To understand the spatial relationships of the different cardiac chambers, three basic rules of cardiac anatomy should be remembered. First, because of the orientation of the cardiac long axis, the ventricles are more or less to the left of their corresponding atrial chambers. Second, the right atrium and the right ventricle are relatively anterior to their left counterparts. Third, because of the anterior position of the chambers of the right heart, the aorta and its valve have a central position in the heart, being wedged between the atrio-ventricular valves and posterior to the infundibulum of the right ventricle. The aortic valve is, therefore, related to all four cardiac chambers.
The right atrial chamber forms the right border of the cardiac silhouette, and is the most superficial chamber to the right of the midline. It has several anatomical components (Ho and Anderson, 2000a). The more posterior component, the venous sinus, or sinus venarum, has smooth walls. As usually viewed by the surgeon at operation, it receives the SVC to the left side and the IVC to the right side of its cavity. It communicates through a wide and unrestricted channel with the second, anterior, component of the chamber, the atrial appendage. The walls of the appendage are rough internally because of the multiple musculi pectinati that cross its surface. It extends forward in a pouchlike manner to clasp the right side of the aorta. When viewed externally in surgical orientation, the appendage is triangular but blunt (Figure 2.10) in comparison with the left atrial appendage, as viewed through a left thoracotomy (Figure 2.11). This distinction between the atrial appendages is of major significance because their shape is the best guide to atrial morphology, and hence to atrial situs. The junction between the venous part of the right atrium and the atrial appendage is marked externally by the sulcus terminalis. This sulcus marks the site of the crista terminalis, the prominent muscular structure that separates the venous sinus and appendage internally. The position of the sinus node as seen by the surgeon is shown in Figure 2.12. A second groove is seen when the heart is tilted leftward from the initial surgical orientation. This second groove is between the SVC and the pulmonary veins and continues toward the IVC, separating it from the left atrium. It marks the site of the interatrial fold, and is known as Waterston's groove (see Figure 2.12). When the right atrium is opened, a muscular prominence is seen in its floor; this prominence overlies the root of the aorta and the central fibrous body (Figure 2.13). To the right of, and superior to, this aortic mound is the tricuspid valve, which leads to the right ventricle. The myocardium inserting into the valvar leaflets is smooth, and forms the atrial vestibule. To the left and below is the septal surface of the chamber, including the fossa ovalis. At some distance beneath the mound is the orifice
of the coronary sinus, which drains most of the venous return from the heart itself. The crista terminalis springs from within the atrial appendage. It extends onto the septal surface to become continuous with the superior rim of the fossa ovalis (Figure 2.14). The main part of the crista, however, swings upward and encircles the orifice of the SVC. It then passes in front of the orifice of the IVC and dips to the right of and beneath the
sulcus terminalis Waterston's groove
sulcus terminalis Waterston's groove
mouth of the coronary sinus. At that point, the muscle bundle widens into a broad strip that runs into the septal leaflet of the tricuspid valve, forming a muscular area of variable dimensions that separates the attachment of the tricuspid leaflet from the coronary sinus and the fossa ovalis. This is the inferior isthmus, which is continuous superiorly with the septal part of the tricuspid vestibule that overlies part of the ventricular septum, and therefore separates the right atrium from the left ventricle. We
used to call this area the ''muscular atrio-ventricular septum''. We now know it is a muscular sandwich, since a superior continuation of the adipose tissue of the inferior atrio-ventricular groove is interposed between the atrial and ventricular muscular layers. Beneath the mouth of the coronary sinus, the transitional area from terminal crest to muscle strip is frequently pouched out to form a diverticulum, the sub-Thebesian sinus. To the left of this sinus, another muscle band, the so-called Eustachian ridge, springs from the crista terminalis and passes toward the aortic mound, separating the orifices of the IVC and the fossa ovalis from the mouth of the coronary sinus. This area also forms the inferior margin of the fossa ovalis (Figure 2.15). Fibrous membranes are attached to the crista terminalis at the entrance of the IVC (the valve of the IVC, or Eustachian valve), and above the entrance of the coronary sinus (the valve of the coronary sinus, or Thebesian valve). Well formed in infancy, these valves regress in adulthood, but frequently persist as filigreed networks that extend across the venous orifices. The junction of these valvar structures, however, is almost always present, forming a fibrous strand that passes forward toward the aortic mound, burying itself in the musculature of the antero-inferior rim of the fossa ovalis, and running forward and upward to insert into the central fibrous body. This structure is the tendon of Todaro, and can easily be brought into prominence by placing tension on the Eustachian valve (Ho and Anderson, 2000b). It is of major significance because it forms one of the boundaries (Figure 2.16) of the triangle that contains the atrio-ventricular node, the so-called
