Key Words
Common atrioventricular junction, trifoliate left atrioventricular valve, ostium primum defect, surgical correction, Down syndrome
There is a group of lesions unified by the anatomic hallmark of a common atrioventricular junction coexisting with deficient atrioventricular septation. The key to their differentiation from other potentially related defects is the presence of the common junction, including the arrangement of the fibrous skeleton of the heart, which is fundamentally different from the morphology found in the normal heart. In clinical terms, it is probably the influence of this abnormal architecture of the atrioventricular valves that is the most prominent feature. In this respect, there is a common atrioventricular junction irrespective of whether there are separate atrioventricular valvar orifices for the right and left ventricles—with the latter lesion traditionally described as the ostium primum defect—or whether the valve itself is also a common structure, often described as the complete form of the malformation. As we show, the abnormal morphology of the ventricular mass and the atrioventricular junction is more or less constant within the overall group, thus calling into question the notion of distinguishing between partial and complete variants. Also we describe the lesions here—described previously as endocardial cushion defects or atrioventricular canal malformations—simply as atrioventricular septal defects. Since the publication of the previous edition of this text, this term has slowly come to achieve recognition as being the most accurate descriptor. Very rarely, hearts with deficient atrioventricular septation can exist with otherwise normal atrioventricular junctions. These are the so-called Gerbode defects, which are described in Chapter 32 , on ventricular septal defect. Patients with all the stigmata described in the present chapter can also be found, again rarely, when the septal structures are intact, presumably due to spontaneous closure of a preexisting atrioventricular septal defect. Notwithstanding these potential caveats, it remains our conviction that atrioventricular septal defect is the best default option for this important group of malformations. For precision, it is necessary also to specify the presence of the common atrioventricular junction and to indicate spontaneous closure in those rare instances when the septal structures are intact despite the presence of the common junction.
Prevalence and Etiology
Early reports of the prevalence of atrioventricular septal defects with a common atrioventricular junction have varied markedly. These differences almost certainly reflect the problems inherent in most epidemiologic studies due to the bias in selection. The most accurate data are those provided by the study of a stable population backed up with verification at clinical study or autopsy. Using such an approach, Samanek et al. calculated a prevalence of 0.19 per 1000 live births, which accounted for 2.9% of the patients examined with congenitally malformed hearts in Bohemia from 1952 to 1979. Of these cases, three-fifths had shunting confined at the atrial level, the so-called ostium primum defects, whereas the remainder had associated ventricular shunting. Regional differences were also found, with most of the afflicted children being born in industrial areas and girls predominating in the overall numbers. The lesion was even more frequent in stillborns, accounting for 6.2% of all congenitally malformed hearts.
There is a strong association between deficient atrioventricular septation with Down syndrome (see Chapter 4 ). In Toronto, about one-third of patients with Down syndrome had an atrioventricular septal defect with a common valvar orifice, whereas only one-twentieth had the so-called ostium primum variant. In the Bohemian population, half of those with deficient atrioventricular septation had Down syndrome. This close association with trisomy 21 is cited as evidence against the usual multifactorial model put forward to explain the inheritance of congenital cardiac disease. Evidence has been found of autosomal dominant inheritance, not linked to chromosome 21, in large families involving many patients with atrioventricular septal defect, and there is appreciable evidence of familial recurrence. It is possible, therefore, to discern at least three different genetic patterns: one found in association with Down syndrome, a second emerging as an autosomal dominant trait, and the third being isolated. There is a high rate of recurrence, particularly in females. Major anatomic differences are present within the overall population of patients having atrioventricular septal defects with or without Down syndrome, such as the atrioventricular valve being common or divided into separate left and right valvar orifices. It is intuitive to suggest that these differences must be a reflection of the different genetic mechanisms involved.
Anatomy
Understanding Atrioventricular Septal Defects With a Common Atrioventricular Junction
As shown in the following text, the abnormal structure and development of atrioventricular junctions are the phenotypic features of the group of lesions forming the focus of this chapter. By atrioventricular junctions, we mean the areas of the heart where the atrial myocardium becomes contiguous with the ventricular myocardium. To understand the abnormalities, it is necessary to emphasize the features of normality. In the normal heart, the myocardial segments within these junctional areas are separated from one another save at the site of penetration of the bundle of His, which is part of the muscular axis responsible for atrioventricular conduction. The separation within the junctions, providing the necessary electrical insulation, is produced largely by the fibrofatty tissues of the atrioventricular grooves. These tissues form the greater part of the so-called valvar annuli, which also support the attachments of the leaflets of the atrioventricular valves. In the normal heart, there are two atrioventricular junctions that surround the tricuspid and mitral valvar orifices. There is a central component present separating the junctions, which also abuts on the subaortic outflow tract ( Fig. 31.1 , left ).
In the normal heart, part of this separating component is made up of a true atrioventricular septum, this being the atrioventricular component of the membranous septum, which is relatively small ( red arrow in Fig. 31.1 , left ). It is the lack of this septal component, along with additional myocardial separating structures, that underscores the morphology of atrioventricular septal defects found in the setting of a common atrioventricular junction ( Fig. 31.1 , right ). In hearts with separate junctions, the septal leaflet of the tricuspid valve is usually attached at a considerably more apical level than is the corresponding leaflet of the mitral valve (see Fig. 31.1 , left ). The posteroinferior part of the area between the hinges of the atrioventricular valvar leaflets, however, is not strictly septal. This is because, in this area, the atrial myocardial wall overlaps the crest of the muscular ventricular mass, with an extension of the insulating inferoposterior fibrofatty atrioventricular groove separating the two muscular masses ( Fig. 31.2 , left ). In effect, it is a sandwich of muscular and fibrofatty tissues that is interposed between the cavities of the right atrium and the left ventricle. The fibrous atrioventricular septum is found anterosuperior to this muscular area (see Fig. 31.2 , right ).
This septal component is also an integral part of the central fibrous body, which forms the rightward wall of the left ventricular outflow tract. In the normal heart, the left ventricular outflow tract interposes between the mitral valvar orifice and the septum ( Fig. 31.3 , left ). In the right ventricular orifice, one of the leaflets of the tricuspid valve is adherent to the ventricular septum over a considerable area. These arrangements are pertinent to the naming of the leaflets of the normal and abnormal valves. In the mitral valvar orifice, the curtain of leaflet tissue is divided by an obliquely orientated line of apposition into a short and square leaflet, best termed the aortic leaflet but often termed the anterior leaflet. The other leaflet is more extensive in terms of its circumferential attachment but much shallower. This is the mural or posterior, leaflet. The two leaflets close over a solitary zone of apposition (see Fig. 31.3 , left ). The normal tricuspid valve has three leaflets, which are located in the anterosuperior, septal, and inferior or mural positions. The dimensions of the septal aspect of the ventricular mass also vary between hearts with separate as opposed to common atrioventricular junctions. In the normal heart, the distance from the attachment of the mitral valve at the crux to the ventricular apex, or the inlet dimension, is much the same as that from the ventricular apex to the anterosuperior attachment of the leaflets of the aortic valve, this being the outlet dimension ( Fig. 31.4A ). The essence of hearts with an atrioventricular septal defect and a common atrioventricular junction is disproportion between these inlet and outlet dimensions.
