Following the first successful repair of atrioventricular septal defects (AVSDs) at the dawn of modern cardiac surgery, significant advances have been made in many aspects of overall patient management. In line with improvements in cardiopulmonary bypass, postoperative care, and refinement of surgical technique together with morphologic understanding,¹ there has been a striking reduction in both postoperative mortality and early morbidity.

Reflecting this increased confidence in the operative management in the last quarter century,² there has been an evolution in the timing of surgery from a staged approach to an era of complete repair in early infancy (Fig. 66-1).^1,3

The prevalence of AVSD has been put at 0.19 per 1000 live births, accounting for 2.9 percent of congenital cardiac malformations. In about 60 percent of these defects, the shunting is confined to the atrial level.

Anatomic Considerations

The principal malformation that defines the AVSD lies at the atrioventricular (AV) junction. The normal arrangement is considered first.

The Normal Atrioventricular Junction

The AV junction is defined as the muscular area that surrounds the orifices of the AV valves and marks the point at which the distal margins of the atrial musculature meet the ventricular myocardium. Thus, there are two AV junctions in the normal heart, one supporting the tricuspid and the other the mitral orifices. Although the musculature of the two chambers abuts over the circumference of this junction, they are nevertheless separable apart from the area of the muscular axis of AV conduction—the bundle of His. Wedged in between the two junctions is the subaortic outflow tract, incorporating the indwelling aortic valve. Beyond this anteriorly is the subpulmonary outflow tract, mounted on its free-standing infundibulum (Fig. 66-2).

In considering junctional morphology, it is possible to distinguish the points of attachment of the AV valvar hinges from the points of muscular AV contiguity. At the right AV junction, where the hinges of the tricuspid valve are anchored by muscle throughout, these two areas are essentially the same apart from the short length where the septal leaflet of the valve crosses the membranous septum. In doing so, it divides the membranous septum into AV and interventricular components (Fig. 66-3).

This disparity between the muscular AV junction and the valvar annulus is most clearly seen at the mitral valve, where the fibrous tissue of the aortomitral fibrous continuity supports one-third of the annulus (Fig. 66-4).

The Atrioventricular Septum and “Sandwich”

Observed in the four-chamber view, the tricuspid annulus is seen to lie at a more apical position than its mitral counterpart. Consequently, in the region immediately caudal and posterior to the area where the septal leaflet traverses the membranous septum, the muscular atrial septum overlaps the crest of the ventricular septum. This area between the offset hinges of the AV valves has previously been called the muscular AV septum,⁴ but it is, in fact, none other than an extension of the fibro-adpose tissue of the posteroinferior AV groove (Fig. 66-5). The term AV septum should therefore be reserved for the AV component of the fibrous membranous septum. It is the absence of these two components—the fibrous membranous septum and the fibro-adipose muscular “sandwich”—that define hearts with AVSDs more than any other anatomic feature.

The Common Atrioventricular Junction in Atrioventricular Septal Defects

The common AV junction is the fundamental feature that sets the AVSD apart from the normal heart (Fig. 66-6).

It occurs irrespective of the number of valvar orifices and is seen even in the absence of septal deficiency. All other morphologic features seen in this defect arise as a direct consequence of this singular arrangement of the junctional structures. Other morphologic markers can be considered under the following subheadings:

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  Leaflets arrangement

  
  
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  Septal deficiency

  
  
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  The ventricular mass

  
  
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  The subaortic outflow tract

  
  
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  The conduction axis

Leaflet Arrangement and the Subvalvar Apparatus

The oval-shaped common AV junction in AVSDs surrounds an orifice guarded by a valve that usually possesses five leaflets. There is some variability in this arrangement, depending on the permutation of leaflet fusion. Variable fusion may result in separate right and left orifices or may produce any number of accessory orifices, depending on the location and extent of fusion.

Lying across the crest of the ventricular septum, to a variable extent, are the superior and inferior bridging leaflets (SBLs and IBLs). These were previously known as the anterior and posterior bridging leaflets, respectively, and have no counterparts in the normally septated heart (left in Fig. 66-6).

Adjacent to the SBL over the orifice of the right ventricle is the anterosuperior leaflet (ASL), which is analogous to its tricuspid counterpart. Supported by the parietal margins of the common junction on both sides are the right and left mural leaflets. The right mural leaflet lies against the ASL and IBL, the left leaflet lies in apposition with the SBL and IBL.

The subvalvar apparatus is similarly abnormal, most importantly in the arrangement of the papillary muscles. The two left-sided papillary muscles normally lie obliquely, assuming anterolateral and posteromedial attitudes. In AVSD, these two muscles are located in the same vertical plane, being positioned superiorly and inferiorly; hence their names (Fig. 66-7).

