The Atrioventricular and Ventriculo-arterial Junctions: A Clinical Perspective for Electrophysiological and Structural Intervention. Part 1: The Atrioventricular Junctions

Recent advances in transcatheter approaches for both electrophysiological and structural intervention have underlined the importance of the atrioventricular and ventriculo-arterial junctions of the heart. Tachycardias may originate from, and be ablated in, the atrioventricular junctions, both tricuspid and mitral.^1,2 A propensity for fibrosis to develop at the base of the heart has been demonstrated, and ‘annular’ ventricular arrhythmias can potentially be an early manifestation of cardiomyopathy related to lamin A/C (LMNA), titin (TTN) and desmoplakin (DSP) gene variants.² Catheter ablation, especially in young patients, might reduce the risk of malignant arrhythmia in the case of arrhythmogenic prolapse of the leaflets of the mitral valve.^3,4 The majority of outflow tract arrhythmias arise from the ventriculo-arterial junctions of the pulmonary and the aortic roots.^5,6 Epicardial or endocardial arrhythmic foci may also be detected in the left ventricular summit and the area of the cardiac crux.^7–9 In some patients with paraseptal superior or crestal pathways, the successful site of ablation can be in the right coronary or non-coronary aortic valvar sinuses, which lie in close anatomical proximity to the superior part of the muscular ventricular septum.¹⁰ Furthermore, a quantitative assessment of the atrioventricular and ventriculo-arterial anatomy is critical for the implementation of transcatheter implantation, a technique that is now becoming feasible for all four valves.^11–17

In a series of two reviews, we revisit the junctional areas from the perspective of their anatomy. This is the first part, devoted to the atrioventricular junctions.

General Principles

The extent of the ventricular septum and its components remains controversial. It can be described simply as having muscular and membranous components, but part of the membranous septum is usually atrioventricular rather than interventricular. And, relative to these septal components, the ventricular cavities and their valves spiral within the overall ventricular mass. These ventricular interrelationships become more significant when considering the junctions between the ventricular cavities and the cavities of their appropriate atrial partners, and with the intrapericardial arterial trunks. These junctions together occupy the base of the ventricular mass. The leaflets of both the atrioventricular and arterial valves are supported within the junctions, but not in comparable fashion.¹⁸ This is because the atrioventricular junctions are both a union and a separation between two myocardial components, namely the atrial and ventricular myocardial walls. The ventriculo-arterial junctions are more complex, given that their larger parts are composed of a union between the fibrous ventricular interleaflet triangles and the arterial walls of the valvar sinuses of the arterial roots.

Reflecting this arrangement, the semilunar hingeing of the arterial valvar leaflets extends distally beyond the margins of the ventricular myocardial cone. This means that there is a fundamental difference at the ventriculo-arterial junctions between the myocardial–arterial interfaces and the lines that form the distal extent of the ventricular cavities. This complexity is not currently clarified by popular drawings, many of which show a fibrous ‘skeleton’ within the junctional areas, which is presumed to provide the support for the leaflets of all of the cardiac valves.¹⁹ The skeleton thus depicted is often presumed also to provide anchorage for the myocardial ventricular walls.²⁰

In reality, the concept of well-formed fibrous rings, or ‘annuli’, within the junctional areas is grossly exaggerated.²¹ Simple dissection of the base of the ventricular mass points to the spurious nature of the drawings (Figure 1 ). Cardiac muscle, furthermore, cannot be compared to skeletal muscle.²² The cardiomyocytes comprising both the atrial and ventricular walls are attached to each other. They are aggregated together to produce a 3D mesh within an overall supporting fibrous matrix. The valvar leaflets are then hinged in various ways from the crest of the ventricular cone or, in the case of the pulmonary root, from the free-standing infundibular myocardial sleeve. The atrioventricular junctions, furthermore, contain the major components of the cardiac vasculature, such as the major coronary arteries, and the coronary sinus and its venous tributaries.^23,24 They also harbour the accessory myocardial connections that provide the substrates for the Wolff-Parkinson-White and Mahaim variants of ventricular pre-excitation.²⁵

Atrioventricular Junctions

The anatomist tends to view the base of the ventricular cone from its atrial aspect (Figure 1 ). Such assessment shows the central location of the aortic root, which is wedged between the right and left atrioventricular junctions. It also shows how the pulmonary root is lifted away from the base of the myocardial cone, placing the leaflets of the pulmonary valve in anterior and left-sided position, despite being supported by the right ventricle (Figure 1 and Supplementary Figure 1A). Assessment from the ventricular aspect (Supplementary Figure 1B) confirms the extent of the subpulmonary infundibulum. It also clarifies the presence of the significant infero-septal recess of the left ventricular outflow tract.²⁶ This space interposes between the inferior part of the muscular ventricular septum and the overall skirt of mitral valvar leaflet tissue. The view obtained by the anatomist is produced by removing the walls of the atrial chambers and by separation of the arterial roots from the intrapericardial arterial trunks at their sinutubular junctions (Figure 1 ). Attention to the trunks themselves shows that, when traced distally from the sinutubular junctions, they spiral as they extend to the margins of the pericardial cavity.

