Atrioventricular Septal Defect (AVSD)

Atrioventricular septal defect (AVSD) — also called AV canal defect, and known by the older, now largely historical term endocardial cushion defect — is a congenital malformation in which both atria connect to a single, common atrioventricular valve annulus rather than two separate tricuspid and mitral annuli. It’s the ninth most common congenital heart lesion (an estimated 34.8 per 100,000 live births) and carries a very strong association with Down syndrome.

The Defining Feature: A Common Atrioventricular Junction

Every AVSD, regardless of subtype, shares one anatomical foundation: a common atrioventricular junction, guarded by a valve with five leaflets — not the normal, separate bicuspid mitral and tricuspid arrangement. Two of these leaflets sit exclusively within the right ventricle (an anterosuperior leaflet and a mural leaflet, broadly comparable to the normal tricuspid valve), one sits exclusively within the left ventricle (a mural leaflet, though one that guards less than a third of the left AV junction’s circumference — much less than the two-thirds a normal mitral mural leaflet covers), and two bridging leaflets (superior and inferior) are shared by both ventricles via a tension apparatus that spans the septum. Whether the common valve has one orifice or is divided into separate right and left orifices, this same five-leaflet arrangement is present — the left component always has three leaflets and the right component always has four.

This has a genuinely important terminological consequence: the “cleft” commonly described in the left AV valve is, in most cases, not a true deficit of tissue. It’s the zone of apposition — a commissure — between the left-ventricular portions of the superior and inferior bridging leaflets, which superficially resembles a cleft in an otherwise normal mitral anterior leaflet but is a fundamentally different structure. This correction is made independently by two separate anatomical traditions, which is worth knowing even though the clinical consequence is similar either way — regurgitation at this commissure is genuinely typical of AVSD.

Several structural features follow from the common junction and are shared across every AVSD subtype:

• The aortic valve is “unwedged” — displaced anteriorly, since it can no longer sit nestled between separate mitral and tricuspid annuli the way it does in a normal heart.
• The LV outflow tract is elongated and narrowed as a direct consequence — often described as a “goose-neck” or “dog’s leg” deformity on longitudinal imaging — making it more vulnerable to obstruction than a normal LVOT.
• Inlet-outlet disproportion: in a normal heart, the ventricular septum’s inlet and outlet dimensions are roughly equal; in AVSD, the outlet dimension is substantially longer than the inlet.
• Papillary muscle orientation for the left valve is superior-inferior, rather than the oblique arrangement supporting a normal mitral valve.

Classification: Two Independent Features, Not One Scale

Clinically, AVSD is usually described as partial, transitional (also called intermediate), or complete — but it’s worth understanding that this single spectrum actually blends two genuinely independent anatomical features, a point made explicitly in the surgical-anatomic literature: (1) whether the common valve has one orifice or two, and (2) the level(s) at which shunting actually occurs, which depends on how the bridging leaflets attach to the septal structures.

• Partial AVSD — a primum ASD (an interatrial communication in the “canal” portion of the septum, between the anteroinferior margin of the fossa ovalis and the AV valves) with a cleft (commissure) in the left AV valve, and no ventricular-level shunt. The common annulus is divided into two orifices by AV valve tissue densely adherent to the crest of the ventricular septum (a “connecting tongue”), which closes off any ventricular communication despite the underlying septal deficiency being anatomically the same as in complete AVSD. Note that the term “ostium primum ASD” — commonly used for this defect — doesn’t fully capture the AV valve abnormality that’s actually present.
• Transitional (intermediate) AVSD — significant chordal attachment of the superior bridging leaflet specifically to the ventricular septum, producing a restrictive ventricular-level shunt through the gaps in that chordal attachment. The VSD component is typically small enough that RV pressure stays subsystemic.
• Complete AVSD — a primum ASD, a common valve with a single orifice, and a large, unrestricted inlet-type VSD — the bridging leaflets are not adherent to the septal crest, allowing free shunting at both atrial and ventricular levels. This is the form most strongly associated with Down syndrome.

