Three-Dimensional Anatomy of the Aortic and Mitral Valves

Wendy Tsang, Benjamin H. Freed and Roberto M. Lang

Key Points

The mitral valve is a complicated three-dimensional (3D) structure made up of multiple, distinct anatomic components including the annulus, commissures, leaflets, chordae tendinae, papillary mucscles, and left ventricle. Optimal interaction of these different elements is crucial for the valve’s functional integrity.

The annulus is a dynamic structure that undergoes 3D deformation in its circumference, excursion, curvature, shape, and size for proper function, which in turn makes it susceptible to ventricular remodeling.

3D echocardiography provides a considerable amount of mechanistic insight into the complex annular alterations that occur in different disease processes, such as degenerative and ischemic mitral valve disease, and the conformational changes that occur to both the mitral and aortic annuli with mitral valve repair.

The leaflet segmentation scheme proposed by Carpentier is the most widely used, dividing the posterior leaflet into three scallops with three apposing anterior segments.

The aortic valve is a part of the aortic root complex, which is composed of the sinuses of Valsalva, the fibrous interleaflet triangles, and the valvular leaflets themselves.

Approximately two thirds of the circumference of the lower part of the aortic root is connected to the septum, and the remaining third is connected via a fibrous continuity known as the aortic-mitral curtain to the mitral valve.

The aortic valve leaflets are attached in a semilunar fashion throughout the entire length of the aortic root, with the highest point of attachment at the level of the sinotubular junction and the lowest in the ventricular myocardium below the anatomic ventriculoarterial junction.

The surgical definition of aortic annulus describes a semilunar crownlike structure demarcated by the hinges of the aortic valve leaflets, whereas the imaging definition refers to the virtual or projected ring that connects the three most basal insertion points of the leaflets.

The aortic annulus and the left ventricular outflow tract are elliptical rather than circular structures.

The aortic annulus changes dynamically during the cardiac cycle. It is largest in the first third of systole and smallest during isovolumic relaxation

Advances in 3D echocardiography technology have ushered its use into mainstream clinical practice. ¹ 3D echocardiography offers a realistic, multiplanar image of both valves and their spatial relationships with adjacent structures, providing anatomic and functional insight that has furthered our understanding of normal spatial relationships and the anatomic and functional abnormalities that develop in patients with valvular heart disease.

Mitral Valve Anatomy

The mitral valve is a complicated 3D structure composed of multiple, distinct anatomical components. Optimal interaction of these different elements comprising the annulus, commissures, leaflets, chordae tendinae, papillary muscles, and left ventricle is crucial for its functional integrity.

Mitral Annulus

The mitral annulus is a fibromuscular ring to which the anterior and posterior mitral valve leaflets attach. The normal mitral valve annulus has a 3D saddle shape with its “lowest points” at the level of both commissures. This allows proper leaflet apposition during systole and minimizes leaflet stress. ² The annulus can be divided into the anterior and posterior annulus based on the insertion of the corresponding leaflets. The anterior portion of the annulus attaches to the right and left fibrous trigones. The right trigone is a fibrous area situated between the membranous septum, the mitral valve, the tricuspid valve, and the noncoronary cusp at the aortic annulus. The left trigone is a fibrous area located at the nadir of the left coronary cusp of the aortic annulus and the left border of the aortic-mitral curtain. The aortic-mitral curtain is the fibrous tissue between the anterior mitral valve leaflet, the left and non-coronary cusps of the aortic valve, and the left and right trigone. ³

The posterior portion of the annulus is less developed owing to the discontinuity of the fibrous skeleton of the heart in this region. This difference explains why the posterior portion of the mitral annulus is more prone to pathologic dilation and the anterior portion is relatively resistant. ⁴ The annulus is a dynamic structure that undergoes 3D deformation in its circumference, excursion, curvature, shape, and size for proper function, which makes it susceptible to ventricular remodeling.^5–9

Mitral Valve Leaflets

The mitral valve has an anterior and a posterior leaflet ( Figure 2-1). The atrial, or smooth, surface is free of any attachments whereas the ventricular, or rough, surface connects to the papillary muscles via the chordae tendinae. The posterior leaflet, which is quadrangular, is attached to approximately three fifths of the annular circumference. The anterior leaflet has a semicircular shape and is attached to approximately two fifths of the annular circumference. ¹⁰ Although the posterior leaflet attaches to a larger portion of the annular circumference, it is shorter than the anterior leaflet.

FIGURE 2-1 Anatomic relationship of the aortic and mitral valves.
A, Schematic diagram and B, en face three-dimensional echocardiographic zoom-mode image of the mitral valve from the left atrial, or the surgeon’s, perspective depicting typical anatomic relationships. In this view, the aortic valve occupies the 12 o’clock position. The aortic mitral curtain separates the anterior leaflet from the aortic valve. A1 to A3, Anterior leaflet segments; cor., coronary; P1 to P3, posterior leaflet segments.

