Complete AVSD Repair with Two-Patch Technique #3: Conduction System for the VSD Patch Closure

1. Why the Conduction Axis Matters in Complete AVSD

Repair of complete atrioventricular septal defect (AVSD) requires closure of the ventricular and atrial septal components while preserving both atrioventricular valve function and atrioventricular conduction. The conduction system is particularly relevant during closure of the ventricular septal defect (VSD) because its course in AVSD differs substantially from that of the normal heart. The AV node is displaced posteriorly and inferiorly, and the penetrating and non-branching components of the conduction axis extend from this displaced nodal region toward the ventricular septum before dividing into the bundle branches.

This displacement is not simply a minor variation of normal anatomy. It reflects the fundamental morphology of AVSD: absence of normal atrioventricular septation, a common atrioventricular junction, and the characteristic “scooped-out” configuration of the ventricular septum. Consequently, the usual landmarks of the normal triangle of Koch cannot be applied without modification. Histologic studies have consistently demonstrated posterior and inferior displacement of the AV node and elongation of the non-branching bundle in AVSD.[1–3]

For the surgeon, the practical implication is straightforward: the inferior and posteroinferior portion of the septal defect is the critical region during VSD patch placement.

2. Normal Versus AVSD Conduction Anatomy

2.1 Normal arrangement

In the normally septated heart, the compact AV node lies within the triangle of Koch. The penetrating bundle crosses the plane of atrioventricular insulation through the central fibrous body and then becomes the non-branching AV bundle. The bundle subsequently divides into the right and left bundle branches along the crest of the ventricular septum.

This relationship depends on normal alignment between the atrial septum, ventricular septum, and central fibrous structures.

2.2 Displacement in AVSD

AVSD lacks the normal atrioventricular septal structures. The common atrioventricular junction and deficient inlet septation alter the relationship between the atrial and ventricular septa, producing a characteristic posterior displacement of the nodal region.[1,2]

The definitive AV node lies close to the coronary sinus ostium and substantially posterior to its usual location. Developmental studies in human hearts with AVSD and trisomy 21 demonstrate persistence of a dorsally situated AV node and formation of a relatively long non-branching bundle extending toward the ventricular septum.[3]

Modern phase-contrast computed tomography has confirmed these classic histologic observations. In 18 AVSD specimens, Yoshitake and colleagues demonstrated the AV node close to the coronary sinus and quantified the elongated conduction axis extending anteriorly toward the septal crest.[4] The mean non-branching bundle length was approximately 6.6 mm, although absolute measurements from fixed specimens should not be translated directly into an operative “safe distance.”

The important concept is therefore geometric rather than numerical:

AV node posterior/inferior → penetrating bundle → elongated non-branching bundle running anteriorly along the inferior septal region → branching bundle → right and left bundle branches.

3. The Conduction Axis in the Surgeon’s View

From the right atrial operative perspective, the displaced AV node lies posteriorly and inferiorly, near the region between the coronary sinus and inferior portion of the common atrioventricular junction. The penetrating bundle leaves this nodal region and enters the fibrous tissues associated with the inferior bridging leaflet.

The conduction axis then continues anteriorly as the non-branching bundle immediately adjacent to the superior aspect of the ventricular septal crest, before reaching the branching point. This anatomy explains why the conduction risk during complete AVSD repair extends farther anteriorly than might be assumed by considering the location of the AV node alone.[1,4]

Once branching occurs, the left bundle-branching system spreads over the left ventricular septal surface, whereas the right bundle branch continues toward the right ventricular aspect. Injury at or proximal to the branching bundle can therefore produce high-grade AV block, while more distal injury may manifest as bundle-branch abnormalities.

The key surgical structure is consequently not simply the AV node. The entire axis from the displaced node to the branching bundle must be mentally projected during VSD closure.