Rare unbalanced forms exist too: the common AV junction can commit predominantly to one ventricle rather than being shared evenly (right- or left-ventricular dominance), which matters significantly for surgical planning, since severe imbalance can push management toward single-ventricle palliation rather than a two-ventricle repair. A related atrial-level variant, “double-outlet atrium,” occurs when the atrial septum is malaligned relative to the ventricular septum rather than the AV valve itself being unbalanced.

A related entity that is not part of the AVSD spectrum at all: an isolated left-ventricle-to-right-atrium communication (a Gerbode-type defect) through a deficient atrioventricular membranous septum is genuinely rare, and structurally the opposite of AVSD — it preserves two separate atrioventricular junctions with otherwise normal valve architecture, rather than a common junction. Most apparent cases of LV-to-RA shunting are actually explained by a deficient or cleft tricuspid septal leaflet adherent to a nearby defect’s margin, not true membranous septal absence.

The Rastelli Classification

Where the common valve has a single orifice, the Rastelli classification further describes how the superior bridging leaflet relates to the ventricular septum — a distinction that matters for surgical planning:

• Type A (most common) — the superior bridging leaflet is divided at the crest of the ventricular septum, with minimal bridging. This type carries the highest association with preoperative LVOT obstruction, in part from more pronounced outflow tract elongation.
• Type B (rarest) — division of the superior bridging leaflet occurs at a right ventricular papillary muscle, often displaced onto the septal surface of the RV, with moderate bridging.
• Type C — the superior bridging leaflet is undivided, or “free-floating,” with no chordal attachment to the septum — the most rudimentary form, and the one characteristically associated with Down syndrome.

Etiology and Associations

Down syndrome (trisomy 21) carries an approximately 1000-fold increased risk of AVSD compared with a normal karyotype. At least 40% of children with Down syndrome have congenital heart disease, and AVSD is among the most common lesions found, alongside isolated VSD. AVSD is also seen frequently with heterotaxy syndrome, particularly the asplenia (right isomerism) variant, and can occur sporadically or with autosomal dominant inheritance patterns independent of any chromosomal abnormality.

Pathophysiology

Shunting is predominantly left-to-right, following the same pressure gradient logic as isolated ASD and VSD, with the specific level(s) of shunting determined by the bridging leaflet anatomy described above. Chronic volume overload affects the right heart and pulmonary circulation, and AV valve regurgitation — common across all subtypes — compounds the volume load further. Children with both Down syndrome and AVSD are specifically more likely to develop elevated pulmonary vascular resistance within the first year of life compared with AVSD patients without Down syndrome — a key driver of the early surgical timing recommended in this population. Untreated, longstanding large shunts progress toward irreversible pulmonary vascular disease and Eisenmenger physiology, just as with isolated ASD/VSD.

Unlike a secundum ASD or a small muscular VSD, the septal defects in AVSD — the primum ASD and the CAVC-type VSD — do not typically close spontaneously, since they reflect a structural deficiency in AV septation itself rather than a growth-related gap.

Clinical Findings

• Auscultation — a pulmonary flow murmur from increased pulmonary valve flow, plus a regurgitant murmur from AV valve incompetence, which is essentially universal to some degree.
• ECG — left axis deviation (a classic, distinctive finding reflecting the abnormal conduction axis around the common junction), biventricular hypertrophy, and a real risk of conduction abnormalities including AV block.
• Chest X-ray — cardiomegaly with increased pulmonary vascular markings from pulmonary overcirculation.

Echocardiographic Findings

Echocardiography is the main tool for diagnosing AVSD and planning repair, and a structured exam matters because so much depends on details — the valve’s leaflet arrangement, how the valve is divided between the ventricles, and whether the left ventricular outflow tract is obstructed.

