|Year : 2021 | Volume
| Issue : 1 | Page : 73-79
The heart valves
Siew Yen Ho
Cardiac Morphology, Royal Brompton Hospital; National Heart & Lung Institute, Imperial College London, UK
|Date of Submission||27-Jan-2021|
|Date of Acceptance||01-Mar-2021|
|Date of Web Publication||30-Mar-2021|
Siew Yen Ho
Royal Brompton and Harefield NHS Foundation Trust, Sydney Street, London SW3 6NP
Source of Support: None, Conflict of Interest: None
The four heart valves are arranged in different planes from each other. The aortic valve is most centrally located being related to all four cardiac chambers. Similar in construction to the pulmonary valve in possessing semilunar leaflets, neither valve has a ring-like annulus. Instead, the semilunar insertions of the leaflets form a crown-like arrangement. The semilunar valves are complexes comprising not only of leaflets but their attachments to arterial walls and adjoining ventricular structures at the ventriculo-arterial junction. The mitral and tricuspid valves guarding the atrioventricular junctions comprise of leaflets hinged to the junction, and have tendinous cords mainly anchored to papillary muscles that are inserted into ventricular walls to the support apparatus. A complete fibrous annulus is lacking, especially in the tricuspid valve and along the hinge line of the posterior mitral leaflet. This article also reviews the relationships of each valvar component and to adjacent cardiac structures.
Keywords: Aortic valve; Atrioventricular junction; Intervention; Mitral; Tricuspid; Pulmonary valve; Ventriculo-arterial junction
|How to cite this article:|
Ho SY. The heart valves. Cardiol Plus 2021;6:73-9
| Introduction|| |
Guarding the inflow and outflow tracts of the ventricles, the four heart valves serve to prevent backflow during the cardiac cycle. As described in a previous article, the four valves are in different planes and levels from one another. While the pulmonary valve is most superior and anterior, it lies nearly horizontally. Posterior and slightly inferior to it is the aortic valve with the plane of its sinuses tilting at an angle of about 30° from the horizontal. The orifice of the aortic valve is directed not only upward but also rightward at an angle of at least 45° to the median plane. It lies in the center of the heart with the mitral and tricuspid valves to each side. Since the plane of the atrioventricular junction is oblique when the heart is viewed from the anterior aspect, the tricuspid and mitral valve orifices are tilted toward the vertical plane [Figure 1].
|Figure 1: Central location of the aortic (A) valve relative to the others. The aortic and mitral (M) valves are adjacent to each other whereas the pulmonary (P) and tricuspid (T) valves are apart.|
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While the semilunar aortic and pulmonary valves appear simple in construction, their efficiency is related to their surrounding structures, particularly the margins of attachment of their leaflets. The atrioventricular valves, on the other hand, are considerably more complex in form, and normal function is dependent on a delicate orchestration of all the components together with the adjacent atrial and ventricular musculature.
| The Aortic Valve|| |
In general construction, the aortic and pulmonary valves have many features in common. Because the aortic valve supports the aortic column, its leaflets are slightly thicker and stronger, yet flexible enough to be moved apart during ventricular ejection.
The valve comprises of the semilunar leaflets and their commissural attachments which form arcs subtended between the struts of a three-pronged crown, the bulging aortic sinuses, the sinutubular junction marking the border between the sinuses and the tubular aorta, and the structures at the ventriculo-arterial junction [Figure 2].
|Figure 2: Diagram of the aortic root comprising of the valvar leaflets and hinge lines, aortic sinuses, interleaflet fibrous triangles, adjoining ventricular structures and junction with the tubular ascending aorta.|
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In closed position, the leaflets meet at the free edges as well as at the lunules to form a zone of apposition that has a height because the total leaflet area is considerably greater than the aortic cross-sectional area. Fenestrations in the lunules are common, especially in the elderly, but the valve remains competent owing to the apposition zone. At the midpoint of the free edge of each leaflet is a fibrous nodule (of Arantius) [Figure 3]A. The nodules come together when the valve closes. Viewed from the aorta, the closed valve has a triradiate appearance of the zones of apposition of the leaflets with the spokes extending out to the peripherally sited commissures [Figure 3]B.
|Figure 3: Aortic leaflets and sinuses.|
A, The right coronary aortic leaflet with its semilunar hinge line (black dashed line). The zone of apposition extends from the blue dashed line to the free edge includes the nodule of Arantius (triangle). B, The bulging sinuses of the aorta are displayed after removing the tubular aorta. The aortic sinuses and leaflets are named left (L) and right® coronary. The noncoronary (N) aortic sinus is the nearest to the plane of the atrial septum. C, This longitudinal section through the aortic root shows disproportion between the right coronary and the noncoronary aortic sinuses in this heart. Consequently, a line joining the nadirs of the sinuses (blue and white dotted line) is not parallel to the blue dashed line at the sinutubular junction.
