Fig. 1.1
This specimen demonstrates the medial and anterior papillary muscles as well as the moderator band extending from the apical trabeculations in the right ventricle. The right ventricular outflow tract has a ‘Y’ shaped muscle bundle called the septomarginal trabeculation (SMT) . The muscle underneath the pulmonary valve is not part of the ventricular septum. This subpulmonary infundibulum can be dissected to liberate the pulmonary valve as in the Ross procedure. AS anterosuperior leaflet, S septal leaflet, PV pulmonary valve, VIF ventriculo-infundibular fold, SMT septomarginal trabeculation, M medial papillary muscle, Ant anterior papillary muscle, MB moderator band, Arrows subpulmonary infundibulum
Fig. 1.2
Viewed from the right atrial aspect the position of the triangle of Koch can be located and the close association of the atrioventricular node (AVN) to the hingeline attachment point of the tricuspid valve at the right atrioventricular annulus. The triangle of Koch is demarcated by dotted lines marking the tendon of Todaro . The solid line marking the tricuspid valve attachment. The third border is the location of the coronary sinus. The hatched oval marks the position of the central fibrous body and the heart shape indicates the location of the atrioventricular node. AS anterosuperior leaflet, S septal leaflet, I inferior papillary muscle, CS coronary sinus, OF oval fossa
Fig. 1.3
In short axis section the position of tricuspid and pulmonary valves can be seen. The two valves are separated by musculature. From the ventricular aspect the orifice of the three tricuspid leaflets can be seen in relation to the mitral valve. The muscular ventricular separating the two atrioventricular valves has been removed in this specimen but a thin part of the septum, the membranous septum, is revealed. AS anterosuperior leaflet, S septal leaflet, I inferior leaflet, PV pulmonary valve, Arrow membranous septum
The tricuspid annulus (or hingeline) within the right ventricle demarcates the junction between the right atrial myocardium and right ventricular myocardium with the attachment of leaflets at this intersection. Although the term ‘annulus’ confers the impression of a well forms fibrous ring-like structure separating atrial from ventricular myocardium, the leaflets of the tricuspid valve in reality are attached to a poorly formed fibrous annulus that is not as clear to delineate as with the mitral valve [1]. Even so, anatomically, the mitral annulus is also not as well defined as commonly thought [2]. Nevertheless, the term, ‘annulus’ continues to be used in the clinical arena in preference to ‘hingeline’. At the tricuspid annulus, musculature of the atrial wall overlaps minimally the atrial surface of the leaflets and the major portion is separated from ventricular wall by fibro-fatty tissues of the atrioventricular groove through which run the right coronary artery and branches of the coronary veins. While the initial course of the right coronary artery is relatively distant from the annulus, there is a gradual shortening of the distance to the endocardial surface toward the inferior segment of the annulus to <3 mm [3]. This has important implications for invasive intracardiac procedures whether it is to ablate accessory atrioventricular pathways for pre-excitation or to repair the tricuspid valve at the annular level.
The tricuspid orifice is larger than the mitral orifice [4] and in this low pressure environment the leaflets are thinner and more translucent [5]. The three dimensional shape of the tricuspid annulus changes not only during the dynamic low pressure cardiac cycle but during disease. This adds to the complexity of understanding the geometry of the valve and right ventricle. Anatomically, the right atrioventricular annulus is located in an almost vertical position and is oval in shape becoming round when dilated [6, 7]. The valve complex is saddle shaped forming a conduit to the right ventricle [8] although it is less saddle shaped than the mitral valve [9]. Owing to the near vertical location of the annulus, valve margins and the leaflets are described as anterosuperior, septal and inferior. There is variation in the annular conformation of the valve between systole and diastole [6] this change is significant and a reduction of 19% in annular circumference during systole has been found [10]. The tricuspid annulus has its highest point at the anteroseptal commissure and its lowest point at the posteroseptal commissure [11]. With enhanced imaging more precise anatomical description may need to be employed to describe the valve during the cardiac cycle.
The normal dimensions of the tricuspid annulus in men and women, have been determined in various clinical studies (3.15 ± 0.4 cm diastolic diameter in males and 3.01 ± 0.47 cm diastolic diameter in females) [12] as well as pathological studies of normal hearts where the annular circumference for males was 11.4 ± 1.1 cm and 10.8 ± 1.3 cm for females [13]. This analysis of the normal morphology is essential in diagnosing altered pathophysiology, to establish if dilatation of the annulus is progressing to regurgitation and to appreciate that the annulus dimensions change during the cardiac cycle. In experimental studies during sinus rhythm the annular size was reduced by 20–39%. Less narrowing of the annulus was also noted with increased heart rates [14].