4. Relationship to the Inferior Bridging Leaflet

The inferior bridging leaflet is an important landmark because the penetrating conduction axis courses beneath this region as it approaches the ventricular septum. The inferior portion of the VSD is therefore different from the anterior and superior portions of the defect.

This has several implications.

First, exposure should be obtained without assuming that the inferior septal crest itself is safe merely because no conduction tissue is directly visible. The specialized tissue is subendocardial and cannot generally be identified visually.

Second, leaflet division, leaflet retraction, or traction on chordal structures should not distort the surgeon's understanding of the underlying septal geometry.

Third, the transition between the inferior bridging leaflet, the ventricular septal crest, and the posteroinferior margin of the defect should be treated as a conduction-sensitive zone, rather than as an ordinary patch anchoring point.

The precise relationship varies with ventricular geometry and associated abnormalities. Malalignment of the ventricular septum, ventricular imbalance, heterotaxy, or unusual atrioventricular connections can alter the expected course.[1] Thus, the classic anatomy should be regarded as a reliable framework, not as a substitute for assessment of the individual heart.

5. VSD Patch Closure in the Two-Patch Repair

In the two-patch technique, the ventricular component is closed separately with a prosthetic or other suitable patch, while a second patch closes the primum atrial component. The VSD patch restores ventricular septation and establishes the level at which the common atrioventricular valve is divided into right and left components.

The VSD patch must satisfy several competing requirements:

1. Complete closure of the ventricular communication.
2. Preservation of the conduction axis.
3. Avoidance of distortion of the bridging leaflets and chordae.
4. Preservation of left ventricular outflow geometry.
5. Creation of an appropriate surface for subsequent atrioventricular valve reconstruction.

The conduction system is most vulnerable during placement of the initial sutures in the inferior and posteroinferior region.

5.1 Inferior sutures

Deep transmural bites into the septal crest near the displaced conduction axis should be avoided. A commonly used principle is to place sutures on the right ventricular aspect of the septal crest, keeping the needle trajectory away from the expected subendocardial conduction tissue.

Near the inferior bridging leaflet, the surgeon may deliberately use leaflet or adjacent fibrous tissue rather than aggressively incorporating the underlying septum when required by the anatomy. Published operative descriptions differ in the exact route used, reflecting institutional and surgeon-specific technique rather than a universally standardized suture line.[5]

The objective is not to leave an arbitrary predetermined distance from the conduction system—because its exact intraoperative location cannot usually be seen—but to construct a suture trajectory based on its predictable anatomical course.

5.2 Progressing anteriorly

As the patch suture line advances anteriorly, the non-branching bundle continues near the septal crest before dividing. The branching point therefore remains relevant even after the surgeon has moved away from the displaced AV node itself.

A useful operative concept is:

posteriorly, avoid the displaced AV node and penetrating bundle; along the inferior septum, avoid the elongated non-branching bundle; anteriorly, remain aware of the transition into the branching system.

This three-dimensional interpretation is more useful than regarding the conduction system as a single localized “danger point.”

6. Patch Geometry and Conduction Preservation

Conduction preservation cannot be separated from patch geometry.

An excessively large VSD patch can distort the atrioventricular valve and alter ventricular geometry. An undersized patch can generate tension, pull the bridging leaflets toward the septum, or compromise valve coaptation. Excessive tension on the inferior patch margin may also transmit force to tissues immediately adjacent to the conduction axis.

The patch should therefore reproduce the native defect geometry without unnecessary redundancy. Chordal attachments crossing the VSD must be incorporated into the reconstruction without forcing an abnormal patch trajectory.

This issue is particularly relevant in Rastelli type A anatomy, in which attachment of the superior bridging leaflet to the ventricular septum can limit exposure and alter the available pathway for the VSD patch. In Rastelli type C anatomy, the free-floating superior bridging leaflet may provide broader access to the ventricular component, although the defect itself can be extensive.

Thus, conduction safety is achieved by integrating septal anatomy, bridging-leaflet morphology, chordal attachments, and patch geometry rather than by following a fixed distance or suture depth.

