Holes between the ventricles can occur as isolated anomalies, but are also seen in association with many other defects. For example, they are found as integral parts of entities such as the tetralogy of Fallot, double-outlet ventricles, and most cases of common arterial trunk. They occur as a component of atrioventricular septal defects with common atrioventricular junction, and are frequently encountered in association with transposition and congenitally corrected transposition. They are an integral part of the functionally univentricular circulation. Taken together, they unequivocally make up the commonest congenital cardiac malformation. In this chapter, we consider only the so-called isolated examples, albeit that we discuss the complications produced by other minor associations. Although holes between the ventricles have a long pedigree, it is with the name of Henri Roger 1 that the anomaly is historically linked. It was Roger who recognised that an isolated ventricular septal defect produced a typical murmur, and could be consistent with prolonged life and good health.
PREVALENCE
If we exclude aortic valves having two leaflets, and also exclude prolapse of the mitral valve, 2 then isolated ventricular septal defect is the most common congenital cardiac malformation. It is difficult to obtain an accurate assessment of prevalence, since most individuals with an isolated defect, being asymptomatic, are not candidates for cardiac catheterisation and angiography. In the past, therefore, there frequently was no objective proof of the presence of a defect. It was the advent of cross sectional echocardiography, and colour flow Doppler, that changed all that. The known high rate of spontaneous closure 3 meant that postmortem data certainly underestimated the incidence of the defect. In series depending heavily on clinical observation, the estimated prevalence has varied markedly. The incidence has been shown to be higher in fetuses dying prenatally, and there is some variation depending on the time of fetal death. Hoffman 4 summarised the literature addressing the proportional distribution of congenital cardiac defects from 22 series, showing that the median distribution for ventricular septal defect was 31%. The most accurate clinical data comes from the prospective Bohemia survival study, 5 which revealed a prevalence of 2.56 per 1000 live births, accounting for two-fifths of all cardiac malformations, similar to the data from Merseyside in the United Kingdom, where the figure was 2.74 per 1000 births. 6 A study from Malta, in contrast, revealed a figure of 3.94, a significant difference. 7 This reflects the increased use of echocardiography in diagnosis. This trend is confirmed by data from the Baltimore–Washington Infant Study, 8 which also used echocardiographic techniques, and showed a remarkable increase in the diagnosis of muscular defects, with a tenfold increase in prevalence. Ventricular septal defects accounted for one-third of the lesions identified in the infants of this study, and made up over two-fifths of the malformations encountered in Malta. A particularly high prevalence of muscular defects has also been reported by those who scanned populations of neonates using colour flow imaging, noting subsequent spontaneous closure of many of the defects. 9–12 None of these studies, however, were population based. 13 Ventricular septal defects are more common in premature infants, and those born with low weight. Incidence, however, is not related significantly to race, sex, maternal age, birth order, or socioeconomic state.
While there does not appear to be a genetic bias towards the incidence of ventricular septal defects, genetics do influence the specific type of defect. The doubly committed subarterial, or juxta-arterial, defect is more common in Asian populations, whereas muscular and multiple defects are less common in the same population. According to Wilkinson, 14 the frequency of the doubly-committed subarterial or juxta-arterial defect requiring repair accounts for at least 30% in an Asian population, compared with the incidence of about 5% of Occidental patients requiring surgery in Western societies. Wilkinson 14 also noted that, while muscular defects account for about three-tenths of defects requiring closure in the Western world, they are uncommon in the Asian population referred for surgical closure, where multiple ventricular septal defects are very rare. Such multiple muscular defects account for one-tenth of operative closures in the west.