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In open position, the free margin of each leaflet straightens, and the aortic orifice becomes more triangular in outline. In anatomical terms, the leaflets are named anterior, left posterior and right posterior. In clinical vernacular, however, the leaflets, and their corresponding aortic sinuses, are named right coronary, left coronary and noncoronary in relation to the origins of the coronary arteries. The left and right coronary sinuses are closest to the pulmonary valve while the noncoronary aortic sinus is immediately anterior to the atrial chambers [Figure 3]B. There is considerable variability in the width and height of each leaflet in the same heart [Figure 3]C.
The aortic valve is more complex than its trifoliate arrangement suggests. The aortic wall behind the semilunar leaflets form part of the valve complex to the level of the commissures. This region bulges out to form the three sinuses of Valsalva [Figure 3]B and [Figure 3]C.
Above the sinus portion, the aortic wall shows a well-defined circumferential ridge which is particularly evident when viewed from the aortic aspect. This prominence, the sinutubular junction, can be traced as a ridge joining the apices of the leaflet commissures [Figure 3]C and [Figure 4]. The orifices of the coronary arteries usually arise immediately beneath or slightly above the sinutubular junction. Because of the crescentic attachments of the valve leaflets to the aortic wall, variability of leaflet and sinus heights, the plane of the aortic valve can be at any level from the zenith to the nadir depending on whether the commissural insertion or the deepest trough of the leaflet attachment is taken as the reference point. As such, sizing of the aortic root varies according to the level measured. Nonuniformity of leaflet sizes results in the plane of a circle joining the apices of the commissures is not parallel but deviates at an angle of approximately 11° from the plane of a circle joining the nadirs of the leaflets [Figure 3]C. Furthermore, measurements on images taken through the longitudinal plane of the valve may not provide the true diameter.
|Figure 4: Crown-shaped aortic annulus.|
A, This dissection made by cutting through the anterior leaflet of the mitral valve into the aortic valve displays the relationship of the aortic valve to the ventricular septum. Removal of the valvar leaflets reveals the depths of the right coronary and part of the left coronary sinuses contain myocardium (arrows). Note the thin interleaflet fibrous triangles (asterisks). The fibrous trigones (green triangles) are thicker. The right trigone adjoins the membranous septum (green dotted line). The red-shaded area represents the region where the atrioventricular conduction bundle emerges from the fibrous area to lie superficially on the ventricular septum. B, Following removal of the aortic sinuses the three peaks (arrows) of the semilunar hinge lines are revealed. This is the crown shape of the annulus as depicted in green on the diagram and the yellow areas represent the interleaflet triangles. With this morphology three rings may be designated.
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The semilunar attachments of the leaflets bring into question the matter of “ring” or “annulus.” In many texts, the valve is suggested to have an “annulus,” although its morphology is such that it does not have an “annulus” that can be conceptualized as a band-like ring. Indeed the “annulus” has been questioned by McAlpine in his exquisite atlas published in 1975. Furthermore, surgical reference to the aortic ring is usually to the region beneath the valve leaflets. In fact, owing to the semilunar shapes of the leaflets, the “annulus” that marks their attachment lines is crown-shaped with the points reaching the sinutubular junction [Figure 4]. In terms of rings, the sinutubular junction may be perceived as one and another may be the ventriculo-arterial junction where ventricular tissues join with aortic wall. A third ring, often used for sizing the valve, is a virtual ring joining the nadirs of the leaflets [Figure 4].
