Anatomy of the Mitral Valve

LEARNING OUTCOMES

• The mitral valve is a complex and highly dynamic structure comprised of various structures that work as a single functional unit.

• The mitral valve annulus is a nonplanar saddle-shaped structure with its highest point at the mid-anterior annulus close to the aortic valve and with a second minor peak at the mid-posterior annulus.

• The fibrous skeleton is concentrated at the base of the ventricular mass and provides electrical insulation at the atrioventricular level and fibrous continuity for the leaflets of the mitral, aortic, and tricuspid valves.

• There are two major types of mitral valve chordae, classified based on their insertion sites to the mitral leaflets. The cords that attach to the leaflet tips are called primary cords, whereas secondary cords are the ones that attach to rough zones of the anterior leaflet and throughout the body of the posterior leaflet.

• The anterior mitral annulus can be divided into three segments (i.e., middle, right, and left). The area of aortic-to-mitral continuity is a dynamic structure that expands during systole and varies in size in response to changes in hemodynamic loading conditions and ventricular contractility.

• The left bundle branch of the cardiac conduction system enters the left ventricular outflow tract (LVOT) posterior to the membranous septum, with the His bundle located at the posteroinferior aspect of the membranous septum and right fibrous trigone.

• With the increasing numbers of mitral valve surgeries, especially reoperations, adequate exposure of the whole mitral valve apparatus through a small left atrium is a key element for successful and expedient intervention.

• A comprehensive knowledge of the mitral valve and its surrounding structure is of utmost importance in order to understand and deal with mitral valve clinical disorders.

INTRODUCTION

Mitral valve is typically 4 to 6 cm ² in area and is a very dynamic structure. Each of the mitral valve’s structural components performs a specific function in order to achieve normal functioning of the valve. In addition to primary mitral valve diseases, the disorders of the left ventricle and left atrium can also lead to abnormal functioning of the mitral valve. In this chapter, we will discuss the normal anatomy of the mitral valve along with the structures that lie around it.

MITRAL VALVE LEAFLETS

The human mitral valve is a highly variable and complex bileaflet structure. Although mitral leaflets are commonly referred to as anterior and posterior but anatomically aortic and mural names, respectively, for these leaflets appear more appropriate. The area where anterior and posterior mitral leaflets come together at their insertion to the mitral annulus is called “commissure.” Mitral valve leaflets are noticeably different from each other in many ways but have nearly identical surface areas. The posterior leaflet is narrow but extends two-thirds around the left atrioventricular junction within the inlet portion of the ventricle. The free edge of this leaflet has indentations dividing the leaflet into three scallops. Infrequently these indentations extend deeper into the leaflet but usually not to the annulus, called clefts. Carpentier’s nomenclature defines the scallop adjacent to the anterolateral commissure as P1, the central scallop as P2, and the most medial scallop as the P3 segment, which is adjacent to the posteromedial commissure ( Fig. 1.1 ). These scallops can be of variable sizes. The distinguishing feature of the posterior leaflet is the continuity of the left atrial wall myocardium on its atrial surface that can possibly make it prone to displacement in conditions like left atrial enlargement.

The anterior mitral leaflet is semicircular in shape, extends one-thirds around the annular circumference but is much wider than the posterior leaflet. This leaflet hangs like a curtain left ventricular inlet and outlet. The distinguishing feature of this leaflet is that it is in fibrous continuity with the aortic valve (left and noncoronary cusps), the interleaflet triangle between the aortic cusps that adjoins the interventricular membranous septum and fibrous trigones. Anterior leaflet does not have clear scallops but is divided into A1 through A3 segments corresponding to the adjacent scallops of the posterior leaflet.

Normal mitral valve leaflets are thin, translucent, and pliable. There are three different zones described in these leaflets; (a) the rough zone is nearer to the leaflet tips where the chords are attached. The leaflets are thicker in this area with nodular atrial surfaces and form the coaptation zone; (b) the clear zone (see Fig. 1.5 ) is next and is devoid of chordal attachments; (c) the basal zone is only present on the posterior leaflet and is the area where basal or tertiary chords are attached.

3D PERSPECTIVE

The four cardiac valvular orifices are not contained within the same plane. In an apical-to-basal direction, the tricuspid valve is the most apical, or most inferior, of the four valves, followed by the mitral valve, aortic valve, and pulmonary valve. The mitral valve is also the most posterior, while the pulmonary valve is the most anterior. The basal planes of the two arterial valves are at right angles to each other.