Assessment of the base of the ventricular mass from its ventricular aspect demonstrates the importance of the attitudinally appropriate approach when naming the leaflets of the atrioventricular valves. Even now, most clinicians continue to describe the valvar leaflets using the Valentine approach. When using this inappropriate approach, the leaflet of the tricuspid valve supported by the diaphragmatic component of the right atrioventricular junction is deemed to be ‘posterior’.²⁷ Structures that are posterior, however, are those that are closest to the spine. The images now available to clinicians directly reveal the relationship of the cardiac components not only to the sternum and the spine, but also to the diaphragm (Supplementary Figure 2A). Such assessment of the junctions, as shown in Supplementary Figure 2 using CT, shows the septal leaflet of the tricuspid valve to be closest to the spine, and hence posterior. The leaflet currently described as being ‘posterior’ is supported by the diaphragmatic margin of the junction. As with the diaphragm, therefore, it is obviously inferiorly positioned. It should be named as being inferior. The third leaflet of the valve is then located antero-superiorly (Supplementary Figures 1B and 2A).

The leaflets of the mitral valve are conventionally described as being anterior and posterior.²⁸ This is reasonably accurate. The leaflet that is located towards the spine, however, wraps itself around the other leaflet (Supplementary Figure 3). This feature, coupled with the obliquity of the valve relative to the body coordinates, offers the alternative approach of naming the leaflets as being aortic and mural. Labelling in this fashion then emphasises the significance of their support within the left atrioventricular junction (Figure 1 ). The leaflet of the mitral valve labelled as being ‘aortic’ receives this title because of its continuity with the non-coronary and left coronary leaflets of the aortic valve (Supplementary Figure 3). Should the ventriculo-arterial connections be discordant, the leaflet would be described as being pulmonary. It is easier to provide an accurate adjective for the other leaflet of the valve, which guards a much greater part of the circumference of the left atrioventricular junction (Figure 1 ). It is mural. The thickened rightward end of the area of continuity between the aortic leaflet of the mitral valve and the leaflets of the aortic valve, described as the right fibrous trigone, is part of the area within the junctions that is formed of fibrous tissues. Called the central fibrous body (Figure 1 ), this area is interpreted conventionally as being composed only of the right fibrous trigone and the membranous septum.^12,20 A significant contribution to the area is also provided by fibrous continuity between the leaflets of the mitral and tricuspid valves. This part of the central fibrous body provides the support for the antero-inferior buttress of the atrial septum (Figure 2).

The expansion of the atrioventricular junctions during the fetal period establishes the definitive arrangement of the area frequently described as the cardiac ‘crux’ (Figure 3). During the process of cardiac septation, the primary atrial septum, which separates the atrial cavities, grows caudally towards the developing muscular ventricular septum. The two septal components come together at the level of the developing atrioventricular junctions. Relative to the plane of the atrioventricular cushions, they form an obvious cross, hence the naming of this part as the ’crux’. The crossing point of the septal components and the junctions becomes distorted during the fetal period, concomitant with inferior expansion of both the developing right and left atrioventricular junctions, and the incorporation of the aortic root into the developing infero-septal recess of the left ventricular outflow tract (Supplementary Figure 2A). When seen in the four-chamber projection in the adult heart, there is then a marked offsetting of the planes of the atrial and ventricular septa (Figure 2). As part of the change produced by expansion of the atrioventricular junctions during fetal life, the atrial wall of the triangle of Koch similarly expands over the underlying ventricular walls to incorporate the fibrofatty epicardial tissues, thus producing the inferior pyramidal space (Supplementary Figure 4). From the stance of the anatomist, it is the epicardial boundary of the space as seen on the diaphragmatic surface of the heart that represents the crux (Figure 3). When viewed in this fashion, the site of the inferior component of the muscular ventricular septum is marked by the inferior interventricular artery and the middle cardiac vein. Electrophysiologists, however, when assessing the focus of ventricular arrhythmias, describe the presence of ‘basal’ and ‘apical’ components of the crux, suggesting that these can be differentiated according to their relationship to the two vascular structures.^2,8,29 It is difficult to understand the logic of this categorisation, more so given that the entirety of the ventricular border of the inferior pyramidal space is part of the basal surface of the ventricular cone.