triangle of Koch. The tendon forms the left boundary of the triangle, meeting the right boundary, which is the septal attachment of the septal leaflet of the tricuspid valve, at the aortic mound. The base of the triangle is the orifice of the coronary sinus. In the view down into the right atrium, an extensive muscular area is seen abutting on the tricuspid and venous orifices. It is important to appreciate that not all of this is atrial septum. The floor of the fossa ovalis, confined by the superior and inferior rims, and the variably developed antero-inferior rim, are indeed septal structures. Much of the superior rim, the so-called ''septum secundum'', however, is a fold between the right pulmonary veins and the SVC. This is evident externally as Waterston's groove. Incisions through this rim take the surgeon outside the heart, as do incisions in the area overlying the aortic mound, which is the superior atrial wall. Frequently, a small muscular fold produces a crevice in the anterior wall in this position. This crevice is of importance because it may catch a catheter passed into the atrium and aimed for the fossa ovalis. It is important to distinguish crevice from fossa ovalis when septal puncture is attempted, because the former overlies the transverse sinus, and is not an interatrial structure. Part of the area of the antero-inferior rim itself is an atrio-ventricular structure rather than an interatrial structure, because the tricuspid valve is attached to the septum more apically than is the mitral valve. The area between the septal attachments of the two atrio-ventricular valves is the atrio-ventricular muscular sandwich, and is described in more detail in the section on the atrio-ventricular junction (Anderson et al., 2000). The area of atrial floor that is truly interatrial is best demonstrated by removal of the interface between the atrial chambers (Figure 2.17). The right atrial myocardium contains and overlies two of the most vulnerable parts of the conduction system: the sinus and atrio-ventricular nodes. The sinus node is a small spindle-shaped structure that lies directly subepicardially in the sulcus terminalis (Figure 2.18). In about 90% of people, the node is entirely lateral to the junction of the SVC with the right atrium, but in the remaining 10% it may pass
across the crest of the atrial appendage, thus sitting on the junction of crest and SVC in a horseshoe manner (Anderson etal., 1979). Because of its immediately subepicardial position, the node is particularly vulnerable to surgical trauma. The considerable variability found in the vascular supply to the node (Figure 2.19), together with the lesser variability in nodal position, compounds the vulnerability of the entire junction of the SVC with the right atrium during cardiac surgery. The nature of the right and interatrial myocardium, and its relationship to interatrial conduction, has been a controversial subject. Some authors have suggested that specialized tracts of conduction tissue extend through the walls of the right atrium and the atrial septum to connect the sinus node with the atrio-ventricular node. We have searched diligently for such tracts by serially sectioning the entire atrial chambers of infant hearts in a single block of tissue. Our results show that the tissue connecting the nodes is the plain atrial myocardium of the right atrial walls and the interatrial septum. There are no tracts of conduction tissue that are insulated from the myocardium in a manner comparable with that of the ventricular conduction pathways (Anderson et al, 1981). The geometrical configuration of the right atrium is such that its walls are divided into bands of muscular tissue by the venous orifices and the fossa ovalis. This atrial morphology more than adequately accounts for the routes of preferential conduction found in electrophysiological mapping. In our opinion, histologically specialized tracts do not exist between the nodes. The atrio-ventricular node, together with its transitional cell zones, is contained entirely within the triangle of Koch (Anderson and Ho, 2001; see Figure 2.16). If all incisions and procedures avoid this triangle, damage cannot be inflicted on the node except in the rare circumstances in which atrio-ventricular conduction occurs through an aberrant conduction system (see Chapter 8). The node, located within the interatrial septum, penetrates into the central fibrous body to become the penetrating atrio-ventricular bundle at the apex of the triangle. The bundle passes from the right atrial tissues into the left ventricular outflow tract on the ventricular aspect of the point of insertion of the tendon of Todaro into the central fibrous body. The detailed anatomy of the penetrating bundle and the ventricular conduction tissues are described in the sections devoted to the atrio-ventricular junction and the left ventricular outflow tract.
