Govindarajan V, Udaykumar H S, Herbertson L H, Deutsch S, Manning K B, Chandran K B
Department of Biomedical Engineering, University of Iowa, Iowa City, IA-52242 USA.
J Med Device. 2010 Mar 1;4(1):11001. doi: 10.1115/1.4000876.
The fluid dynamics during valve closure resulting in high shear flows and large residence times of particles has been implicated in platelet activation and thrombus formation in mechanical heart valves. Our previous studies with bi-leaflet valves have shown that large shear stresses induced in the gap between the leaflet edge and the valve housing results in relatively high platelet activation levels whereas flow between the leaflets results in shed vortices not conducive to platelet damage. In this study we compare the result of closing dynamics of a tilting disc valve with that of a bi-leaflet valve. The two-dimensional fluid-structure interaction analysis of a tilting disc valve closure mechanics is performed with a fixed grid Cartesian mesh flow solver with local mesh refinement, and a Lagrangian particle dynamic analysis for computation of potential for platelet activation. Throughout the simulation the flow remains in the laminar regime and the flow through the gap width is marked by the development of a shear layer which separates from the leaflet downstream of the valve. Zones of re-circulation are observed in the gap between the leaflet edge and the valve housing on the major orifice region of the tilting disc valve and are seen to be migrating towards the minor orifice region. Jet flow is observed at the minor orifice region and a vortex is formed which sheds in the direction of fluid motion as observed in experiments using PIV measurements. The activation parameter computed for the tilting disc valve, at the time of closure was found to be 2.7 times greater than that of the bi-leaflet mechanical valve and was found to be in the vicinity of the minor orifice region mainly due to the migration of vortical structures from the major to the minor orifice region during the leaflet rebound of the closing phase.
瓣膜关闭过程中的流体动力学导致高剪切流和颗粒的长时间停留，这与机械心脏瓣膜中的血小板活化和血栓形成有关。我们之前对双叶瓣膜的研究表明，在瓣叶边缘与瓣膜外壳之间的间隙中产生的大剪切应力会导致相对较高的血小板活化水平，而瓣叶之间的流动会产生不利于血小板损伤的脱落涡流。在本研究中，我们比较了倾斜盘式瓣膜与双叶瓣膜的关闭动力学结果。使用具有局部网格细化的固定网格笛卡尔网格流动求解器对倾斜盘式瓣膜关闭力学进行二维流固耦合分析，并使用拉格朗日粒子动力学分析来计算血小板活化的可能性。在整个模拟过程中，流动保持在层流状态，通过间隙宽度的流动以剪切层的发展为特征，该剪切层在瓣膜下游与瓣叶分离。在倾斜盘式瓣膜主要孔口区域的瓣叶边缘与瓣膜外壳之间的间隙中观察到再循环区域，并且这些区域朝着次要孔口区域迁移。在次要孔口区域观察到射流，并形成一个涡流，该涡流在流体运动方向上脱落，这与使用粒子图像测速（PIV）测量的实验中观察到的情况一致。在关闭时为倾斜盘式瓣膜计算的活化参数被发现比双叶机械瓣膜的活化参数大2.7倍，并且主要由于在关闭阶段瓣叶回弹期间涡旋结构从主要孔口区域向次要孔口区域的迁移，该活化参数在次要孔口区域附近被发现。