The Tricuspid Leaflets and Commissures
In our study of 50 heart specimens only 62% had three readily identifiable leaflets whereas 30% could be described as having two leaflets and 8% had four leaflets. But, in the same study, a review of a series of cross sectional echocardiograms showed three definite lines of closure in all cases [15]. The trifoliate configuration of the leaflets can be seen from the right atrial aspect when the valve is closed in systole thus preventing back flow of blood back into the right atrium (Fig. 1.4). Complete closure and apposition of the leaflets is essential in preventing regurgitation and maintaining ventricular pressure. Proper valve function is controlled by a number of factors including the contraction of the papillary muscles and ventricular wall. But there is also a ‘spectrum of normality’ in the variation of tendinous cord attachment to the tricuspid valve. The leaflets are tethered down by an elegant arrangement of cords attached to papillary muscles and the septum; this prevents ballooning of the leaflets back into the right atrium.
Fig. 1.4
Viewed from the right atrium the trifoliate arrangement of the three leaflets when in closed position can be seen
The leaflets of the valve do not all lie in the same plane [16] and the attachment is described as non-planar [17]. Each leaflet is unique and all have variation in shape, scallop indentations along the elongated free edge and attachment in the normal valve [18, 19] as well as in disease conditions. Normal variations as well as remodelling due to disease need to be appreciated.
The three leaflets of the tricuspid valve should be designated according to their location as anterosuperior , inferior and septal . The anterosuperior leaflet has been labelled anterior while the inferior leaflet has been labelled posterior or mural in other reports. All leaflets are attached like a hinge at the annulus of the right atrioventricular junction, leading to the use of the term hinge-line. All three are noticeably dissimilar in shape and size to each other. The posteroseptal portion of the leaflets is near to the coronary sinus. Taking the plane of the annulus as a whole, the anterolateral portions of the leaflets are nearest to the apex and posterolateral nearer the right atrium. Each leaflet has variable number of scallops along the free edge and is attached within the ventricle via the cords and papillary muscles. Some papers have previously defined the tricuspid valve as having more than three leaflets or cusps [20, 21] and subdivision by scallops of the leaflets has also been described [13]. The general agreement is that there are three tricuspid leaflets which may or may not have scallops.
Viewed from the right atrial aspect the tricuspid valve orifice is roughly triangular in appearance with the three leaflets closing together resulting in a large area of coaptation. The anterosuperior leaflet is the largest and most extensive leaflet that extends curtain-like around approximately half the free wall of the right ventricle from the infundibular outflow region to the inferior part of the right ventricle. Anatomically, the inferior/mural leaflet occupies an inferior location and originates from the diaphragmatic parietal wall of the right ventricle and is also described as the inferior or in older literature as the posterior leaflet.
The septal leaflet is more complex than the anteroseptal and inferior leaflets. This leaflet has been described as occupying the smallest portion of the annulus [8, 22] while other studies describe the mural leaflet as the smallest [23]. From the septal leaflet tendinous cords attach directly to the muscular ventricular septum. This is one of the characteristic features of the tricuspid valve seen with diagnostic imaging that distinguishes it from the mitral valve which does not normally have cordal attachments to the septum. The restricted size of the septal leaflet and the multiple tendinous cord attachments along the septal leaflet directly to the septum results in a different range of movement compared to the other two tricuspid leaflets whose cord attachments have a different morphology [8, 17].
The septal leaflet attachment across the membranous septum divides it into two; a right atrial to right ventricular component which is superior (atrial) to the tricuspid valve and an interventricular component between both ventricles and inferior to the tricuspid valve leaflet. The septal leaflet has one other distinct feature seen with cross sectional imaging and that is its more apically located annular attachment in contrast to the mitral valve. This offset attachment is a key feature seen in diagnostic cross-sectional imaging in the four-chamber plane that aids identification of ventricular morphology in congenital heart disease.