The leaflet attachments cross the ventriculo-arterial junction resulting in the lower portions of each semilunar attachment line crossing either muscle or fibrous tissues. Viewed from the ventricular perspective, approximately half the ventriculo-arterial junction is muscle and half is fibrous tissue. The right coronary leaflet and the adjacent half of the left coronary leaflet cross into muscle of the ventricular septum resulting in muscle enclosed in the nadirs of these sinuses [Figure 4]A. The fibrous part is made up of the extensive area of aortic-mitral fibrous continuity where the noncoronary and medial part of the left coronary aortic leaflets border the anterior leaflet of the mitral valve forming the aorto-mitral curtain.
In addition to the obvious region of fibrous tissue in the area of aortic-mitral fibrous continuity, there are interleaflet fibrous triangles located between adjacent aortic sinuses. The crown-like arrangement of the so-called annulus becomes apparent when the aortic wall and sinuses are removed along the semilunar hingelines of the leaflets leaving the leaflets and interleaflet triangles [Figure 4]B.
Situated above the level of ventricular structures, interleaflet triangles are potentially in communication with extracardiac space and potential sites for aneurysmal formation. The triangle between the right and noncoronary leaflets is continuous with the interventricular part of the membranous septum which is a good guide to the location of the atrioventricular conduction bundle [Figure 4]A that emerges from the central fibrous body to become sandwiched between the membranous septum and the crest of the muscular ventricular septum. From here, the left bundle branch takes off and descends on the endocardial surface of the ventricular septum, radiating into its fascicles whereas the right bundle branch passes through the muscular septum to emerge on the septal surface of the right ventricle. It is important to avoid trauma to this region of the valve during valve surgery and transcatheter valve implants.
The triangle between the noncoronary and left coronary leaflets is along the region of aortic-mitral fibrous continuity. Its upper part abuts on the transverse pericardial sinus. Least extensive is the triangle between left and right coronary leaflets and this faces the pulmonary trunk.
| The Pulmonary Valve|| |
Like the aortic valve in having semilunar leaflets and hinge lines, the leaflets of the pulmonary valve are more delicate as they are subjected to less extreme pressure profiles. Unlike the aortic valve, the leaflets are supported from beneath by the complete muscular infundibulum of the right ventricular outflow tract. The sinuses of the pulmonary trunk meet ventricular myocardium at the ventriculo-arterial junction. The hingelines of the leaflets cross this junction. Hence, the depths of the pulmonary sinuses consist of segments of ventricular myocardium [Figure 5]A.
|Figure 5: Pulmonary leaflets and sinuses.|
A, The leaflets have been removed to show the border between the muscle of the infundibulum and the wall of the pulmonary trunk. Note the interleaflet fibrous triangles (asterisks) and segments of myocardium in the depths of the sinuses. B, The infundibulum raises the pulmonary valve above the ventricular septum and anterior to the proximal courses of the main coronary arteries.
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The semilunar attachments rise to the commissures, which are linked in annular fashion forming a mild ridge at the sinutubular junction. Like the aortic valve, three interleaflet fibrous triangles can be discerned. Their apices extend toward the fibro-elastic wall of the pulmonary trunk. Supported by the muscle of the subpulmonary infundibulum, the pulmonary valve is raised above the ventricular septum [Figure 5]B. Owing to this arrangement, the pulmonary valve can be harvested intact during the Ross procedure without incising into the left ventricle. However, care should be taken to avoid damage to the first septal perforating artery arising from the left anterior descending coronary artery. Moreover, the proximal courses of the right and left coronary arteries may run very close to the pulmonary valve putting them at potential risk during transcatheter valve implant [Figure 5]B.
| The Mitral Valve|| |
The Renaissance anatomist Andreas Vesalius in his publication in 1543 described this valve as resembling a bishop's miter, a type of cap that has two peaks. Its two-leaflet arrangement is characteristic [Figure 6] but the valve comprises of more than its leaflets.
|Figure 6: Mitral valve.|
A, The mitral valve viewed from the left atrium showing the leaflets in closed position and two clefts (asterisks) in the posterior leaflet. A1, A2, A3 and P1, P2, P3 denote the segments of the anterior and posterior leaflets respectively. B, The two groups of papillary muscles are pulled apart in this display to show the intact anterior leaflet and the commissures supported by fan-shaped cords.
