13 Atrioventricular Septal Region
The anatomy of the base of the ventricular mass is markedly complex. Several important structures, including the hinges of the leaflets of the cardiac valves, the inferior pyramidal space, the membranous septum, the basal components of the atrial septum, the paraseptal areas, the atrioventricular conduction axis, and the arterial roots are all concentrated around or within the base of the ventricles. 1 , 2 , 3 , 4 , 5 , 6 , 7 Even with the heart in one’s hands, it can be remarkably difficult to provide a simple description of the relationships of the various components. For example, because the hinge of the septal leaflet of the tricuspid valve is located apically relative to the mitral valvar attachment, the cavity of the right atrium faces that of the left ventricle. There is, therefore, seemingly an extensive atrioventricular septal area. 1 In fact, because a cranial extension of the inferior atrioventricular groove interposes between the right atrial wall and the crest of the muscular ventricular septum, the larger part of the area is a sandwich rather than a true septum. 7 It has recently been shown that these subtle relationships are better illustrated using virtual dissection of three-dimensional datasets obtained during life using computed tomography (CT). 8 , 9 , 10 Such virtual dissections also place the related structures into the heart as seen in the living individual, providing attitudinally appropriate images. 11 , 12 In this chapter, we use such datasets to show the precise anatomical relationships of all these cardiac components. We begin, however, by illustrating the advantages to be gained in the clinical setting by describing the components using attitudinally appropriate nomenclature.
Anatomical Arrangement of the Base of the Ventricular Mass
The base of the ventricular mass surrounds and supports the atrioventricular and ventriculoarterial junctions (Fig. 13‑1). Within the roof of the left ventricle, the atrioventricular and ventriculoarterial junctions are adjacent to one another, sharing a common area of aortic-to-mitral valvar continuity. The aortic root is deeply wedged between the orifice of the mitral valve and the basal extent of the muscular ventricular septum (Fig. 13‑1 , Fig. 13‑2). Within the right ventricle, in contrast, the atrioventricular and arterial valves are supported by discrete and separate junctions, with the muscular subpulmonary infundibulum lifting the leaflets of the pulmonary valve away from the remainder of the ventricular base (Fig. 13‑1). When viewed from the right and from posterior, it can be seen that the inferior components of the atrioventricular junctions diverge from one another, with the inferior pyramidal space filling the area between the junctions (Fig. 13‑1). The inferior pyramidal space is an extracardiac space separating the atrial and ventricular myocardial components (Fig. 13‑3). It is filled with epicardial fibroadipose tissue. The anteroinferior muscular buttress of the atrial septum provides the cranial and posterior margin of the inferior pyramidal space and the right atrial vestibular myocardium continues apically to overlap the crest of the muscular ventricular septum. At the apex of the inferior pyramidal space, the orifices of the atrioventricular valves and the aortic root are in conjunction (Fig. 13‑1 , Fig. 13‑4). This area, known as the central fibrous body, is made up of the right fibrous trigone and the membranous septum (Fig. 13‑4). The right fibrous trigone is the rightward end of the area of fibrous continuity between the leaflets of the mitral and aortic valves (Fig. 13‑4). The central fibrous body produces the most obvious and strongest part of the so-called cardiac skeleton. Understanding its relations is key to the appreciation of the overall arrangement. The hinge of the septal leaflet of the tricuspid valve divides the membranous septum into its atrioventricular and interventricular components (Fig. 13‑5). On the ventricular aspect of the tricuspid valvar hinge, and inferior to the interventricular portion of the membranous septum, by virtue of the wedged position of the subaortic outflow tract, the basal part of the muscular ventricular septum interposes between the right ventricular inlet and the left ventricular outlet (Fig. 13‑2). The atrioventricular conduction axis 13 , 14 , 15 , 16 , 17 is carried on the crest of this part of the muscular ventricular septum, with the wedged location of the aortic root making it possible for the penetrating part of the axis to extend directly from the apex of the inferior pyramidal space to the septal crest (Fig. 13‑6).