The left atrium is the most posterior of the four cardiac chambers. Like the right atrium, it has several components: a smooth-walled part that receives the pulmonary veins, an appendage with muscli pectinati on its inner surface, a smooth vestibule inserting to the left atrio-ventricular junction, a septal component, and then an obvious body that supports the other components (Anderson and Ho, 2000a). Figure 2.20 shows the dissected chamber as it may be viewed through a left thoracotomy. In contrast to the right atrium, there is no prominent muscle band between the smooth component and the pectinate part, and the appendage is much smaller and not as well trabeculated. Its shape is characteristic, being hooked and usually constricted at several points along its extensive but narrow length (Figure 2.11). The appendage extends around the left border of the pulmonary trunk, and is the only part of the left atrium to appear on the cardiac silhouette. The smooth-walled
roof of the left atrium receives a pulmonary vein at each of its four corners. Running inferiorly beneath the posterior surface, and occupying the left inferior aspect of the atrio-ventricular junction, is the coronary sinus, which drains to the right atrium. When a left SVC is present, it usually drains to the coronary sinus, extending downward between the left atrial appendage and the left pulmonary veins. More usually, the left SVC is not present, being represented by its vestige, the oblique vein of the left atrium, itself related to a fold of pericardium in the oblique sinus. Surgical access to the left atrium is usually obtained either through the atrial septum, in which case its confines must be remembered (Figure 2.17), or through the left atrium between the interatrial groove and right pulmonary veins. A discrete furrow exists between the entrance of the right pulmonary veins and the SVC. This furrow, known as Waterston's or Sondergaard's groove, forms the greater part of the superior rim of the fossa ovalis. It is no more than an infolding of the atrial roof and, if necessary, can be dissected to produce additional atrial tissue, as, for example, for use in the Senning procedure. The atrial roof itself is extensive between the entrances of the right and left pulmonary veins. Having opened the left atrium through an incision in either the septum or the roof, the surgeon is confronted with the vestibule of the mitral valve in the floor of the atrium, and with the much smaller orifice to the appendage on the left. The septal surface of the left atrium is not visible. When seen in isolation, the characteristic roughened aspect is, in reality, part of the anterior wall rather than the septum and overlies the transverse sinus. It is not the left atrial counterpart of the fossa ovalis.
Although the right and left ventricles exhibit important differences, they are comparable units. Each ventricle is composed of an inlet portion, which contains an atrio-ventricular valve, and is limited by the attachments of its tension apparatus, an apical trabecular portion, and an outlet portion, which supports the leaflets of an arterial valve. The atrio-ventricular and arterial valves similarly are of comparable structure in each ventricle, albeit that each valve possesses significant individual differences. Both sets of valves are complex structures. The leaflets are the primary units of the valve, meeting centrally when closed. The leaflets do not meet at their free edges; rather, they close at about one-third of the distance from the free edge to the attached margin. In the atrio-ventricular valves, the leaflets originate at the atrio-ventricular junction, in which fibrous tissue is formed to a variable extent, and which is usually termed the annulus (Angelini etal., 1988). Supporting the ventricular aspect of the leaflets are the chordae tendineae, which attach mostly to the so-called ''rough zones''. In some places, however, a single chorda divides in fan-like manner, and the subsidiary cords are then attached to the free edge of two adjacent leaflets. Each of the single chorda branching in a fanlike manner usually arises from the apex of a papillary muscle, and they tend to mark the ends of the zones of apposition between the leaflets. The chordae that are attached to the free edges of the leaflets may also branch in fan-like fashion to support the spaces between scallops in some of the leaflets. These are referred to as cleft chordae. In addition to the chordae supporting the free edges and rough zones, the leaflets are also supported by chordae that pass directly from the ventricular myocardium to the underside of the leaflets. These are the basal chordae. In contrast to the atrio-ventricular valves, the arterial valves lack any tension apparatus. The overall architecture, nonetheless, is surprisingly complex. The interrelationship between the aortic sinuses and the leaflets of the arterial valves is such that a marked discordance is produced between the haemodynamic and anatomical ventriculo-arterial junctions (Anderson, 2000; Figure 2.21). The haemodynamic junction is dictated by the attachment of the leaflets. This attachment is arranged in a semilunar manner, in which the attachment of each leaflet arises to the peripheral attachment of the zone of apposition with the adjacent leaflet, and sinks to the mid-point of the leaflet. As the attachment rises toward the sinutubular junction, it crosses the anatomical ventriculo-arterial haemodynamic ventrículo-arterial junction (semilunar)
/ artery as part of ventricle
arterial wall arterial wall
ventricle as part of great artery anatomical ventrículo-arterial junction (circular)
ventricle as part of great artery anatomical ventrículo-arterial junction (circular)
junction. This latter junction is the circle over which the fibrous wall of the arterial trunk is supported by the respective ventricle. As a consequence of this arrangement, three triangles of arterial wall, thinner than the adjacent walls of the arterial sinuses, are incorporated within the structure of the ventricular outflow tracts, and three scallops of ventricle are incorporated at the base of each arterial sinus (Figures 2.22 and 2.23). It follows from this complex geometrical arrangement that there is no collagenous ring, or annulus, supporting the semilunar leaflets of the arterial valves in any sense comparable with the way in which the leaflets of the atrio-ventricular valves are attached in a circular manner at the atrio-ventricular junction. The only ringlike structure at the ventriculo-arterial junction is the line of fusion of the fibrous arterial wall with the supporting ventricle, viz. the anatomical ventriculo-arterial junction (Anderson, 2000). It is the crown-like configuration of the attachments of the leaflets, nonetheless, that is referred to by surgeons as the ''annulus''.
