Cardiac Fibrous Skeleton #4: Septal Anatomy

1. Overview

The septal region of the cardiac fibrous skeleton is a compact three-dimensional junction where the atrial septum, muscular ventricular septum, aortic root, atrioventricular junctions, and valve-supporting fibrous tissues converge. Its central component is the membranous septum, a small but surgically critical fibrous structure connecting the central fibrous body to the crest of the muscular ventricular septum.

The membranous septum defines important relationships among the aortic root, septal tricuspid leaflet, ventricular septum, and atrioventricular conduction axis. It is therefore directly relevant to perimembranous ventricular septal defect closure, repair of left ventricle-to-right atrium communications, subaortic membrane resection, tricuspid valve surgery, aortic annular procedures, and transcatheter aortic valve implantation.

The septal atrioventricular junction should not be considered a single flat septal plane. It is a complex region containing the membranous septum, central fibrous body, triangle of Koch, inferior pyramidal space, base of the muscular ventricular septum, and adjacent portions of the aortic, mitral, and tricuspid valves [1]. Safe intervention requires reconstruction of these structures in three dimensions rather than reliance on a single surface landmark.

2. The Septal Junction Within the Fibrous Skeleton

The cardiac fibrous skeleton provides mechanical support for the atrioventricular and arterial valves while electrically insulating the atrial myocardium from the ventricular myocardium. Most of the atrioventricular junction is occupied by fibrofatty tissue that interrupts direct myocardial continuity. Normal electrical conduction between the atria and ventricles is concentrated through the atrioventricular conduction axis.

At the septal aspect of the heart, the right fibrous trigone merges with adjacent fibrous tissues to form the central fibrous body. This structure is continuous with:

• The rightward end of the aorto-mitral curtain
• The aortic root near the right and noncoronary leaflets
• The membranous septum
• The septal atrioventricular junction
• The fibrous attachment of the septal tricuspid leaflet

The left fibrous trigone supports the opposite end of the aorto-mitral curtain. The right fibrous trigone and central fibrous body, however, are more directly related to the penetrating atrioventricular bundle and therefore carry greater relevance to conduction injury.

The atrial and ventricular septa are offset rather than aligned in a continuous plane. Because the tricuspid valve inserts more apically than the mitral valve, a small portion of the left ventricular outflow tract is separated from the right atrium by fibrous tissue. This arrangement creates the atrioventricular component of the membranous septum.

3. The Membranous Septum

The membranous septum is the nonmuscular component of the septal complex immediately beneath the aortic root. It is composed predominantly of dense fibrous tissue and is considerably thinner than the adjacent muscular ventricular septum.

Its superior border merges with the central fibrous body. Inferiorly, it approaches the crest of the muscular ventricular septum. Its right-sided surface is related to the hinge of the septal tricuspid leaflet, while its left-sided surface contributes to the posteroseptal boundary of the left ventricular outflow tract.

The membranous septum is variable in shape, thickness, orientation, and dimensions. In computed tomographic analysis of 107 candidates for transcatheter aortic valve implantation, its mean height was 6.6 ± 2.0 mm and its mean width was 10.2 ± 3.1 mm. Its dimensions were associated with the size of the aortic root, and the conduction-axis surrogate distance beneath the virtual basal ring was 5 mm or less in 91% of patients [2].

These measurements were derived from an elderly transcatheter-valve population and should not be transferred directly to infants or children. They nevertheless demonstrate that the membranous septum is not a constant structure and that its relationship to the aortic annulus can be closer than suggested by simplified anatomical diagrams.

4. Atrioventricular and Interventricular Components

The hinge of the septal tricuspid leaflet divides the membranous septum into atrioventricular and interventricular components.

4.1 Atrioventricular Membranous Septum

The atrioventricular membranous septum lies above the septal tricuspid hinge. It separates the left ventricular outflow tract from the right atrium rather than separating the two ventricles.

This component exists because of the apical displacement of the tricuspid valve relative to the mitral valve. A defect through this region creates a direct left ventricle-to-right atrium communication. Such communications are commonly categorized as Gerbode-type defects and may be congenital or acquired after surgery, infective endocarditis, trauma, or transcatheter intervention.

The tricuspid leaflet may partially cover the right atrial opening, modify the Doppler appearance, or direct the high-velocity jet toward the right atrial wall. Accurate differentiation from tricuspid regurgitation is important because both may produce a systolic jet within the right atrium.