Basic Morphology of Atrioventricular Septal Defects
Apart from the Gerbode defect (see later), the essential morphologic ingredient of hearts now usually described as having atrioventricular septal defects is the presence of a common atrioventricular junction. This junction is common to all four cardiac chambers in all hearts. There are no intermediate or transitional arrangements. The junctional arrangements are matched by differences in the arrangement of the fibrous skeleton, with the continuity between the membranous septum and the right fibrous trigone fundamentally disturbed when there is a common atrioventricular junction ( Fig. 31.5 ).
These differences in junctional architecture underscore departures from normality in at least four of the features already enumerated as characteristic of hearts with separate right and left atrioventricular junctions. First, almost always but not universally, there is a defect at the anticipated site of the structures that usually interpose between the right atrium and the left ventricle. The septal deficiency is comparable irrespective of whether there is a common valvar orifice within the common junction ( Fig. 31.6 , right ) or there are separate atrioventricular valvar orifices for the right and left ventricles ( Fig. 31.6 , left ). The arrangement with separate valvar orifices, but a common atrioventricular junction, is also known as an “ostium primum defect.” The septal deficiency is between the leading edge of the atrial septum and the crest of the muscular ventricular septum, to which the valvar leaflets are attached (see Fig. 31.1 , right ).
The second feature is the oval shape of the common atrioventricular junction (see Fig. 31.3 , right ). Irrespective of the number of atrioventricular valvar orifices within the junction, the left ventricular outflow tract is not wedged between the left and right atrioventricular annuli, as seen in the normal heart. Instead, the aortic orifice is anterosuperior to the common junction. The junction is just as common in the hearts having separate valvar orifices for the right and left ventricles ( Fig. 31.7 , left ) as when there is a common valvar orifice ( Fig. 31.7 , right ).
Since there is a common atrioventricular junction and a lack of any septal atrioventricular muscular contiguity, the third feature that differs from the normal is the arrangement of the valvar leaflets that guard the common junction. These leaflets, irrespective of whether they guard a common atrioventricular orifice or separate orifices for the right and left ventricles, bear scant resemblance to the arrangement of the leaflets of the normal mitral and tricuspid valves (see Fig. 31.7 ). When describing the valves and comparing them with the arrangement seen in the normal heart, the key feature is the boundaries of the individual leaflets. These are best assessed when the valve is seen in its closed position. Studied in this fashion, the overall curtain of leaflet tissue in hearts having the phenotypic feature of an atrioventricular septal defect with a common atrioventricular junction can be separated into five discrete components ( Fig. 31.8 ).
These five leaflets are seen to their best advantage when the common junction is guarded by a common valvar orifice (see Fig. 31.7 , right ). When a tongue of valvar tissue joins together the two leaflets of the common valve that bridge the ventricular septum, the effect is to produce separate orifices within the common junction for the inlets to the right and left ventricles (see Fig. 31.7 , left ). This tongue is usually attached directly to the musculature along the length of the crest of the ventricular septum. The essence of the so-called ostium primum defect, therefore, is that the fused bridging leaflets are depressed into the ventricular cavity and also fused to the crest of the scooped-out ventricular septum (see Fig. 31.6 , left ). This produces a double-orifice common atrioventricular valve. When considering the basic arrangement, three of the leaflets are confined to one or other of the ventricles, with one being exclusive to the left ventricle and two to the right ventricle. It is the left mural leaflet, which is much less extensive than the mural leaflet of the normal mitral valve, that is tethered between the superior and inferior papillary muscles of the left ventricle. The anterosuperior leaflet and the right mural leaflet are confined to the right ventricle. The bridging leaflets have no counterparts in the normal heart. The space between them is their zone of apposition (see Fig. 31.7 ). Although often still called a “cleft” for brevity, there is no question that the space is their zone of apposition. Reflecting the zones of apposition, the left valve closes in trifoliate fashion ( Fig. 31.9 ).
Such trifoliate closure is markedly different from the pattern of closure of the mitral valve, the two leaflets of which come together along a solitary zone of apposition (see Fig. 31.3 , left ). Today the surgeon seeking to correct the lesion will usually close the space between the left ventricular components of the bridging leaflets. This surgical maneuver, however, never produces an arrangement of leaflets for the newly constructed left atrioventricular valve that replicates the arrangement seen in the normal mitral valve. The arrangement of the left ventricular papillary muscles reflects the location of the leaflets. The muscles are paired, as in the normal heart. But instead of being located in anteroinferior and posterosuperior positions, they are in more-or-less direct superoinferior positions. On occasion they may be even more abnormally arranged, producing the so-called parachute arrangement. In reality, the abnormal valve looks more like a funnel, representing the spatial inverse of a parachute. The arrangement of the right ventricular muscles is comparable with that of the normal heart, although the position of the medial papillary muscle is variable. This variability has significance, since it reflects the extent to which the superior leaflet bridges into the right ventricle, underscoring the variability seen in the Rastelli classification (see later).
As already emphasized, another difference characterizing the morphology of atrioventricular septal defects with a common atrioventricular junction is the dimensions of the ventricular mass. In the normal heart, the inlet and outlet dimensions of the left ventricle are approximately the same (see Fig. 31.4A ). In atrioventricular septal defects with a common junction, the dimension of the outlet is considerably greater than that of the inlet (see Fig. 31.4B ). It is of little consequence morphologically whether this is because the inlet is shorter than normal or because the outlet is longer. Probably it is a combination of the two. The disproportion is within the same range, be there a common valvar orifice or separate valvar orifices at the inlets to the right and left ventricles. Indeed, once the curtain of valvar leaflets is removed from the atrioventricular junctions in any individual heart, it is not possible to judge simply from examination of the ventricular mass whether there had initially been a common valvar orifice or separate right and left atrioventricular orifices. The extent of deficiency of the midpoint of the septum, which we describe as the degree of “scooping,” is usually greater in those hearts having a common atrioventricular valvar orifice.
Categories of Atrioventricular Septal Defect
All the lesions discussed in this chapter are unified by presence of a common atrioventricular junction. Despite the commonality of the junctional arrangements, as already discussed, it is still usual for clinicians to recognize two major categories. These are considered to be “partial” and “complete,” despite their anatomic comparability. Two anatomic features underscore this clinical stratification. The first is the arrangement of the individual leaflets within the overall curtain of valvar tissue guarding the common atrioventricular junction. The second is the relationship between the bridging leaflets of the common valve and the atrial and ventricular septal structures. Problems are produced when these two features are combined so as to identify the presumed complete and partial variants, since the two aspects of anatomy are mutually independent. Attempts to combine descriptions of the two features to give one all-embracing categorization have produced bewildering arrays of definitions for so-called transitional or intermediate lesions. If the two features are described separately, the need to nominate intermediate categories is avoided. Other features, furthermore, are important in clinical presentation. Examples are the extent of the sharing of the common atrioventricular junction between the atrial and ventricular chambers and the presence of associated lesions. All of these features require description.
Arrangement of the Valvar Leaflets Relative to the Atrioventricular Orifices.