Preoperative Assessment: Key Elements

A complete study, with 2D (and 3D where useful) plus color and spectral Doppler, should cover:

• The AV valve: morphology and function, including the leaflet arrangement (Rastelli type) and the number and spacing of the left ventricular papillary muscles.
• Balance: the relationship of the atrial septum to the inlet and ventricular septum, how the AV valve is distributed over the two ventricles, annulus size and inflow by color Doppler, and the severity of any ventricular hypoplasia.
• Left ventricular outflow tract obstruction, and its cause if present.
• Hemodynamics: flow direction across the atrial and ventricular defects by color and spectral Doppler; RV systolic pressure from the peak instantaneous gradient across the VSD by CW Doppler, or — when the VSD is restrictive or absent — from the right AV valve regurgitant jet velocity; severity of AV valve regurgitation by color mapping; the effect on the ventricles (dilation, increased flow across the pulmonary outflow, left AV valve inflow); and ventricular septal position as a marker of elevated RV pressure.
• Biventricular systolic and diastolic function, and associated lesions — particularly PDA, coarctation, and tetralogy of Fallot or other conotruncal anomalies.
• 3D assessment of the AV valve anatomy and the cause of any regurgitation.

A View-by-View Approach: Complete AVSD

• Subxiphoid frontal (long-axis) view and the “in-between” left anterior oblique (LAO) sweep. These show the size of the ostium primum-type atrial defect, any additional atrial communications, and (by color Doppler) the atrial shunt direction. The frontal view also shows the displaced aortic outflow and the abnormally elongated LV outflow tract with its resulting curve into the ascending aorta — the “goose-neck deformity.”
• Subxiphoid LAO view. The ideal view for seeing the common AV valve en face, to judge how much of the valve is apportioned to each ventricle (balance). It’s also a good view for Rastelli classification, because the attachment of the superior bridging leaflet — or the lack of one — can be seen.
• Subxiphoid sagittal (short-axis) view. Shows AV valve anatomy and chordal attachments, and so also helps with Rastelli type. The papillary muscles can be identified and are often located counterclockwise to their position in a normal heart; a solitary papillary muscle with an eccentric valve orifice is worth flagging as a complicating factor for left AV valve repair. The VSD size is hard to judge from here, because the probe sweeps through the defect without highlighting its edges.
• Apical four-chamber view. Not ideal for the atrial septum (parallel beam, false dropout), but extremely useful in AVSD because it shows the AV valve sitting within the AV septal defect, which helps establish the size of the atrial and ventricular communications. Tilting the transducer from posterior to anterior often reveals the full scope of the VSD and any additional septal defects. It’s also the best view for AV valve inflow and regurgitation, which can be graded subjectively and quantitatively. In rare cases the AV valve is attached to the atrial septum, leaving only a VSD component.
• Apical five-chamber view. Shows LV outflow tract obstruction, with severity quantified by PW and CW Doppler.
• Parasternal long-axis view. A sweep from anterior to posterior shows the atrial and ventricular communications; atrial-level and ventricular-level shunting are distinguished by the location and timing of the color Doppler flow (atrial left-to-right shunting occurs in diastole).
• Parasternal short-axis views. A low left parasternal view can image the atrial septum when the subxiphoid window is poor. A plane at the level of the ventricular septum shows the VSD and its flow and identifies additional muscular defects; it also shows the superior and inferior bridging leaflets of the left AV valve and the papillary muscle orientation.

Partial and Transitional AVSD

The imaging goals are the same, but the anatomy differs: partial AVSD has no ventricular-level shunt, and transitional AVSD has a restrictive VSD.