This chamber forms the greater part of the anterior surface of the ventricular mass, extending from its more or less vertically positioned atrio-ventricular valve, the tricuspid valve, and running behind the sternum to the horizontal arterial valve, the pulmonary valve, at the left border of the cardiac base (Anderson etal., 2004). The characteristic morphological features of the right
ventricle, apart from the features of its atrio-ventricular valve, are that, first, the apical trabecular zone has coarse trabeculations, and second, the atrio-ventricular and arterial valves are separated by a prominent muscular shelf in the ventricular roof, the supraventricular crest (crista supraventricularis) (Figure 2.24). The inlet portion of the right ventricle extends from the junctional attachment of the leaflets of the tricuspid valve to the ventricular attachments of their papillary muscles. The funnel thus formed is more extensive inferiorly and laterally than septally and superiorly. The three leaflets of the tricuspid valve are positioned septally, anterosuperiorly and inferiorly. The zones of apposition separating the leaflets have anteroseptal, supero-inferior and inferior ends. The leaflets are illustrated in Figure 2.25, with the heart in anatomical position, but with wide retraction of the valve opened through its inferior commissure. When the surgeon approaches the valve through the right atrium, the orientation is altered (Figure 2.26). The anteroseptal zone of apposition is on the left, the inferior zone of apposition is on the right, and the septal leaflet extends between them. The anteroseptal zone of apposition is supported by the almost constant, but small, medial papillary muscle, known also as the muscle of Lancisi, which springs from the inferoposterior limb of the trabecula septomarginalis (Figure 2.27). A cleft extending to the area of the membranous septum is frequently seen in the septal leaflet of the valve (Figure 2.28). The area around both the cleft and the zone of apposition should be avoided at all costs because it is the location of the atrio-ventricular conduction tissues contained within the triangle of Koch (Figure 2.28). The morphology of anterosuperior and inferior leaflets and their zone of apposition is more variable. This zone of apposition is frequently supported by the prominent anterior papillary muscle, which originates from the apical body of the trabecula septomarginalis. In many hearts, however, this muscle supports the middle of the anterosuperior leaflet,
and a second, smaller, anterior papillary muscle supports the zone of apposition. The inferior zone of apposition is supported by a small inferior papillary muscle, but several other muscles spring like a sheaf from the apical ramification of the trabecula septomarginalis. When approached from the infundibulum, it is the anterosuperior leaflet of the tricuspid valve that is visible to the surgeon. This leaflet is supported superiorly and to the left by the medial papillary muscle and to the right by the anterior papillary muscle (Figure 2.27). A muscular ring demarcates the approximate limits of the ventricular inlet portion. In the ventricular roof, the ring is formed by the supraventricular crest, which represents the inner curvature of the heart wall (Vricella et al., 2004). Septally, the crest inserts between the limbs of the trabecula septomarginalis. The trabeculation is extensive, and is characteristic of the right ventricle. The anterocephalad limb of the trabeculation runs upward toward the attachments of the pulmonary valve, and the posterocaudal limb extends variably into the inlet portion, giving rise to the medial papillary muscle (pm in Figure 2.29), and other chordae that support the leaflets of the tricuspid valve. The body of the trabeculation runs apically, and ramifies at the apex into the leash of papillary muscles that support the inferior and septal leaflets of the tricuspid valve. It also gives rise to the prominent moderator band that crosses to the anterior papillary muscle, forming the effective floor of the muscular ring. The parietal rim of the ring is another prominent trabeculation, viz. the downward continuation of the crest opposite the trabecula septomarginalis. The trabecular zone of the ventricle extends from this muscular ring, confining the inlet portion toward the ventricular apex, and being characteristically coarse. It extends well up toward the infundibulum, and additional bands from the trabecula septomargionalis, the trabeculae septoparietales, cross freely through it. The infundibulum, or ventricular outlet portion, is the muscular funnel that supports the leaflets of the pulmonary valve and the wall of the pulmonary trunk. Its margin with the trabecular zone is indistinct. The walls become smoother immediately beneath the valve. When viewed from the anterior aspect, the posterior wall of the infundibulum is made up for the larger part by the supraventricular crest. A small part of the crest that inserts between the limbs of the trabecula septomarginalis can be identified by dissection to represent a true muscular outlet septum,