MORPHOLOGY AND MORPHOGENESIS
Categorisation of Defects
It is, perhaps, surprising that, as we start the 21st century, there is still no consensus concerning the best way to categorise and describe holes between the ventricles. The lesion, however, is not always as simple as it first appears. For example, when the hole between the ventricles is overridden by an arterial valve, it is open to debate as to which plane in space represents the defect. 15 This is because the septum that interposes between the outflow tracts is no longer an interventricular structure. This fact, in itself, negates the possibility of defining a ventricular septal defect as a simple hole in the substance of the ventricular septum. This situation is magnified in the setting of double-outlet right ventricle. In this setting, the hole in the ventricular septum, or the interventricular communication, functions as the outlet from the left ventricle. Surgical correction of patients with double outlet right ventricle, therefore, is based on the concept of tunneling this hole to one or other of the ventricular outflow tracts. In this setting, closure of the hole between the ventricles would wall off the left ventricle from both arterial trunks. Because of these subtle differences in the potential definition of septal defects, different investigators have chosen different features on which to base their criterions for classification. It is hardly surprising, therefore, that currently there is lack of concensus on the best options for categorisation. Indeed, as yet there is no agreement on what defines a ventricular septal defect. Our proposal for definition and classification is based upon several principles ( Table 28-1 ).
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First, it is necessary to define clearly and unambiguously the plane of space chosen to represent the defect. Having defined this plane, the second principle is to account for all its various anatomical features. Most important of these are its boundaries. As we will show, it is the structure of the boundaries that permits phenotypic categorisation. Having established the phenotype of the defect, it is then necessary to describe the position of the hole relative to the landmarks and components of the ventricular septum, including the surgically significant atrioventricular conduction axis. If present, it is also necessary to take account of valvar overriding. When either an atrioventricular or an arterial valve overrides the crest of the muscular ventricular septum, then of necessity there is malalignment between the different septal components. Attention must also be given to the size of the hole chosen to represent the defect. The system, to be used throughout our volume, and forming the basis for the categorisation used in the European Paediatric Cardiac Code, 16 caters for all these features. As we will subsequently show, this system is applicable not only to isolated defects to be described in this chapter, but for those in all the various settings described elsewhere in these pages.
What Is the Defect?
When there is a simple hole, punched as it were into the substance of the muscular ventricular septum, then there is no problem in defining its margins, nor in agreeing that the margins are exclusively muscular ( Fig. 28-1 ). Many holes, however, have the crest of the muscular ventricular septum as one of their margins, but abut directly upon the hingepoints of the leaflets of either the atrioventricular or the arterial valves ( Fig. 28-2 ), or on occasion the attachments of both the arterial and atrioventricular valvar leaflets. Some of these lesions are associated with marked overriding of an arterial or atrioventricular valvar orifice. An example of overriding of the arterial valve is demonstrated in Figure 28-3 . In this particular circumstance, it is much harder to define the precise borders and margins of hole between the ventricles. This is because an inverted cone of space, with an elongated base, extends from the attachments of the leaflets of the overriding arterial valve to the crest of the muscular ventricular septum ( Fig. 28-4 ). Within this inverted cone, any of a number of planes can justifiably be nominated to represent the defect. When viewed in a single section, as seen by the echocardiographer, then at least three of these planes are important (see Fig. 28-4 ). One is the continuation of the long axis of the ventricular septum to the underside of the overriding valvar leaflets, the green oval as shown in Figure 28-4 . This particular choice for defining the defect, which without question represents the plane of the superior continuation of the muscular ventricular septum, exists only when the valvar leaflets are closed. Even though this plane represents the geometric interventricular communication, it is never the locus for placement of a patch by the surgeon during operative repair of an isolated defect. Indeed, when the arterial valvar leaflets open during ventricular systole, the superior margin of this hole is the underside of the aortic arch! The second plane, shown by the yellow oval in Figure 28-4 , marks the boundary between the cone of space subtended beneath the overriding valve and the left ventricle. This plane is the opening from the left ventricle to the subaortic outflow tract, presuming that it is the aortic valve which is overriding the septal crest. Although unequivocally important, this plane is also unlikely to be chosen as the septal defect. The third plane, shown as the red oval in Figure 28-4 , has features in common with the second, in that it marks a ventricular border of the inverted cone of space. This plane, to our eyes, is of greatest practical importance, since it is the border around which the surgeon will insert sutures to secure a patch placed to prevent shunting between the ventricles. It is also the position where the interventionist will place a device with the same goal in mind. It is this third plane, therefore, that we choose to define as the ventricular septal defect, considering the hole as the plane of space that must be closed so as to restore septal integrity. It is the difference in the anatomical make-up of the margins of this hole, as viewed from the morphologically right ventricle, which provide the phenotypic features of the different types of defect. This also is the view obtained by the surgeon when restoring septal integrity in the operating room. In many instances, the hole closed by the surgeon to restore septal integrity is also the obvious hole within the ventricular septum ( Fig. 28-5 ). In these situations, therefore, the septal defect and the interventricular communication are one and the same. In the situation of double outlet right ventricle, this is obviously not the case ( Fig. 28-6 ). When both arterial trunks arise from the right ventricle, the hole closed so as to restore septal integrity is the plane from the crest of the muscular ventricular septum to the underside of the right ventricular outlet septum. This hole is now markedly different from the interventricular communication, which is the outlet for the left ventricle. In the setting of double outlet right ventricle, therefore, as already discussed, it would be a disaster if the surgeon chose to close the interventricular communication, as opposed to the hole which, when closed, restores septal integrity (see Fig. 28-6 ).
