Robert Anderson and colleagues simplified this scheme by presenting an approach that highlights phenotypic variability assessed on the basis of the anatomic borders of the defects. This approach condenses the categories to three types. It does not propose to ignore the equally important feature of the location of the defect relative to the landmarks of the right ventricle, nor septal malalignment. But whereas the initial numeric surgical classification proposed four classes that combined features of anatomic position and borders, the approach based on the anatomic borders can reduce the classes to three: defects that are directly juxtapulmonary, those that are perimembranous, and those with exclusively muscular borders when viewed from the right ventricle. The specificity provided by concentration on these phenotypic features reinforces the simplicity of the initial approach. These modifications were proposed to provide for the needs of modern surgery because providing information relative to the anatomic borders of the defect permits inferences to be made with considerable accuracy regarding the likely location of the atrioventricular conduction axis.
The Type I defectas defined in the initial classification opens to the outlet of the right ventricle immediately beneath the pulmonary valve. Its phenotypic feature is fibrous continuity between the leaflets of the aortic and pulmonary valves in the roof of the defect owing to failure of muscularization of the subpulmonary infundibulum. For this reason, the defect is considered by some to represent conal hypoplasia. Indeed, it is frequently possible to recognize a fibrous raphe in fibrous continuity between the arterial valves; this is the hypoplastic outlet septum and can be malaligned in either a cranial direction (potentially obstructing the subpulmonary outflow tract) or a caudal direction (potentially obstructing the subaortic outflow tract). Because of failure of formation of the subpulmonary infundibular sleeve, the defect opens to the right ventricle between the limbs of the septal band.
The left-hand panel of Fig. 6.2 shows the phenotypic feature of the so-called Type IVSD. It is directly juxtapulmonary but also doubly committed because of its phenotypic feature and fibrous continuity between the leaflets of the aortic and pulmonary valves owing to failure of formation of the free-standing subpulmonary infundibular sleeve. In most instances, as shown in this figure, the defect has a muscular posteroinferior rim. The drawing in the right-hand panel shows how this muscular rim protects the atrioventricular conduction axis. In some instances, however, the defect can extend so as to open centrally when it is also perimembranous.
Fig. 6.2
In some classifications, this defect is considered to be supracristal. As shown in the left-hand panel, the defect opens to the right ventricle between the limbs of the septomarginal trabeculation, or septal band and is the defining feature of the infracristal defect. Thus, in this system of classification, it was the structure taken to represent the crista that changed, rather than the location of the defect itself. The defect is better described as being doubly committed and juxtapulmonary. In most cases, its posteroinferior rim is muscular, formed by fusion between the caudal limb of the septal band and the ventriculoinfundibular fold. This formed muscle bar protects the atrioventricular conduction axis, as seen in the right-hand panel. In a minority of cases, the defect can extend to the central area of the ventricular base. It is then bordered posteroinferiorly by an area of fibrous continuity between the leaflets of the aortic and tricuspid valves. This is the phenotypic feature of the Type II defect in the original surgical classification and is of major surgical significance because the conduction axis is at greater risk when the defect extends to reach the area of aortic-to-tricuspid fibrous continuity. It is the Type II defectthat is encountered most frequently during cardiac surgery, exists because of failure to close the embryonic interventricular communication, and opens to the right ventricle at the center of the cardiac base.
Figure 6.3 (left-hand panel) shows a defect opening centrally at the base of the right ventricle. As shown in the right-hand diagram, its phenotypic feature is fibrous continuity at its posteroinferior margin between the leaflets of the aortic valve and the septal leaflet of the tricuspid valve. It opens inferior and posterior to the caudal limb of the septomarginal trabeculation and behind the septal leaflet of the tricuspid valve, with the supraventricular crest inserted in normal fashion between the limbs of the trabeculation. As shown, the atrioventricular conduction axis runs posteroinferior to the defect but is at risk in this corner.
Fig. 6.3
The atrial component of the membranous septum is incorporated within this fibrous border, as seen in the right-hand panel of Fig. 6.3. The defect was initially interpreted on the basis of failure of formation of the membranous septum, so it was considered a membranous defect. A remnant of the interventricular part of the membranous septum, known as the membranous flap, is frequently found reinforcing the posteroinferior margin of the defect. The defect does not exist because the membranous septum is absent but rather because its muscular margins are deficient. For this reason, it was described as being perimembranous. Depending on the deficiency of its muscular borders, the central defect can also extend to open more to the inlet of the right ventricle, rather than being beneath the septal leaflet of the tricuspid valve.
Figure 6.4 (left-hand panel) shows a four-chamber section through a heart with a defect located centrally within the ventricular base but extending inferiorly so as to open to the inlet of the right ventricle, being shielded by the septal leaflet of the tricuspid valve. Because the defect retains the phenotypic feature of fibrous continuity between the leaflets of the aortic valve and the septal leaflet of the tricuspid valve, the conduction axis continues to run posteroinferiorly, relative to the defect when viewed from the right ventricle (right-hand panel).