but this is of minimal dimensions in the normal heart (Figure 2.30) and cannot be identified with certainty without resorting to dissection. This anatomical structure, however, is of considerable importance with regard to controversies concerning the nature and description of the supraventricular crest. The crest in the normal heart is the extensive muscular structure that separates the attachments of the leaflets of the tricuspid and pulmonary valves in the roof of the right ventricle (Figure 2.27). Some authors categorize the trabecula septomarginalis (sm in Figure 2.30) as the ''septal band'' of the normal crest, but the trabeculation is purely a septal structure, and is in no way ''supraventricular''. The crest itself has two distinct components that are discrete structures only in malformed hearts. The larger part, the ventriculo-infundibular fold, represents the inner curvature of the heart wall (Figure 2.30). Incisions through this part take the surgeon outside the heart. Only a very small part of the crest, the muscular outlet septum, is located between the subpulmonary and subaortic outflow tracts.
The leaflets of the pulmonary valve are delicate structures attached in a semilunar way in part to the muscular wall of the infundibulum and in part to the wall of the pulmonary trunk. This arrangement (Figure 2.22) means that, as in the aortic root, part of the right ventricular muscular infundibulum is incorporated within the arterial sinus, while triangles of arterial wall are included within the ventricular outlet. Furthermore, careful dissection (Figure 2.31) reveals that the entire subpulmonary infundibulum is a free-standing structure that can be removed in one piece from the ventricle
without disturbing the musculature of the left ventricle (Merrick et al., 2000). This has important consequences concerning the morphology of the ventricular septum (to be described). When the sinuses of the pulmonary trunk, and the leaflets of the pulmonary valve that they contain, are named and described, advantage can be taken of the fact that two sinuses of the pulmonary trunk almost always ''face'' the sinuses of the aorta that give rise to the coronary arteries. These sinuses can, therefore, be termed the right and left facing sinuses. The zone of apposition between the leaflets that they support is almost always opposite the zone of apposition between the corresponding facing leaflets of the aortic valve. The other leaflet of the pulmonary valve is thereby described as the non-facing leaflet, and is supported by the non-facing sinus.
The left ventricle is located almost entirely behind the right ventricle; only its most leftward rim is visible on the cardiac silhouette. The major features that differentiate it from the right ventricle (Figures 2.24 and 2.32) are that: first, the apical trabecular zone of the left ventricle has characteristically fine trabeculations; second, the leaflets of its atrio-ventricular and arterial valves are in fibrous continuity, whereas in the right ventricle the supraventricular crest separates them; third, the septal surface is smooth, lacking any structure comparable with the trabecula septomarginalis of the right ventricle. The inlet portion of the left ventricle extends from the atrio-
ventricular junction to the attachment of the papillary muscles, and contains the mitral valve. This valve has two major leaflets, separated by the solitary zone of apposition between them, and is supported by paired papillary muscle groups (Kanani and Anderson, 2003). Although usually described as being ''anterolateral'' and ''posteromedial'', in reality they are positioned anteroseptally and posterolaterally. Surgeons usually gain access to the mitral valve through the left atrium behind the interatrial groove, via the right atrium and the atrial septum, or superiorly through the left atrial roof. When seen through any of these approaches, the aortic, or anterior, leaflet is superior and to the right of the mural, or posterior, leaflet (Figure 2.33). The aortic leaflet, so-called because it is in fibrous continuity with the leaflets of the aortic valve, occupies only one-third of the circumference of the mitral orifice, but the leaflet is much deeper than the mural one, so that the area occupied by the leaflets is about the same. The mural leaflet is usually divided into a series of mini-leaflets, or scallops, by marked slits. Three scallops are usually seen, along with two further components adjacent to the ends of the zone of apposition between the leaflets. The inferoseptal end of the zone of apposition, along with its so-called commissural leaflet, abuts against the atrial septum. This is the area of the valve most vulnerable to surgical trauma because the atrio-ventricular (av in Figure 2.33) node and bundle occupy the septum at this point. The attachment of the valve laterally may be closely related to the left coronary artery, a feature that is discussed in the section on the atrio-ventricular junction. The apical trabecular zone extends from the attachments of the papillary muscles, which are adjacent when the heart is unopened, to the ventricular apex. The trabeculations are characteristically fine, and a moderator band is not found. The outflow tract of the ventricle extends from the apical trabecular component to the attachments of the leaflets of the aortic valve. The margins between the apical trabecular and outlet portions are indistinct.