7. The Atrial Patch and the Posterior Conduction Region

Although this chapter focuses on VSD closure, the conduction risk does not end when the ventricular patch is completed.

Closure of the primum ASD brings the suture line back toward the displaced AV nodal region and coronary sinus. Surgeons therefore use different strategies for the posteroinferior atrial suture line. Some techniques keep the coronary sinus draining to the right atrium and route the patch sutures carefully around the conduction region; others may place the coronary sinus on the left atrial side when local anatomy makes the nodal area particularly vulnerable.[5]

Neither configuration should be considered intrinsically mandatory. The underlying principle is to recognize that the posteroinferior atrial septal margin is another conduction-sensitive region because of the displaced AV node.

8. Intraoperative Warning Signs

New conduction abnormalities during or immediately after repair should prompt consideration of direct injury, compression, ischemia, edema, or excessive traction near the conduction axis.

Particular concern is warranted for:

• persistent complete AV block;
• new advanced second-degree AV block;
• marked PR prolongation accompanied by broader conduction abnormalities;
• new bifascicular patterns suggesting substantial injury to the branching system.

Transient abnormalities may resolve as edema decreases, but persistent high-grade block requires postoperative pacing support and serial reassessment.

If complete AV block appears immediately after separation from cardiopulmonary bypass and is associated with concern regarding the inferior VSD patch suture line, the possibility of surgically correctable mechanical injury should be considered. Whether patch revision is appropriate depends on the operative anatomy, rhythm pattern, and likelihood that a discrete suture is compromising the conduction tissue.

9. Postoperative and Long-Term Conduction Outcomes

Contemporary complete AVSD repair generally has excellent survival, but conduction abnormalities remain a recognized complication. In the Society of Thoracic Surgeons Congenital Heart Surgery Database, 2,399 patients undergoing complete AVSD repair across 101 centers demonstrated the broad contemporary safety of repair, although outcomes varied with patient size and complexity.[6]

A large single-center experience of 406 patients, 97% of whom underwent two-patch repair, likewise demonstrated low contemporary perioperative mortality and durable overall outcomes.[7] These series emphasize that meticulous reconstruction can achieve excellent results even though AVSD inherently places specialized conduction tissue close to operative suture lines.

Long-term rhythm surveillance remains important. In a cohort of 522 patients with repaired partial or complete AVSD followed over several decades, 4.98% ultimately required permanent pacing for AV block: 2.6% for early and 2.2% for late AV block.[8] These data combine different forms of AVSD and several surgical eras, so they should not be interpreted as the expected contemporary rate after isolated two-patch complete AVSD repair. Nevertheless, they demonstrate that conduction disease may appear years after the original operation.

A separate multicenter cohort of 74 patients with repaired complete AVSD followed into young adulthood also demonstrated progression of conduction abnormalities in a minority of patients, supporting continued ECG surveillance beyond the immediate postoperative period.[9]

10. Completion Assessment

Before leaving the operating room, assessment should address both structural repair and electrical integrity.

The surgeon and imaging team should confirm:

• no important residual VSD adjacent to the inferior patch margin;
• no patch-related distortion of the common AV valve reconstruction;
• acceptable left and right AV valve function;
• unobstructed left ventricular outflow;
• preserved ventricular function;
• stable sinus rhythm with satisfactory AV conduction.

A tiny residual VSD must be interpreted in relation to its location. A residual jet adjacent to the conduction-sensitive posteroinferior patch margin creates a different reintervention problem from a small muscular or more remote residual communication, because attempts at additional suturing may themselves increase conduction risk.