Features Requiring Description
As shown, the holes to be closed so as to restore septal integrity are either found within the muscular septum, or else at its margins. The holes at the margins of the muscular septum can be related directly to the hinges of the leaflets of the atrioventricular, or those of the arterial valves, or in some circumstances to the leaflets of both arterial and atrioventricular valves. It is an appreciation of these relationships, as viewed from the right ventricle, which we use as the primary criterion for description. When considered from this stance, all defects can be placed into one of four groups, namely those which are muscular, those which are perimembranous, those which are juxtatricuspid (and non-perimembranous), and those which are both doubly committed and juxta-arterial ( Fig. 28-7 and Table 28-2 ).
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Muscular Defects
Those holes within the substance of the septum of necessity have exclusively muscular rims, and hence are described as muscular defects ( Fig. 28-8 ).
Perimembranous Defects
The second group of holes are those related to the aortic root, opening into the right ventricle in the area where the subpulmonary outflow tract turns superiorly relative to the atrioventricular junction. We describe this area as the inner curvature of the right side of the heart ( Fig. 28-9 ). When the ventricular septum is intact, this is the area occupied by the membranous part of the septum, with the supraventricular crest separating the area from the leaflets of the pulmonary valve (see Chapter 2 ). The membranous septum, on its left ventricular aspect, is directly continuous with the area of fibrous continuity between the leaflets of the aortic and mitral valves. The right end of this zone of continuity is called the right fibrous trigone. It is fusion of this trigone with the membranous septum that produces the central fibrous body ( Figs. 28-10 and 28-11 ).
This entire area is then part of the fibrous root of the aorta, and is itself continuous with the triangle of fibrous tissue which ascends to occupy the space between the non-coronary and right coronary leaflets of the aortic valve. On the right side, the membranous septum itself is crossed by the hinge of the septal leaflet of the tricuspid valve, dividing it into atrioventricular and interventricular components. The axis of atrioventricular conduction tissue penetrates through this septum from the apex of the triangle of Koch (see Chapter 2 ). Having penetrated, when the septum is intact, the conduction bundle runs on the crest of the muscular septum, being sandwiched between the fibrous and muscular septal components, along the postero-inferior margin of the interventricular part of the membranous septum. When the ventricular septum is deficient in this area, it springs apart between these two septal components. In consequence, the postero-inferior margin of the defects opening from the aortic root is formed by fibrous continuity between the leaflets of the aortic and tricuspid valves (see Fig. 28-9 ). In the past, such defects were often called membranous defects, since it was presumed that they existed because of a deficiency in the membranous septum. It was pointed out long since 17 that this is unlikely, since the defects are always larger than the normal dimensions of the membranous septum. Furthermore, the membranous septum is not divided into its interventricular and atrioventricular components until after the embryonic septum has closed. 18 It seems far more likely that the defects persist because the muscular ventricular septum is deficient in the environs of the developing membranous septum. Taking account of this fact, we suggested that the defects should be considered perimembranous. 19 Their anatomical hallmark in the otherwise normally constituted heart is the presence, as seen from the right side, of fibrous continuity between the leaflets of the aortic and tricuspid valves (see Fig. 28-9 ). When viewed from the left side, the continuity is seen to include also the aortic leaflet of the mitral valve (see Fig. 28-11 ). Some of these defects, because of associated anomalies of tricuspid valve, such as clefts or perforations of the septal leaflet, or deformity or adherence of valvar tissue to the margins of the septal defect and widening of the anteroseptal commissure, 20 produce the potential for ventriculo-atrial shunting. Defects of this type are the commonest examples of so-called Gerbode defects. 