Posteriorly, the aortic leaflet of the mitral valve separates the inlet and outlet portions. Laterally, the left septal leaflet bundle branches av bundle septal leaflet bundle branches av bundle
scallops of mural leaflet postera-mediaI
scallops of mural leaflet postera-mediaI
commrsure margin of the inner heart curvature supports the aortic valve, and the ventricular parietal wall forms the margin of the outflow tract, merging anteriorly with the ventricular septum. The septal surface of the outflow tract is characteristically smooth. If the surgeon requires direct access to the outflow tract through the anterior wall, as in corrected or complete transposition, care must be taken not to damage the papillary muscle when the ventricular incision is made. A discrete plane usually exists between the septum, marked by the anterior descending coronary artery, and the supero-lateral papillary muscle, through which good access can be gained to the outflow tract (Figure 2.34). The leaflets of the aortic valve, forming the distal margin of the left ventricular outflow tract, have partly muscular and partly fibrous support. The leaflets are supported over half their circumference by muscular tissues, viz. by the ventricular septum anteriorly and by the ventricular free wall and inner heart curvature laterally. This can be seen by transecting the outflow tract and viewing its anterior part from behind (Figure 2.35a). The rest of the circumference of the leaflets is supported by fibrous tissue, specifically the membranous septum and right fibrous trigone, together making up the central fibrous body, and the area of continuity between the leaflets of the aortic and mitral valves, with its thickened leftward extremity, forming the left fibrous trigone. This is best demonstrated by viewing the posterior part of the transected outflow tract from the front (Figure 2.35b). The three leaflets of the aortic valve are named the right coronary, left coronary and non-coronary leaflets. When the aortic valve is approached through the aorta, as seen
by the surgeon, the left coronary leaflet is on the left, the right coronary leaflet is anterior, and the non-coronary leaflet is posterior and to the right (Figure 2.36). The zone of apposition between the leaflets arising from the two aortic sinuses giving rise to the coronary arteries always faces a zone of apposition between the leaflets of the pulmonary valve (PV in Figure 2.36). The right coronary leaflet extends from this point to the area of
the central fibrous body. The peripheral attachment of the zone of apposition between it and the non-coronary leaflet usually overlies the penetrating atrio-ventricular bundle. The non-coronary leaflet, at its attachment to the membranous septum, is in potential connexion with the right atrium and right ventricle. The attachment of its zone of apposition with the left coronary leaflet is normally positioned along the area of fibrous continuity, but this positioning is variable. At the peripheral attachment of the zones of apposition, the outflow tract is tented up, and the attachments reach onto the wall of the aorta. As in the right ventricle, this arrangement incorporates part of the arterial wall within the ventricular outflow tract. This is well shown by a dissection in which the left atrium and the tubular part of the aorta are removed (Figure 2.37). The tip of the tent in the area of fibrous continuity is separated from the transverse sinus of the pericardium. Aneurysms in this area can herniate directly into the pericardial cavity. Of equal significance, the apices ofthe other zones of apposition extend upward, so that the interleaflet triangles are again in potential continuity with the pericardial cavity. The site of the triangle separating the non-coronary and right coronary leaflets of the aortic valve is particularly important because it incorporates the area of the membranous septum. This, in turn, overlies the point of penetration of the atrio-ventricular conduction axis (Figure 2.38). When approached from the aorta, therefore, the area most vulnerable, from the standpoint of the conduction tissues, to surgical trauma is the area related to the zone of apposition between the right coronary and non-coronary
leaflets. The apex of the zone of apposition between the facing, or coronary, leaflets of the aortic valve is also related to the outside of the heart, specifically to an extensive tissue plane separating the front of the aorta from the back of the pulmonary trunk (Figure 2.39). The extent of this interleaflet triangle is best seen anatomically after the subpulmonary infundibulum is removed from the right ventricle.
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