The curtain of valvar tissue guarding the junction in all hearts with atrioventricular septal defect and common atrioventricular junctions can be described in terms of five leaflets. The relationship between the two bridging leaflets themselves permits all of these hearts to be placed into one of two groups with no intermediate categories. The majority have a space between the two bridging leaflets and hence have a common atrioventricular valvar orifice (see Fig. 31.7 , right ). The component of this common valve committed to the left ventricle almost always closes in a trifoliate pattern (see Fig. 31.9 ). In a minority of hearts, the two bridging leaflets are themselves joined to each other by a connecting tongue of valvar tissue. This divides the common atrioventricular junction into right and left components ( Fig. 31.7 , left ), essentially producing dual orifices within the common junction. In most instances, shunting across the defect in this setting is confined to the atrial level, since the leaflets are also fused to the crest of the muscular ventricular septum ( Fig. 31.6 , left ). Such fusion to the ventricular septum, however, is not a universal feature of hearts in which the common junction is divided into right and left ventricular components by the union between the bridging leaflets. In some instances, the bridging leaflets can be fused to each other, thus dividing the valvar orifice, but also fused to the underside of the atrial septum so that the potential for shunting is restricted at ventricular level. This is the “atrioventricular canal” type of ventricular septal defect, in reality an atrioventricular septal defect with shunting confined exclusively to the ventricular level ( Fig. 31.10 , left ). In very rare instances, the bridging leaflets can be fused, producing separate valvar orifices for the right and left ventricles but with the tongue of leaflet tissue joining the leaflets attached to neither the atrial nor ventricular septa ( Fig. 31.10 , right ). In terms of the overall morphology of the curtain of valvar tissue, therefore, all hearts can be divided into those with a common atrioventricular valvar orifice guarding the common junction and those in which the common junction is guarded by separate right and left atrioventricular valves for the right and left ventricles, respectively. This feature is an “all-or-none” phenomenon.
Potential for Shunting Across the Atrioventricular Septal Defect.
It is the potential for shunting across the septal defect that is probably the single most important anatomic variant influencing clinical presentation. This anatomic variability depends on the relationship of the bridging leaflets and of the connecting tongue, if present, to the leading edge of the atrial septum on the one hand, and, on the other hand, to the crest of the scooped-out ventricular septum. In most cases the two bridging leaflets are attached directly to neither septal component as they bridge between the ventricles (see Fig. 31.6 , right ). In this arrangement, therefore, the potential exists for shunting at both the atrial and ventricular levels, as it does even if fused bridging leaflets float freely within the septal defect (see Fig. 31.10 , right ). The extent of ventricular shunting depends on the proximity of the bridging leaflets to the ventricular septal crest. If the leaflets float freely, there will be an extensive interventricular communication. Should cords from the septal crest tightly tether one or the other leaflet or both leaflets, the potential for ventricular shunting can be severely limited. Very rarely, in the presence of separate superior and inferior bridging leaflets, both leaflets may be fused to the ventricular septal crest. Shunting will occur only at the atrial level, even though there is a common valvar orifice. The arrangement in which the bridging leaflets are attached directly to the ventricular septum, however, is typically seen when they are additionally attached to each other by the connecting tongue. As discussed, this arrangement gives separate valvar orifices for the right and left ventricles, and produces the ostium primum defect (see Fig. 31.6 , left ). In a proportion of patients with separate right and left atrioventricular valvar orifices, the bridging leaflets may be attached to each other by the connecting tongue, but intercordal spaces beneath the tongue and beneath the bridging leaflets can permit small interventricular communications. On occasion the tissue of the bridging leaflets can also entirely close the septal deficiency. Such spontaneous closure produces a heart with a common atrioventricular junction, but lacking the opportunity for shunting across the preexisting atrioventricular septal defect. In terms of overall shunting across the atrioventricular septal defect, therefore, the key feature is the relationship between the bridging leaflets and the septal structures. This feature is independent of the arrangement of the valvar leaflets that guard the junction.
Abnormalities of the Left Atrioventricular Valve.
The essence of the left atrioventricular valve in hearts with atrioventricular septal defect and a common atrioventricular junction is that it closes in trifoliate fashion (see Fig. 31.9 ). Apart from its residence within the morphologic left ventricle, the only anatomic affinities with the morphologic mitral valve are the fabric of the valve, and even this shows significant differences. The left valve, like the mitral valve, is itself liable to be congenitally malformed. The presence of additional connecting tongues between the leaflets of the valve can produce dual orifices. Rarely, should bridges be found between the mural leaflet and both bridging leaflets, the left valve can have three orifices. The presence of these tongues of leaflet tissue extending between adjacent leaflets, usually between one of the bridging leaflets and the mural leaflet, and separating the orifice into subordinate compartments, is comparable to the way the connecting tongue itself divides the common orifice into separate right and left atrioventricular components (see Fig. 31.7 , left ). As we have emphasized, the ostium primum defect is no more than a heart with a common atrioventricular valve with dual orifices. The abnormalities can also involve the papillary muscles. Hypoplasia of one or other papillary muscle supporting the left atrioventricular valve, or fusion of the muscles, produces an arrangement replicating a funnel, albeit usually, and misleadingly, described in terms of a “parachute.” In this entity, the orifice of the valve is represented by the space between the bridging leaflets. In severe cases the valve can take on a bifoliate configuration. Such a left valve with two leaflets remains anatomically different from the arrangement of the normal mitral valve.
Rastelli Classification.
In the past, it was conventional to subdivide atrioventricular septal defects with a common valvar orifice depending on the morphology of the papillary muscle supporting the right ventricular extremity of the superior bridging leaflet. Such variability was first noted and highlighted by Rastelli and colleagues. They described three major types. In the first, which they dubbed “type A,” the bridging leaflet was mostly contained in the left ventricle and was usually tightly tethered by tendinous cords to the crest of the ventricular septum. In this arrangement, the zone of apposition of the superior bridging leaflet with the anterosuperior leaflet of the right ventricle is supported by the medial papillary muscle, which arises in relatively normal fashion from the right side of the ventricular septum. In the second type, the superior bridging leaflet extends more into the right ventricle, usually being unattached to the ventricular septum as it crosses the septal crest but supported by an anomalous right ventricular papillary muscle arising from the body of the septomarginal trabeculation. In the third type, the free-floating bridging leaflet—again unattached to the septum—extends even further into the right ventricle and is attached to an anterior papillary muscle. In this spectrum, as the superior bridging leaflet becomes increasingly committed to the right ventricle, the zone of apposition with the anterosuperior leaflet of the right ventricle also moves into the right ventricle, with corresponding diminution in size of the anterosuperior leaflet. The spectrum can be extended, therefore, to include the so-called ostium primum defect. In this lesion, the bridging leaflets are usually fused to the ventricular septal crest, but with minimal bridging of the superior leaflet ( Fig. 31.11 ). Variability is also found in the arrangement of the inferior bridging leaflet, but this is not taken into account in terms of the Rastelli classification. The variation in the inferior bridging leaflet relates not so much to the extent of bridging, since almost always the leaflet extends well into both ventricles, but more to its tethering. Sometimes the inferior bridging leaflet is separated into right and left ventricular components by a well-formed raphe, which is firmly attached to the ventricular septum. In other hearts, the bridging leaflet is tethered by short tendinous cords as it crosses the septum, whereas in still others it can float freely. Thus far, no obvious relationship has been discovered between the degree of tethering of the two bridging leaflets, but the extent to which either leaflet does bridge is of obvious surgical significance.
