Conduction System in Perimembranous VSD with Outlet/Inlet Extension

Date
September 23, 2025
URL
https://x.com/WithAScalpel/status/1970455950311018590
Categories
Cardiac SurgeryVSD

Conduction System in Perimembranous VSD with Outlet/Inlet Extension — Surgical Map

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1) Core orientation: AV node → His → bundle branches

The atrioventricular (AV) node at the apex of the triangle of Koch gives rise to the penetrating His bundle, which traverses the central fibrous body to the septal crest as the non-penetrating bundle. Distally, the bundle bifurcates into the left bundle branch (LBB), fanning across the LV septal surface, and the right bundle branch (RBB), coursing toward the septomarginal trabeculation–moderator band complex [1].

2) Why perimembranous VSDs are special

Perimembranous VSDs abut the membranous septum; thus the AV conduction axis hugs the posteroinferior rim of the defect. When approached through the tricuspid valve, the axis lies to the surgeon’s right hand in perimembranous inlet defects, a practical cue when planning suture trajectories and leaflet detachment [2]. The precise surface relationship of the RBB depends on whether the defect extends toward the outlet (conal/infundibular) or the inlet (AV-valvar septum), and is further modified by the topography of the trabecula/septomarginalis (SMT) [3]. Modern phase-contrast CT quantification places the penetrating bundle a median ~1.4 mm from the septal crest and the origin of the RBB ~0.5–2.6 mm from reference landmarks—underscoring how little margin exists for deep, transmural passes near the posteroinferior rim [4].

3) Outlet-extension PM-VSD — “RBB hidden by the posterior SMT limb”

  • The posterior limb of the SMT runs inferior to the defect and can overlie the emerging RBB as it surfaces on the RV septum; the conduction tissue is therefore not on the VSD edge and is often sheltered under the posterior SMT limb (commonly surfacing posterior to the muscle of Lancisi) [3,4].
  • Stitching implication. The dominant hazard remains the posteroinferior rim (His bundle). Use short, shallow, slightly RV-eccentric bites on the patch and avoid deep passes beneath the SMT overhang [3,4].

4) Inlet-extension PM-VSD — “RBB on the rim; MPM is not your landmark”

  • With inlet extension, the posterior SMT limb courses superior to the defect, and the RBB lies at—or very near—the VSD margin on the RV surface. The medial papillary muscle (MPM) is not a reliable landmark for the RBB in this configuration [2,4].
  • Stitching implication. Along the inlet/posteroinferior edge, adopt even shorter, more superficial bites, steering clear of the visible RBB trajectory [2,4].

5) Evidence-based operative strategy and pearls

  • Outline the danger rim first. Track from the septal tricuspid hinge across the posteroinferior circumference—this follows the penetrating His and proximal RBB [1,2].
  • SMT awareness saves bundles. The SMT limb’s position (inferior vs superior to the defect) predicts whether the RBB is hidden (outlet) or exposed at the rim (inlet) [3,4].
  • Tricuspid valve detachment (TVD) judiciously. Selective septal/anterior leaflet detachment improves exposure; reattach with superficial annulus–patch sutures to avoid the conduction corridor [2].
  • Stitch choreography.
    • Anterosuperior/muscular rims: standard depth, orthogonal bites.
    • Posteroinferior/inlet rim: short, shallow, closely spaced bites on the patch side, intentionally RV-eccentric [1–4].
  • Avoid “edge traps.” In inlet extension, confirm under magnification whether the RBB is on the rim before placing corner stitches. In outlet extension, remember the RBB may be concealed under the SMT—do not pass deep beneath the overhang [2–4].
  • Hemostasis without harm. Prefer additional superficial patch bites or topical agents over deep septal passes along the danger rim [1,3,4].
  • Technique matters. Comparative data show that shallow stitches taken close to the rim markedly reduce complete RBBB after pmVSD closure (6.3% vs 43.8%) [5].
  • Why you care. Postoperative complete RBBB is linked to ventriculotomy approach and can carry functional consequences; preventing conduction injury is not merely cosmetic ECG hygiene [6].

6) Field landmarks you can trust

  • Posteroinferior VSD rim = His/proximal RBB neighborhood (highest-risk zone) [1,2].
  • Posterior SMT limb:
    • Inferior to defect (outlet) → may cover the RBB [3,4].
    • Superior to defect (inlet) → RBB exposed at the rim [2,4].
  • MPM: unreliable predictor for RBB position in inlet extension [2].

All perimembranous VSDs threaten the conduction axis along the posteroinferior margin; the extension pattern dictates how the RBB reaches the RV surface—concealed by the posterior SMT limb in outlet-type vs on the edge in inlet-type—and safe closure hinges on recognizing these patterns, tailoring exposure, and using shallow, rim-hugging sutures to minimize conduction injury [1–5, 7].

References

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

[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] Kurosawa H, Becker AE. Modification of the precise relationship of the atrioventricular conduction bundle to the margins of the ventricular septal defects by the trabecula septomarginalis. J Thorac Cardiovasc Surg. 1984;87(4):605-615.

[4] 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.

[5] 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.

[6] van Lier TA, Harinck E, Hitchcock JF, et al. Complete right bundle branch block after surgical closure of perimembranous ventricular septal defect: relation to type of ventriculotomy. Eur Heart J. 1985;6(11):959-962.

[7] Yoneyama F, Kato H, Mathis BJ, Suetsugu F, Hiramatsu Y. Right bundle branch in ventricular septal defects. Eur J Cardiothorac Surg. 2025 Mar 28;67(4):ezaf105.