The superior muscle supports the zone of apposition at the SBL–mural leaflet interface, and the inferior muscle supports the zone of apposition between the IBL and mural leaflets. Unlike the case on the right side, the position of these left-sided muscles is consistent. However, they may be found in clusters, as in the so-called parachute arrangement, where the chords from all leaflets converge onto only one papillary muscle (Fig. 66-8).

Rastelli Classification and Right Ventricular Papillary Muscles

The papillary muscle arrangement of the right side depends on the degree of bridging of the SBL across the VSD. This forms the basis for the Rastelli classification.^2,5

When there is minimal bridging of the SBL into the left ventricle, or Rastelli class A, the edge of the leaflet is tethered to the crest of the septum (Fig. 66-9A). Here, the ventricular septum is analogous to a papillary muscle, supporting the zone of apposition between the SBL and IBL. In this situation, the ASL of the right AV valve is well developed and morphologically similar to its tricuspid counterpart. At operation, this arrangement gives the appearance of the SBL being divided over the septum (Fig. 66-10A). Subsequent anatomic observation determined that this “division” of the SBL over the septum merely represents the zone of apposition between the SBL and well-formed ASL in the class A defect and that the SBL is undivided over the ventricular septum in all classes of the defect.^6–8

In the class B defect, the SBL extends even more into the right ventricle, with further reduction in the size of the ASL (Fig. 66-10B). It is usually unattached to the ventricular septum but at its right margin is supported solely by an anomalous right ventricular papillary muscle that arises from the septomarginal trabeculation. In the so-called type C defect (Fig. 66-9B), the SBL extends even further into the right ventricle and floats freely above the ventricular septum, extending from the anterior papillary muscle of the right ventricle to the superior papillary muscle of the left ventricle (Fig. 66-10C). The ASL is necessarily smaller, accommodating the greater encroachment of the SBL into the right side. Lack of chordal attachments to the septum ensures unhindered access to the aortic valve beneath.^9,10 In the type C defect, there is a high association with tetralogy of Fallot (ToF). The reasons for this are developmental, as rightward displacement of the outlet septum during the development may limit chordal attachment to the septum.^11,12

Epidemiologically, the type A defect has the greatest prevalence, both in morphologic and clinical series, ranging from 50 to 75 percent of cases.^7,13–15 Variability in the degree of bridging of the SBL may also be seen in cases of AVSD with separate orifices, or the “partial defect,” where minimal bridging of the SBL is the most common pattern.

The Nature of the Zone of Apposition: “Cleft” or “Commissure?”

The area between left sides of the bridging leaflets has been the subject of much controversy. The principal question is whether it represents a cleft, as in the so-called isolated cleft of the anterior leaflet of the mitral valve,¹⁶ or whether it is the commissure between the leaflets of a left AV valve. A commissure is the functional division between the leaflets of a valve that is supported by a fan-shaped tendinous chord atop a papillary muscle. A cleft is defined as a space or opening made by splitting the anterior leaflet.¹⁷ Given these definitions, this space between the bridging leaflets may be best thought of as septal commissure in a trifoliate leaflet AV valve.

Morphology of the Atrial and Ventricular Septal Defects

Although septal deficiency is almost uniformly seen in hearts with this defect, the pattern of blood flow between the chambers is largely determined by the chordal and leaflet anatomy.

The ventricular septum in AVSD is “scooped,” with a gentle curve extending from the crux of the heart to the left ventricular outflow tract (LVOT) (Fig. 66-11). The depth of the scoop and the resulting deficiency are very variable, being more extensive in hearts with common orifices.^7,18,19

Above the plane of the annulus is the so-called ostium primum defect (Fig. 66-12), which, in some instances, may be obliterated through attachments of the leaflets to its edge, giving rise to obligatory ventricular shunting. If combined with a secundum atrial septal defect (ASD), it may produce a common atrium.

Common Versus Separate Orifices

There are two varieties of AVSD, depending on the level of intra-cardiac shunting caused by the pattern of leaflet fusion. However, when the leaflets are stripped from both variations, their junctional morphology is identical.⁴

In the defect with separate orifices, also known as the partial AVSD, there is a tongue of leaflet connecting the bridging leaflets over the ventricular septal crest. Therefore, the septal communication beneath the leaflets is obliterated, limiting shunting to the atrial level (Figs. 66-13 and 66-14).

