Perimembranous VSD with Inlet Extension

Surgical anatomy, conduction pathway, and patch-closure strategy

1. Anatomic setting

• Location. Perimembranous VSDs with inlet extension lie in the membranous septum and extend posteriorly toward the inlet septum, closely related to the septal leaflet of the tricuspid valve and its supporting papillary muscles.
• Subvalvular apparatus. Several accessory papillary muscles and dense chordae insert into the septal leaflet, partially masking the true VSD margin and creating a complex three-dimensional field.
• “Triangular septum.” This region can be conceptualized as a triangular wedge bounded by:
• the tricuspid annulus
• the muscular septum
• the conal septum

Each corner represents a transition zone where the course of the conduction tissue changes and the risk of injury increases [1].

2. Conduction axis in inlet-type perimembranous VSD

• AV node. Sits at the posteroinferior apex of the triangular septum, near the hinge of the septal tricuspid leaflet.
• His bundle. Penetrates the central fibrous body and runs immediately along the inferior rim of the VSD as it enters the muscular septum [2].
• Right bundle branch (RBB).
• Emerges near the medial papillary muscle, essentially on the edge of the defect.
• Then tracks along the trabecular septum.
• Risk zone. Compared with outlet-type extensions, inlet extension brings the conduction axis closer to the postero-inferior border, making this arc of the suture line the critical danger zone for iatrogenic heart block [2].

3. Surgical exposure and surface landmarks

• Approach. Standard right atriotomy, viewing the VSD through the tricuspid valve.
• Inspection.
• Carefully define the true defect margin by retracting (not dividing) chordae.
• Identify accessory papillary muscles and chordal insertions that may cross or skirt the VSD.
• Key landmarks for the conduction system [2]:
• Medial papillary muscle
• Septal leaflet hinge and adjacent tricuspid annulus

These structures mark the expected location of the AV node, His bundle, and proximal RBB.

4. Patch suturing strategy along the inferoposterior arc

• General sequence.
• Begin in the anterosuperior “safe” zone, where the conduction tissue is remote.
• Parachute the patch onto the septal crest using a running or interrupted technique, following contemporary teaching principles [1].
• Progress gradually toward the inlet/posteroinferior rim.
• Needle depth and trajectory

Evidence and clinical experience support shallow, rim-hugging sutures rather than deep bites into the septum [1,3].

1. Keep bites 1–2 mm from the true rim, biased toward the LV side.
2. Avoid deep penetration into the right ventricular myocardium along the inferior margin.
3. Allow the patch to bow slightly toward the LV, keeping the suture line off the conduction corridor.
• Protecting the conduction bundle [3]:
• Across the critical inferoposterior arc, use superficial stitches close to the VSD rim, rather than deep, remote bites.
• Where the rim is indistinct, it is safer to incorporate:
• tricuspid valve tissue, or
• fibrous tissue near the annulus

rather than thin septal myocardium.

• Tricuspid valve preservation.
• Prefer gentle chordal retraction; avoid cutting secondary/tertiary chordae unless absolutely necessary.
• Keep the patch low-profile beneath the septal leaflet, preventing leaflet distortion and preserving coaptation.

5. Outcomes and risk of heart block

• When the triangular septal anatomy and inlet-type conduction pattern are respected and shallow, offset suturing is used, the incidence of conduction injury is very low.
• A large series of isolated VSD closures reported permanent complete heart block in only 0.7% of patients, reflecting the safety of contemporary techniques [4].
• The goal is a repair that yields:
• secure, hemostatic VSD closure
• preserved atrioventricular conduction
• intact tricuspid valve function without stenosis or regurgitation.

Key points for practice

• Visualize the triangular septum and its three transition zones when planning the suture line [1].
• Remember that in inlet-extension PM-VSD, the AV node, His bundle, and proximal RBB hug the postero-inferior rim [2].
• Start suturing in the anterosuperior safe zone, then approach the inlet side with:
• shallow, LV-biased, rim-hugging bites
• avoidance of deep septal stitches, particularly near the medial papillary muscle [1,3].
• Use tricuspid/annular fibrous tissue to move the suture line away from the conduction plane when the inferior rim is ill-defined.
• Meticulous adherence to these principles keeps the risk of permanent heart block <1% in isolated VSD repair [4].

References

[1] Manning PB. Ventricular septal defect closure: how I teach it. Ann Thorac Surg. 2018;106(2):324-326.

[2] Spicer DE, Anderson RH, Backer CL. Clarifying the surgical morphology of inlet ventricular septal defects. Ann Thorac Surg. 2013;95(1):236-241.

[3] Fukuda T, Suzuki T, Kashima I, Sato M, Morikawa Y. Shallow stitching close to the rim of the ventricular septal defect eliminates injury to the right bundle branch. Ann Thorac Surg. 2002;74(2):550-555.

[4] Andersen HØ, de Leval MR, Tsang VT, Elliott MJ, Anderson RH, Cook AC. Is complete heart block after surgical closure of ventricular septum defects still an issue? Ann Thorac Surg. 2006;82(3):948-956.