Where is the atrioventricular (AV) node located in a ventricular septal defect (VSD)?

AV Node Location in Ventricular Septal Defects

The location of the AV node in a VSD depends critically on the type of defect, with the most important surgical distinction being that in perimembranous VSDs (80% of all VSDs), the AV conduction axis runs along the posteroinferior rim of the defect, while in muscular inlet VSDs it courses on the opposite side, and in AVSDs with ventricular-level shunting, the node is displaced posteriorly and inferiorly to the coronary sinus 1, 2.

Type-Specific AV Node Locations

Perimembranous VSD (Type 2 - Most Common, ~80%)

• The AV node and conduction axis are positioned along the posteroinferior margin of the defect 2, 3
• From the surgeon's perspective operating through the tricuspid valve, the conduction tissue lies to the right hand 2
• The defect is in the membranous septum, adjacent to the septal leaflet of the tricuspid valve and the aortic valve 1
• Critical surgical implication: Sutures must be placed away from the posteroinferior rim to avoid heart block 2, 4

Muscular Inlet VSD (Type 3)

• The conduction axis is positioned on the opposite side compared to perimembranous defects 2
• From the surgeon's perspective through the tricuspid valve, the axis lies to the left hand 2
• These defects occur in the lower right ventricle, adjacent to the tricuspid valve 1
• More common in patients with Down syndrome 1

Outlet VSD (Type 1 - Subarterial)

• Located in the outflow portion of the RV 1
• The conduction tissue is typically remote from the defect margins 3
• More common in Asian populations (up to 33% vs 6% in non-Asian) 1
• Lower risk of conduction system injury during closure 3

Muscular VSD (Type 4)

• Can be central, apical, or marginal 1
• The conduction tissue is well separated from these defects 5, 3
• Lowest risk for conduction system injury 3

Special Case: AVSD with Ventricular-Level Shunting

In atrioventricular septal defects (distinct from isolated VSDs), the AV node is positioned posterior and inferior to the coronary sinus, with the bundle of His and left bundle branch displaced posteriorly 6, 7. The node arises at the crux of the heart from an inferiorly displaced nodal triangle 2.

Critical Surgical Considerations

Risk of Heart Block

• Patients developing bifascicular or transient trifascicular block after VSD closure are at risk for late complete AV block (can occur up to 15 years post-surgery) 6, 7
• Regular ECG monitoring is essential in follow-up 7
• The risk is highest with perimembranous and inlet VSDs where the conduction axis is intimately related to the defect margins 2, 4

Device Closure Implications

For transcatheter closure of perimembranous VSDs:

• Type 5 defects (inferior-posterior extension) lie closest to the conduction system 8
• Deploying devices within an aneurysmal pouch when present helps prevent heart block 8
• Understanding the specific anatomical variant guides device selection and deployment strategy 8

Common Pitfalls to Avoid

1. Assuming all VSDs have the same conduction anatomy - The AV node location varies dramatically by defect type 2, 3
2. Placing sutures on the posteroinferior rim of perimembranous defects - This is where the conduction axis runs 2, 4
3. Confusing AVSD anatomy with isolated VSD - AVSDs have a distinctly different and more posterior/inferior nodal position 6, 7, 2
4. Underestimating late heart block risk - Conduction disturbances can develop years after initially successful repair 6, 7