Forming the atrial walls of the pyramidal space, the vestibules diverge markedly when traced inferiorly from the central fibrous area. They can be removed to reveal the ventricular surface of the space (Figure 1 and Supplementary Figure 5B). This ventricular component, made up of the diverging inferior ventricular walls, has usually been considered to be part of the septum. It has been labelled as the ‘posterior septal process’.³⁰ Sections taken across the base of the left ventricle, however, show that the ventricular myocardium in question is neither septal nor posterior. It is the diverging inferior wall of the left ventricle (Supplementary Figure 4). It is in this area that echocardiographers observe the characteristic ‘offsetting’ of the leaflets of the mitral and tricuspid valves. At no point, however, are the leaflets hinged from either side of the muscular ventricular septum. They are separated from each other by the myocardial floor of the inferior pyramidal space, with the leaflets of the mitral valve additionally lifted away from the septal surface by the extension of the left ventricular outflow tract now known to form the infero-septal recess.²⁶ Hence, the true offset seen at the so-called crux is between the planes of the atrial and the ventricular septal structures (Figure 2).

It is because of the presence of the fibroadipose epicardial tissues in the walls of the inferior pyramidal space that accessory muscular atrioventricular connections found in this area are correctly described as being paraseptal rather than septal.³¹ They are conventionally described as being located posteriorly. When assessed in attitudinally appropriate fashion, the pathways are located inferiorly. In the same way, pathways currently supposed to be ‘antero-septal’ are located superiorly and in paraseptal location.³² They are directly adjacent to the right coronary sinus of the aortic root (Supplementary Figure 6A). The only septal component of the atrioventricular junctions is the atrioventricular component of the membranous septum, although this is often overlaid by vestibular myocardium of the right atrium. In such circumstances the septum is fibromyocardial rather than purely fibrous. This area includes the atrioventricular conduction axis itself at its inferior margin (Supplementary Figure 6B). It is unlikely that true septal accessory muscular connections could exist in this area. Those previously labelled as being ‘mid-septal’ are better considered to be mid-paraseptal (Supplementary Figure 6A).³³

It follows that appreciation of the composition of the atrioventricular junctions provides the basis for understanding the arrangement of the various pathways producing the different forms of pre-excitation. Neither junction contains a well-formed and continuous ‘annulus’ supporting the hinges of the leaflets of the atrioventricular valves (Figure 1 ). Throughout the larger parts of the circumference of the right junction, for example, it is the epicardial fibroadipose tissues that separate the atrial vestibular myocardium from the underlying summit of the right ventricular walls. In areas within the circumference it is possible, in the same heart, to find shelves of fibrous tissue separating the atrial and ventricular walls. Such ‘disjunction’ is part of the normal arrangement.^34,35 Such prominent disjunction between the atrial and ventricular components of the left atrioventricular junction has been suggested to be associated with mitral valve prolapse and sudden cardiac death.³⁶ The finding of abnormal rhythms, however, particularly when severe, is known to be associated independently with long-term excess mortality and lower event-free survival, irrespective of the presence or absence of mitral regurgitation and the severity of the so-called disjunction, which is a ubiquitous finding in the normal heart.^18,37 When excessive, it remains to be established whether the disjunction is the consequence of the valvar disease or vice versa.

When found in the right junction, the accessory pathways traverse within the fibroadipose tissues on the epicardial aspect of the hinges of the valvar leaflets. An additional feature of importance, nonetheless, is to be found in the right atrioventricular junction. This is the presence within the vestibular myocardium of remnants of the primary ring of conduction tissue found during embryonic development. This ring initially surrounded the primary interventricular foramen. With expansion of the atrioventricular junction, part of the ring is incorporated in the right atrial vestibule. It then encircles the parietal part of the orifice of the tricuspid valve. Most pathways for accessory conduction are made of working myocardium.³⁸ These pathways can traverse the epicardial tissues of the junction at any site from the hinge of the leaflet to the epicardial covering, and can be found anywhere in the parietal parts of the right junction. On occasion, however, the atrial origin of right-sided pathways can be formed by remnants of the initial embryonic primary ring (Figure 4A). These are the so-called ‘atriofascicular tracts’.³⁹ The ventricular parts of the tracts can insert into the right bundle branch. They are insulated as they join the ventricular walls, usually adjacent to the hinge of the antero-superior leaflet of the tricuspid valve at the acute margin of the right atrioventricular junction. The atriofascicular tracts conduct in decremental fashion. Such decrementally conducting tracts can also be found producing pre-excitation in the left atrioventricular junction.⁴⁰ Whatever is responsible for the decremental features of the left-sided pathways, it cannot be because they originate in remnants of the primary ring, given that the primary ring is related only to the right atrioventricular junction. The vestibular myocardium of the left atrioventricular junction, nonetheless, is derived from the embryonic atrioventricular canal. As such, it does have the capacity to delay atrioventricular conduction. During its development, the primary ring is terminated in a second nodal remnant formed within the base of the atrial septum. This is the retroaortic node.⁴¹ As yet, the function of the node during normal atrioventricular conduction has yet to be established. It is likely to be involved in some of the arrhythmias that are cured by ablation from the non-coronary sinus of the aortic root.⁴²