Left Ventricular Outflow Tract.
By virtue of its anterior and unwedged position, the left ventricular outflow tract is particularly susceptible to obstruction. This is true irrespective of whether there is a common valvar orifice or separate right and left atrioventricular valves within the common junction. On anatomic examination, the tract almost always seems narrowed as compared with the diameter of the aortic valve. Its length, and the extent of the apparent narrowing, are more marked in defects in which the superior bridging leaflet is firmly fused to the septal crest—in other words, in ostium primum defects. Additional lesions compromising the already narrowed channel are responsible for hemodynamically significant obstruction or, alternatively, the effects of corrective surgery. Any of the lesions that produce subaortic stenosis in the normal heart can rapidly produce similar problems in the setting of atrioventricular septal defects with a common atrioventricular junction, particularly when there is tethering of the superior bridging leaflet to the septum or separate valvar orifices for the two ventricles.
Dominance of Chambers.
Most commonly in atrioventricular septal defects, whether with separate right and left atrioventricular valvar orifices or a common valvar orifice, the right and left components of the common atrioventricular junction are of comparable circumference and the ventricles are of similar size. This produces the so-called balanced arrangement. The common atrioventricular junction can be committed in its larger part to the right ventricle, producing right ventricular dominance, or to the left, giving a dominant left ventricle. Right ventricular dominance is usually associated with clinically significant hypoplasia or abnormality of the left ventricular and aortic structures but it is most often found with normal alignment between the atrial and ventricular septal structures. In the presence of left ventricular dominance, in contrast, it is the right ventricular and pulmonary arterial structures that are hypoplastic, typically in association with malalignment between the atrial and the muscular ventricular septal structures. Such hearts with atrioventricular septal malalignment constitute part of a spectrum that extends to double-inlet left ventricle, but through a common atrioventricular valve. In these variants, the ventricular septum no longer meets the atrioventricular junction at the crux, with important consequences for the disposition of the atrioventricular conduction axis (see later). A similar spectrum obviously exists when the right ventricle is dominant, with the extreme end of this spectrum being double-inlet right ventricle with a common atrioventricular valve. The concept of chamber dominance can also be extended to include the atria. When one of the atria is dominant, the common atrioventricular junction is more or less equally shared by the ventricles but is mostly connected to the dominant atrium. The only exit for the other atrium is across the atrial component of the atrioventricular septal defect. This arrangement is often termed double-outlet atrium, albeit this description is applicable to hearts having absence of one atrioventricular connection and straddling of the other atrioventricular valve (see Chapter 49 ).
Associated Malformations.
If not ruled out by its anatomy, any lesion must be anticipated to exist in hearts having an atrioventricular septal defect with a common atrioventricular junction. We have already mentioned some of the more frequent malformations, notably obstructions within the left ventricular outflow tract, and those affecting the left atrioventricular valve. Additional deficiencies of the atrial septum are important and are sometimes described in terms of a common atrium. Common atrioventricular valves can also be found in hearts with abnormal segmental connections, such as double-inlet ventricle and discordant or ambiguous atrioventricular connections. In these settings, the patients frequently also exhibit abnormal ventriculoarterial connections. Double-inlet ventricle, however, is usually not classified as an atrioventricular septal defect. Discordant ventriculoarterial connections, for example, are the rule in association with either double-inlet left ventricle or discordant atrioventricular connections. A double outlet from the right ventricle is frequently found, particularly when there is isomerism of the atrial appendages (see Chapter 27 ). Of the other associated lesions, tetralogy of Fallot or pulmonary stenosis is particularly important, occurring in up to one-tenth of patients with atrioventricular septal defect and a common atrioventricular junction. Presence of a second muscular ventricular septal defect is also significant. In those hearts with obstruction of the left ventricular outflow tract, right ventricular dominance, along with coarctation or interruption of the aortic arch, should be anticipated.
Atrioventricular Conduction Tissues.
In most instances the atrial and ventricular septal structures are appropriately aligned in the setting of atrioventricular septal defect with a common atrioventricular junction. The arrangement of the atrioventricular conduction axis is different from normal but comparable in all the phenotypic variants with septal alignment. The difference from the normal arrangement reflects the lack of the atrioventricular septal structures and the concomitant lack of a normal central fibrous body. Because of the deficient atrioventricular septation, the inferior edge of the margin of the atrial septum usually makes contact with the ventricular septum only at the crux. It is at the crux, therefore, that the atrioventricular conduction axis usually penetrates from the atrial tissues to reach the crest of the muscular ventricular septum. In consequence of this arrangement, the entire nodal area is displaced posteriorly and inferiorly. Although a well-formed triangle can be seen at this location, this nodal triangle is not the same as the normal triangle of Koch ( Fig. 31.12 ).
The area of union between the posteroinferior extremity of the ventricular septum and the atrioventricular junction provides the most reliable guide to the position of the conduction axis. This landmark is to be found even when the coronary sinus is unroofed or opens to the left atrium. It is also present when there is malalignment between the atrial and ventricular septal structures, but in this latter setting the atrioventricular node will be positioned away from the cardiac crux (see Fig. 31.12B ). Having taken origin from the atrioventricular node, either in the nodal triangle or along the inferior atrioventricular junction, the elongated nonbranching bundle runs either on the crest of the muscular ventricular septum or to its left side, being covered by the inferior bridging leaflet. The bundle branches are found more posteriorly than in the normal heart. Only the right bundle branch extends along the crest in the bare area found in the presence of a common orifice. In hearts with separate valvar orifices for the right and left ventricles, this part of the axis is covered over by the connecting tongue of leaflet tissue. This feature is of major surgical importance, since it permits sutures to be secured on the left side of the fibrous raphe or within the bridging leaflets themselves without courting damage to the underlying conduction tissues. The left ventricular outflow tract, by virtue of its unwedged location, is unrelated to the conduction axis. This feature eliminates the risk of surgical damage compared with the normal heart should removal be attempted of the obstructing lesions.
Related Lesions
Knowledge of the anatomic hallmarks of atrioventricular septal defect with a common atrioventricular junction permits recognition of other lesions with similar features that do not fit into the group as defined because they do not possess a common atrioventricular junction. The most obvious candidate in this regard is the heart with a normally located subaortic outlet but a deficient atrioventricular component of the membranous septum. Such lesions, known as direct Gerbode defects, permit shunting from the left ventricle to the right atrium. This is possible only because there are separate right and left atrioventricular junctions (see Chapter 32 ). Patients with these defects have a normally wedged subaortic outflow tract and are very rare. Closely related are the hearts with separate right and left atrioventricular junctions and a normally wedged aorta but with direct shunting across a perimembranous inlet ventricular septal defect into the right atrium because of anomalous attachment of the leaflets of the tricuspid valve. These are the indirect Gerbode defects. The presence of a mitral valve guarding a discrete left atrioventricular junction distinguishes them from atrioventricular septal defects with a common atrioventricular junction. This distinction holds equally good for hearts having perimembranous inlet defects along with a cleft in the aortic leaflet of an otherwise normally structured mitral valve. This lesion, the “isolated” cleft, which can also be found in the setting of intact septal structures, is often considered to be directly related to atrioventricular septal defects. It has obvious developmental affinities. The anatomic basis of the two lesions, however, is quite different. It also used to be considered that hearts with straddling of the tricuspid valve had an atrioventricular canal type of ventricular septal defect. Although these lesions share with atrioventricular septal defect the presence of valvar leaflets that override the septum, they have separate right and left atrioventricular junctions as opposed to the common atrioventricular junction (see Chapter 32 ). In summary, therefore, the presence of the common atrioventricular junction is the anatomic hallmark of all the hearts with deficient atrioventricular septation that are discussed further in this chapter.
