The Atrioventricular and Ventriculo-arterial Junctions: A Clinical Perspective for Electrophysiological and Structural Intervention. Part 2: The Ventriculo-arterial Junctions

As was emphasised in the introduction to the first part of this review, the junctions of the ventricular mass with the arterial roots are arguably more complex anatomically than are the junctions between the atrial and ventricular myocardial components. As was shown in the first part, although the leaflets of the atrioventricular valves are not supported by a continuous fibrous ‘annulus’, they are hinged in their junctions in relatively circular fashion. This is not the case when considering the components of the arterial valves that open and close to produce valvar competence. They are hinged in their supporting roots in semilunar fashion (Figure 1). When assessed in three dimensions, this produces a crown-like configuration. This arrangement creates significant problems when attempts are made to define an annulus. Further problems with nomenclature arise when considering the moving flaps of tissue that come together to close both the atrioventricular and the ventriculo-arterial junctions. These entities traditionally were described as ‘cusps’. A cusp, of course, is a point or elevation. The word is used by anatomists to describe the surfaces of the molar and premolar teeth. When the closed arterial roots are viewed from their ventricular aspect, it becomes possible to appreciate why anatomists might have thought it appropriate also to describe the moving parts as ‘cusps’ (Supplementary Figure 1). Such usage, however, has become compromised with the passage of time. Thus, some now suggest that it is only the leaflets of the arterial valves that should be nominated as representing ‘cusps’, with ‘leaflets’ reserved for description of the moving parts of the atrioventricular valves.¹ This is obviously less than satisfactory, given that one of the atrioventricular valves is itself said to be ‘tricuspid’. There is now a greater problem. Electrophysiologists, when seeking to cure outflow tract arrhythmias, frequently describe their procedures as ablating the valvar cusps. Even those who favour the distinction between leaflets and cusps have described the coronary arteries as arising from the ‘cusps’ of the aortic root.² All of these problems can be avoided simply by avoiding the use of ‘cusp.’³ The moving parts can then be described as leaflets in both the atrioventricular and arterial valves. For the arterial valves, the leaflets are then supported by the valvar sinuses.

The Complexity of the Anatomical Arrangements

Opening the pulmonary root illustrates the anatomical complexity of a seemingly simple structure. The semilunar hinges of the leaflets, which come together distally at the sinotubular junction, cross the anatomical junction between the distal extent of the myocardial right ventricular infundibulum and the arterial walls of the valvar sinuses (Figure 2). The myocardial–arterial junction, which represents the distal extent of the ventricular myocardium, can be shown by removing the leaflets to form an obvious ring. The junctions between the walls of the ventricles and the valvar sinuses they support extend much more distally beyond the myocardial–arterial junction. These fibrous distal walls separate the valvar sinuses, which come together at the sinotubular junction to form the distal extent of the root (Supplementary Figure 2). The points at which the leaflets, supported by their sinuses, join at the sinotubular junction are recognised as the valvar commissures. They can be joined together to form a discrete ring. The situation then becomes still more complicated, given that the walls of the valvar sinuses extend further distally than do the commissures themselves. Another virtual ring, described as the tubular ring, can then be created by joining together these distal margins of the sinuses.⁴ This means that the tubular ring, along with the part of the root recognised by echocardiographers as the valvar annulus, with an annulus itself being a little ring, have no anatomic counterparts (Figure 2). They are virtual entities. The myocardial–arterial junction, in contrast, which is a true anatomical ring, is not currently described as being an ‘annulus’. Still further confusion with regard to the use of ‘annulus’ is created when it is appreciated that many surgeons define the semilunar hinges of the leaflets themselves as the ‘annuli’ of both arterial valves. Indeed, the anatomical study that emphasised the significance of the tubular ring suggested that the semilunar hinges should be considered as the ‘true’ annulus of the pulmonary root.⁴ Irrespective of these linguistic niceties, there are no morphological entities that correspond either to the planes created by joining together the nadirs of the semilunar leaflets, or the zeniths marking the distal extent of the sinus walls (Figure 2). They are best described, therefore, as the virtual basal ring and the tubular ring.^4,5