• The subxiphoid sagittal view often clinches the diagnosis. A normal anterior mitral leaflet lies parallel to the ventricular septum, whereas the “cleft” of a partial AVSD is oriented perpendicular to it. See Mitral Valve Prolapse for how a true isolated cleft mitral valve is distinguished from this.
• The LAO views show the ostium primum defect well, but it can be hard to tell complete from partial there, because the VSD component isn’t highlighted.
• Apical four-chamber view. Shows the AV valve’s position and adherence to the ventricular septum. Often AV valve tissue under the valve “fills in” the VSD; some consider this a transitional form even though no VSD flow is present. AV valve regurgitation is usually seen through the left-sided cleft. In the transitional form, restrictive ventricular-level shunting can be seen here — it may be minimal, so use color Doppler to find a small defect.
• Parasternal views. Show the ostium primum defect when the subxiphoid window is poor. The short-axis view assesses ventricular septal position (flattening suggests elevated RV pressure — useful when there isn’t enough right AV valve regurgitation to estimate it), can align the Doppler beam for the VSD gradient, and — in a sweep — shows the trifoliate left AV valve and where the left and right AV valve regurgitation arises.

Left Ventricular Outflow Tract Obstruction

Because the LV outflow tract is elongated, obstruction can occur before and after surgery, and is more common in the partial form and in Rastelli type A. Causes include abnormal chordal attachments to the left ventricular side of the septum, a discrete subaortic membrane, septal hypertrophy, and an anomalous or prominent anterolateral papillary muscle. The subxiphoid LAO view and the apical five-chamber view show the outflow tract well and give an appropriate angle for color and spectral Doppler; the parasternal long-axis view also shows it and identifies the cause.

Unbalanced AVSD

• Atrial unbalance is unusual. When suspected it’s best seen on the apical four-chamber view, where color Doppler shows how the atrial inflow is partitioned.
• Ventricular unbalance. The subxiphoid LAO or sagittal view shows how much of the valve is assigned to each ventricle. The AV valve index (AVVI = left AV valve area ÷ total valve area) grades it: under 0.4 is right dominant, over 0.6 is left dominant, and 0.4 to 0.6 is considered balanced.
• The apical four-chamber view usually best shows the severity of ventricular hypoplasia, and diastolic color flow shows how much inflow (if any) crosses into the hypoplastic ventricle, by measuring the secondary color inflow diameter. In right-dominant AVSD, limited inflow into the left ventricle is a risk factor for a poor outcome of biventricular repair. A newer predictor is the RV/LV inflow angle, measured on the same view.
• Other signs of LV adequacy: right-to-left flow at the VSD in systole, and retrograde flow in the transverse aortic arch, are both risk factors for a poor outcome.
• Right-dominant AVSD is often accompanied by a hypoplastic left mural leaflet, best seen on the subxiphoid LAO or sagittal view. When it’s small, the “cleft” makes up the primary commissure of the left AV valve, which can lead to significant regurgitation after biventricular repair — though recent work suggests it’s not a major risk factor.
• Also look for distal obstruction, particularly in the LV outflow tract and the aortic arch.

Additional Defects to Look For

• Tetralogy of Fallot. The subxiphoid sagittal view shows the anterior malalignment of the conal septum as well as the common AV valve, and subxiphoid, apical, and parasternal views help confirm the association. See Tetralogy of Fallot.
• Patent ductus arteriosus should always be ruled out, particularly in children with Down syndrome; the high parasternal “ductal” view confirms it. See Patent Ductus Arteriosus.
• Suprasternal imaging determines arch sidedness, rules out coarctation (see Coarctation of the Aorta), and confirms normal systemic and pulmonary venous connections — particularly important with heterotaxy. In polysplenia (left isomerism), complete heart block with noncompaction of both ventricles can occur, a combination with a poor prognosis.