Morphogenesis
In the past the starting point in considering the morphogenesis of atrioventricular septal defects was usually the assumption that the atrioventricular endocardial cushions contributed markedly to the formation of the septal structures that are lacking in the malformed hearts and also to the atrioventricular valvar leaflets. It was because of these assumptions that “endocardial cushion defect” came to enjoy some popularity as a descriptive term. The concepts underscoring this approach do not bear rigorous examination, neither from the stance of embryologic evidence nor on the basis of the anatomy of the definitive lesions. It is very attractive to argue that complete failure of fusion of the endocardial cushions will result in the “complete” defect. Partial failure of fusion could arguably result in the ostium primum or partial, defect and, in similar fashion, account for the cleft in the aortic leaflet of the mitral valve. But something much more fundamental must happen to produce the group of hearts under discussion. We have already seen how their anatomy differs from the normal state in far more respects than the presence of a simple hole in the septum coupled with a cleft in the aortic leaflet of the mitral valve. Indeed, the fact that isolated clefts in the aortic leaflet of the mitral valve do exist but have markedly different anatomy from the group under discussion is additional strong evidence that the abnormal development is more than simple failure of fusion of the embryonic endocardial cushions.
It could well be that an initial failure of fusion of the cushions is the first step in the production of an atrioventricular septal defect. But the cushions do not go on to produce all the valvar leaflets, nor do they form the entirety of the separating atrioventricular junctional structures. Formation of the valvar leaflets is a very late developmental event. It occurs predominantly by undermining of ventricular myocardium, a fact well known to German embryologists and anatomists in the 19th century. Thus the abnormal atrioventricular valvar leaflets found in atrioventricular septal defects with a common atrioventricular junction will be produced and sculpted in part from the grossly abnormal ventricular mass. It is the abnormal architecture of the ventricles, specifically their junction with the atriums, that underscores the characteristic phenotypic morphology. When searching for the mechanism that results in the development of a common atrioventricular junction, therefore, it is necessary to direct attention to the formation of the junctional regions. This is where it is important to understand the initial function of the atrioventricular endocardial cushions. They act as the forerunner of the atrioventricular valves in early development, serving also to glue together the central point of the developing atrioventricular junctions. Only after the superior and inferior aspects of the junction have been stuck together can the aortic outflow tract be incorporated into the developing left ventricle (see Chapter 3 ). The leaflets of the mitral and tricuspid valves are subsequently delaminated from the ventricular walls or else sculpted in part from the fused cushions, but in the setting of a normally wedged subaortic outflow tract. If the endocardial cushions do not fuse together or if there is an intrinsic abnormality in the formation of the atrioventricular junctions, there is loss of this focal point for the subsequent development of the ventricles. The atrioventricular junctions will not grow as separate left and right components with a septum and the subaortic outflow tract interposed between them. Instead, the superior and inferior margins of the junctions will retain their initial common appearance. Indeed, there is remarkable affinity between the arrangement of the developing atrioventricular junction with the anatomy as previously described prior to the incorporation, during development, of the subaortic outflow tract into the developing left ventricle ( Fig. 31.13A ). It is the persistence of the initial common arrangement of the atrioventricular junction, therefore, that will produce the prototypic ventricular mass of an atrioventricular septal defect with a common atrioventricular junction. The maldevelopment will also explain the presence of the basic septal defect itself. This is because, in the normal heart, the fusing cushions also provide the focal point for appropriate development and alignment of the atrial and ventricular septal structures. In the absence of such a keystone, the precise type of atrioventricular septal defect formed will depend on the way in which the atrioventricular valvar leaflets are delaminated and developed from the abnormal ventricular mass. In this respect, it should be remembered that a tongue of valvar tissue joins together the bridging leaflets in the ostium primum defect. Paradoxically, therefore, a lesion often presumed to develop because of failure of fusion of endocardial cushions has continuity of valvar tissue through the center of the common junction.
As yet the tissues derived from the cushions have still to be determined with certainty. The abnormality, furthermore, involves more than the atrioventricular endocardial cushions themselves. Additional mesenchymal tissues surrounding the embryonic primary atrial foramen are involved in its closure. These include the mesenchymal cap carried on the leading edge of the primary atrial septum and the mound of tissue, with its own mesenchymal covering, that grows into the heart from the posterior mediastinum (see Fig. 31.13B ). This latter structure, the vestibular spine, was initially discovered and described by His the elder in the latter part of the 19th century. It disappeared from consideration for some considerable time before its recent resurrection. We now know that in several mouse models the vestibular spine is lacking, such as the Tbx1 knockout; it is also absent in some genetically normal mice ( Fig. 31.14 , left ). The genetically normal mice have typical “ostium primum” defects, with the findings confirming that the common atrioventricular junction is present despite the fusion of the atrioventricular cushions ( Fig. 31.14 , right ).
Yet other genetically perturbed mice have now been discovered with common atrioventricular junctions and with shunting through the atrioventricular septal defect exclusively at the ventricular level because the atrial septum is fused to the atrioventricular cushions. The vestibular spine has been shown to be normally formed in mice with this arrangement. Ongoing studies of these mice, and other mouse models, are likely to identify the specific genes involved in normal and abnormal development. It will be necessary to take careful note of the precise structure of the cardiac abnormalities produced in these experimental animals to make appropriate inferences for the situation pertaining in human maldevelopment.
Anatomy
Understanding Atrioventricular Septal Defects With a Common Atrioventricular Junction
As shown in the following text, the abnormal structure and development of atrioventricular junctions are the phenotypic features of the group of lesions forming the focus of this chapter. By atrioventricular junctions, we mean the areas of the heart where the atrial myocardium becomes contiguous with the ventricular myocardium. To understand the abnormalities, it is necessary to emphasize the features of normality. In the normal heart, the myocardial segments within these junctional areas are separated from one another save at the site of penetration of the bundle of His, which is part of the muscular axis responsible for atrioventricular conduction. The separation within the junctions, providing the necessary electrical insulation, is produced largely by the fibrofatty tissues of the atrioventricular grooves. These tissues form the greater part of the so-called valvar annuli, which also support the attachments of the leaflets of the atrioventricular valves. In the normal heart, there are two atrioventricular junctions that surround the tricuspid and mitral valvar orifices. There is a central component present separating the junctions, which also abuts on the subaortic outflow tract ( Fig. 31.1 , left ).