As discussed above, the semilunar hinges of the valvar leaflets come together at the sinotubular junction as the valvar commissures (Supplementary Figure 3). The use of this word then creates yet another problem for the anatomist. This is because anatomists usually define commissures as the zones of apposition between adjacent structures, such as the commissures between the lips or the eyelids. In the setting of arterial valvar function, therefore, given that clinicians define the commissures for only the distal ends of the areas of conjunction between the leaflets, it is necessary to describe the remaining apposing surfaces as the zones of apposition. They come together in a snug fashion in each arterial root when the leaflets are closed, meeting at the valvar centroid (Supplementary Figure 3). It should not then be forgotten that, at the base of each of the sinuses supporting a leaflet of the pulmonary valve, there is a myocardial crescent. Only two such myocardial crescents are to be found in the aortic root. These myocardial crescents are the remnants of the outflow myocardium that, during development, supported the entirety of both of the developing arterial roots (Supplementary Figure 4A).⁶ During cardiac development such myocardium initially extends to the margins of the pericardial cavity. With the formation of the intrapericardial arterial trunks, the level of myocardium regresses to what will become the sinotubular junction.⁷ During ongoing embryonic and later fetal life, there is then continuing attenuation and regression of the myocardium supporting the epicardial aspect of the developing arterial valvar sinuses. Remnants of this epicardial myocardium can still be recognised postnatally in the pulmonary root (Supplementary Figure 4B). The myocardium found uniformly at the bases of all three pulmonary valvar sinuses (Figure 2 and Supplementary Figure 2), and in two of the sinuses of the aortic root (Supplementary Figure 5), shows that the regression of the outflow myocardium does not reach the level of the virtual basal ring. It is the myocardium remaining at the bases of the sinuses that is a target of electrophysiologists as they seek to ablate outflow tract arrhythmias.^8,9 Whether the myocardium remaining in the epicardial tissues could also serve as a substrate for the arrhythmias remains to be determined.¹⁰ The finding of the myocardium incorporated at the bases of all three sinuses of the pulmonary root reflects the fact that, in the postnatal heart, the root itself is lifted away from the base of the ventricular cone by the free-standing infundibular sleeve of the right ventricle. This means that, in the postnatal heart, there is no ‘outlet septum’. The sinuses of the pulmonary root adjacent to the aortic root are separated from the aortic valvar sinuses by extracavitary fibroadipose tissue (Supplementary Figure 6).

The Aortic versus the Pulmonary Root

The uniform inclusion of myocardium in the bases of the valvar sinuses as seen in the pulmonary root is not the case for the aortic root. It is only the bases of the two aortic valvar sinuses that give rise to the coronary arteries that contain myocardium (Supplementary Figure 5). A significant part of the base of the root is supported by the fibrous continuity that exists between the non-coronary and right coronary leaflets of the aortic valve and one of the leaflets of the mitral valve. It is this feature that permits the involved leaflet of the mitral valve to be described as being aortic. On occasion, the aortic root may be supported by a complete muscular infundibulum, but this is rare.¹¹ It is presumably for this reason that it is even rarer to find accessory myocardial pathways in relation to the aortic leaflet of the mitral valve. It is of note, nonetheless, that patients with cardiomyopathy have been observed to retain myocardium in the inner heart curvature, separating the hinges of the leaflets of the aortic and mitral valves.¹² Our own preliminary investigations are in keeping with this finding (Supplementary Figure 7).

The fibrous parts of the root adjacent to the septum are the most important because they constitute the so-called central fibrous body. In most instances, part of this fibrous area is produced by continuity between the leaflets of the mitral and tricuspid valves. This area forms the roof of the infero-septal recess of the left ventricular outflow tract.¹³ It is not an integral part of the aortic root. Another part of the fibrous support at the base of the root, however, is provided by the membranous septum, which is another part of the central fibrous body.

As was the case for the pulmonary root, the proximal boundary of the aortic root is the virtual basal plane that can be constructed by joining together the nadirs of the semilunar hinges of the aortic valvar leaflets (Supplementary Figure 1). Again, in keeping with the arrangement found in the pulmonary root, the sinotubular junction forms its distal boundary. Not surprisingly, therefore, the junction is not a planar entity. A tubular ring can also be identified in the aortic root. It is created by joining the zeniths of the walls of the valvar sinuses (Figure 3).¹⁴ When the aortic root is assessed in its opened state in the autopsy room, fibrous interleaflet triangles are much more obvious when compared with the triangles seen in the pulmonary root.¹⁵ During development, the epicardial surfaces of all of the triangles include the outflow tract myocardium. Removal of the fibrous walls of the triangles shows that, subsequent to regression of the myocardium, a potential communication is produced between the extensions of the left ventricular outflow tract and extracavitary spaces (Figure 3). The triangles between the non-coronary sinus and its neighbours both open potentially to the transverse sinus of the pericardium (Figure 3A and B). Opening the triangle between the two sinuses giving rise to the coronary arteries, in contrast, would create a communication with the extracavitary area of tissue separating the free-standing infundibular sleeve from the aortic root (Supplementary Figure 6 and Figure 3C).

It is the semilunar hingeing of the leaflets in the arterial valvar sinuses that permits them to come together snugly under the pressure of the column of arterial blood to ensure valvar closure during ventricular diastole (Supplementary Figure 2). When closed, the centroid of the valvar skirt is well below the level of the sinotubular junction (Figure 4). The ability, when using CT images, to measure the dimensions of the leaflets, the features of their coaptation, and their relationship to the overall arrangement of the aortic root (Figure 5), has now revolutionised the approach to surgical repair of the aortic valve.^16,17 Such multimodality imaging is similarly critical for those contemplating transcatheter replacement of the aortic valve. It permits insertion of the correct device and guides the most appropriate route for access.^18,19

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