There are two major terminology classifications for the segmental anatomy of the mitral leaflets, which help with the description of the localization of specific mitral valve lesions. The leaflet segmentation scheme proposed by Carpentier ¹¹ is the most widely used. In this scheme, the posterior leaflet has two well-defined indentations dividing it into three separate sections or “scallops.” The anterolateral scallop is defined as P1, the middle scallop as P2, and the posteromedial scallop as P3. The anterior leaflet typically has a smoother surface and is devoid of indentations. The segment of the anterior leaflet opposing P1 is designated A1 (anterior segment), the segment opposite to P2 is A2 (middle segment), and the segment opposite to P3 is A3 (posterior segment) ( Figure 2-2).

FIGURE 2-2 Classification of mitral valve segmental anatomy.
A, The Carpentier system divides the posterior leaflet into three scallops (P1, P2, P3) on the basis of leaflet indentation. The anterior leaflet is then divided and classified into three segments on the basis of their relationship to the posterior leaflet (A1, A2, A3). B, The Duran system divides the leaflet on the basis of the chordal insertion of the papillary muscles. Thus the anterior leaflet is divided into two segments (A1, A2) and the posterior into four segments (P1, PM1, P2, PM2). Segments A1, P1 and PM1 attach to the anterolateral papillary muscle, and segments A2, P2, and PM2 to the posteromedial papillary muscle. C, The modified Carpentier system divides A2 and P2 into lateral (A₂L, P₂L) and medial (A₂M, P₂M) segments, allowing grouping of leaflet segments according to papillary muscle attachment. Thus, segments A1, A₂L, P1, and P2L are attached to the anterolateral papillary muscle, and A3, A₂M, P3, and P₂M to the posteromedial papillary muscle.

The modified Duran nomenclature is based on the chordal insertion of the papillary muscles. ¹² In this classification, the posterior leaflet is divided into P1, PM₁, PM₂, and P2 and the anterior leaflet is divided into A1 and A2. If a line were drawn directly down the center of the mitral valve, then P1, PM₁, and A1 would be grouped together because they all attach to the anterolateral papillary muscle, and P2, PM₂, and A2 would be grouped together because they attach to the posteromedial papillary muscle (see Figure 2-2).

A final proposed classification is the modified Carpentier classification, which is a combination of both the Carpentier and the modified Duran nomenclature. ¹³ This classification scheme divides A2 and P2 into medial (M) and lateral (L) segments, grouping P1, P₂L, A1, and A₂L because they are attached to the anterolateral papillary muscle and P₂M, P3, A₂M, and A3 because they are attached to the posteromedial papillary muscle. This last terminology converts the well-known Carpentier leaflet segmentation into anatomically relevant groupings (see Figure 2-2).

Mitral Valve Commissures

During systole, the margins of the two mitral leaflets oppose each other for several millimeters to ensure valve competency against normal left ventricular (LV) end-systolic pressure.14,15 The distinct area where the anterior and posterior leaflets appose each other during systole is known as the commissure. Carpentier divides the commissures into anterolateral and posteromedial commissures. ¹¹ The amount of tissue in the commissures varies from several millimeters of leaflet tissue to distinct leaflet segments.

Mitral Valve Chordae

The chordae tendinae are responsible for determining the position and tension on the anterior and posterior leaflets at LV end-systole. The chordae are fibrous extensions originating from the heads of the papillary muscles and infrequently from the inferolateral ventricular wall. They are named according to their insertion site on the mitral leaflets. Marginal or primary chordae insert on the free margins of the mitral leaflets and prevent marginal prolapse. Intermediate or secondary “strut” chordae insert on the ventricular surfaces of the leaflets, preventing billowing while reducing tension on the leaflet tissues.16,17 These chords may also play a role in determining dynamic ventricular shape and function through their contribution to ventricle-valve continuity.^18,19 Basal or tertiary chordae insert on the posterior leaflet base and mitral annulus. Their specific function is unclear.

Papillary Muscles

The two papillary muscles—the anterolateral and the posteromedial—originate from the area between the apical and middle thirds of the LV free wall. The anterolateral papillary muscle has an anterior head and a posterior head, whereas the posteromedial papillary muscle usually has anterior, intermediate, and posterior heads. ²⁰ The anterolateral papillary muscle has a dual blood supply from both the left anterior descending and left circumflex arteries, and the posteromedial papillary muscle receives its single blood supply from the right coronary artery when the right coronary is dominant, which is the situation in 90% of individuals. When the left coronary is dominant, the posteromedial papillary muscle is supplied by the left circumflex. Because the papillary muscles connect directly to the left ventricle, any geometric alteration of the ventricle can change the axial relationship of the chordae and leaflets, resulting in poor leaflet coaptation.

