroinferior rim near the septal leaflet hinge, but it does not run within the membranous flap itself. Therefore, when the flap is clearly identified and sufficiently robust, it may be incorporated into the suture line during patch closure.

This concept is surgically useful because the flap may provide a fibrous structure for patch anchoring. It may allow the surgeon to avoid deeper bites into the adjacent high-risk posteroinferior muscular rim.

However, the presence of a membranous flap should not be interpreted as complete protection from conduction injury. The base of the flap and the adjacent posteroinferior rim remain close to the atrioventricular conduction axis.

6. Surgeon’s View Through the Tricuspid Valve

Most perimembranous VSDs are approached through the right atrium and tricuspid valve.

From the surgeon’s view, the following landmarks should be identified before placing sutures:

• septal leaflet of the tricuspid valve
• anterior leaflet of the tricuspid valve
• medial papillary muscle
• septomarginal trabeculation
• posteroinferior rim of the VSD
• aortic valve relationship, especially when there is outlet extension
• membranous flap, if present

The defect should not be understood as a simple circular hole. It is a three-dimensional opening whose margins have different surgical risks.

The superior and anterosuperior margins may be close to the aortic valve. The posteroinferior margin is close to the conduction axis. The tricuspid valve leaflets and chordae may partially cover the defect and can either assist exposure or obscure the true margin.

7. Surgical Implications for Patch Closure

The principal surgical objective is complete closure of the VSD while preserving the conduction axis, aortic valve competence, and tricuspid valve function.

7.1 Posteroinferior Margin

The posteroinferior margin is the highest-risk region for conduction injury.

Sutures should be placed with precise awareness of the expected course of the atrioventricular conduction axis. Deep or aggressive bites near the septal leaflet hinge and posteroinferior rim may injure the penetrating or non-branching bundle.

Quantitative anatomic data showing the penetrating bundle within approximately 1–1.5 mm of the septal crest reinforces the need for shallow, controlled suture placement in this region [5].

When a membranous flap is present, it may be used as part of the suture line rather than taking deep bites into the adjacent muscular rim.

7.2 Septomarginal Trabeculation and Right Bundle Branch

The right bundle branch emerges onto the right ventricular septal surface and is related to the septomarginal trabeculation. Sutures placed deep in the valley of the septomarginal trabeculation may risk right bundle branch injury.

This is particularly relevant when closing large perimembranous defects from the right-sided approach. The surgeon should distinguish the conduction-risk zone of the posteroinferior rim from the more distal course of the right bundle branch on the right ventricular septal surface.

7.3 Suture Technique

Surgical technique influences conduction outcomes.

A modified shallow mattress suturing technique for large perimembranous VSD closure in infants was associated with no complete atrioventricular block in the reported cohort and a lower incidence of complete right bundle branch block compared with conventional suturing [7].

The surgical principle is not that a specific stitch pattern is universally mandatory. Rather, the important concept is that suture depth, direction, and tissue purchase must respect the conduction axis. Along the posteroinferior rim, sutures should secure the patch without deep penetration into the conduction tissue plane.

7.4 Superior and Outlet Margin

When there is outlet extension, the patch must close the full superior and outflow component of the defect. Failure to recognize outlet extension may leave a residual VSD.

At the same time, sutures near the aortic valve must avoid distortion of the right coronary cusp or non-coronary cusp. Aortic cusp prolapse or postoperative aortic regurgitation may occur when the relationship between the defect, outlet septum, and aortic valve is not respected.

7.5 Tricuspid Valve Relationship

The septal leaflet of the tricuspid valve often overlies part of a perimembranous VSD.

Exposure may require careful retraction, chordal preservation, and, in selected cases, limited tricuspid leaflet detachment. The patch and suture line should preserve leaflet mobility, chordal attachments, and coaptation.

Tricuspid regurgitation after VSD closure may result from leaflet distortion, chordal injury, excessive traction, or incorporation of leaflet tissue into the patch suture line.

8. Device Closure and Conduction Risk

The close relationship between the perimembranous VSD margin and the atrioventricular conduction axis is also relevant to device closure.