The plane of the mitral valve is not flat, and it resembles a saddle, just like the origin of its name from “mitra,” meaning “bishop’s hat” ( Fig. 1.2 ). The highest point is the mid-point of the anterior leaflet. This shape of the mitral valve plays an important role in the distribution of forces during the cardiac cycle as the mitral valve area reduces by 10% to 15% during systole due to annular contraction. The shape of the saddle is further exaggerated by elevation of the highest points in systole when annulus contracts and commissural areas are moved towards the cardiac apex. This function of the mitral valve is affected by left ventricular dilatation leading to mitral annular dilation. The line of coaptation of mitral leaflets is a U-shaped curve. Along the coaptation line, the leaflet tips curl towards the left ventricle leading to an overlap of leaflet’s surfaces called coaptation length. This allows persistently adequate leaflet coaptation even in the settings of moderate mitral annular dilation pulling the leaflets apart. In a normal mitral valve, the coaptation line is always below the plane of the mitral annulus, called coaptation depth ( Fig. 1.3 ).

SUB-VALVULAR APPARATUS

Papillary Muscles

There are two papillary muscles classified into anterolateral and posteromedial muscles based on their relationship to the lateral and medial mitral commissures, respectively. The bodies of the papillary muscles originate from the apical third of the left ventricular wall. The anterolateral papillary muscle usually contains a single head and has a dual blood supply from the diagonal branch of the left anterior descending artery and the obtuse marginal branch of the left circumflex artery. The posteromedial papillary muscles usually have two heads and receive blood supply from a single vessel, either the right coronary artery or the obtuse marginal branch of the left circumflex artery.

Papillary muscles play an integral role in the proper functioning of the complex mitral valve apparatus ( Fig. 1.4 ). During the first half of the ventricular systole, the entire papillary muscle moves closer to each other and concurrently towards the mitral annulus due to longitudinal contraction of the ventricle base. This coordinated and symmetric movement prevents the distortion of the mitral leaflets when the leaflets move towards the left atrium during the first half of the systole. Additionally, at the same time, annular contraction allows early systolic leaflet coaptation by early saddle-shape accentuation. Whereas during late systole, isolated papillary muscle contraction shortens the muscle leading to an increase in distance between papillary muscle tip and the annulus, keeping the mitral leaflets under directed tension and posterior restrain to prevent leaflet prolapse and systolic anterior motion of the anterior leaflet leading to dynamic left ventricular outflow tract obstruction, respectively.

Chordae Tendineae

The chordae tendineae are the fibrous chords that originate from the tips of papillary muscles and attach to the ventricular aspect of the mitral leaflets in a hand held, fan-like pattern. Rarely, the chordae can stem from the basal posterior segment of the left ventricle and attach directly to the basal segments of the posterior mitral leaflet.

There are two major types of the chordae that are classified based on their insertion sites to the mitral leaflets ( Fig. 1.5 ). The chords that attach to the leaflet tips are called primary chords, whereas secondary chords are the ones that attach to rough zones of the anterior leaflet and throughout the body of the posterior leaflet. These chords are formed of tight collagen and elastin network. The primary chords are thinner with limited extensibility to prevent mitral leaflet inversion or flail. Whereas the secondary chords are thick, containing more elastin that makes them more extensible and are less likely to break compared to the primary chords. The anatomy and branching pattern of the chords are highly variable. These chords have the ability to adapt and can lengthen in response to the altering loading conditions.

LEFT ATRIUM

The left atrium is the most posteriorly situated of the cardiac chambers when viewed from the front side of the chest. With the interatrial septum being an oblique structure and mitral orifice higher than the tricuspid, the left atrial chamber is more posteriorly and superiorly situated relative to the right atrial chamber. Accordingly, the pulmonary veins that enter the posterior part of the left atrium, the left veins located more superior than the right veins ( Fig. 1.6 ). The left atrium has two distinct segments: the main body and the left atrial appendage (LAA). The main body can be further divided into the pulmonary venous segment, the septal segment, and the vestibule, which is the outflow part of the left atrial chamber just above and surrounding the mitral valve. Although the LAA has a well-defined opening called “os,” the segments of the left atrial main body do not have any clearly demarcated outlines. Anterior to the left atrium lies the transverse pericardial sinus, and in front of the sinus is the aortic root ( Fig. 1.7 ). Whereas, the tracheal bifurcation, esophagus, and descending thoracic aorta are immediately behind the pericardium overlying the posterior wall of the left atrium.