The Cardiac Valvar Attachments
When describing the location of the hinges of cardiac valves within the ventricular base, we prefer to use the term “attachment” instead of the more popular “annulus,” since there are no discrete anatomical rings to be found encircling the orifices of the arterial valves. In recognition of common usage, nonetheless, when labeling our images, we have used “annulus” to describe the attachments of the leaflets of the mitral valve. Even the rings representing the attachments of the atrioventricular valves, however, are no more than the valvar hinges, with fibroadipose tissues occupying the larger parts of the so-called atrioventricular valvar annuluses. The reconstructions made possible by analysis of the three-dimensional datasets, furthermore, demonstrate the inadequacy of illustrating the attachments of all four valvar leaflets in the same plane. As is shown, the attachments of the leaflets guarding all four valvar orifices occupy markedly different planes (Fig. 13‑1). The attachments of the arterial valvar leaflets reconstruct in the form of three-pointed coronets, rather than little rings. The pulmonary valvar coronet is located most superiorly, having a marked obliquity relative to the aortic valvar coronet. The hinges of the mitral and tricuspid valvar leaflets, in contrast, do produce ring-like configurations. They are positioned close together in relatively planar fashion, although the mitral valvar hinges produce a saddle-like configuration, and the tricuspid valvar attachment is located slightly inferiorly and apically (Fig. 13‑7). It is the resulting inferior gap between the mitral and tricuspid valvar attachments that encloses the inferior pyramidal space, with the membranous septum at its apex (Fig. 13‑7).
As we have already emphasized, the aortic root is deeply wedged between the mitral valvar orifice, the muscular ventricular septum, and the tricuspid valvar attachment (Fig. 13‑7). The basal part of the ventricular septum, therefore, is interposed between the right ventricular inlet and left ventricular outlet (Fig. 13‑2). Even though the leaflets of the aortic and mitral valves are in fibrous continuity, the planes of the two valvar orifices are obtuse rather than parallel (Fig. 13‑1), producing the so-called aortoseptal angle between the aortic root and the left ventricle, as routinely visualized in echocardiography. 18 It is the fibrous continuity between the leaflets of the aortic and mitral valves that makes up the roof of the left ventricle (Fig. 13‑4), with the ends of the fibrous area thickened to form the right and left fibrous trigones. The leaflets of the tricuspid valve are hinged from the right atrial vestibule at all points except where the septal leaflet crosses the membranous septum (Fig. 13‑5 , Fig. 13‑7). As we have discussed, both valvar orifices are relatively planar, although that of the mitral valve is somewhat saddle-shaped (Fig. 13‑7). This shape of mitral ring is considered important in reducing the stress placed on the valvar leaflets during the cardiac cycle. 19 , 20
The Inaccurate Notion of a “Cardiac Skeleton”
It is the central fibrous body that constitutes the strongest part of the so-called cardiac skeleton. As we have shown, this structure is the combination of the membranous septum and the right fibrous trigone (Fig. 13‑4). Aside from the central fibrous body, there is no “skeleton.” Even the central fibrous body is part of the insulating tissues of the heart, rather than functioning as part of a “skeleton” in the sense of the musculoskeletal system. The notion of the so-called cardiac skeleton, therefore, has been greatly exaggerated. The hinges of the valvar leaflets are supported by the ventricular myocardium, rather than providing origins and insertions for the ventricular cardiomyocytes. It is the fibroadipose tissue contained within the atrioventricular junctions, furthermore, which serves to separate the atrial and ventricular muscular masses at all points except the penetration of the atrioventricular conduction axis. As far as the arterial valves are concerned, it is the semilunar attachments of the leaflets that conventionally are considered to represent part of the “skeleton.” These lines are no more than the hinges of the semilunar leaflets, with the free-standing subpulmonary infundibular sleeve serving to lift the leaflets of the pulmonary valve away from the cardiac base (Fig. 13‑1).
The Inferior Pyramidal Space
The triangular inferior pyramidal space is filled by the cranial extension of fibroadipose tissue from the inferior atrioventricular groove (Fig. 13‑7 , Fig. 13‑8). In certain individuals, during the fluoroscopic examination, it is sometimes possible to recognize the space as filled by an extensive fat pad. Its location can also be recognized in some patients during routine coronary angiography, when an inverted U-turn of the distal part of the right coronary artery gives rise to the artery of the atrioventricular node (Fig. 13‑7). The sides of the pyramid are bordered by the inferior rim of the atrial septum, the inferior rim of the ventricular septum, and the inferior attachments of the tricuspid and mitral valves (Fig. 13‑7 , Fig. 13‑8). As we have already established, the hinge of the septal leaflet of the tricuspid valve is positioned apically relative to the attachments of the mitral valve (Fig. 13‑1 , Fig. 13‑7). Between these two hinges, the septal component of the right atrial vestibule, overlapping the crest of the basal part of the ventricular septum, forms the atrial surface of the triangle of Koch (Fig. 13‑6 , Fig. 13‑9). And, as we have already discussed, this overlapping was previously considered to represent a muscular atrioventricular septum. 21 The area, however, cannot be removed without creating a communication with the extracardiac adipose tissue contained within the inferior pyramidal space. That is why the “septum” is best considered as an atrioventricular sandwich, with “meat” of the sandwich provided by the insulating tissues which interpose between the atrial and ventricular muscular layers (Fig. 13‑10).