Fig. 6.4
Defects incorporating a deficiency at the central part of the ventricular base at the site of the embryonic interventricular communication can also be found extending to the outlet of the right ventricle. Such defects retain the phenotypic feature of fibrous continuity between the leaflets of the aortic and tricuspid valves, and hence are still perimembranous. They have the additional phenotypic feature of separation between the subpulmonary infundibulum and the crest of the muscular septum by virtue of malalignment of the muscular outlet septum. As with the fibrous outlet septum, there can be either cranial or caudal malalignment. Cranial malalignment is most frequent, in association with overriding of the aortic root, producing the lesion known as the Eisenmenger defect that is shown in Fig. 6.5.
Fig. 6.5
The left-hand panel of Fig. 6.5 shows a defect with malalignment of the muscular outlet septum and associated overriding of the aortic root. It extends centrally to become perimembranous (the so-called Eisenmenger defect). As shown in the right-hand panel, because the defect extends to become perimembranous, the conduction axis continues to extend posteroinferiorly relative to the margins of the defect. It is in this setting that the muscular outlet septum becomes recognizable as forming the cranial margin of the defect. This defect is described by some as being conoventricular. Its right ventricular opening, like the juxtapulmonary or Type I defect, is cradled between the limbs of the septal band. In this arrangement, the fibrous continuity forming the posteroinferior margin is between the leaflets of the aortic valve and the anterosuperior (rather than the septal) leaflet of the tricuspid valve. In terms of its physical location, it is an outlet defect, and hence can be classified as a Type I defect by those using the initial surgical classification. Like the central defect, nonetheless, and also the central defect extending to open to the inlet of the right ventricle, the defect extending to open to the right ventricular outlet shares the feature of fibrous continuity between the leaflets of the aortic and tricuspid valves; hence, it is perimembranous. Because of this feature, the atrioventricular conduction axis again penetrates directly through the posteroinferior border of the defect and is at risk of damage during surgical closure.
The Type III defectin the initial surgical classification is considered to represent an inlet defect. It is described in this fashion because of its location relative to the landmarks of the right ventricle. As seen in Fig. 6.4, however, a perimembranous defect can open to the inlet of the right ventricle when it is associated with deficiency of the inferior and basal part of the muscular ventricular septum. In fact, the phenotypic feature of the Type III defect is malalignment between the apical muscular septum and the atrial septum, as shown in the left-hand panel of Fig. 6.6. This image shows a defect unequivocally opening to the inlet of the right ventricle. Its phenotypic feature, however, is malalignment of the muscular ventricular septum relative to the atrial septum, with straddling and overriding of the tricuspid valve. As is shown in the right-hand panel, because of the malalignment of the muscular septum, which carries the atrioventricular conduction axis, the axis arises from an anomalous atrioventricular node formed at the point where the malaligned ventricular septum meets the inferior component of the atrioventricular junction. The regular atrioventricular node is hypoplastic, but it remains in its expected position within the triangle of Koch.
Fig. 6.6
It exists because of incomplete expansion of the atrioventricular canal during embryonic development. Because of the abnormal development of the inlet component, there is overriding and straddling of the tricuspid valve. By virtue of this feature, the malaligned ventricular septum extends across the full circumference of the tricuspid valvar orifice (right-hand panel). This appearance is similar to that seen in the atrioventricular septal defect with common valvar orifice. It was this finding that presumably prompted some investigators to categorize the lesion as an atrioventricular canal defect. Patients with straddling and overriding tricuspid valve, however, do not exhibit a common atrioventricular junction, which is the essence of an atrioventricular canal.
In a minority of patients having an atrioventricular septal defect in the setting of a common atrioventricular junction—in other words an unequivocal atrioventricular canal—it is possible for shunting across the defect to be confined at the ventricular level because the bridging leaflets of the common valve are attached to the leading edge of the atrial septum, as shown in Fig. 6.7. This drawing shows the features of an atrioventricular septal defect with exclusively ventricular shunting, by virtue of the attachment of the bridging leaflets of the common valve to the leading edge of the atrial septum. In this setting, the conduction axis arises from an anomalous node located at the cardiac crux, rather than at the apex of the triangle of Koch.