Corresponding to the three leaflets configuration, there are three commissures or peaks of apposition between adjacent leaflets. It is important, however, to appreciate that although ‘tricuspid’ denotes three leaflets, there is no separation of one leaflet from another in that the three leaflets are arranged like a skirt and commissures between the leaflets do not extend all the way to the level of the annulus. Instead, continuity between adjacent leaflets is preserved by at least 5 mm of leaflet tissue along the annulus. This continuity is crucial for maintaining integrity of the valve when the leaflets come together in apposition. As mentioned above, the septal leaflet crosses the membranous septum. It swings ‘around the corner’, away from the septum into the antero-superior leaflet. Thus, the antero-septal commissure attached to the medial papillary muscle is away from the septum. Interestingly, at the membranous septum, it is not uncommon to find a gap in the septal leaflet. Leaflet tissue around this area is usually adequate to provide apposition to close the orifice.
From the attachment of the leaflet at the annulus to the free edge, the tricuspid leaflet has specific zones [13]. The clear zone comprises the majority of the leaflet and is the thin translucent central portion and generally comprises two-thirds of the leaflet length from the annulus toward the free margin.
The basal zone is the portion of the leaflet attached to the annulus at the atrioventricular junction at this connection point there is vascularisation and innervation to the leaflet. This is the junction where atrial myocardium inserts at the annulus. The rough zone forms the free edge and the region of apposition, forming approximately a third of the leaflet length. This thicker portion of the leaflet contains more glycosaminglycans allowing cushioning of the leaflet edges as they oppose during closure. These zones emphasise that the morphology is not uniform macroscopically or microscopically throughout the leaflet.
In cross section, each leaflet extends from the annulus hinge line proximally to the free edge distally. The normal tricuspid valve has a layered leaflet arrangement of the atrialis, spongiosa, fibrosa and ventricularis layers surrounded by a layer of endothelial cells which is continuous with the luminal surface of the atrium and the ventricle. In the tricuspid valve the leaflet comprises interstitial fibroblasts and connective tissue fibres within an extracellular matrix.
The atrialis is the uppermost layer. When the valve is in closed position, the atrialis faces directly the right atrium. This layer is composed of mainly aligned elastic and collagen fibres covered with overlying endothelium. Of the layers, the atrialis has the most elastic fibres [24]. Beneath the atrialis is the spongiosa layer. This layer is composed largely of an extracellular matrix of proteoglycans and glycosaminglycans, along with elastic fibres. The glycosaminglycans and proteoglycans are hydrophilic and attract water molecules. This causes the extracellular matrix to expand and swell at the free edge, providing a natural physical protective buffer to the leaflet. The spongiosa functions as a physiological ‘shock absorber’. It provides a structural cushion along the point of apposition to offset the effect of leaflet closure at the free edge.
Beneath the spongiosa is the fibrosa layer which is the major load-bearing layer, comprising the central structural collagenous core [5] of compact and aligned collagen fibres to the leaflet. The fibrosa layer extends from the annulus into two thirds of the leaflet and is absent at the free edge. The final layer of the tricuspid leaflet is the ventricularis , this layer is covered by a continuous sheet of endothelial cells that overlie elastic fibres and collagen fibres. The thickness of each layer varies from the attachment site at the annulus to the free edge. At the proximal region of the leaflet, near the annulus, the fibrosa is the thickest layer providing the structural core of the leaflet. The spongiosa and atrialis in contrast are relatively thin at the annulus attachment point of the leaflet, but increase in thickness distally, becoming the main component of the leaflet at the free edge.
Histologically, the extracellular matrix surrounds the connective tissue fibres in the leaflet and is a dynamic substrate of neutral pH [25]. The leaflet is composed of the glycosaminoglycan hyaluronic acid and proteoglycans such as aggrecan, decorin, veriscan [26–28] as well as fibroblast cells, some with variable phenotypes exhibiting actin filaments [29].
The maintenance of connective tissue integrity is fundamental in determining the overall strength and competence of the leaflet. This depends upon a continued balance between synthesis of the matrix components, their deposition and repair within the leaflet and the degradation of the tissue elements. Metalloproteinase enzymes, synthesised from fibroblasts, regulate the renewal and turnover of matrix components in connective tissues [30] within the human cardiac valves [31].
Under normal physiological conditions, the extracellular matrix comprising fibrillar proteins of the normal leaflet undergoes constant turnover of constituents to maintain both structure and function with the expression various proteins, including the contractile proteins troponin [32] and myosin [33]. These factors contribute in determining the overall structural integrity of the leaflets. External factors , such as mechanical stresses exerted on the valve leaflet, may also trigger secondary physiological responses affecting the structure and function of the leaflet.