4.2 Interventricular Membranous Septum

The interventricular membranous septum lies below the septal tricuspid hinge and separates the left and right ventricles. It extends toward the crest of the muscular ventricular septum beneath the right–noncoronary aortic interleaflet triangle.

Its extent depends on the relationship between the tricuspid hinge and the muscular septal crest. A relatively high tricuspid attachment leaves a larger interventricular fibrous component. Conversely, when the leaflet attaches near the muscular crest, the interventricular component may be extremely small or absent.

Histological and clinical analysis has confirmed marked variability in this arrangement. In almost three-fifths of examined hearts, an identifiable interventricular component of the fibrous membranous septum was absent [3]. Thus, the common diagram showing a broad and consistently divided membranous septum represents only one anatomical pattern.

5. Relationship to the Aortic Root

The membranous septum lies beneath the aortic root, principally near the junction between the right and noncoronary leaflets. The right–noncoronary interleaflet triangle is particularly important because the penetrating conduction axis enters the central fibrous body near this region before continuing along the ventricular septal crest.

The attachment of the aortic leaflets is crown-shaped rather than a flat circular ring. The interleaflet triangles extend between adjacent sinuses and continue inferiorly toward ventricular or fibrous supporting structures. The membranous septum is related to the basal portion of the right–noncoronary interleaflet triangle.

Aortic root rotation varies substantially. In one anatomical study, the root was centrally positioned in 54% of cadaveric specimens, clockwise-rotated in 15%, and counterclockwise-rotated in 31%. In the corresponding computed tomographic cohort, the distribution was 56%, 27%, and 17%, respectively [4].

Rotation changes the relationship among the aortic root, central fibrous body, membranous septum, and muscular ventricular septum. Clockwise rotation was associated with greater fibrous continuity between the membranous septum and right fibrous trigone, producing a wider central fibrous body. Such variation may alter the location at which a prosthesis, annular suture, or resection plane approaches the conduction axis.

6. Relationship to the Atrioventricular Conduction Axis

The compact atrioventricular node is located within the triangle of Koch. The conduction axis then transitions into the penetrating atrioventricular bundle, which traverses the insulating fibrous tissues of the central fibrous body. Distally, the nonbranching bundle courses near the crest of the muscular ventricular septum and divides into the right and left bundle branches.

The axis is closely related to the membranous septum, but the membranous septum should be regarded as a surrogate landmark rather than the structure containing the entire conduction axis. The bundle may course immediately beneath its inferior margin, within adjacent fibrous tissue, or along the muscular septal crest.

Histological examination of 20 human hearts found that the superior edge of the left bundle branch was, on average, 3.3 mm from the hinge of the right coronary aortic leaflet. The range was wide, from 0.4 to 10.2 mm [5]. The axis was usually located near the ventricular septal crest but could be deviated toward either the left or right side.

The point at which the conduction axis becomes penetrating is also variable relative to the triangle of Koch and septal tricuspid hinge. In just over half of anatomical specimens and patients undergoing electrophysiological evaluation, penetration occurred on the atrial aspect of the septal tricuspid hinge. Additional variation occurred relative to the apex of the triangle of Koch and the right-sided endocardial surface [3].

Consequently, no universal distance from the tricuspid annulus, aortic annulus, or visible membranous septum can define an absolutely safe operative zone.

7. Right-Sided Surgical Landmarks

From the right ventricle, the membranous septal region is surrounded by valvar and muscular structures that help define the margins of a ventricular septal defect.

The septomarginal trabecula contributes to the architecture separating the inlet, trabecular, and outlet portions of the right ventricle. Its limbs are related to the outlet septum and the septal papillary muscle complex. The muscle of Lancisi, commonly corresponding to the medial papillary muscle or a related septal papillary structure, arises near the posteroinferior perimembranous region.

Chordae from the medial papillary muscle to the septal or anterior tricuspid leaflet may cross or obscure a perimembranous defect. Accessory tricuspid tissue may partially close the defect, produce an aneurysmal pouch, or divide the communication into multiple right ventricular openings.

These structures are useful for operative orientation, but none precisely identifies the conduction axis. The septal tricuspid hinge, central fibrous body, and posteroinferior defect margin must be considered together.

8. Perimembranous Ventricular Septal Defects

A perimembranous ventricular septal defect is defined by fibrous continuity between part of the defect margin and the central fibrous body or membranous septal region. The defect may extend toward the inlet, trabecular, or outlet septum. Its complete description should therefore include both its perimembranous relationship and its direction of extension.