21 Ventriculo-atrial shunting can also be produced when there is deficiency of the atrioventricular component of the membranous septum, producing a true atrioventricular septal defect with separate right and left atrioventricular junctions, but these lesions are exceedingly rare (see Chapter 27 ). They form the rarer variant of the Gerbode defect. 21
Doubly Committed and Juxta-arterial Defects
There is then a third group of defects that is discrete from the first two groups. The feature of these defects is that they occupy the region in the normal heart formed by the free-standing component of the muscular subpulmonary infundibulum. The infundibulum is the sleeve of free-standing muscle that lifts the leaflets of the pulmonary valve away from the base of the heart. In our initial description, 19 we interpreted this part of the infundibulum as being formed by the muscular outlet septum. We now know this to be incorrect, since as emphasised above, the leaflets of the pulmonary valve, in the normal heart, are supported by a complete sleeve of free-standing infundibular musculature. Defects of this third type, therefore, which are both doubly committed and juxta-arterial, and characterised by fibrous continuity in their roof between the leaflets of the aortic and pulmonary valves ( Fig. 28-12 ), cannot exist in hearts in which the free-standing muscular subpulmonary infundibulum has developed in normal fashion. In other words, the presence of this type of defect implies abnormal formation of the subpulmonary infundibulum in such a way as to permit a hole to exist directly beneath the arterial roots. When viewed from the right ventricle, most defects of this type are seen to have a muscular postero-inferior rim separating the leaflets of the aortic and tricuspid valves. They are, therefore, described as doubly committed and juxta-arterial (see Fig. 28-12 , upper panel). On occasion, nonetheless, they can extend so that the postero-inferior margin is formed by fibrous continuity between the aortic and tricuspid valvar leaflets, the roof continuing to be made up of aortic to pulmonary valvar continuity. Defects with such margins are both doubly committed and juxta-arterial and, at the same time, perimembranous (see Fig. 28-12 , lower panel).
Juxta-tricuspid and Non-perimembranous Defects
The juxta-tricuspid, and non-perimembranous, defect is very rare. The hole between the ventricles is roofed by fibrous continuity between the leaflets of the tricuspid and mitral valves, but is separated from the area of the membranous septum. 22 The atrioventricular valvar leaflets are then in direct contact through the posterior margin of the defect. In contrast to perimembranous defects, the tricuspid valve is not in direct contact with the aortic valve. It was defined on the basis of the four-chamber echocardiographic cut, and its angiographic equivalent. 22 Those describing the defect commented that it may be difficult to distinquish from a perimembranous defect, and indeed, one of us (RHA) has never encountered an autopised specimen with such a defect. The conduction axis will be located at the anterior aspect of the defect, coursing along the postero-inferior margin of the intact membranous septum.
Position of Defects
Having accounted for the margins of the various defects, as viewed from the right side, and having used this as our primary criterion for definition, we then describe the position of the defect relative to the landmarks and components of the right ventricle. For the muscular defects, this creates no problems, since these holes are punched within the substance of the muscular septum so as to open directly to the inlet, the outlet, or the apical parts of the right ventricle. For the perimembranous defects, the situation is more complicated.
