The papillary muscles are usually organized into two groups, the posteromedial and anterolateral papillary muscles, and are attached to the left ventricle wall approximately one third distance from the apex and two thirds from the annulus. The anterolateral papillary muscle usually attaches to the left ventricle at the junction between the septum and the posterior wall while the posteromedial papillary muscle usually attaches on the lateral wall of the left ventricle [1].
Each of the two mitral leaflets are further divided by two indentations at their free edges into three scallops, termed A1, A2 and A3 in the anterior leaflet, and P1, P2 and P3 in the posterior leaflet (Fig. 1.1). The area between the two leaflets at the annulus is termed the commissures. A small commissural leaflet is present in this area which forms the continuity between the anterior and posterior leaflets. The anterior and posterior leaflets approximate and overlap each other at their free edges and commissures, forming a surface of coaptation of about 7–9 mm. This surface of coaptation between the free edges of the two leaflets is essential for valve competency. The normal coaptation line lies parallel to the posterior annulus [1].
Mitral Valve Motion and Geometry
The normal motion and geometry of the mitral valve has been previously studied mainly using radio-opaque markers in animals and also by echocardiography [2–5]. More recently, Chan, et al., used cardiovascular magnetic resonance to study the motion and geometry of the normal mitral valve apparatus in normal healthy volunteers; the results of this study are described here [6]. Six phases of mitral annular and leaflet motion can be identified during the normal cardiac cycle (Figs. 1.2 and 1.3):
Fig. 1.2
The six phases of mitral annular and leaflet motion during the cardiac cycle in a normal individual. The motion of the mitral valve leaflets, annulus and left ventricle at its inferolateral wall were traced from individual cardiovascular magnetic resonance cine images and superimposed on each other to produce the image. Phase I represents left ventricular systole, Phases II–V represent left ventricular diastole, Phase VI represents the end of left ventricular diastole and the onset of left ventricular systole (Reprinted from Chan, et al. with permission. From Chan et al. [6])
Fig. 1.3
“Two-dimensional M-Mode” display of mitral annular and leaflet motion during the cardiac cycle in a normal individual. The figure was produced by superimposing and tracing the images from individual cardiovascular magnetic resonance cine images. The view is that of the septo-lateral dimension of the mitral annulus. Phase I represents left ventricular systole, Phases II–V represent left ventricular diastole, Phase VI represents the end of left ventricular diastole and the onset of left ventricular systole. Arrows indicate maximal early and late diastolic opening of the leaflet tips, and arrow heads the diastolic leaflet approximation. Horizontal axis represents time in msec. Numbers at the top is the septo-lateral diameter in mm (From Chan et al. [6])
Phase I: Ventricular systolic excursion
Phase II: Leaflet opening and annular recoil
Phase III: Leaflet approximation
Phase IV: Mid diastolic pause
Phase V: Atrial systolic excursion
Phase VI: Leaflet closure
Left ventricular systole starts in Phase VI with leaflet closure and occurs throughout Phase I. Left ventricular diastole starts in Phase II with leaflet opening, continues throughout Phases III to V, and ends in Phase VI with leaflet closure.
Phase I: Ventricular Systolic Excursion
Left ventricular systole occurs in this phase and as the left ventricle contracts, the mitral annulus moves towards the left ventricular apex. This excursion of the mitral annulus towards the left ventricular apex is asymmetrical with greater excursion of the posterior annulus and leaflets compared to the anterior annulus and leaflets (Fig. 1.2) [6]. The septo-lateral diameter of the mitral annulus increases in size during this phase but the commissure-commissure diameter remains relatively unchanged in size. The mitral leaflets remain approximated throughout this phase. During this phase of left ventricular systole, the papillary muscles contract, becoming shorter in length, and also approximate medially towards each other, towards the mitral annulus, and towards the left ventricular septum. The direction of greatest approximation is towards each other. Although both the papillary muscles and the mitral annulus move towards each other during this phase, contraction of the papillary muscles with corresponding reduction in their length means that the distance between the papillary muscle tips and the mitral annulus is relatively unchanged. Therefore, during this phase of left ventricular systole, both the anterolateral and posteromedial papillary muscles approximate towards each other and towards the long-axis midline of the left ventricle, and the anterolateral papillary muscle approximates towards the left ventricular septum. The distance between the tips of the papillary muscles and the mitral annulus remains relatively constant throughout this phase despite the mitral annulus and the left ventricular wall moving towards each other, accounted for by the 30–40 % contraction in its length.
Phase II: Leaflet Opening and Annular Recoil
This phase corresponds to the end of left ventricular systole and the start of left ventricular diastole. At the start of this phase, the mitral leaflets open and this is followed closely by recoil of the mitral annulus back towards the left atrium (Figs. 1.2 and 1.3). The septo-lateral diameter of the mitral annulus continues to increase in size reaching its maximal size in the middle of this phase, corresponding to maximal separation of the mitral leaflets. The left ventricle starts to relax and the papillary muscles move apart from each other, away from the long axis midline of the left ventricle and from the left ventricular septum.
Phase III: Leaflet Approximation
Phase IV: Mid Diastolic Pause
Left ventricular diastole continues throughout this phase. The mitral leaflets drift about in a neutral position during this phase. This represents the phase of diastolic inactivity and is the true neutral position of the heart.
Phase V: Atrial Systolic Excursion
Left ventricular diastole continues throughout this phase. Left atrial systole pulls the mitral annulus back towards the left atrium from its neutral position. The separation of the mitral leaflet edges increases (Figs. 1.2 and 1.3). The annulus begins to contract (pre-systolic contraction) and its septo-lateral diameter starts to decrease in size during this phase.
Phase VI: Leaflet Closure
This phase marks the end of left ventricular diastole and the onset of left ventricular systole. The leaflets close. The annulus continues to contract and reduce in size at its septo-lateral dimension.
The ability of cardiovascular magnetic resonance to image the entire mitral valve apparatus (mitral leaflets, annulus and papillary muscles) and the left ventricle, and to visualise these together in a single image, has given unique insights into the function, motion and geometry of the mitral valve apparatus. The entire mitral valve apparatus changes in size, shape and position during the cardiac cycle.
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