Ann Thorac Surg 2006;82:1377-1378
© 2006 The Society of Thoracic Surgeons
Original Articles: Cardiovascular
Invited commentary
Tomasz A. Timek, MD
Department of Cardiothoracic Surgery, Stanford University, CVRB Building 2nd Floor, 300 Pasteur Drive, Stanford, CA 94305
(Email: kotek1{at}stanford.edu).
The complexities of the mitral valve continue to challenge and marvel simultaneously. Although the bulk of surgical research has ignored leaflet tissue in the pathophysiology of mitral regurgitation, a rising tide of experimental literature suggests that mitral leaflets have intricate ultrastructure and exhibit non-uniform mechanical behavior and can no longer be dismissed as "inert flaps." The current study by Sacks and colleagues [1] examines the mechanical deformation of the central region of the ovine anterior mitral leaflet during valve closure. The authors show that the anterior leaflet undergoes substantially greater radial than circumferential deformation in vivo, thus confirming their earlier findings of nonhomogenous leaflet strain in vitro. Furthermore, the anterior leaflet undergoes high strain rates during valve closure with no subsequent systolic deformation. These data corroborate prior in situ experiments of other investigators finding that the gradient of radial stretch along the anterior mitral leaflet increases from the mitral annulus to the line of coaptation and follows collagen fiber orientation. With gradual fiber orientation change in the leaflet-strut chordae transition zone, radial extensibility decreases underscoring the influence of the ultrastructure on mechanical properties of the mitral valve. It is reasonable to conjecture that final collagen fiber alignment confers the systolic leaflet stiffness observed in the current study, although analysis of collagen orientation was not performed. Yet reducing leaflet deformation to a simple structural response to a hemodynamic stimulus is itself too limiting. The mitral leaflets have a compound interstitial matrix with glycosaminoglycan and water content being differentially distributed in the central and free edge portions of the human anterior leaflet. Similarly, anterior leaflet remodeling in functional mitral regurgitation has been found to be nonhomogenous with greater collagen deposition and tissue lengthening near the leaflet edge. Arborization of neural networks across the leaflet has also been found to be location specific. Thus, regional variations in leaflet mechanics may be due to differences in local geometry, ultrastructure, load bearing, innervation, or even humoral milieu as growth factors such as transforming growth factor beta 1 have been shown to activate valvular myofiberblasts and extracellular matrix remodeling in vitro. Such structural and functional heterogeneity may have clinical importance as surgical interventions on the leaflet edge, such as chordal transposition or edge-to-edge leaflet approximation would be expected to affect leaflet dynamics differently than centrally oriented techniques such as leaflet extension. The study at hand provides important in vivo mechanical data with clinical implications, and simultaneously challenges the surgeon to embrace an integrated biomechanical, histological, and ultrastructural approach in the clinical evaluation of mitral pathophysiology. It is through such multidisciplinary efforts that the intricate puzzle of the mitral valve will continue to take its proper form.
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- Sacks MS, Enomoto Y, Graybill JR, et al. In-vivo dynamic deformation of the mitral valve anterior leaflet Ann Thorac Surg 2006;82:1369-1378.[Abstract/Free Full Text]
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