Relation of Perimembranous Defects to Components of the Right Ventricle
In our initial account, 19 we described perimembranous defects as excavating into the inlet, outlet, or apical trabecular components of the septum. This is incorrect. Although much of the septum, when viewed from the right side, seems to be positioned to separate the inlet of the right from the inlet of the left ventricle, this is not the case. Because of the deeply wedged location of the normal subaortic outflow tract, the majority of the septum as viewed from the right side in relation to the septal leaflet of the tricuspid valve separates the inlet of the right from the outlet of the left ventricle. As discussed, it is also the case that, in the normal heart, the seemingly septal surface of the right ventricular outflow tract is formed by the free-standing muscular subpulmonary infundibulum. The posterior wall of this muscular subpulmonary infundibulum separates the cavity of the right ventricle from the aortic root, rather than forming a septum between the cavities of the right and left ventricles. Indeed, in the normal heart it is not possible to recognise the boundaries of a discrete muscular outlet septum (see Chapter 2 ). When perimembranous defects open anteriorly towards the pulmonary valve, they do not excavate directly into the free-standing muscular subpulmonary infundibulum. Instead, the presence of the defect now makes it possible to recognise a discrete muscular outlet septum, which forms the superior margin of the hole between the ventricles. This true outlet septum, located within the right ventricle, can be extensively malaligned relative to the remainder of the muscular septum. The arrangement permits shunting between the subaortic and subpulmonary outlets ( Fig. 28-13 ). Because of the close relationship of the aortic valve to the tricuspid valve, some of the shunting will still be directed towards the inlet of the right ventricle. Large perimembranous defects, therefore, can be considered confluent, shunting to all parts of the right ventricle.
Valvar Overriding and Septal Malalignment
The third important feature that requires description, when present, is valvar overriding, along with its associated feature of malalignment of septal components. Malalignment can involve the muscular outlet septum, as discussed above. In the situation illustrated thus far (see Fig. 28-13 ), the outlet septum is a right ventricular structure, as is also the case in tetralogy of Fallot (see Chapter 36 ). The outlet septum can be deviated posteriorly to become a left ventricular structure, then obstructing the outflow tract of the left ventricle ( Fig. 28-14 ). There can also be malalignment between the muscular ventricular septum and the atrial septum. This is the phenotypic feature of hearts with overriding of the orifice of the tricuspid valve and straddling of its tendinous cords ( Fig. 28-15 ).
Size and Shape of the Defect
Size is obviously important in determining the haemodynamic consequences of the various types of defects, and also in determining those defects most likely to diminish in size or close spontaneously. Size can be described according to taste, either using subjective adjectives such as large, medium or small, or by relating the different dimensions of the plane of space measured as the defect to the diameter of the aortic root. The shape of the defect may also vary; muscular defects are typically round or oval when seen from the right ventricle, although they can be crescentic or cashew-like. Defects that are bordered by atrioventricular or arterial valves are typically half-moon shaped. The shape of the defect may appear different when it is encroached upon by prolapsing leaflets of the aortic valve. When it is not round, the defect may appear larger on one plane than on another as seen using cross sectional and angiographic imaging.
Having discussed the general principles of categorisation, and the system by which we describe ventricular septal defects, we will now consider each of four morphological types of defects in more detail, emphasising their relationships to the atrioventricular conduction axis.