The Membranous Septum
The atrioventricular component of the membranous septum, located at the apex of the inferior pyramidal space, where the cavity of the right atrium faces directly with the left ventricle, is a true atrioventricular septal structure (Fig. 13‑5 , Fig. 13‑11). Since the membranous septum, in most individuals, is crossed by the hinge of the septal leaflet of the tricuspid valve, its anterior component is an interventricular septal structure (Fig. 13‑5 , Fig. 13‑11 , Fig. 13‑12). The entirety of the membranous septum, when viewed from its left side, is seen to occupy the base of the triangle between the right coronary and nonadjacent aortic sinuses. It forms part of the medial wall of the left ventricular outflow tract (Fig. 13‑5 , Fig. 13‑12).
The Atrial Septum
At first sight, the atrial septum seems extensive, being made up of the oval fossa and its surrounding rims. Of these structures, however, it is only the floor of the oval fossa, derived from the primary atrial septum, and the anteroinferior muscular buttress, which anchors the oval fossa to the atrioventricular junctions, which are true septums (Fig. 13‑3 , Fig. 13‑13). Only these parts can be removed without exiting from the cardiac cavities. 22 The posterosuperior rim of the oval fossa is often described as the “secondary septum.” This is not the case, since the rim is no more than an extensive fold between the attachments of the caval veins to the right atrium and the right pulmonary veins to the left atrium (Fig. 13‑3 , Fig. 13‑13). The anterosuperior rim of the fossa is also a fold, separated by the transverse sinus of the pericardium from the nonadjacent aortic sinus, also known as the noncoronary aortic sinus, although very rarely it can give rise to a coronary artery. The nonadjacent aortic sinus, therefore, when images are viewed in the right anterior oblique projection, serves as a useful landmark in recognizing the anterosuperior margin of the atrial septum. The oval fossa itself has been measured as having average dimensions of 14.7 by 12.0 mm. 23 In up to one-third of normal individuals, there is persistent patency of the oval foramen, which is seen as a slit-like tunnel. 24 When assessed using cardiac CT in an American population, persistent patency of the foramen was found in just under one-quarter of individuals. 24 Assessment of autopsied specimens, in contrast, revealed prevalence of only one-eighth in the Japanese population. 25 A comparable assessment of autopsied specimens in North America had revealed probe patency in up to one-third, 26 so these differences in prevalence are likely to be real.
When viewing the anatomy in right anterior oblique fluoroscopy projections, note should be taken of the relations of an obvious radiolucent band to the orifices of the coronary sinus and the right coronary artery (Fig. 13‑14 a). This band is produced by the right atrioventricular groove. Its position is useful when locating the site of the right atrial appendage (Fig. 13‑14 c), and in distinguishing the right coronary from the nonadjacent aortic sinuses (Fig. 13‑14 b). These relationships are significant when assessing the progress of a catheter or pacing lead from the right atrium into the right ventricle (Fig. 13‑14 c).
The Atrioventricular Conduction Axis
The atrioventricular node is located at the apex of the inferior pyramidal space, with the atrial surface of the space being recognized in the opened heart as the triangle of Koch (Fig. 13‑6 , Fig. 13‑9). The location of the node can be determined by tracing the course of the nodal artery, which ascends in the insulating fibroadipose tissues of the inferior pyramidal space, 27 usually taking its origin from the distal right coronary artery (Fig. 13‑8). Having reached the central fibrous body, the conduction axis becomes insulated from the atrial myocardium as it enters the atrioventricular portion of the membranous septum. Although there are individual variations, 16 the nonbranching component of the conduction axis, having entered the subaortic outflow tract, is usually sandwiched between the inferior rim of the membranous septum and the crest of the muscular ventricular septum. The axis then gives rise to the left and right bundle branches, with the left branch cascading in fan-like fashion down the smooth left ventricular surface of the ventricular septum. 13 The right bundle branch, in contrast, is a thin cord-like fascicle, which passes through the muscular crest, surfacing in the right ventricle beneath the medial papillary muscle (Fig. 13‑6). A line drawn from the apex of the triangle of Koch to the medial papillary muscle serves as the landmark to the location of the conduction axis. 17 The inferior rim of the membranous septum, therefore, can serve as the landmark for electrophysiologists aiming to locate the His-bundle electrode, marking as it does the penetrating portion of the axis. When viewed in left anterior oblique projection, this site is seen inferior to the aortic root at the dimple of the medial tricuspid valvar attachment (Fig. 13‑15).