Key Surgical Principles

• Expect posterior-inferior displacement of the AV node. The normal triangle of Koch does not provide the usual relationship seen in a normally septated heart.
• Think of an elongated conduction axis, not a single danger point. The penetrating and non-branching bundle extend anteriorly toward the septal crest before bifurcation.
• Treat the inferior and posteroinferior VSD margins as conduction-sensitive.
• Control needle depth and trajectory. Avoid unnecessarily deep bites close to the expected course of the conduction axis.
• Integrate leaflet and septal anatomy. Inferior bridging-leaflet position, chordal attachments, and septal geometry determine the safest patch pathway.
• Avoid patch tension or distortion. Conduction preservation, valve competence, and residual-shunt prevention are interdependent.
• Remain cautious during primum ASD closure. The displaced AV node remains relevant near the coronary sinus and posteroinferior atrial margin.
• Continue long-term rhythm surveillance. Late AV block and sinus-node dysfunction occur in a minority of repaired AVSD patients.

Figure. Conduction system during complete AVSD repair. In the surgeon’s view, the AV node is displaced posteriorly and inferiorly. The penetrating bundle leaves the nodal region beneath the inferior bridging leaflet, and the elongated non-branching bundle courses anteriorly immediately adjacent to the ventricular septal crest before dividing into the right and left bundle branches.

References

1. Ho SY, Anderson RH. Conduction tissue in congenital heart surgery. World J Surg. 1985;9:550-567. doi:10.1007/BF01656057. PMID: 4036149.
2. Henry GW, Wilcox BR. Surgical anatomy of the atrioventricular septum and atrioventricular septal defects. Cardiol Young. 1991;1:306-314. doi:10.1017/S1047951100010374.
3. Blom NA, Ottenkamp J, DeRuiter MC, Wenink ACG, Gittenberger-de Groot AC. Development of the cardiac conduction system in atrioventricular septal defect in human trisomy 21. Pediatr Res. 2005;58:516-520. doi:10.1203/01.PDR.0000179388.10921.44. PMID: 16148066.
4. Yoshitake S, Kaneko Y, Morita K, Hoshino M, Nagashima M, Takahashi M, Anderson RH, SPring-8 Cardiovascular Structure Analyzing Research Group. Reassessment of the location of the conduction system in atrioventricular septal defect using phase-contrast computed tomography. Semin Thorac Cardiovasc Surg. 2020;32:960-968. doi:10.1053/j.semtcvs.2020.03.011. PMID: 32450213.
5. Backer CL, Mavroudis C, Alboliras ET, Zales VR. Repair of complete atrioventricular canal defects: results with the two-patch technique. Ann Thorac Surg. 1995. doi:10.1016/0003-4975(95)00468-Z. PMID: 7677476.
6. St Louis JD, Jodhka U, Jacobs JP, He X, Hill KD, Pasquali SK, Jacobs ML. Contemporary outcomes of complete atrioventricular septal defect repair: analysis of the Society of Thoracic Surgeons Congenital Heart Surgery Database. J Thorac Cardiovasc Surg. 2014;148:2526-2531. doi:10.1016/j.jtcvs.2014.05.095. PMID: 25125206.
7. Mery CM, Zea-Vera R, Chacon-Portillo MA, Zhu H, Kyle WB, Adachi I, Heinle JS, Fraser CD Jr. Contemporary outcomes after repair of isolated and complex complete atrioventricular septal defect. Ann Thorac Surg. 2018;106:1429-1437. doi:10.1016/j.athoracsur.2018.06.006. PMID: 30009807.
8. Di Mambro C, Calvieri C, Silvetti MS, et al. Bradyarrhythmias in repaired atrioventricular septal defects: single-center experience based on 34 years of follow-up of 522 patients. Pediatr Cardiol. 2018;39:1590-1597. doi:10.1007/s00246-018-1934-4. PMID: 29948031.
9. Houck CA, Evertz R, Teuwen CP, et al. Dysrhythmias in patients with a complete atrioventricular septal defect: from surgery to early adulthood. Congenit Heart Dis. 2019;14:280-287. doi:10.1111/chd.12724. PMID: 30485659.