How to Diagnose It: A Practical Sequence

1. Start subxiphoid (frontal view and LAO sweep) and look for the ostium primum defect and how the AV valve relates to the septum.
2. Confirm a single AV junction on the apical four-chamber view: the AV valves are coplanar — inserting at the same level on the septum rather than showing the normal offset — with the valve sitting within the AV septal defect. Tilt posterior to anterior to see the full extent of the VSD.
3. Decide which type it is. Complete: an atrial defect, a VSD component, and a common valve. Transitional: a restrictive VSD, often partly closed by valve tissue. Partial: no ventricular-level shunt, with the left-valve cleft oriented perpendicular to the septum on the subxiphoid sagittal view.
4. Define the valve. En face on the LAO view: how it’s apportioned between the ventricles, and the superior bridging leaflet’s attachment (Rastelli type). Then papillary muscle number and position, and the regurgitation — where the jets arise and how severe — plus any stenosis (mean gradient).
5. Grade balance with the AVVI, and look at ventricular size and inflow on the apical four-chamber view.
6. Examine the LV outflow tract: the goose-neck deformity, and any obstruction and its cause.
7. Estimate RV pressure from the VSD gradient (or, with a restrictive or absent VSD, the right AV valve jet), and the septal position.
8. Search for associated lesions: PDA, coarctation and arch anatomy, tetralogy of Fallot, and the venous connections.
9. Add 3D or TEE when valve anatomy or the cause of regurgitation needs clarifying.

Prenatal, Adult, and Perioperative Imaging

• Prenatal. The four-chamber view readily shows most anatomic features. If AVSD is diagnosed before birth, serial fetal echocardiography is suggested to watch for progression, and amniocentesis is suggested given the strong association with Down syndrome.
• Adults. Imaging can be quite challenging, particularly with Down syndrome. The subxiphoid window is typically not feasible because of limited penetration, so the primum defect may be hard to assess; the apical window gives a lot of information — ventricular size, biventricular shortening, the valve’s position within the defect, and regurgitation severity, plus an RV pressure estimate in partial or transitional forms. Most adults with unrepaired complete AVSD have pulmonary vascular disease and may show Eisenmenger physiology with right-to-left ventricular-level shunting and RV hypertrophy (see Eisenmenger Syndrome). Partial AVSD is occasionally diagnosed in adulthood, since symptoms may not appear until the third or fourth decade. TEE is an excellent alternative when windows are poor.
• Intraoperative and postoperative. TEE is typically used, with epicardial imaging if TEE is contraindicated. After weaning from bypass, look for residual atrial or ventricular defects (or a new left ventricle-to-right atrium shunt), residual AV valve regurgitation, and any AV valve stenosis, along with ventricular function. A residual VSD under 3 mm generally doesn’t need reintervention, while one over 4 mm needs immediate reoperation; in borderline cases or very small infants, Qp:Qs from oxygen saturations can help. Intraoperative TEE commonly underestimates AV valve regurgitation compared with later transthoracic assessment, probably because of differences in preload, afterload, and inotropic support just after bypass — still a useful screen for whether immediate reintervention is needed.

Treatment Considerations

• Surgical repair addresses the septal defect(s) and reconstructs the AV valve(s) — typically patch closure of the atrial and/or ventricular components, with division and closure of the common valve’s commissure/cleft to reduce regurgitation and re-create two functional AV valve orifices.
• Early surgical timing is specifically emphasized in Down syndrome patients, given their documented tendency toward earlier-onset elevated pulmonary vascular resistance — the goal is repair before pulmonary vascular disease becomes established.
• Unbalanced forms may require a different surgical strategy altogether — single-ventricle palliation rather than biventricular repair — determined largely by the degree of AV junction commitment to one ventricle.
• Postoperative follow-up should track residual shunting, AV valve regurgitation or stenosis (particularly at the reconstructed commissure), LVOT obstruction (especially relevant in Rastelli type A anatomy), and conduction abnormalities.

Clinical Importance

AVSD is a spectrum, not a single fixed lesion, and its clinical course — timing of intervention, surgical complexity, and long-term valve durability — depends heavily on where a given heart falls across the classification features above, rather than on a single “type” label. See Atrial Septal Defect and Ventricular Septal Defect for the isolated forms of the septal deficiencies that AVSD combines, and Right Ventricle Evaluation for how the resulting volume and pressure changes are assessed.