Einstein Daniel R, Kunzelman Karyn S, Reinhall Per G, Nicosia Mark A, Cochran Richard P
Department of Bioengineering, University of Washington, Seattle, Washington 98195, USA.
J Biomech Eng. 2005 Feb;127(1):134-47. doi: 10.1115/1.1835359.
Many diseases that affect the mitral valve are accompanied by the proliferation or degradation of tissue microstructure. The early acoustic detection of these changes may lead to the better management of mitral valve disease. In this study, we examine the nonstationary acoustic effects of perturbing material parameters that characterize mitral valve tissue in terms of its microstructural components. Specifically, we examine the influence of the volume fraction, stiffness and splay of collagen fibers as well as the stiffness of the nonlinear matrix in which they are embedded.
To model the transient vibrations of the mitral valve apparatus bathed in a blood medium, we have constructed a dynamic nonlinear fluid-coupled finite element model of the valve leaflets and chordae tendinae. The material behavior for the leaflets is based on an experimentally derived structural constitutive equation. The gross movement and small-scale acoustic vibrations of the valvular structures result from the application of physiologic pressure loads. Material changes that preserved the anisotropy of the valve leaflets were found to preserve valvular function. By contrast, material changes that altered the anisotropy of the valve were found to profoundly alter valvular function. These changes were manifest in the acoustic signatures of the valve closure sounds. Abnormally, stiffened valves closed more slowly and were accompanied by lower peak frequencies.
The relationship between stiffness and frequency, though never documented in a native mitral valve, has been an axiom of heart sounds research. We find that the relationship is more subtle and that increases in stiffness may lead to either increases or decreases in peak frequency depending on their relationship to valvular function.
许多影响二尖瓣的疾病都伴随着组织微观结构的增殖或退化。对这些变化进行早期声学检测可能有助于更好地管理二尖瓣疾病。在本研究中，我们研究了扰动二尖瓣组织微观结构成分特征的材料参数所产生的非平稳声学效应。具体而言，我们研究了胶原纤维的体积分数、刚度和展布以及它们所嵌入的非线性基质的刚度的影响。
为了模拟浸泡在血液介质中的二尖瓣装置的瞬态振动，我们构建了一个动态非线性流体耦合有限元模型，用于模拟瓣膜小叶和腱索。小叶的材料行为基于实验得出的结构本构方程。瓣膜结构的整体运动和小规模声学振动是由生理压力负荷的施加引起的。发现保持瓣膜小叶各向异性的材料变化能保持瓣膜功能。相比之下，发现改变瓣膜各向异性的材料变化会深刻改变瓣膜功能。这些变化在瓣膜关闭声音的声学特征中表现出来。异常的是，变硬的瓣膜关闭得更慢，且伴随的峰值频率更低。
刚度与频率之间的关系，尽管在天然二尖瓣中从未有过记录，但一直是心音研究的一个公理。我们发现这种关系更为微妙，刚度的增加可能导致峰值频率增加或减少，这取决于它们与瓣膜功能的关系。