The pathways found in the left atrioventricular junction are mostly in its mural component. It was suggested for quite some time that the pathways passed through ‘gaps’ in the fibrous annulus of the mitral valve.⁴³ But, as with the right atrioventricular junction, there is no continuous ‘annulus’ supporting the mural leaflet of the mitral valve. As in the right junction, the accessory myocardial pathways extend between the atrial and ventricular muscle masses by running within the fibroadipose tissues of the atrioventricular groove (Figure 4B and Supplementary Figure 7 ).²⁸ The mural leaflet of the mitral valve, as was the case for the tricuspid valve, can in the same heart be hinged from a shelf of fibrous tissue, or directly from the crest of the ventricular wall. Irrespective of its composition, the pathways producing pre-excitation in the left junction skirt the hinge of the mural leaflet of the mitral valve (Supplementary Figure 7 ). As such, they can be closely related to the circumflex artery and the coronary sinus. The pathways would be particularly close to the circumflex artery when the artery itself is dominant, supplying the inferior interventricular artery and the artery to the atrioventricular node. Although the pathways are known to exist throughout the parietal left atrioventricular groove, pathways have sometimes been described in relation to the aortic leaflet of the mitral valve. In almost all individuals, this leaflet is in fibrous continuity with two of the leaflets of the aortic valve. Therefore, it is difficult to envisage how an accessory myocardial pathway could extend from the atrial to the ventricular myocardial masses in this area. The obvious explanation is that the pathways are found in those rare circumstance where the aortic root remains supported by the myocardium of the inner heart circumference. Such myocardium is present during the early stages of embryonic life, but attenuates during the initial stages of fetal development.⁴⁴ Should such myocardium persist postnatally, then it would provide a potential substrate for an accessory atrioventricular pathway.⁴⁵ It is also known that the accessory pathways can be associated with diverticuli of the coronary sinus.⁴⁶

Key components of both atrioventricular junctions are the vascular channels that supply and receive blood from the walls of the cardiac chambers. The blood supply to the walls is provided by the right and left coronary arteries.²³ These major arteries emerge from the sinuses of the aortic root adjacent to the pulmonary trunk (Supplementary Figure 5B). The right coronary artery enters the right atrioventricular groove directly above the supraventricular crest of the right ventricle. It then encircles the parts of the junction hingeing the antero-superior and inferior leaflets of the tricuspid valve. In nine-tenths of individuals, the right coronary artery is dominant, supplying the inferior interventricular artery and the artery to the atrioventricular node, and then continuing into the left atrioventricular groove to supply blood to the diaphragmatic wall of the left ventricle (Supplementary Figure 5B). In around half of individuals the first branch of the artery supplies the sinus node. At the acute margin of the junction it gives rise to the acute marginal artery. As it enters the inferior right atrioventricular junction it is accompanied by the small cardiac vein, with the venous tributary then draining to the coronary sinus (Figure 3).

The left coronary artery, having exited from the aortic root as a solitary vessel, enters the space directly behind the subpulmonary infundibulum, with the tubular left atrial appendage forming the roof of the space. This area of the ventricular base was named the left ventricular summit (Supplementary Figure 8). It received this name because, when viewed attitudinally, it is the most superior part of the myocardium making up the ventricular cone.⁴⁷ The crest of the left ventricular wall extending inferiorly and posteriorly from the area of the summit is the mural component of the left atrioventricular junction. As the left coronary artery enters this area, now described as the summit, it branches into the anterior interventricular and circumflex arteries, often giving off a third intermediate branch.

Also entering the summit is the great cardiac vein.⁴⁸ It ascends alongside the anterior interventricular artery to reach the summit, exiting to enter the inferior left atrioventricular groove, where it joins with the oblique vein of the left atrium to form the coronary sinus (Figure 5). The vascular components of the summit have become important, given that they can be catheterised so as to obtain access to the crest of the muscular ventricular septum. This part of the septum supports part of the left coronary leaflet of the aortic valve, with a varying extent of myocardium incorporated at the base of the left aortic coronary sinus.⁴⁹ As already discussed, the venous coronary sinus itself is an integral part of the left junction, along with the circumflex artery in the one-tenth of the population in whom it is dominant. And, on occasion, diverticuli of the sinus can be the substrates for Wolff-Parkinson-White syndrome.⁴⁶

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