Perimembranous Ventricular Septal Defects
Perimembranous defects are those opening in the region of the inner curvature of the right ventricle, having as their diagnostic feature continuity between the leaflets of the tricuspid and aortic valves, this continuity incorporating also the central fibrous body. In this situation, the crucial postero-inferior part of the margin of the defect as seen from the right ventricle is formed by the area of fibrous continuity ( Fig. 28-16 ). When such defects open to the inlet of the right ventricle, and are viewed from their right side, the crucial distinguishing part is often curtained from view by the leaflets of the tricuspid valve ( Fig. 28-17 ). On occasion, a defect opening between the ventricular inlets can be part of a heart having a common atrioventricular junction ( Figs. 28-18 and 28-19 ). In this setting, the left atrioventricular valve will have three leaflets, and the heart will show all the other features of an atrioventricular septal defect with common atrioventricular junction (see Chapter 27 ), albeit with shunting occurring only at the ventricular level because the bridging leaflets of the valve guarding the common junction are firmly attached to the under surface of the atrial septum. Such defects should be distinguished from perimembranous ones opening to the inlet of the right ventricle. Irrespective of their size, the latter defects have separate atrioventricular junctions, with a mitral valve guarding the left junction, even when it is cleft. 23 The hearts already discussed with atrioventricular septal malalignment, and with the tricuspid valve straddling and overriding a defect between the ventricular inlets (see Fig. 28-15 ), should also be distinguished from hearts with atrioventricular septal defect and common atrioventricular junction. The ones with overriding of the tricuspid valve constitute a particular subset of perimembranous defect, since they still possess the phenotypic feature of aortic–tricuspid valvar fibrous continuity. The septal malalignment, however, predicates that there will be an anomalous location of the atrioventricular conduction axis ( Fig. 28-20 ). Similar septal malalignment can also be found in the setting of a common atrioventricular junction (see Chapter 27 ). When perimembranous defects permit shunting primarily to the inlet of the right ventricle, the septal leaflet of the tricuspid valve is frequently divided or deficient. If the two components of the abnormal leaflet are at all bound down to the margins, then shunting can occur from left ventricle to right atrium. Such shunting is frequently held to be caused by absence of the atrioventricular membranous septum, but this is very unusual. Deficiency of the atrioventricular component of the membranous septum can occur, very rarely, in the setting of separate right and left atrioventricular junctions ( Fig. 28-21 ). More usually when there is ventriculo-atrial shunting, this is first across a perimembranous defect, and only subsequently to the right atrium ( Fig. 28-22 ). As already discussed, the two defects permitting ventriculo-atrial shunting are also decribed as Gerbode defects. 21
The other frequent type of perimembranous defect is the one that extends so as to open primarily beneath the ventricular outlets. The outlet compoment of the ventricular septum is not normally in the same plane as the remainder of the muscular septum. Defects involving the junction of the inlet and outlet components of the ventricular septum, therefore, are characterised by a degree of overriding of the aortic valve. The feature of these defects, as shown in Figure 28-13 , is that the muscular outlet septum, supporting the free-standing muscular subpulmonary infundibulum, becomes recognisable as a discrete entity, being malaligned relative to the rest of the muscular septum, and most usually occupying a position exclusively within the right ventricle, although the outlet septum can also be deviated into the left ventricle. In these settings, the orifice of the aortic valve overrides the muscular ventricular septum ( Fig. 28-23 ). Such defects opening between the outlets in the presence of aortic overriding and with the outlet septum deviated into the right ventricle are closely related to tetralogy of Fallot (see Chapter 36 ). The distinction between the two entities depends upon the presence or absence of muscular infundibular stenosis. The lesion with valvar overriding in the absence of obstruction to pulmonary flow is often described as the Eisenmenger ventricular septal defect.
As we have also discussed, the outlet septum can be deviated posteriorly into the left ventricular outflow tract. This usually produces subaortic obstruction (see Fig. 28-14 ), and is almost always associated with obstructive lesions of the aortic arch, either severe tubular hypoplasia or interruption. It would seem that the pulmonary valvar orifice should override the muscular ventricular septum in such defects. Because of the length of the free-standing muscular subpulmonary infundibulum, however, valvar overriding is rare, usually being seen only when the defect is doubly committed and juxta-arterial. 24
Irrespective of whether a perimembranous defect extends to open between the inlet or outlet ventricular components, or is large enough to open to all parts of the ventricle, a situation described as being confluent, the basic distribution of the axis for atrioventricular conduction is the same. In the normal heart, the penetrating component of the atrioventricular conduction axis passes through the central fibrous body, and branches on the crest of the muscular ventricular septum. Since the central fibrous body forms the postero-inferior margin of perimembranous defects, this will be the site of penetration of the axis. The landmark of the atrioventricular node, as in the normal heart, is the apex of the triangle of Koch. When the defect opens to the right ventricular inlet, the triangle may itself be displaced posteriorly, but its apex still serves as the guide to the site penetration of the conducting bundle. When the axis has penetrated through the central fibrous body, it is related to the postero-inferior rim of perimembranous defects. The precise relationship of its non-branching component to the septal crest depends upon the location of the defect. It is much closer to the crest when a defect opens to the inlet, becoming more remote as the defect extends to open between the outlets.
