Partial Atrioventricular Septal Defect (AVSD) — #2 Conduction System

Date
October 22, 2025
URL
https://x.com/WithAScalpel/status/1980967556224626775
Categories
AVSD

Partial AVSD — #2 Conduction System

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Overview

In partial atrioventricular septal defect (partial AVSD), the same endocardial cushion maldevelopment that produces a primum ASD and a “cleft” of the left AV valve also remodels the conduction axis. The AV node is displaced posteroinferiorly from the apex of Koch’s triangle toward the AV annulus, and the penetrating bundle traverses the inferior membranous septum before assuming a long, anteriorly running non-branching segment along the ventricular septal crest—features that define the surgical danger zone during primum patching and cleft closure [1, 2].

AV node — posteroinferior and annular

Compared with normal hearts, partial AVSD places the compact AV node adjacent to the AV valve hinge within a nodal triangle bounded by the inferior bridging/septal leaflet, coronary sinus orifice, and tendon of Todaro. This posterior shift of the AV junction is a hallmark of AV canal morphology and correlates with the relationship of the bridging leaflets to the septal structures [1].

His bundle and the non-branching segment

From this ectopic node, the penetrating His bundle perforates the inferior membranous septum and enters the muscular septum. It then courses anteriorly beneath the endocardium as a non-branching bundle that hugs the posterior–inferior rim of the primum ASD before dividing into right and left bundle branches. Imaging–anatomic reviews emphasize that this axis lies perilously close to low posteroinferior septal margins—knowledge that should shape suture placement during septal/valvar repair [2]. Electro-anatomic work in primum AVSD underscores the distinctive activation patterns produced by this rearranged axis and links surface ECG axes to anatomic variation, reinforcing the need for conduction-aware surgery [3].

Surgical implications and “safe-zone” heuristics

  • Primum ASD closure. The highest risk resides at the posterior–inferior margin near the coronary sinus; place patch sutures on the atrial side of the true annulus, take shallow, short bites, and keep slightly away from the rim in this quadrant to avoid the non-branching bundle [1, 2].
  • Left AV valve cleft repair. The cleft itself is usually remote from the conduction axis; risk increases only if bites migrate posterior–inferiorly toward the annular hinge close to the coronary sinus [1, 2].
  • When the coronary sinus is atypical. AVSDs commonly exhibit coronary-sinus variability (including unroofing or left-sided drainage); importantly, the conduction axis position tracks the junction of the inlet septum with the AV junction rather than the sinus itself [4]. If exposure is tight at a very low primum rim, coronary-sinus unroofing into the LA can safely create patch space without altering the axis location [4].
  • Landmarks when usual cues are distorted. If Koch’s triangle or the sinus orifice is malformed or malaligned, use the fusion point of the ventricular septum with the AV junction as the most reliable guide to the axis [5].
  • Team habits. Avoid traction injury from retractors/suckers at the inferior septum; monitor rhythm and pacing thresholds during weaning so transient conduction insults are recognized and relieved promptly [3].

Bottom line: Picture a posteroinferior annular node, an inferior membranous penetration, and an anteriorly directed non-branching bundle that skirts the posterior–inferior primum rim. Atrial-side, shallow, slightly offset stitches in this quadrant make primum closure and cleft repair both effective and conduction-safe [1, 2, 4, 5].

References

[1] Adachi I, Uemura H, McCarthy KP, Ho SY. Surgical anatomy of atrioventricular septal defect. Asian Cardiovasc Thorac Ann. 2008;16(6):497-502.

[2] Saremi F, Hassani C, Sánchez-Quintana D. Septal Atrioventricular Junction Region: Comprehensive Imaging in Adults. Radiographics. 2016;36(7):1966-1986.

[3] Hakacova N. Electrophysiologic and anatomical relationships studied in primum atrioventricular septal defect. J Electrocardiol. 2010;43(2):155-160.

[4] Ho SY, Gerlis LM, Toms J, Lincoln C, Anderson RH. Morphology of the posterior junctional area in atrioventricular septal defects. Ann Thorac Surg. 1992;54(2):264-270.

[5] Chiu IS, Hegerty A, Anderson RH, de Leval M. The landmarks to the atrioventricular conduction system in hearts with absence or unroofing of the coronary sinus. J Thorac Cardiovasc Surg. 1985;90(2):297-300.