Ellis J T, Healy T M, Fontaine A A, Saxena R, Yoganathan A P
Cardiovascular Fluid Mechanics Laboratory, School of Mechanical Engineering, Georgia Institute of Technology, Atlanta 30332-0100, USA.
J Heart Valve Dis. 1996 Nov;5(6):591-9.
During recent clinical trials the Medtronic Parallel bileaflet mechanical heart valve was found to have an unacceptable number of valves with thrombus formation when implanted in the mitral position. Thrombi were observed in the hinge region and also in the upstream portion of the valve housing in the vicinity of the hinge. It was hypothesized that the flow conditions inside the hinge may have contributed to the thrombus formation.
In order to investigate the flow structures within the hinge, laser Doppler anemometry (LDA) measurements were conducted in both steady and pulsatile flow at approximately 70 predetermined sites within the hinge region of a 27 mm Medtronic Parallel mitral valve with transparent housing. The pulsatile flow velocity measurements were animated in time using a graphical software package to visualize the hinge flow field throughout the cardiac cycle.
The LDA measurements revealed that mean forward flow velocities through the hinge region were on the order of 0.10-0.20 m/s. In the inflow channel, a large vortical structure was present during diastole. Upon valve closure, peak reverse velocity reached 3 m/s close to the housing wall in the inflow channel. This area also experienced high turbulent shear stresses (> 6000 dynes/cm2) during the leakage flow phase. A disturbed, vortical flow was again present in the inflow channel after valve closure, while slightly above the leaflet peg and relief the flow was essentially stagnant. The high turbulent stresses near the top of the inflow channel, combined with a persistent vortex, implicate the inflow channel of the hinge as a likely region of thrombus formation.
This experimental investigation revealed zones of flow stagnation in the inflow region of the hinge throughout the cardiac cycle and elevated turbulent shear stress levels in the inflow region during the leakage flow phase. These fluid mechanic phenomena are most likely a direct result of the complex geometry of the hinge of this valve. Although the LDA measurements were conducted at only a limited number of sites within the hinge, these results suggest that the hinge design can significantly affect the washout capacity and thrombogenic potential of the Medtronic Parallel bileaflet mechanical heart valve. The use of LDA within the confines of the hinge region of a mechanical heart valve is a new application, made possible by recent advances in manufacturing technologies and a proprietary process developed by Medtronic that allowed the production of a transparent valve housing. Together, these modalities represent a new method by which future valve designs can be assessed before clinical trials are initiated.
在近期的临床试验中，发现美敦力双叶机械心脏瓣膜植入二尖瓣位置时，出现血栓形成的瓣膜数量多得令人无法接受。在铰链区域以及铰链附近瓣膜外壳的上游部分均观察到血栓。据推测，铰链内部的流动状况可能促成了血栓的形成。
为了研究铰链内部的流动结构，在一个带有透明外壳的27毫米美敦力双叶二尖瓣的铰链区域内，于大约70个预定位置进行了稳定流和脉动流状态下的激光多普勒测速（LDA）测量。利用图形软件包对脉动流速测量结果进行实时动态显示，以观察整个心动周期内的铰链流场。
LDA测量显示，通过铰链区域的平均正向流速约为0.10 - 0.20米/秒。在流入通道中，舒张期存在一个大的涡旋结构。瓣膜关闭时，流入通道靠近外壳壁处的反向峰值速度达到3米/秒。在泄漏流阶段，该区域还经历了高湍流剪切应力（> 6000达因/平方厘米）。瓣膜关闭后，流入通道中再次出现紊乱的涡旋流，而在瓣叶销稍上方和减压处，流动基本停滞。流入通道顶部附近的高湍流应力，加上持续的涡旋，表明铰链的流入通道可能是血栓形成的区域。
该实验研究揭示了在整个心动周期中铰链流入区域存在流动停滞区域，以及在泄漏流阶段流入区域的湍流剪切应力水平升高。这些流体力学现象很可能是该瓣膜铰链复杂几何形状的直接结果。尽管LDA测量仅在铰链内的有限数量位置进行，但这些结果表明，铰链设计可显著影响美敦力双叶机械心脏瓣膜的冲洗能力和血栓形成潜力。在机械心脏瓣膜的铰链区域内使用LDA是一种新的应用，这得益于制造技术的最新进展以及美敦力开发的一项专有工艺，该工艺使得生产透明瓣膜外壳成为可能。总之，这些方法代表了一种新的方式，通过它可以在临床试验启动之前对未来的瓣膜设计进行评估。