In the normal heart, part of this separating component is made up of a true atrioventricular septum, this being the atrioventricular component of the membranous septum, which is relatively small ( red arrow in Fig. 31.1 , left ). It is the lack of this septal component, along with additional myocardial separating structures, that underscores the morphology of atrioventricular septal defects found in the setting of a common atrioventricular junction ( Fig. 31.1 , right ). In hearts with separate junctions, the septal leaflet of the tricuspid valve is usually attached at a considerably more apical level than is the corresponding leaflet of the mitral valve (see Fig. 31.1 , left ). The posteroinferior part of the area between the hinges of the atrioventricular valvar leaflets, however, is not strictly septal. This is because, in this area, the atrial myocardial wall overlaps the crest of the muscular ventricular mass, with an extension of the insulating inferoposterior fibrofatty atrioventricular groove separating the two muscular masses ( Fig. 31.2 , left ). In effect, it is a sandwich of muscular and fibrofatty tissues that is interposed between the cavities of the right atrium and the left ventricle. The fibrous atrioventricular septum is found anterosuperior to this muscular area (see Fig. 31.2 , right ).
This septal component is also an integral part of the central fibrous body, which forms the rightward wall of the left ventricular outflow tract. In the normal heart, the left ventricular outflow tract interposes between the mitral valvar orifice and the septum ( Fig. 31.3 , left ). In the right ventricular orifice, one of the leaflets of the tricuspid valve is adherent to the ventricular septum over a considerable area. These arrangements are pertinent to the naming of the leaflets of the normal and abnormal valves. In the mitral valvar orifice, the curtain of leaflet tissue is divided by an obliquely orientated line of apposition into a short and square leaflet, best termed the aortic leaflet but often termed the anterior leaflet. The other leaflet is more extensive in terms of its circumferential attachment but much shallower. This is the mural or posterior, leaflet. The two leaflets close over a solitary zone of apposition (see Fig. 31.3 , left ). The normal tricuspid valve has three leaflets, which are located in the anterosuperior, septal, and inferior or mural positions. The dimensions of the septal aspect of the ventricular mass also vary between hearts with separate as opposed to common atrioventricular junctions. In the normal heart, the distance from the attachment of the mitral valve at the crux to the ventricular apex, or the inlet dimension, is much the same as that from the ventricular apex to the anterosuperior attachment of the leaflets of the aortic valve, this being the outlet dimension ( Fig. 31.4A ). The essence of hearts with an atrioventricular septal defect and a common atrioventricular junction is disproportion between these inlet and outlet dimensions.
Basic Morphology of Atrioventricular Septal Defects
Apart from the Gerbode defect (see later), the essential morphologic ingredient of hearts now usually described as having atrioventricular septal defects is the presence of a common atrioventricular junction. This junction is common to all four cardiac chambers in all hearts. There are no intermediate or transitional arrangements. The junctional arrangements are matched by differences in the arrangement of the fibrous skeleton, with the continuity between the membranous septum and the right fibrous trigone fundamentally disturbed when there is a common atrioventricular junction ( Fig. 31.5 ).
These differences in junctional architecture underscore departures from normality in at least four of the features already enumerated as characteristic of hearts with separate right and left atrioventricular junctions. First, almost always but not universally, there is a defect at the anticipated site of the structures that usually interpose between the right atrium and the left ventricle. The septal deficiency is comparable irrespective of whether there is a common valvar orifice within the common junction ( Fig. 31.6 , right ) or there are separate atrioventricular valvar orifices for the right and left ventricles ( Fig. 31.6 , left ). The arrangement with separate valvar orifices, but a common atrioventricular junction, is also known as an “ostium primum defect.” The septal deficiency is between the leading edge of the atrial septum and the crest of the muscular ventricular septum, to which the valvar leaflets are attached (see Fig. 31.1 , right ).
The second feature is the oval shape of the common atrioventricular junction (see Fig. 31.3 , right ). Irrespective of the number of atrioventricular valvar orifices within the junction, the left ventricular outflow tract is not wedged between the left and right atrioventricular annuli, as seen in the normal heart. Instead, the aortic orifice is anterosuperior to the common junction. The junction is just as common in the hearts having separate valvar orifices for the right and left ventricles ( Fig. 31.7 , left ) as when there is a common valvar orifice ( Fig. 31.7 , right ).
Since there is a common atrioventricular junction and a lack of any septal atrioventricular muscular contiguity, the third feature that differs from the normal is the arrangement of the valvar leaflets that guard the common junction. These leaflets, irrespective of whether they guard a common atrioventricular orifice or separate orifices for the right and left ventricles, bear scant resemblance to the arrangement of the leaflets of the normal mitral and tricuspid valves (see Fig. 31.7 ). When describing the valves and comparing them with the arrangement seen in the normal heart, the key feature is the boundaries of the individual leaflets. These are best assessed when the valve is seen in its closed position. Studied in this fashion, the overall curtain of leaflet tissue in hearts having the phenotypic feature of an atrioventricular septal defect with a common atrioventricular junction can be separated into five discrete components ( Fig. 31.8 ).
These five leaflets are seen to their best advantage when the common junction is guarded by a common valvar orifice (see Fig. 31.7 , right ). When a tongue of valvar tissue joins together the two leaflets of the common valve that bridge the ventricular septum, the effect is to produce separate orifices within the common junction for the inlets to the right and left ventricles (see Fig. 31.7 , left ). This tongue is usually attached directly to the musculature along the length of the crest of the ventricular septum. The essence of the so-called ostium primum defect, therefore, is that the fused bridging leaflets are depressed into the ventricular cavity and also fused to the crest of the scooped-out ventricular septum (see Fig. 31.6 , left ). This produces a double-orifice common atrioventricular valve. When considering the basic arrangement, three of the leaflets are confined to one or other of the ventricles, with one being exclusive to the left ventricle and two to the right ventricle. It is the left mural leaflet, which is much less extensive than the mural leaflet of the normal mitral valve, that is tethered between the superior and inferior papillary muscles of the left ventricle. The anterosuperior leaflet and the right mural leaflet are confined to the right ventricle. The bridging leaflets have no counterparts in the normal heart. The space between them is their zone of apposition (see Fig. 31.7 ). Although often still called a “cleft” for brevity, there is no question that the space is their zone of apposition. Reflecting the zones of apposition, the left valve closes in trifoliate fashion ( Fig. 31.9 ).
Such trifoliate closure is markedly different from the pattern of closure of the mitral valve, the two leaflets of which come together along a solitary zone of apposition (see Fig. 31.3 , left ). Today the surgeon seeking to correct the lesion will usually close the space between the left ventricular components of the bridging leaflets. This surgical maneuver, however, never produces an arrangement of leaflets for the newly constructed left atrioventricular valve that replicates the arrangement seen in the normal mitral valve. The arrangement of the left ventricular papillary muscles reflects the location of the leaflets. The muscles are paired, as in the normal heart. But instead of being located in anteroinferior and posterosuperior positions, they are in more-or-less direct superoinferior positions. On occasion they may be even more abnormally arranged, producing the so-called parachute arrangement. In reality, the abnormal valve looks more like a funnel, representing the spatial inverse of a parachute. The arrangement of the right ventricular muscles is comparable with that of the normal heart, although the position of the medial papillary muscle is variable. This variability has significance, since it reflects the extent to which the superior leaflet bridges into the right ventricle, underscoring the variability seen in the Rastelli classification (see later).