Mitral Valve Apparatus Quantification

In addition to its ability to provide detailed and multidimensional images of the mitral valve, 3D echocardiography also provides accurate and reproducible quantification of mitral valve geometry and dynamics throughout the cardiac cycle. With the advent of 3D imaging, new parameters of annular, coaptation, leaflet, and subvalvular geometry are easily obtained.3,10 These measurements have generated new insights into the mechanics of the mitral valve. A detailed description of the most commonly used parameters is shown in Figures 2-3 and 2-4.

FIGURE 2-3 Volumetric reconstruction of the mitral valve.
3D echocardiography–based software provides measurements of mitral annular, leaflet, coaptation line, intervalvular relationships, and subvalvular geometry. A, Anterior; AL, anterolateral; Ao, aortic valve; P, posterior; PM, posteromedial.

FIGURE 2-4 Functional anatomy of the mitral annulus during the cardiac cycle.
A, Three-dimensional transesophageal echocardiography image of the mitral valve with a three-dimensional mitral valve model of the annulus, leaflets, and coaptation line imposed on the image. B, three-dimensional transesophageal echocardiographic dataset of the mitral valve with a three-dimensional mitral valve model of the mitral annulus and papillary muscles superimposed on the image. C, Results from specialized three-dimensional echocardiographic software tracking mitral annular displacement, height, and area during systole. AP, Anteroposterior; CC, inter-commisural; RR, R-R interval.

3D echocardiography provides a considerable amount of mechanistic insight into the complex annular alterations that occur in different disease processes, such as degenerative and ischemic mitral valve diseases and the conformational changes that occur to both the mitral and aortic annuli with mitral valve repair. These new insights can potentially lead to improved surgical techniques that could eventually lead to better patient outcomes.

Mechanisms of Mitral Valve Dysfunction

Diseases that affect the mitral valve are best described by defining the etiology of the disease, the specific lesions caused by the disease, and the dysfunction it creates on the mitral valve apparatus. This “pathophysiologic triad,” first described by Carpentier et al ²¹ in the early 1980s, is still extremely useful today in characterizing different types of mitral valve disorders.

Mitral valve disease is due to either primary (direct) or secondary (indirect) causes. Examples of diseases that directly affect the mitral valve are congenital malformations, rheumatic disease, valvular tumors, and degenerative diseases. Diseases that indirectly affect the mitral valve include ischemic and nonischemic dilated cardiomyopathy, hypertrophic cardiomyopathy, and myocardial infiltrative diseases.

No matter what the etiology of the mitral valve disease, each disease process frequently results in one or more lesions. For example, dilated cardiomyopathy can result in mitral annular dilation in what is commonly referred to as functional mitral regurgitation (MR). Degenerative diseases such as Barlow disease and fibroelastic deficiency result in multiple types of lesions, including excess myxomatous leaflet tissue, chordal elongation, thinning, and rupture. Rheumatic heart disease results in commissural fusion, leaflet thickening, and chordal agglutination. Myocardial infarction can lead to lesions such as papillary muscle displacement, leaflet tethering, and mitral annulus dilation.

These lesions, in turn, lead to mitral valve dysfunction. Instead of classifying this dysfunction as simply mitral stenosis (MS) or MR, Carpentier ¹¹ developed a classification scheme to aid in the surgical strategy on the basis of the type of leaflet motion ( Table 2-1). Patients with mitral annular dilation or leaflet perforation usually have normal leaflet motion and are categorized as having type I dysfunction. Type II dysfunction consists of prolapse (free edge of one or both leaflets overriding the plane of the annulus during valve closure) and flail (excessive motion of the leaflet margin above the plane of the annulus) due to excessive and redundant leaflet tissue and chordal rupture, respectively ( Figure 2-5). Leaflet restriction during valve closure due to fusion of various components of the mitral valve apparatus is defined as type IIIa dysfunction, whereas leaflet restriction during valve opening resulting from leaflet tethering, is defined as type IIIb dysfunction.