Although this fusion results in separate AV orifices, the connecting tongue is not a continuation of the common annulus across the septum. The morphologic extent of this tongue again shows variability. In some instances, the bridging leaflets are fused with each other, but multiple inter-chordal spaces beneath the leaflets leave the potential for interventricular shunting (Fig. 66-15).

Hearts exhibiting this latter phenomenon have been termed the “intermediate” or “transitional” AVSD, emphasizing separate orifices in the context of persisting ventricular shunting, as if it were embryologically a transitional form between two extremes. We believe that this term is misleading and loses sight of the fact that the defining feature of hearts with this defect is deficiency of AV septation and not leaflet morphology. As such, septation may be deficient or otherwise, with no intermediate arrangement.²⁰ Despite the clinically more benign course of the defect with separate orifices, there is a greater prevalence of subaortic obstruction, papillary muscle anomalies, and leaflet dysplasia. Nevertheless, the association with other complex malformations is more common in hearts with a common orifice.

Ventricular Morphology and Balance

In the normal heart, the inlet/outlet ratio is the same (Fig. 66-16, left panel). This is also seen in hearts with perimembranous VSDs and those with an isolated “cleft” in the anterior leaflet of the mitral valve, which have all been previously described as the “forme fruste” of AVSD.²¹ In the latter, the outlet length is significantly longer than the inlet and can be appreciated from the “gooseneck” deformity of the subaortic outflow tract (Fig. 66-16, right panel). This has been explained in terms of a shorter inlet²² or a longer outlet.²³ These ratios are the same in AVSDs with common and separate orifices.

The diameters of the inlets of both ventricles as well as the volumes of the ventricles are the same, or balanced, with the atrial and ventricular septa being in line. In the era of two-staged repair, a reduction in the volume of the right ventricle was occasionally observed after pulmonary trunk banding, owing to hypertrophy of the trabecular layer of the myocardium. Aside from this specific situation, left dominance may occur with hypoplasia of the right ventricular and pulmonary arterial components, typically in association with malalignment of the atrial and ventricular septa. This leads to the most extreme case of double-inlet left ventricle with common AV valve.²⁴ In the case of right dominance, there is usually hypoplasia of the left ventricular and aortic structures, with normal alignment of the atrial and ventricular structures.²⁵

The Subaortic Outflow Tract and Subaortic Stenosis

Although the subaortic outflow tract is narrower²⁶ and longer¹⁰ than normal (as seen on angiography), obstruction is surprisingly uncommon (Fig. 66-16, right panel).²⁷

The principal source of obstruction derives from the anterior and unwedged position of the subaortic outflow tract, which is a direct product of failure of AV septation. This exaggeration of the subaortic outflow tract is also more pronounced in hearts with the Rastelli type A leaflet morphology, where the SBL is tethered to the crest of the ventricular septum, narrowing the outflow tract even further. This also explains why obstruction is more frequent in the setting of separate orifices.

In addition, the superior papillary muscle or an anomalous portion of it may extend into the subaortic outflow tract, or there may be a prominent anterolateral trabecular muscle bundle. Similarly, there may be accessory tissue tags or chordal attachments from the SBL, further obstructing the path. Such hearts may also become obstructed through mechanisms that affect normally septated hearts, such as fibrous subaortic shelves.

There may also be a further, dynamic component to the potential for obstruction in these hearts. The elongated outflow tract is a muscular tube that constricts during systole, heard as a persisting systolic murmur in the absence of clinically significant left AV valve regurgitation. Obstruction is also well recognized in the postoperative setting, as after left AV valvar replacement, where it accounts in part for the high mortality.

The Morphology of the Atrioventricular Conduction Axis

The displacement of the conduction axis is the direct result of deficient septation, with absence of the central fibrous body that normally marks the point at which the AV node continues as the AV bundle. There is some variability among hearts with the AVSD, depending on the alignment of the atrial and ventricular septa. Thus, the nodal triangle is displaced posteriorly and inferiorly and lies in the posterior right atrial wall, between the orifice of the coronary sinus and the crux of the heart.

From this position, the bundle of His passes to the crest of the ventricular septum through the crux, which is the first point of contact of the atrial and ventricular musculature. It runs along the crest of the septum under the cover of the IBL, giving off left bundle branches. At about the midpoint of the ventricular septum, it becomes the right bundle branch, which descends to the medial papillary muscle of the right ventricle (Fig. 66-17). Thus, at operation, the greatest danger is at the time of securing the atrial septal patch, when the node is approached at the crux.

Aside from synchronous congenital malformations, the presentation and clinical findings of patients with AVSD are determined by several factors, among which are the level of shunt and the competence of the left AV valve. Both variables may be the cause of acute and severe presentation.