1. Mitral Valve Structure & Function
David J McCormack
BSc(Hons) MBBS MFSTEd FRCSEd(CTh) FRCS (CTh)
Consultant Surgeon Waikato Cardiothoracic Unit

2. Topics to be covered… Location of the mitral valve
Relation to the other valves and important structures
Mitral valve components
Annulus
Leaflets
Subvalvular apparatus
Functional anatomy of the mitral valve

7. Annulus A ‘D’ shaped hinge line of the valvular leaflets
Circumference: ♂ 9cm, ♀ 7.2cm
NOT a solid ring
Fibrocollagenous elements of varying consistency
Allows major changes in shape and dimensions during the cardiac cycle

12. Effect of annular shape on leaflet curvature in reducing mitral leaflet stress.
Salgo IS, Gorman JH 3rd, Gorman RC, Jackson BM, Bowen FW, Plappert T, St John Sutton MG, Edmunds LH Jr.
Philips Medical Systems, Andover, Mass, USA.
Abstract
BACKGROUND: Leaflet curvature is known to reduce mechanical stress. There are 2 major components that contribute to this curvature. Leaflet billowing introduces the most obvious form of leaflet curvature. The saddle shape of the mitral annulus imparts a more subtle form of leaflet curvature. This study explores the relative contributions of leaflet billowing and annular shape on leaflet curvature and stress distribution.
METHODS AND RESULTS: Both numerical simulation and experimental data were used. The simulation consisted of an array of numerically generated mitral annular phantoms encompassing flat to markedly saddle-shaped annular heights. Highest peak leaflet stresses occurred for the flat annulus. As saddle height increased, peak stresses decreased. The minimum peak leaflet stress occurred at an annular height to commissural width ratio of 15% to 25%. The second phase involved data acquisition for the annulus from 3 humans by 3D echocardiography, 3 sheep by sonomicrometry array localization, 2 sheep by 3D echocardiography, and 2 baboons by 3D echocardiography. All 3 species imaged had annuli of a similar shape, with an annular height to commissural width ratio of 10% to 15%.
CONCLUSION: The saddle shape of the mitral annulus confers a mechanical advantage to the leaflets by adding curvature. This may be valuable when leaflet curvature becomes reduced due to diminished leaflet billowing caused by annular dilatation. The fact that the saddle shape is conserved across mammalian species provides indirect evidence of the advantages it confers. This analysis of mitral annular contour may prove applicable in developing the next generation of mitral annular prostheses.

13. Leaflets Anterior leaflet Posterior leaflet Semicircular
1/3 of annular circumference
Fibrous continuity with NCC and LCC
Defines boundary LVIT and LVOT
Posterior leaflet
Quadrangular
2/3 of annular circumference
Height is less than AMVL

16. Leaflets – Histological Structure
Three layers:
The fibrosa, the solid collagenous core that is continuous with the chordae tendineae
The spongiosa, which is on the atrial surface and forms the leaflet leading edge (it consists of few collagen fibers but has abundant proteoglycans, elastin, and mixed connective tissue cells)
A fibroelastic covering of most of the leaflets
The atrialis is thin and rich in elastin
The ventricularis is much thicker, is confined mostly to the anterior leaflet, and is densely packed with elastin

17. Subvalvular Apparatus
Chordae Tendineae
Papillary Muscles
Left Ventricle

18. Chordae Tendineae
String like structures attaching the ventricular surface or the free edge of the leaflets to the papillary muscles
Chordal fibroblast are metabolically active
Convey blood to the leaflets
Withstand repetitive contractile stress generated by papillary muscles

19. Primary chords Secondary cords Tertiary cords Free margin
Ventricular surface
Tertiary cords
PMVL only
From ventricle to basal zone (near hinge)

20. Chordae Tendineae Commissural chords Cleft chords Strut chords
Originating as a single chord but fanning out to span the commissural region
Cleft chords
Strut chords
one originating from each papillary muscle
Inserting into the AMVL
Strongest and thickest

22. Papillary Muscles Location is variable
Usually arise from apical and middle thirds of left ventricular wall
Commonly described as two muscle groups
Anterolateral (70% single muscle)
Posteromedial (60% two to three muscles)

24. Papillary Muscles Blood Supply
Vessels are deep and mid aspects of muscles
Anterolateral – LAD and Cx
Posteriormedial – RCA or CX
(more vulnerable to ischaemia)

25. Left Ventricle
Lateral wall of left ventricle provides attachment for papillary muscles
Within the left ventricle bridges exist between PM and AL papillary muscles
Direct origin of tertiary chordae

26. Mitral Valve Function Regulated blood flow from LA to LV
No significant gradient
Prevents systolic regurgitation
Integrety is essential to maintain normal LV size, geometry and function

27. Mitral Annulus
Anterior annulus mechanically coupled to aortic annulus (aortomitral curtain)

28. Mitral Annulus Sphincteric contracture Translation
Conformational change during cardiac cycle
Sphincteric contracture
Reduction in annular area by 25%
Maximal in late diastole
Minimal in mid systole
Substantial presystolic contraction is attributable to atrial contraction
Translation
Reduction in LV long axis dimension
Increases LA filling

30. Chordae Tendinae Primary chords Secondary chords
Facilitate valve closure
Maintain leaflet apposition
Sectioning generates acute mitral regurgitation
Variable tension throughout cardiac cycle
Secondary chords
Involved in marinating normal LV size and geometry
Sectioning does not generate mitral regurgitation
Taught ‘like a rubber band’ throughout cardiac cycle

31. Mitral Leaflets Leaflets coapt along their rough zones
Zone of coaption ~ 1cm
Scallops act like pleats so as to allow the leaflets to accommodate the curved line of valve closure.
Smooth surface of AMVL is suited to laminar projection of blood through the LVOT

32. Thank you