TABLE 2-1

Carpentier Functional Classification for Mitral Valve Dysfunction

FIGURE 2-5 Myxomatous disease with mitral valve prolapse.
A, Schematic demonstrating anterior leaflet prolapse. B, Imaging from two-dimensional transesophageal echocardiography. C, three-dimensional transesophageal echocardiographic image of the mitral valve with anterior leaflet prolapse as viewed en face from the left atrium. Leaflet prolapse is diagnosed when the free edge of the leaflet overrides the plane of the mitral annulus during systole. D, Schematic demonstrating bileaflet billowing of the mitral valve due to chordae elongation. E, Images of bileaflet mitral valve billowing from two-dimensional transesophageal echocardiography and F, three-dimensional transesophageal echocardiography as viewed en face from the left atrium. Leaflet billowing is diagnosed when there is systolic excursion of the leaflet body into the left atrium as a result of excess leaflet tissue, with the leaflet free edge remaining below the plane of the mitral annulus. G, Schematic demonstrating anterior mitral leaflet prolapse and posterior mitral leaflet flail due to chordal rupture. H, A two-dimensional transesophageal echocardiographic example of P2 flail segment. I, Corresponding three-dimensional transesophageal echocardiographic en face image as viewed from the left atrium. The dashed black and solid red lines in parts A to H represents the mitral annular plane.

It is important to emphasize that the different components of the pathophysiologic triad are not mutually exclusive and can be clinically combined in different ways. For example, the typical lesions seen in type IIIa dysfunction can also occur in conjunction with the lesions of type II dysfunction. Type IIIb dysfunction is the result of ventricular remodeling, with the primary lesion being leaflet tethering due to papillary muscle displacement as occurs in ischemic MR. Associated annular dilation is a common finding in patients with chronic degenerative MR, but the classification of dysfunction should differentiate the primary lesion causing the regurgitation (i.e., chordal rupture) from secondary lesions (i.e., annular dilation).

Degenerative Mitral Valve Disease

Mitral valve prolapse is now recognized as the most common cause of MR in developed countries. ²² It results primarily from two distinctive types of degenerative diseases, Barlow disease and fibroelastic deficiency ( Table 2-2). There is considerable overlap between these two entities, and it is difficult to reliably distinguish them on the basis of either the gross or histologic appearance of the valve. Some valves may represent a forme fruste of Barlow disease and demonstrate myxoid infiltration on subsequent histologic examination. ⁶

TABLE 2-2

Key Differences Between Barlow Disease and Fibroelastic Deficiency

DIFFERENTIATING CHARACTERISTICS	BARLOW DISEASE	FIBROELASTIC DEFICIENCY
Pathology	Excess of myxomatous tissue	Impaired production of connective tissue
Typical age at diagnosis	Younger (<40 years)	Older (>60 years)
Duration of disease	Years to decades	Days to months
Physical findings	Midsystolic click and late systolic murmur	Holosystolic murmur
Leaflet involvement	Multisegmental	Unisegmental
Leaflet lesions	Leaflet billowing and thickening	Thin leaflets with a thickened involved segment
Chordae lesions	Chordal thickening and elongation	Chordal elongation and chordal rupture
Carpentier classification	Type II	Type II
Type of dysfunction	Bileaflet prolapse	Prolapse and/or flail
Complexity of valve repair	More complex	Less complex

Barlow Disease

Barlow disease results from an excess of myxomatous tissue, which is an abnormal accumulation of mucopolysaccharides in one or both of the leaflets and many or few of the chordae. ²³ This myxoid infiltration results in thick, bulky, redundant, billowing leaflets and elongated chordae, which often lead to bileaflet, multisegmental prolapse ( Figure 2-6). Barlow disease is usually diagnosed in young adulthood, and patients are typically monitored for many decades with well-preserved LV size until criteria for surgery are met in the fourth or fifth decade of life.

FIGURE 2-6 Barlow disease and fibroelastic deficiency.
A, En face three-dimensional transesophageal echocardiographic image of a mitral valve as viewed from the left atrium, demonstrating bileaflet prolapse in a patient with Barlow disease. B, The corresponding parametric map, in which the color gradations towards orange indicate the distance of the leaflet from the mitral annular plane toward the left atrium. C, En face three-dimensional transesophageal echocardiographic image of a mitral valve as viewed from the left atrium, demonstrating a flail P2 segment due to fibroelastic deficiency. D, The corresponding parametric map demonstrates the abnormal P2 segment. A, anterior; AL, anterolateral; Ao, aortic valve; P, posterior.

Fibroelastic Deficiency

In contrast, fibroelastic deficiency results from acute loss of mechanical integrity due to abnormalities of connective tissue structure and/or function. ²³ It usually leads to either a localized or unisegmental prolapse due to elongated chordae or flail leaflet due to ruptured chordae (see Figure 2-6). Patients most commonly present in the sixth decade of life with a relatively short history of MR. This entity is the most common form of organic mitral valve disease for which mitral valve repair surgery is required. There is considerable overlap between these two entities and it is difficult to reliably distinguish them based on either the gross or histologic appearance of the valve. Some valves may represent a forme fruste of Barlow disease and will demonstrate myxoid infiltration on subsequent histological examination. ⁶