This document discusses atrioventricular canal defects (AVSDs), including their embryogenesis and pathophysiology. It describes the anatomy and classification of partial and complete AVSDs. Partial AVSDs involve a primum atrial septal defect with two distinct but contiguous AV valves, while complete AVSDs have a single common AV valve. The embryogenesis of AVSDs involves faulty development of the endocardial cushions. The document provides detailed descriptions and images of the anatomy and features of partial and complete AVSDs. It discusses the clinical aspects of AVSDs including prevalence, association with Down syndrome, surgical repair outcomes, and lifelong surveillance needs.

1. AV Canal Defect
Embryogenesis and Pathophysiology
Saurabh Bhardwaj MD,
Cardiology Trainee,
National Heart Institute,
New Delhi

2. Introduction
• Atrioventricular canal defects or Endocardial cushion defects or Atrioventricular septal defects (AVSDs)
are a group of anomalies that share a defect of the atrioventricular septum and abnormalities of
the atrioventricular (AV) valves.
• Divided into partial and complete forms.
• In Partial- primum atrial septal defect (ASD) with 2 distinct, but focally contiguous, right and left
AV valve orifices. The left AV valve invariably is cleft.
• In Complete- a primum ASD, but it is contiguous with a large inlet ventricular septal defect
(VSD) and the common AV valve has a single orifice.

3. Anatomic Features Shared by All Forms of AVSD
▪AV valve leaflets insert at the same level at
the cardiac crux
▪Absence of the AV septum
▪Unwedged and anterior displacement of
the aortic valve
▪ Elongated LVOT
▪Counterclockwise rotation of the LV
papillary muscles
▪Cleft left AV valve component, directed
toward the ventricular septum

4. Embryogenesis
• Due to faulty development of the AV endocardial cushions.
• Dorsal mesenchymal protrusion that may act alone or in concert with development of the
endocardial cushions to create AVSDs.
• The primum atrial septal component of this defect is variable in size and can be quite large
resulting in downward displacement of the anterior left AV valve leaflet to the level of the septal
right AV valve leaflet.
• In AVSDs, the AV valves have the same septal insertion level in contrast to the leaflet
arrangement in the normal heart. The distance from the cardiac crux to the left ventricular apex is
foreshortened, and the distance from the apex to the aortic valve is increased.
• In AVSDs, the disproportion between the two distances causes anterior displacement of the
LVOT resulting in elongation and narrowing of the LVOT producing the
characteristic “gooseneck” deformity.

5. Partial AVSD
• Incomplete fusion of the superior and inferior endocardial cushions results in a cleft
in the midportion of the left AV valve anterior leaflet often associated with
regurgitation.
• Right and left AV valve orifices are separated by a tongue of tissue, frequently
consists of a primum ASD and a cleft left AV valve anterior leaflet.
• Most primum ASDs are large and located anterior and inferior to the fossa ovalis.
• The defect is bordered by a crescentic rim of atrial septal tissue posterosuperiorly and
by AV valve continuity anteroinferiorly.
• These defects are not amenable to transcatheter device closure because of their
proximity to the AV valves.
• The most common associated anomalies with partial AVSD are secundum ASD,
patent ductus arteriosus (PDA), and persistent left superior vena cava connecting to
the coronary sinus

6. Partial AVSD with atrioventricular valve anatomy
• Top left: Systolic apical four-chamber image demonstrating that both right and left atrioventricular valves insert onto the crest of
the ventricular septum at the same level.
• Top right: Corresponding diastolic frame revealing a large primum ASD. Systolic frames often understate the size of the
interatrial communication. There is significant right atrium (RA) and right ventricle (RV) enlargement.
Bottom panels: These are transthoracic parasternal short-axis scans focused at the alve leaflet level in the left
ventricular inflow.
• Left panel demonstrates the cleft in the anterior leaflet of the left atrioventricular valve (asterisk). The anterior leaflet is made of
two separate components that move independently. This creates the diastolic gap in the leaflet (asterisk).
• Color Doppler imaging on the right panel shows considerable regurgitation through the cleft.

7. Complete Atrioventricular Septal Defect
• Lack of fusion between the superior and inferior cushions and, consequently, with the formation
of separate anterior and posterior bridging leaflets along the subjacent ventricular septum.
• Characterized by a large septal defect with interatrial and interventricular components and a
common AV valve that spans the entire septal defect.
• The septal defect extends to the level of the membranous ventricular septum, which is usually
deficient or absent.
• Two lateral leaflets correspond to the posterior tricuspid and posterior mitral leaflets in a normal
heart.
• The rightsided anterior leaflet, represents the normal anterior tricuspid leaflet, and the so-called
anterior bridging leaflet corresponds to the superior half of the anterior mitral leaflet

8. Pathologic specimens of complete AVSD.
A. Right anterior oblique view with right atrial and
right ventricular free walls removed,
demonstrating a large septal defect.
B. Left posterior oblique view (same specimens as in
A) with left atrial and left ventricular free walls
removed, showing the same septal defect.
C. Short-axis view, illustrating a type A common
atrioventricular valve with five leaflets.
D. Four-chamber view, showing secondary right
ventricular hypertrophy and right atrial dilation.
E. Short-axis view of a biventricular specimen
removed during cardiac transplantation, showing
an unbalanced form of AVSD with dilation of a
common inlet right ventricle, leftward septal
bowing, and a hypoplastic left ventricle.
F. Four-chamber view of a complete AVSD associated
with right atrial isomerism, mirror-image
ventricles (L-loop ventricular inversion), and
asplenia.

9. Rastelli Classification for Complete AVSD
Type Anterior Bridging
Leaflet and Chordae
A Divided and attached to
crest of ventricular
septum. Multiple
chordae.
B Partly divided, not
attached to crest of the
septum. Chordae
attach to papillary
muscle in RV
usually on septal
surface.
C Not divided and not
attached to the crest of
the septum (“free
floating”). Chordae
attach to
papillary muscle on
RV free wall.

10. Clinical Compendium
1. AVSDs account for 4% to 5% of congenital heart defects with an estimated
occurrence of 0.19 in 1,000 live births.
2. Approximately 40% to 45% of children with Down syndrome have congenital
heart disease, and among these, 45% have an AVSD.
– Most patients with Down syndrome, who have congenital heart disease,
have the complete form of AVSD (>75%).
– Conversely, 50% of patients with AVSD have Down syndrome.
3. In the modern era, adult survival with excellent quality of life is expected for
children born with AVSD. However, 15% to 20% of these patients may face
reoperation in their lifetime.
4. The repair of AVSD has been one of the success stories in the field of congenital
heart surgery over the last 5 decades.
5. All of these patients require lifelong surveillance for development of LVOT
obstruction and left AV valve regurgitation.

11. Thanks
“The saddest aspect of life right now is that science gathers knowledge faster
than society gathers wisdom.”
― Isaac Asimov