The conduction axis generally passes along the posteroinferior margin of a typical perimembranous defect [1]. It may be protected by a narrow fibrous rim, covered by the septal tricuspid leaflet, or displaced by malalignment of the ventricular septal components.

Aneurysmal transformation of membranous or tricuspid tissue may reduce the effective shunt diameter. However, such tissue can complicate closure by creating multiple fenestrations, a winding tunnel, tricuspid chordal attachments, or proximity to the aortic valve. It may also contribute to tricuspid regurgitation, right ventricular obstruction, or progressive subaortic obstruction.

The conduction axis differs among morphologically distinct inlet defects. Inlet perimembranous defects, muscular inlet defects, atrioventricular septal defects, and defects associated with straddling or overriding atrioventricular valves do not share an identical conduction pathway [6]. The expected posteroinferior course of a conventional perimembranous VSD should not be applied indiscriminately to all inlet communications.

9. Surgical Principles for VSD Closure

Safe closure begins with complete definition of the defect circumference. The surgeon should identify:

• The septal tricuspid hinge
• The aortic valve and subaortic margin
• The muscular ventricular septal crest
• Inlet or outlet extension
• Chordal attachments crossing the defect
• The posteroinferior conduction-sensitive margin

At the posteroinferior margin, sutures should avoid deep penetration into the septal crest or central fibrous body. Controlled right-sided bites through fibrous tissue, adjacent endocardium, or selected tricuspid leaflet tissue may be safer than deep full-thickness bites.

The patch should completely exclude the communication without distorting the septal tricuspid leaflet or aortic valve. The suture line must also avoid narrowing the left ventricular outflow tract or creating a residual tunnel around accessory tricuspid tissue.

When tricuspid tissue prevents adequate visualization, limited detachment of the septal leaflet may be considered. Detachment should improve exposure of the defect margin and permit accurate placement of the conduction-sensitive sutures. The leaflet must subsequently be reattached without producing stenosis, significant regurgitation, or injury to the adjacent conduction tissue.

Postrepair assessment should include evaluation for residual shunting, aortic regurgitation, tricuspid regurgitation, left or right ventricular outflow obstruction, and conduction abnormalities. New complete atrioventricular block may reflect direct bundle injury, suture entrapment, local hematoma, edema, traction, or ischemia.

10. Septal Anatomy in Congenitally Malformed Hearts

In congenitally malformed hearts, the conduction axis follows the architecture of the atrioventricular junctions and septal structures rather than a fixed coordinate within the heart.

In atrioventricular septal defects, the normal central fibrous body is deficient or reorganized. The atrioventricular node is displaced posteriorly and inferiorly, and the conduction axis follows the inferior margin of the ventricular component of the defect.

In congenitally corrected transposition, malalignment between the atrial and ventricular septa and the abnormal atrioventricular connections may produce an anterior conduction axis. In double-outlet right ventricle, tetralogy of Fallot, and other conotruncal malformations, the relationship among the outlet septum, ventriculo-infundibular fold, septomarginal trabeculation, and ventricular septal crest is highly variable [7].

These changes alter the margins of the VSD and the expected location of the conduction tissue. A defect that appears “perimembranous” from one ventricular perspective may have a different relationship to the atrioventricular conduction axis because of septal malalignment or abnormal atrioventricular junction morphology.

11. Procedures Near the Aortic Root

During subaortic membrane resection, deep excision near the right–noncoronary commissural region can injure the conduction axis, create a ventricular septal defect, or destabilize the aortic valve. Fibrous obstructive tissue must be distinguished from the normal membranous septum and central fibrous body.

Aortic annular sutures placed near the right–noncoronary region may enter the central fibrous body. The risk is particularly relevant in small congenital aortic annuli, where the operative field is restricted and the conduction axis may lie only a few millimeters beneath the leaflet hinge [5].

Deep tricuspid annuloplasty sutures near the septal annulus may similarly injure the penetrating bundle. The surgeon should avoid treating the septal tricuspid annulus as equivalent to the anterior or posterior muscular portions of the annulus.

12. Transcatheter Intervention and Conduction Injury

The membranous septum has been used as an imaging surrogate for the position of the conduction axis during transcatheter aortic valve implantation. A short membranous septum, a shallow inferoseptal recess, or close proximity of the conduction tissue to the right coronary leaflet may indicate limited separation between the prosthesis and the conduction axis [2,5].