Because perimembranous defects are closely related to the septal leaflet of the tricuspid valve, there is always the possibility that they may be closed by plastering down of the leaflet across the defect, with small ones being those most likely to close. Defects between the outlet components, particularly when complicated by malalignment, are unlikely to close by this mechanism. Neither are extensive inlet defects. A more frequent, and closely related, mechanism of closure is aneurysmal enlargement of fibrous tissue in the environs of the defect. Although often described as aneurysms of the membranous septum, it is unusual for the remnant of the membranous septum itself to be involved. In most cases, the grape-like lesions are sculpted from the underside of the leaflets of the tricuspid valve. On occasion, the tags can be more extensive hammock-like lesions, with cordal attachments to the septum ( Fig. 28-24 ). The presence of tissue tags is an indication that defects may close spontaneously, and hence that there will be less necessity for surgical closure. On rare occasions, however, the tags can become sufficiently large as to balloon into the right ventricular outflow tract and produce subpulmonary obstruction.
Aneurysmal prolapse of the right coronary leaflet of the aortic valve can be found in the setting of perimembranous defects, particularly those opening between the outlets, when the outlet septum is markedly deficient. Sometimes the non-coronary aortic leaflet, and rarely the left, can also prolapse through the defect. Rarely, such valvar prolapse may produce partial or complete plugging of the defect, and result in spontaneous closure or a false sense of security that the defect is small when, in reality, it is a large defect. The prolapse can also result in aortic regurgitation. In all of these possible mechanisms of closure or diminution in size, contraction and fibrosis around the edges of the defect are additional contributory factors.
Muscular Defects
Defects with entirely muscular rims are also distinguished according to whether they open mostly to the inlet, mostly to the apical trabecular component, or mostly to the subpulmonary outlet as seen from the right ventricular side. Such defects can be multiple, particularly when in the apical trabecular septum, or can co-exist with perimembranous or juxta-arterial defects. Those opening to the right ventricular inlet will be covered to some extent by the septal leaflet of the tricuspid valve, and can then be difficult to distinguish from perimembranous defects opening to the inlet. For the defect to be muscular, a bar of muscle, which may be quite small, must interpose between the defect and the hingepoints of the leaflets of the mitral and tricuspid valves (see Fig. 28-1 ). When a defect is perimembranous, or juxta-tricuspid and non-perimembranous, the leaflets of the mitral and tricuspid valves are in fibrous continuity in its postero-inferior margin (see Fig. 28-16 ). When the defect is muscular, or juxta-tricuspid and non-perimembarnous, the atrioventricular conduction axis passes above, or anterosuperior, and will be to the left hand when viewed by the surgeon in the operating room. The bundle is always located postero-inferiorly when the defect is perimembranous, and hence will be to the right hand as viewed by the surgeon ( Fig. 28-25 ).
Muscular defects in the apical trabecular septum are frequently large holes that tend to be found to one or other side of the septomarginal trabeculation. Not infrequently, a single opening, as viewed from the left ventricular aspect, is crossed by trabeculations to produce two, or more, openings when viewed from the right ventricle ( Fig. 28-26 ). Multiple smaller muscular defects give the so-called Swiss-cheese septum. These may be particularly difficult to define, even in the postmortem specimen. Although the atrioventricular conduction axis is itself unrelated to muscular defects in the apical septum, the distal bundle branches may pass through the muscular septum between holes, producing pseudobifurcations.
