Complete AVSD: Basic Anatomy and Conduction

Date
August 25, 2025
URL
https://x.com/WithAScalpel/status/1960092150433247414
Categories
AVSDCardiac Surgery

Complete Atrioventricular Septal Defect

Definition. Complete AVSD is a conotruncal alignment lesion characterized by a single common atrioventricular (AV) junction and deficiency of the atrial and ventricular septa at the inlet. The primum atrial septal defect (ASD) is continuous with an inlet ventricular septal defect (VSD), creating a large interatrial–interventricular communication bordered by a common AV valve [1,2].

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Septal Anatomy

  • Primum ASD. A deficiency of the inferior (primum) portion of the atrial septum; distinct from secundum defects at the fossa ovalis.
  • Inlet VSD. A defect of the inlet muscular septum immediately beneath the AV junction. In complete AVSD the primum ASD and inlet VSD are contiguous, forming a single atrioventricular septal gap [1].
  • Common AV annulus. Instead of separate mitral and tricuspid annuli, there is one common annulus spanning both ventricles. Partitioning of this annulus (and the common valve) is the central task at repair [1,2].

Common AV Valve Morphology

The valve consists of five leaflets arranged around the common annulus: (1) superior bridging leaflet (SBL), (2) inferior bridging leaflet (IBL), (3) left mural (lateral) leaflet, (4) right mural (lateral) leaflet, and (5) right anterior leaflet [1,2]. Zones of apposition between the bridging leaflets create the left-sided cleft, a frequent source of AV valve regurgitation if not addressed. Subvalvar attachments (papillary muscles/chordae) vary and underpin the Rastelli classification (Types A–C), which guides surgical strategy [3].

Rastelli Classification (Types A–C)

Complete AVSD is classically subtyped by SBL insertion/mobility, with direct surgical implications for septation and outflow interactions:

  • Type A (most common). SBL with chords to the septal crest—facilitates septation but can predispose to LVOT narrowing if patch geometry is restrictive.
  • Type B (rare). SBL attaches to an anomalous RV papillary muscle (straddling); partitioning is more complex.
  • Type C (free-floating). SBL without septal attachments; large common orifice with wide mobility—avoid leaflet prolapse and LVOT encroachment during patching [3].

Conduction System

The conduction system is displaced relative to normal AV canal anatomy:

  • AV node. Posterior–inferior, in a nodal triangle adjacent to the common annulus (not classic Koch’s triangle) [1].
  • His bundle. Arises from this node, courses beneath the IBL, penetrates the septal crest, then continues as a non-penetrating, non-branching bundle that runs anteriorly along the septal crest before bifurcating into right/left bundle branches [1,4].

Surgical implication. Conduction tissue is most vulnerable near the IBL footprint and posteroinferior VSD rim—VSD patch sutures and cleft closure should respect this course to avoid complete heart block [1,4].

Hemodynamics and Clinical Features

  • Shunt physiology. Large atrial and ventricular left-to-right shunts → pulmonary overcirculation, tachypnea, failure to thrive, and progressive pulmonary vascular disease if uncorrected.
  • AV valve regurgitation (AVVR). Common via the left cleft or malcoaptation; may dominate hemodynamics.
  • Ventricular interaction. Common annulus alters AV coupling; LV preload is often high, but forward output can be limited by AVVR.
  • Associations. Strong link with trisomy 21; ECG often shows left-axis deviation with superior QRS axis [2,3].

Imaging Essentials

  • Echocardiography (2D/3D) delineates the common junction, bridging leaflets, chordal insertions, and AVVR, and helps anticipate LVOT interactions [5].
  • Key views: apical four-chamber (common annulus, bridging leaflets), subcostal coronal (primum ASD–inlet VSD continuity), parasternal long-axis (inlet extension/LVOTO risk); 3D datasets refine leaflet and cleft geometry for surgical planning [5,6].

Surgical Principles (at a glance)

  • Septation. Partition the common valve into right/left orifices and close atrial/ventricular components—two-patch (separate VSD/ASD) vs (modified) single-patch approaches tailored to valve and chordal anatomy [2,3].
  • Cleft management. Primary or partial closure of the left cleft to restore coaptation while preserving area.
  • Conduction protection. Posteroinferior VSD sutures superficial and away from IBL; avoid deep bites along the septal crest [1,4].
  • Avoiding LVOTO. Prevent SBL tethering into LVOT; ensure patch geometry does not impinge on outflow [3].
  • Timing/outcomes. Repair is typically undertaken around 3–6 months (earlier if heart failure/pulmonary hypertension or significant AVVR), with improved outcomes in contemporary series due to refined anatomical understanding and perioperative care [7,8].

Summary

Complete AVSD combines a common AV junction, contiguous primum ASD–inlet VSD, and a five-leaflet common valve. The AV node lies posterior–inferiorly, and the His bundle courses beneath the IBL before running anteriorly along the septal crest—defining a surgical hazard zone. Accurate echo definition (including 3D) and anatomy-driven septation with cleft repair achieve durable results while preserving rhythm and outflow [1–3,5–8].

References

[1] Adachi I, Uemura H, Ho SY, et al. Morphologic spectrum and surgical anatomy of complete atrioventricular septal defect. Semin Thorac Cardiovasc Surg Pediatr Card Surg Annu. 2008.

[2] Anderson RH, Becker AE, Wilcox BR, et al. Surgical anatomy of atrioventricular septal defects with common atrioventricular junction. J Thorac Cardiovasc Surg. 1991.

[3] Calabrò R, Limongelli G, et al. Complete atrioventricular canal: anatomical variants, classification, and surgical implications. Heart. 2006.

[4] Henry GW, Wilcox BR, et al. The atrioventricular conduction axis in atrioventricular septal defects: surgical relevance. Circulation. 1991.

[5] Taqatqa A, Vettukattil JJ, et al. Three-dimensional echocardiography for preoperative planning in atrioventricular septal defects. Echocardiography. 2021.

[6] Nayak VM, Shetty K, et al. Echocardiographic assessment of complete atrioventricular septal defect: key views and measurements. Ann Pediatr Cardiol. 2020.

[7] Backer CL, Stewart RD, et al. Complete atrioventricular septal defect repair: timing, techniques, and outcomes. Ann Thorac Surg. 2007.

[8] Calkoen EE, Hazekamp MG, et al. Contemporary outcomes of repair for complete atrioventricular septal defect. Eur J Cardiothorac Surg. 2016.