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In the open state, the valve is like a funnel extending from the hinge line of its leaflets, the “annulus,” to the free margins of the leaflets. Tendinous cords anchor the leaflets to two closely arranged groups of papillary muscles. Owing to this arrangement, the intercordal spaces serve as important pathways for blood flow. The valvar complex comprises the hinge line, the leaflets, the tendinous cords, and the papillary muscles. Furtermore, important for its functioning is the left atrial musculature overlying the leaflets and the ventricular wall to which the papillary muscles are inserted.
The mitral hinge line is shaped nearly like a letter D which in its anterior straight part has no muscle to separate it from the aortic valve. Instead, there is aortic-mitral fibrous continuity. At either extreme of the continuity are expanded portions of the mitral hinge line known as the left and right fibrous trigones [Figure 4]A. The right fibrous trigone and its adjoining membranous septum forms the central fibrous body through which the his bundle of the conduction system penetrates to reach the ventricles.
Although the so-called mitral annulus or ring is better formed than that of the tricuspid, it is by no means a continuous fibrous cord., It is less well formed along the hinge line of the posterior leaflet along the posterolateral border. There are many gaps and variability in fibrous tissue component that could account for the tendency of this region to dilate more than other parts. Instead of a distinct cord, often there is a thin fibrous membrane underneath the leaflet hinge that McAlpine demonstrated in his elegant dissections. This separates the atrial wall from the corresponding part of the ventricular wall while the leaflet hinges to the end of the atrial wall, accounting for the entity termed mitral disjunction. Disjunction is suggested to be a particular feature for mitral prolapse but it is a feature of normal mitral anatomy[Figure 7].,, In some hearts, the leaflet is hinged to the atrial wall or the ventricular wall with the gap filled by fibro-fatty tissues of the atrioventricular groove. On the epicardial side of this part course the circumflex artery and the great cardiac vein continuing into the coronary sinus. However, there is considerable variability in their distances from the hinge line.
|Figure 7: Mitral hinge and disjunction.|
A, This heart cut in a plane simulating a transthoracic echocardiographic plane through the left heart displays the apposition of the mitral leaflets and the hinge (asterisk) of the posterior leaflet to the atrial wall. A thin membrane (red arrow) separates atrial myocardium from ventricular myocardium. B and C, are histologic sections from two hearts taken in similar planes to A. The stain (Masson's trichrome) colors myocardium magenta and fibrous tissue blueish green. The valvar hinge in B appears like a robust fibrous nodule interposing between atrial and ventricular walls. The valvar hinge in C is like the arrangement in A, showing separation (curved arrow) between atrial and ventricular walls or disjunction.
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The two leaflets of the mitral valve can be described as the anterior (or aortic being adjacent to the aortic valve) and posterior (or mural) leaflets although in strict anatomic terms they are not precisely anterior or posterior [Figure 6]. Atrial wall extends over a millimeter or so of the atrial aspect of both leaflets. The anterior leaflet has considerable height, but its hinge line is less than half the perimeter of the mitral orifice. It hangs like a curtain between the left ventricular inflow and outflow tracts. When the valve is closed it becomes the greater part of the atrial floor [Figure 7].
The mural leaflet, by contrast, has a longer hinge line and less height. Its free margin is characterized by three or more scallops [Figure 6]. The indentations, or clefts, between scallops do not reach to the hinge line. Similarly, the anterolateral and posteromedial commissures separate the two leaflets, but the separation is not complete, stopping short about 5 mm from the hinge line in the adult heart. It is conventional to assign three portions to each leaflet from anterolateral commissure to posteromedial commissure. Thus, A1, A2, A3 and P1, P2, P3 labels are applied to thirds of the anterior and posterior leaflets respectively [Figure 6].
An opaque rough zone at the free margins of both leaflets marks the region of leaflet coaptation during ventricular systole. A clear translucent zone extends from the rough zone to the hinge line. The leaflets are supported by cords which mainly insert into the free edge and rough zone on the ventricular aspect [Figure 7].
Each leaflet is supported by cords arising from both sets of papillary muscle groups. There are considerable variations in structure, number and distribution of cords. Although various classifications have been used, the cords may be described simply as interleaflet (commissural) cords and leaflet cords., Interleaflet cords supporting the anterolateral and posteromedial commissures arise from the tip of the anterolateral and posteromedial papillary muscles respectively as a short stem that branches almost immediately like the struts of a fan. Leaflet cords that insert into the rough zone provide the maximal support for both leaflets. They branch soon after their origin from the papillary muscles. In the anterior leaflet further support is provided by one or two strut cords which are thicker tendinous structures that insert into specific “critical” leaflet positions. The mural leaflet has leaflet cords which comprise of rough-zone cords, cleft cords and basal cords. Cleft cords arise from the papillary muscles to support the breaches between the scallops. Basal cords arise directly from the mural ventricular myocardium to provide support mainly to the clear zone of the leaflet.