As already emphasized, another difference characterizing the morphology of atrioventricular septal defects with a common atrioventricular junction is the dimensions of the ventricular mass. In the normal heart, the inlet and outlet dimensions of the left ventricle are approximately the same (see Fig. 31.4A ). In atrioventricular septal defects with a common junction, the dimension of the outlet is considerably greater than that of the inlet (see Fig. 31.4B ). It is of little consequence morphologically whether this is because the inlet is shorter than normal or because the outlet is longer. Probably it is a combination of the two. The disproportion is within the same range, be there a common valvar orifice or separate valvar orifices at the inlets to the right and left ventricles. Indeed, once the curtain of valvar leaflets is removed from the atrioventricular junctions in any individual heart, it is not possible to judge simply from examination of the ventricular mass whether there had initially been a common valvar orifice or separate right and left atrioventricular orifices. The extent of deficiency of the midpoint of the septum, which we describe as the degree of “scooping,” is usually greater in those hearts having a common atrioventricular valvar orifice.
Categories of Atrioventricular Septal Defect
All the lesions discussed in this chapter are unified by presence of a common atrioventricular junction. Despite the commonality of the junctional arrangements, as already discussed, it is still usual for clinicians to recognize two major categories. These are considered to be “partial” and “complete,” despite their anatomic comparability. Two anatomic features underscore this clinical stratification. The first is the arrangement of the individual leaflets within the overall curtain of valvar tissue guarding the common atrioventricular junction. The second is the relationship between the bridging leaflets of the common valve and the atrial and ventricular septal structures. Problems are produced when these two features are combined so as to identify the presumed complete and partial variants, since the two aspects of anatomy are mutually independent. Attempts to combine descriptions of the two features to give one all-embracing categorization have produced bewildering arrays of definitions for so-called transitional or intermediate lesions. If the two features are described separately, the need to nominate intermediate categories is avoided. Other features, furthermore, are important in clinical presentation. Examples are the extent of the sharing of the common atrioventricular junction between the atrial and ventricular chambers and the presence of associated lesions. All of these features require description.
Arrangement of the Valvar Leaflets Relative to the Atrioventricular Orifices.
The curtain of valvar tissue guarding the junction in all hearts with atrioventricular septal defect and common atrioventricular junctions can be described in terms of five leaflets. The relationship between the two bridging leaflets themselves permits all of these hearts to be placed into one of two groups with no intermediate categories. The majority have a space between the two bridging leaflets and hence have a common atrioventricular valvar orifice (see Fig. 31.7 , right ). The component of this common valve committed to the left ventricle almost always closes in a trifoliate pattern (see Fig. 31.9 ). In a minority of hearts, the two bridging leaflets are themselves joined to each other by a connecting tongue of valvar tissue. This divides the common atrioventricular junction into right and left components ( Fig. 31.7 , left ), essentially producing dual orifices within the common junction. In most instances, shunting across the defect in this setting is confined to the atrial level, since the leaflets are also fused to the crest of the muscular ventricular septum ( Fig. 31.6 , left ). Such fusion to the ventricular septum, however, is not a universal feature of hearts in which the common junction is divided into right and left ventricular components by the union between the bridging leaflets. In some instances, the bridging leaflets can be fused to each other, thus dividing the valvar orifice, but also fused to the underside of the atrial septum so that the potential for shunting is restricted at ventricular level. This is the “atrioventricular canal” type of ventricular septal defect, in reality an atrioventricular septal defect with shunting confined exclusively to the ventricular level ( Fig. 31.10 , left ). In very rare instances, the bridging leaflets can be fused, producing separate valvar orifices for the right and left ventricles but with the tongue of leaflet tissue joining the leaflets attached to neither the atrial nor ventricular septa ( Fig. 31.10 , right ). In terms of the overall morphology of the curtain of valvar tissue, therefore, all hearts can be divided into those with a common atrioventricular valvar orifice guarding the common junction and those in which the common junction is guarded by separate right and left atrioventricular valves for the right and left ventricles, respectively. This feature is an “all-or-none” phenomenon.
Potential for Shunting Across the Atrioventricular Septal Defect.
It is the potential for shunting across the septal defect that is probably the single most important anatomic variant influencing clinical presentation. This anatomic variability depends on the relationship of the bridging leaflets and of the connecting tongue, if present, to the leading edge of the atrial septum on the one hand, and, on the other hand, to the crest of the scooped-out ventricular septum. In most cases the two bridging leaflets are attached directly to neither septal component as they bridge between the ventricles (see Fig. 31.6 , right ). In this arrangement, therefore, the potential exists for shunting at both the atrial and ventricular levels, as it does even if fused bridging leaflets float freely within the septal defect (see Fig. 31.10 , right ). The extent of ventricular shunting depends on the proximity of the bridging leaflets to the ventricular septal crest. If the leaflets float freely, there will be an extensive interventricular communication. Should cords from the septal crest tightly tether one or the other leaflet or both leaflets, the potential for ventricular shunting can be severely limited. Very rarely, in the presence of separate superior and inferior bridging leaflets, both leaflets may be fused to the ventricular septal crest. Shunting will occur only at the atrial level, even though there is a common valvar orifice. The arrangement in which the bridging leaflets are attached directly to the ventricular septum, however, is typically seen when they are additionally attached to each other by the connecting tongue. As discussed, this arrangement gives separate valvar orifices for the right and left ventricles, and produces the ostium primum defect (see Fig. 31.6 , left ). In a proportion of patients with separate right and left atrioventricular valvar orifices, the bridging leaflets may be attached to each other by the connecting tongue, but intercordal spaces beneath the tongue and beneath the bridging leaflets can permit small interventricular communications. On occasion the tissue of the bridging leaflets can also entirely close the septal deficiency. Such spontaneous closure produces a heart with a common atrioventricular junction, but lacking the opportunity for shunting across the preexisting atrioventricular septal defect. In terms of overall shunting across the atrioventricular septal defect, therefore, the key feature is the relationship between the bridging leaflets and the septal structures. This feature is independent of the arrangement of the valvar leaflets that guard the junction.
Abnormalities of the Left Atrioventricular Valve.
The essence of the left atrioventricular valve in hearts with atrioventricular septal defect and a common atrioventricular junction is that it closes in trifoliate fashion (see Fig. 31.9 ). Apart from its residence within the morphologic left ventricle, the only anatomic affinities with the morphologic mitral valve are the fabric of the valve, and even this shows significant differences. The left valve, like the mitral valve, is itself liable to be congenitally malformed. The presence of additional connecting tongues between the leaflets of the valve can produce dual orifices. Rarely, should bridges be found between the mural leaflet and both bridging leaflets, the left valve can have three orifices. The presence of these tongues of leaflet tissue extending between adjacent leaflets, usually between one of the bridging leaflets and the mural leaflet, and separating the orifice into subordinate compartments, is comparable to the way the connecting tongue itself divides the common orifice into separate right and left atrioventricular components (see Fig. 31.7 , left ). As we have emphasized, the ostium primum defect is no more than a heart with a common atrioventricular valve with dual orifices. The abnormalities can also involve the papillary muscles. Hypoplasia of one or other papillary muscle supporting the left atrioventricular valve, or fusion of the muscles, produces an arrangement replicating a funnel, albeit usually, and misleadingly, described in terms of a “parachute.” In this entity, the orifice of the valve is represented by the space between the bridging leaflets. In severe cases the valve can take on a bifoliate configuration. Such a left valve with two leaflets remains anatomically different from the arrangement of the normal mitral valve.