Collagen fibres are the major component of atrioventricular leaflets and have an important role in defining shape, integrity and mechanical strength. Collagen fibres are produced from interstitial fibroblasts and endothelial cells [34, 35]. The alignment of collagen provides mechanical strength required for the repeated opening and closing of the valve. Elastic fibres interconnect with the collagen fibrils and bundles and promote recoil ensuring the leaflet and tendinous cord can return to the resting state.
The pH of the extracellular matrix environment affects the morphology of the fibres. Normal collagen fibres in the tissues are tightly packed and interspersed with smaller homogenous fibrils [36]. These are stable within a near neutral pH [37] but if the pH is altered, as seen in studies on collagen fibre development in chicks where the pH was more acidic, the collagen fibres synthesised can have variable diameters [38].
Matrix enzymes such as matrix metalloproteinases (MMP) and tissue inhibitor of matrix metalloproteinases (TIMP) can also act on collagen fibres [30] to remodel the leaflet tissue.
Tendinous Cords and Papillary Muscles
The slender and fibrous tendinous cords are the key interconnecting structure tethering the leaflets to the papillary muscles ensuring a functional and efficient valve. In the normal valve, leaflet cords and interleaflet cords have been identified. Fan-shaped tendinous cords insert at the junction between each leaflet, facilitating the bringing together and separation of adjacent leaflets as the valve closes and opens. These are the interleaflet cords and they have multiple connections between adjacent leaflet connecting to the papillary muscles. Leaflets cords include rough zone tendinous cords which insert directly to the ventricular surface of the leaflet. The thicker ones are sometimes referred to as strut cords as they bear the mechanical load during valve opening and closing. On the ventricular aspect of the leaflet there are also attachments of tendinous cords to the rough and clear zones of the leaflet and these are termed deep cords. Basal cords can be found attaching the underside of the leaflets close to the annulus directly to the ventricular wall. Further leaflet cords also attach to the free edge of the leaflet and these are simply described as free-edge cords. The multiple attachments of the tendinous cords along the leaflets result in greater support and control of the valve.
Although the papillary muscles are not as uniformly distributed as in the mitral valve, the anterior papillary muscle sited in the right ventricle is usually well-defined, supporting the antero-superior leaflet in its midportion. Usually, there is a cluster of smaller papillary muscles are located laterally or inferiorly in the right ventricle and these support the inferior as well as the antero-superior leaflets. Supporting the commissure between the septal and antero-superior leaflets is a small papillary muscle called the medial (or conal) papillary muscle, also known as the muscle of Lancisi . Variability in number of papillary muscles has been noted as well as the papillary muscle group having multiple heads [39, 40].
The papillary muscles are extensions of trabecular myocardium extending from the apical portion of the right ventricle and are highly innervated, highly vascularised with a central artery, and carry the distal ramifications of the purkinje fibre network. The most common pattern of coronary supply to the right ventricle and papillary muscles is via the dominant right coronary artery running within the right atrioventricular groove. Histologically, the cords are formed of a collagen core with elastic fibres surrounded by endothelial cells. The mechanical load is supported by the collagen fibres during systole.
The anterior papillary muscle is the largest of the three papillary muscles and usually is composed of a distinct conical muscle with multiple heads with cords attached, hence it is described as bifid or trifid. Sometimes, there are several papillary muscles fused together or there is a dominant papillary muscle with adjoining smaller papillary muscles. They are located between the anteroseptal leaflet and the inferior leaflet. Additionally, further support from smaller anterior muscles is provided to the anterior segment of the leaflet (Fig. 1.5). The anterior papillary muscle extends from the apical portion of the right ventricle. It usually arises or continues from the moderator band as the latter muscle band crosses the right ventricular cavity. The origin of the anterior papillary muscle from the moderator band can be midway along the band or close to where the band itself fuses with the right ventricular free wall, on cross sectional imaging, recognising the moderator band is one way of distinguishing the morphologically right ventricle from a left ventricle . Apart from the distinctive moderator band, the right ventricular chamber is criss-crossed by further muscle bundles termed trabeculations and these mainly occupy the apical portion. They interconnect extending from the septal to the apical and parietal walls and these trabeculations are course in comparison to those in the left ventricle.