Outlet, or infundibular, muscular defects are small and usually single, although they can be larger ( Fig. 28-27 ). The superior rim of such outlet defects is the outlet septum, often attenuated, along with the sleeve of free-standing subpulmonary infundibulum, while the inferior muscular rim, which separates the defect from the membranous septum, is formed by fusion of the posterior limb of the septomarginal trabeculation with the ventriculo-infundibular fold. This muscular tissue separates the conduction tissue axis from the crest of the septum, thus providing protection from surgical damage by sutures placed in the postero-inferior edges of the defect. Small muscular defects close spontaneously simply by growth of the muscular structures surrounding them. As already discussed, they are almost certainly the most common defects undergoing spontaneous closure. 9–12
Doubly Committed Juxta-arterial Defects
The defect is termed juxta-arterial because it is directly related to both aortic and pulmonary valves (see Fig. 28-12 ), this feature underscoring echocardiographic recognition ( Fig. 28-28 ). It is also because of this feature that both arterial valves frequently override the septum, giving a progressive spectrum of anomalies the end-point of which is double outlet left ventricle. Such hearts have also been termed double outlet both ventricles. 25 Prolapse of the aortic valvar leaflets is frequent with the juxta-arterial defect, albeit that, as already discussed, such prolapse can also lead to valvar insufficiency when a defect is perimembranous. The relationship of the atrioventricular conduction axis to the doubly committed defect depends on the morphology of its postero-inferior margin. A well-formed muscular rim ( Fig. 28-29 ) separates the conduction axis from the postero-inferior margin of the defect. In contrast, the atrioventricular bundle is more at risk when the doubly committed defect is also perimembranous ( Fig. 28-30 ).
Juxta-tricuspid and Non-perimembranous Defects
This rarest type of ventricular septal defect involves the inlet muscular septum along the tricuspid annulus but does not reach the membranous septum and therefore the aortic valve. 22 In a four-chamber view, the defect may not show any features different from a perimembranous defect extending toward the inlet along the tricuspid valvar annulus. The tricuspid and mitral valves are in direct contact above the defect in a four-chamber view. The major distinguishing feature of the juxta-tricuspid and non-perimembranous defect from the perimembranous variety is seen in a parasternal long axis cut, which shows that the defect is some distance from the aortic valve. As discussed, the conduction axis is located at the anterior aspect of the defect along the postero-inferior margin of the intact membranous septum.
Morphogenesis
It is impossible to achieve normal closure of the embryonic interventricular communication until the right atrioventricular junction is connected to the right ventricle, and the subaortic outflow tract transferred to the left ventricle. Thereafter, the persisting interventricular communication is closed by tissue derived from various sources (see Chapter 3 for details). It is not closed specifically, however, by the interventricular membranous septum. When the interventricular communication is closed, the septal leaflet of the tricuspid valve has yet to be delaminated from the muscular septum. 18 There cannot, therefore, at this stage, be an interventricular component to the membranous septum. Perimembranous defects cannot be explained simply on the basis of failure of closure of the embryonic interventricular communication by the interventricular component of the membranous septum. It is more likely that the muscular septum is deficient in the environs of the closing plug of tissue, which is a consequence of insufficient area or volume. Deficiency of the different parts of the muscular septum then accounts for the diversity in position and orientation of perimembranous defects.
The formation of muscular defects is more easily explained. It is now established that, at least in the chick, 26 the muscular septum is produced by coalescence of embryonic trabeculations. Muscular defects, therefore, likely result from failure of the trabeculations to coalesce. This notion is now supported by the fact that, increasingly, such an arrangement is seen in association with ventricular noncompaction. When extensive, and involving the septum, such noncompaction produces the Swiss-cheese septum. Alternatively, there may be failure of fusion of the muscular septum with the free-standing septal component of the subpulmonary infundibulum, derived by muscularisation of the outflow cushions. Such failure of fusion of the infundibulum with the muscular septum and the septomarginal trabeculation gives a good explanation for the muscular defect opening between the outlets.
The doubly committed juxta-arterial defect is well explained simply on the basis of failure of muscularisation of proximal outflow cushions during division of the embryonic outflow tract. Because of the failure of formation of the muscular subpulmonary infundibulum, the defect is closely related developmentally to common arterial trunk (see Chapter 41 ).
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