Intraoperative or transcatheter device closure of perimembranous VSD can be complicated by atrioventricular block because device discs, waist expansion, local compression, inflammation, or mechanical irritation may affect the conduction tissue adjacent to the posteroinferior margin [8].

This reinforces the same anatomic principle: the perimembranous region is not simply a septal deficiency. It is a conduction-adjacent defect located within a fibrous and valvar complex.

9. Clinical and Perioperative Significance

The major complications related to perimembranous VSD closure are directly explained by the anatomy:

• complete atrioventricular block from injury to the penetrating or non-branching bundle
• right bundle branch block from injury to the right-sided bundle branch course
• residual VSD from incomplete recognition of the true defect margin or outlet extension
• aortic regurgitation from cusp distortion or associated cusp prolapse
• tricuspid regurgitation from leaflet or chordal injury
• patch dehiscence or residual shunting if the suture line is placed on fragile tissue

Postoperatively, rhythm monitoring is essential because conduction injury may be immediate or delayed. Echocardiography should evaluate residual shunt, tricuspid valve competence, aortic valve function, ventricular function, and outflow tract anatomy.

10. Practical Surgical Message

Perimembranous VSD closure is not simply patch closure of a septal hole. It is an operation performed at the intersection of the membranous septum, tricuspid valve, aortic valve, central fibrous body, and atrioventricular conduction axis.

The membranous flap, when present, is an important landmark. It represents a remnant of the interventricular membranous septum and may be usable for the suture line. However, the conduction axis remains close to the posteroinferior rim near the septal leaflet hinge.

A safe repair depends on recognizing three structures separately:

1. The true margin of the VSD
2. The membranous flap
3. The expected course of the conduction axis

Understanding these relationships allows the surgeon to close the defect completely while minimizing the risk of heart block, residual shunt, and valve injury.

Key Takeaways

• The membranous septum is divided by the septal tricuspid leaflet hinge into atrioventricular and interventricular components.
• The atrioventricular conduction axis penetrates the fibrous septal region and courses along the inferior margin of the interventricular membranous septum.
• In perimembranous VSD, the conduction axis is most relevant along the posteroinferior rim near the septal leaflet hinge.
• Anatomic studies show that the penetrating bundle may lie within approximately 1–1.5 mm of the septal crest.
• A membranous flap may be present as a remnant of the interventricular membranous septum.
• The membranous flap does not contain the conduction axis itself and may be useful for the suture line when clearly identified.
• Shallow, controlled suture placement along the posteroinferior rim is essential to reduce conduction injury.
• Outlet extension requires careful patch positioning to avoid residual VSD and aortic valve distortion.
• Successful closure requires simultaneous protection of the conduction axis, aortic valve, and tricuspid valve.

References

[1] Soto B, Becker AE, Moulaert AJ, Lie JT, Anderson RH. Classification of ventricular septal defects. Br Heart J. 1980;43(3):332-343.

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

[3] Anderson RH, Wilcox BR. The surgical anatomy of ventricular septal defect. J Card Surg. 1992;7(1):17-35.

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

[5] Yoshitake S, Kaneko Y, Morita K, Hoshino M, Oshima Y, Takahashi M, Anderson RH, SPring 8 Cardiovascular Structure Analyzing Research Group. Visualization and quantification of the atrioventricular conduction axis in hearts with ventricular septal defect using phase contrast computed tomography. J Thorac Cardiovasc Surg. 2020;160(2):490-496.

[6] Anderson RH, Wilcox BR. The surgical anatomy of ventricular septal defects associated with overriding valvar orifices. J Card Surg. 1993;8(2):130-142.

[7] Shi G, Chen H, Sun Q, Zhang H, Zheng J. Mattress stitch: a modified shallow stitching in the surgical closure of large perimembranous ventricular septal defect in infants. Ann Thorac Cardiovasc Surg. 2015;21(3):282-288.

[8] Chen Q, Cao H, Zhang GC, Chen LW, Li QZ, Qiu ZH. Atrioventricular block of intraoperative device closure perimembranous ventricular septal defects; a serious complication. BMC Cardiovasc Disord. 2012;12:21.