However, gross anatomy alone does not fully predict postprocedural conduction injury. In a study of 200 transcatheter aortic valve recipients, 20.5% required permanent pacemaker implantation. Gross variation in aortic root position and membranous septal dimensions was not independently associated with permanent pacemaker implantation or new left bundle branch block [8].

Instead, procedural characteristics—including implantation depth and the mechanical properties or radial force of the implanted valve—were more closely associated with conduction damage [8]. The practical implication is that anatomical assessment should guide procedural planning, but technical execution ultimately determines how strongly the prosthesis compresses the vulnerable septal tissues.

This distinction also applies conceptually to congenital surgery. Anatomical vulnerability identifies where injury may occur, whereas suture depth, patch orientation, traction, resection extent, and device position determine whether that vulnerability becomes a clinical conduction complication.

13. Imaging and Operative Orientation

Echocardiography should define the relationship among the defect, aortic root, septal tricuspid leaflet, and muscular septal crest in multiple imaging planes. Computed tomography and three-dimensional reconstruction are particularly useful when there is complex outlet malalignment, prior patch material, unusual aortic root rotation, or suspected left ventricle-to-right atrium communication.

Current clinical imaging rarely demonstrates the conduction tissue directly. Its position must be inferred from the membranous septum, atrioventricular junction, aortic leaflet hinges, ventricular septal crest, and morphology of the congenital defect.

Operative orientation should therefore proceed sequentially:

1. Identify the aortic root and subaortic margin.
2. Define the hinge of the septal tricuspid leaflet.
3. Locate the muscular ventricular septal crest.
4. Determine whether the defect extends toward the inlet, trabecular, or outlet septum.
5. Identify accessory tricuspid tissue and chordal attachments.
6. Infer the conduction axis along the vulnerable fibrous or posteroinferior margin.

14. Practical Summary

The membranous septum is the fibrous junction among the central fibrous body, aortic root, atrioventricular septal region, and muscular ventricular septum. The septal tricuspid hinge divides it into an atrioventricular component separating the left ventricle from the right atrium and an interventricular component separating the ventricles.

Both the dimensions of the membranous septum and its relationship to the aortic root vary considerably. The interventricular fibrous component may be minimal or absent, and the conduction axis may lie less than 1 mm or more than 10 mm from an aortic leaflet hinge [2–5].

These variations explain why simplified anatomical diagrams cannot define a universally safe suture line. Nevertheless, anatomy alone does not determine clinical injury. Conduction complications result from the interaction between anatomical vulnerability and procedural factors such as suture depth, patch placement, resection extent, implantation depth, and device-generated compression.

The membranous septum, tricuspid hinge, muscular septal crest, central fibrous body, aortic root, septomarginal trabecula, and muscle of Lancisi should therefore be interpreted as an integrated three-dimensional landmark complex. Safe surgery depends on defining the specific morphology of each heart and adapting the intervention accordingly.

References

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

[2] Mori S, Tretter JT, Toba T, Izawa Y, Tahara N, Nishii T, et al. Relationship between the membranous septum and the virtual basal ring of the aortic root in candidates for transcatheter implantation of the aortic valve. Clin Anat. 2018. doi:10.1002/ca.23071.

[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] Tretter JT, Mori S, Saremi F, Chikkabyrappa S, Thomas K, Bu F, et al. Variations in rotation of the aortic root and membranous septum with implications for transcatheter valve implantation. Heart. 2017. doi:10.1136/heartjnl-2017-312390.

[5] 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.

[6] Spicer DE, Anderson RH, Backer CL. Clarifying the surgical morphology of inlet ventricular septal defects. Ann Thorac Surg. 2013. doi:10.1016/j.athoracsur.2012.08.040.

[7] Hosseinpour AR, Jones TJ, Barron DJ, Brawn WJ, Anderson RH. An appreciation of the structural variability in the components of the ventricular outlets in congenitally malformed hearts. Eur J Cardiothorac Surg. 2007. doi:10.1016/j.ejcts.2007.01.043.

[8] Tretter JT, Mori S, Anderson RH, Taylor MD, Ollberding NJ, Truong V, et al. Anatomical predictors of conduction damage after transcatheter implantation of the aortic valve. Open Heart. 2019. doi:10.1136/openhrt-2018-000972.