The two papillary muscle groups, in anterolateral and posteromedial position, arise from the ventricular wall at a level approximately one third of the ventricular length from the apex. They can be traced as extensions from the ventricular trabeculations. The papillary muscles are situated close together and any apparent wide separation in pictures is artefactual [Figure 6]B. A single pillar-like muscle usually occupies the anterolateral position, but there is more variation in posteromedial position. The anterolateral muscle is supplied by an artery derived from the circumflex or anterior descending branch of the left coronary artery. It is the right coronary artery that frequently supplies the posteromedial papillary muscle since right dominance of the coronary pattern is most common.
| The Tricuspid Valve|| |
Unlike the mitral valve, all leaflets of the tricuspid valve are hinged to the atrioventricular junction. At the hinge of the leaflets, atrial myocardium may overlap the leaflet surface by 0.5–2 mm. Normal function of the valve requires normal valvar components and adjoining right atrial and ventricular walls, as well as muscular interaction with the left ventricle.
Like the mitral valve, the concept of a robust fibrous ring attaching to the leaflets is misleading. Instead, fibrous tissue at the hinge line is frequently deficient in many places, especially at the segment corresponding to the right ventricular “free wall” accounting for its greater potential for dilatation compared to the septal part. Epicardial to the hinge line, the right coronary artery, its branches, and coronary veins course within the fibro-fatty tissues of the atrioventricular groove at variable distances and levels.
When viewed from the right atrium the three leaflets occupy septal, anterosuperior and inferior positions. Measured along the hinge line, the antero-superior leaflet is the most extensive whereas the inferior leaflet is least. The commissures between the leaflets are termed the anteroseptal anteroinferior and inferior commissures [Figure 8]A. In some hearts, the commissure between the anterior and posterior leaflets is indistinct, giving the impression of a bi-leaflet tricuspid valve.
|Figure 8: Tricuspid valve.|
A, The tricuspid valve leaflets, commissures and ventricular insertions are displayed. B, The hinge line (dashed line) crosses the membranous septum (dotted shape). The shaded orange area denotes the atrioventricular node of the conduction system and its extension into the His bundle.
cs: coronary sinus orifice
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The medial papillary muscle (muscle of Lancisi) supporting the anteroseptal commissure is the smallest. It originates either singly or as a tiny cluster from the posterior-inferior limb of the septomarginal trabeculation [Figure 8]. Various cords arise directly from the septum to the free margin of the septal leaflet as well as to the rough zone of the major portion of the septal leaflet tethering and limiting its excursion during valvar closure. The hinge line of the septal leaflet crosses the membranous septum. Frequently, there is a cleft in the septal leaflet in the membranous septal area which may be mistaken for a commissure [Figure 8]B.
Notably, the fan-like commissural cord demarcating the border between the septal and the anterosuperior leaflet is superior and lateral to the membranous septum [Figure 8B]. This portion of the septal leaflet, together with the anterosuperior leaflet, is suspended like a curtain over the back of the ventriculo-infundibular fold. Flow from inlet to outlet of the right ventricle passes beneath the free margin of the leaflets. The major anterior papillary muscle arises from the ventricular free wall close to or at the insertion of the moderator band. Its cords often insert to the midpoint of the anterosuperior leaflet. The antero-inferior commissure is usually supported by a smaller anterior muscle arising in the vicinity. The inferior papillary muscle, frequently bifid or trifid, provides commissural support to the septal and inferior leaflets, as well as support to the leaflet scallops via cleft cords to the inferior leaflet. The basal leaflet area is supported by basal cords of variable morphology arising from the ventricular wall.
| Conclusions|| |
Each heart valve comprises of several more components than its leaflets. All component parts need to work in coordinated fashion for normal valvar function. Furthermore, understanding the spatial relationships of the valves to one another and to other cardiac structures is fundamental knowledge when carrying out imaging or interventions on the valves.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]