Rastelli Classification.
In the past, it was conventional to subdivide atrioventricular septal defects with a common valvar orifice depending on the morphology of the papillary muscle supporting the right ventricular extremity of the superior bridging leaflet. Such variability was first noted and highlighted by Rastelli and colleagues. They described three major types. In the first, which they dubbed “type A,” the bridging leaflet was mostly contained in the left ventricle and was usually tightly tethered by tendinous cords to the crest of the ventricular septum. In this arrangement, the zone of apposition of the superior bridging leaflet with the anterosuperior leaflet of the right ventricle is supported by the medial papillary muscle, which arises in relatively normal fashion from the right side of the ventricular septum. In the second type, the superior bridging leaflet extends more into the right ventricle, usually being unattached to the ventricular septum as it crosses the septal crest but supported by an anomalous right ventricular papillary muscle arising from the body of the septomarginal trabeculation. In the third type, the free-floating bridging leaflet—again unattached to the septum—extends even further into the right ventricle and is attached to an anterior papillary muscle. In this spectrum, as the superior bridging leaflet becomes increasingly committed to the right ventricle, the zone of apposition with the anterosuperior leaflet of the right ventricle also moves into the right ventricle, with corresponding diminution in size of the anterosuperior leaflet. The spectrum can be extended, therefore, to include the so-called ostium primum defect. In this lesion, the bridging leaflets are usually fused to the ventricular septal crest, but with minimal bridging of the superior leaflet ( Fig. 31.11 ). Variability is also found in the arrangement of the inferior bridging leaflet, but this is not taken into account in terms of the Rastelli classification. The variation in the inferior bridging leaflet relates not so much to the extent of bridging, since almost always the leaflet extends well into both ventricles, but more to its tethering. Sometimes the inferior bridging leaflet is separated into right and left ventricular components by a well-formed raphe, which is firmly attached to the ventricular septum. In other hearts, the bridging leaflet is tethered by short tendinous cords as it crosses the septum, whereas in still others it can float freely. Thus far, no obvious relationship has been discovered between the degree of tethering of the two bridging leaflets, but the extent to which either leaflet does bridge is of obvious surgical significance.
Left Ventricular Outflow Tract.
By virtue of its anterior and unwedged position, the left ventricular outflow tract is particularly susceptible to obstruction. This is true irrespective of whether there is a common valvar orifice or separate right and left atrioventricular valves within the common junction. On anatomic examination, the tract almost always seems narrowed as compared with the diameter of the aortic valve. Its length, and the extent of the apparent narrowing, are more marked in defects in which the superior bridging leaflet is firmly fused to the septal crest—in other words, in ostium primum defects. Additional lesions compromising the already narrowed channel are responsible for hemodynamically significant obstruction or, alternatively, the effects of corrective surgery. Any of the lesions that produce subaortic stenosis in the normal heart can rapidly produce similar problems in the setting of atrioventricular septal defects with a common atrioventricular junction, particularly when there is tethering of the superior bridging leaflet to the septum or separate valvar orifices for the two ventricles.
Dominance of Chambers.
Most commonly in atrioventricular septal defects, whether with separate right and left atrioventricular valvar orifices or a common valvar orifice, the right and left components of the common atrioventricular junction are of comparable circumference and the ventricles are of similar size. This produces the so-called balanced arrangement. The common atrioventricular junction can be committed in its larger part to the right ventricle, producing right ventricular dominance, or to the left, giving a dominant left ventricle. Right ventricular dominance is usually associated with clinically significant hypoplasia or abnormality of the left ventricular and aortic structures but it is most often found with normal alignment between the atrial and ventricular septal structures. In the presence of left ventricular dominance, in contrast, it is the right ventricular and pulmonary arterial structures that are hypoplastic, typically in association with malalignment between the atrial and the muscular ventricular septal structures. Such hearts with atrioventricular septal malalignment constitute part of a spectrum that extends to double-inlet left ventricle, but through a common atrioventricular valve. In these variants, the ventricular septum no longer meets the atrioventricular junction at the crux, with important consequences for the disposition of the atrioventricular conduction axis (see later). A similar spectrum obviously exists when the right ventricle is dominant, with the extreme end of this spectrum being double-inlet right ventricle with a common atrioventricular valve. The concept of chamber dominance can also be extended to include the atria. When one of the atria is dominant, the common atrioventricular junction is more or less equally shared by the ventricles but is mostly connected to the dominant atrium. The only exit for the other atrium is across the atrial component of the atrioventricular septal defect. This arrangement is often termed double-outlet atrium, albeit this description is applicable to hearts having absence of one atrioventricular connection and straddling of the other atrioventricular valve (see Chapter 49 ).
Associated Malformations.
If not ruled out by its anatomy, any lesion must be anticipated to exist in hearts having an atrioventricular septal defect with a common atrioventricular junction. We have already mentioned some of the more frequent malformations, notably obstructions within the left ventricular outflow tract, and those affecting the left atrioventricular valve. Additional deficiencies of the atrial septum are important and are sometimes described in terms of a common atrium. Common atrioventricular valves can also be found in hearts with abnormal segmental connections, such as double-inlet ventricle and discordant or ambiguous atrioventricular connections. In these settings, the patients frequently also exhibit abnormal ventriculoarterial connections. Double-inlet ventricle, however, is usually not classified as an atrioventricular septal defect. Discordant ventriculoarterial connections, for example, are the rule in association with either double-inlet left ventricle or discordant atrioventricular connections. A double outlet from the right ventricle is frequently found, particularly when there is isomerism of the atrial appendages (see Chapter 27 ). Of the other associated lesions, tetralogy of Fallot or pulmonary stenosis is particularly important, occurring in up to one-tenth of patients with atrioventricular septal defect and a common atrioventricular junction. Presence of a second muscular ventricular septal defect is also significant. In those hearts with obstruction of the left ventricular outflow tract, right ventricular dominance, along with coarctation or interruption of the aortic arch, should be anticipated.
Atrioventricular Conduction Tissues.
In most instances the atrial and ventricular septal structures are appropriately aligned in the setting of atrioventricular septal defect with a common atrioventricular junction. The arrangement of the atrioventricular conduction axis is different from normal but comparable in all the phenotypic variants with septal alignment. The difference from the normal arrangement reflects the lack of the atrioventricular septal structures and the concomitant lack of a normal central fibrous body. Because of the deficient atrioventricular septation, the inferior edge of the margin of the atrial septum usually makes contact with the ventricular septum only at the crux. It is at the crux, therefore, that the atrioventricular conduction axis usually penetrates from the atrial tissues to reach the crest of the muscular ventricular septum. In consequence of this arrangement, the entire nodal area is displaced posteriorly and inferiorly. Although a well-formed triangle can be seen at this location, this nodal triangle is not the same as the normal triangle of Koch ( Fig. 31.12 ).
