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Mitral Valve Structure & Function
David J McCormack BSc(Hons) MBBS MFSTEd FRCSEd(CTh) FRCS (CTh) Consultant Surgeon Waikato Cardiothoracic Unit
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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
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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
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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.
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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
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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
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Subvalvular Apparatus
Chordae Tendineae Papillary Muscles Left Ventricle
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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
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Primary chords Secondary cords Tertiary cords Free margin
Ventricular surface Tertiary cords PMVL only From ventricle to basal zone (near hinge)
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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
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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)
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Papillary Muscles Blood Supply
Vessels are deep and mid aspects of muscles Anterolateral – LAD and Cx Posteriormedial – RCA or CX (more vulnerable to ischaemia)
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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
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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
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Mitral Annulus Anterior annulus mechanically coupled to aortic annulus (aortomitral curtain)
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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
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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
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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
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Thank you
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