Fig. 6.7
These lesions found with common atrioventricular junctions are the true defects of atrioventricular canal type that produce ventricular shunting because the hole is the ventricular component of an atrioventricular septal defect, rather than representing a VSD. In the lesion with overriding of the tricuspid valvar orifice, or the initial Type III defect(Fig. 6.6, left-hand panel), it is the malalignment between the atrial septum and the muscular ventricular septum that is the major phenotypic finding. Because of septal malalignment, the atrioventricular conduction axis no longer takes its origin from the regular node located at the apex of the triangle of Koch. Instead, it arises from an anomalously located atrioventricular node, as shown in the right-hand panel of Fig. 6.6. This important surgical feature reflects the phenotype rather than the anatomic location of the defect. And, as we have already shown, other defects with different phenotypic features can open to the inlet of the right ventricle, such as the central perimembranous defect with inlet extension (Fig. 6.4) or the atrioventricular septal defect with exclusively ventricular shunting (Fig. 6.7). It follows, therefore, that to provide the specificity required for surgical description, it is necessary to account not only for the physical location but also the phenotypic features of the various types of VSD. This is even more important when considering defects that open to the inlet of the right ventricle, as defects with exclusively muscular borders can share this feature (Fig. 6.8, left-hand panel). When a defect opening to the right ventricular inlet has exclusively muscular anatomic borders, the conduction axis is at risk anterosuperiorly, or to the left of the surgeon operating through the tricuspid valve (Fig. 6.8, right-hand panel).
Fig. 6.8
The heart shown in the left-hand panel of Fig. 6.8 has a defect opening to the inlet of the right ventricle, being seen subsequent to retraction of the septal leaflet of the tricuspid valve. In this heart, however, the defect has exclusively muscular borders, which means that the conduction axis is located anterosuperiorly relative to the defect. As shown in the right-hand panel, the axis is to the left of the surgeon working in the operating room.
In terms of physical location, therefore, the central perimembranous defect extending inferiorly is also an inlet defect (Fig. 6.9, left-hand panel). With this variant, however, unlike the Type III defect (Fig. 6.9, right-hand panel), the atrioventricular conduction axis retains its anticipated origin from the atrioventricular node located at the apex of the triangle of Koch, although both triangle and node are deviated inferiorly within the right atrium.
Fig. 6.9
Figure 6.9 drawings show the location of the conduction axis in the setting of defects extending inferiorly from the central perimembranous area so as to open primarily to the inlet of the right ventricle, but in the settings of atrioventricular septal alignment (left-hand panel) and atrioventricular septal malalignment (right-hand panel). When the septal components are aligned, as in the left-hand panel, the atrioventricular node remains located at the apex of the triangle of Koch, but the triangle and the node are displaced inferiorly, although they are not located at the cardiac crux as is the case for the atrioventricular septal defect with exclusive ventricular shunting (compare Fig. 6.7). When there is septal malalignment, in contrast, the conduction axis arises from an anomalous atrioventricular node (right-hand panel). Because the defect found with straddling and overriding of the tricuspid valve is also associated with fibrous continuity between the leaflets of the aortic and tricuspid valves, it remains a perimembranous defect, but with the additional essential feature of malalignment between the atrial septum and the muscular ventricular septum.
The unifying feature of the final types of defect, which were classified as being Type IVin the initial system, is that they all have exclusively muscular borders. Figure 6.10 shows how defects with exclusively muscular borders, defined as Type IV in the initial surgical classification, can open to any part of the morphologically right ventricle, so that they vary with regard to their relationship to the atrioventricular conduction axis. They can show marked variability in terms of their physical location. As already described, some open to the inlet of the right ventricle (Fig. 6.8), which might justify their classification as Type III defects.
Fig. 6.10
Others are found opening to the outlet of the right ventricle, making them describable as Type I defects, the more so because they open between the limbs of the septal band (Fig. 6.11). In terms of classification, they are well described as muscular outlet defects. The image in Fig. 6.11 shows the location of a muscular defect opening to the outlet of the right ventricle. It exists because of separation between the subpulmonary infundibular sleeve and the crest of the muscular ventricular septum. The posteroinferior muscular rim of the defect protects the atrioventricular conduction axis (yellow dotted line). The muscular outlet defects exist because of separation between the muscular septum and the subpulmonary infundibulum, so they can also be considered to represent conal hypoplasia. Muscular defects can also be found at various locations within the basal, middle, or apical part of the muscular ventricular septum, and can be located anteriorly or posteriorly relative to the septal band. The latter lesions are produced because of incomplete compaction of the developing apical part of the septum. Larger muscular defects can be crossed by right ventricular trabeculations, so that a defect that appears solitary when viewed from the left ventricular aspect seemingly has multiple orifices when viewed from the right ventricle. There also can be multiple small defects percolating through the apical septum, producing the arrangement known as the Swiss-cheese septum. Any of the muscular defects can also coexist with perimembranous defects. If the muscular defect in such a combination opens to the inlet of the right ventricle, the conduction axis runs within the muscular strand interposing between the defects. This strand can be very thin, making it difficult to close the defects independently without producing atrioventricular block.
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