Subramanian A, Mu H, Kadambi J R, Wernet M P, Brendzel A M, Harasaki H
Department of Mechanical and Aerospace Engineering, Case Western Reserve University, Cleveland, Ohio 44106, USA.
J Heart Valve Dis. 2000 Sep;9(5):721-31.
Our previous studies of bileaflet mechanical heart valves (MHV) explanted from sheep revealed patterns of localized platelet aggregation on valve surfaces, which may have clinical relevance. Since flow phenomena may promote localized platelet aggregation, an evaluation of flow within a valve lumen was conducted.
Phase-locked particle image velocimetry (PIV) measurements were obtained within the lumen of a 'mitral' model bileaflet MHV with transparent acrylic leaflets and housing, in a pulsatile flow loop. Instantaneous, two-dimensional flow maps of a central plane, perpendicular to the flow and leaflet pivot axes, were obtained at discrete times during the simulated cardiac cycle. Flow conditions were cardiac output, 3.5 l/min; rate, 72 beats/min; and systolic duration, 300 ms, using blood analog fluid refractive index-matched to acrylic. Leaflet closing velocities and angles were found using double-exposure imagery, and maximum leaflet closing velocity was extrapolated from regression analysis.
During full opening, flow within the three lumenal orifices formed a three-peak axial velocity profile. Vorticity was concentrated in shear layers adjacent to downstream leaflet surfaces and in downstream wakes. Forward flow peak velocity was 90 cm/s, with a steep velocity gradient in the central orifice. During closing, the central-gap regurgitant flow formed a jet (peak velocity, 144 cm/s). High vorticity occurred near leaflet leading and trailing edges. During full closure, first a transient (<3 ms) 'stopping vortex' developed near the leaflet trailing edge, followed by a wall jet which formed at the leaflet-housing junction. Maximum leaflet closing velocity was 1.4 m/s.
Localized jets, steep velocity gradients, high vorticity and vortex recirculation have been observed in vitro near model MHV surfaces. In vivo, each of these flow phenomena, when occurring near valve surfaces, may promote localized platelet aggregation. For the acrylic leaflets, maximum velocity was comparable with results reported for pyrolytic carbon leaflets. PIV of fully transparent models is a promising method for evaluating lumenal flows.
我们之前对从绵羊体内取出的双叶机械心脏瓣膜（MHV）的研究揭示了瓣膜表面局部血小板聚集的模式，这可能具有临床意义。由于流动现象可能促进局部血小板聚集，因此对瓣膜腔内的流动进行了评估。
在脉动流回路中，对具有透明丙烯酸瓣叶和外壳的“二尖瓣”模型双叶MHV的腔内进行锁相粒子图像测速（PIV）测量。在模拟心动周期的离散时间获取垂直于流动和瓣叶枢轴的中心平面的瞬时二维流图。流动条件为心输出量3.5升/分钟、心率72次/分钟、收缩期持续时间300毫秒，使用与丙烯酸折射率匹配的血液模拟流体。通过双曝光图像确定瓣叶关闭速度和角度，并通过回归分析推断最大瓣叶关闭速度。
在完全打开期间，三个腔口内的流动形成了三峰轴向速度分布。涡度集中在下游瓣叶表面附近的剪切层和下游尾流中。向前流动的峰值速度为90厘米/秒，中心腔口处速度梯度陡峭。在关闭期间，中心间隙反流形成一股射流（峰值速度为每秒144厘米）。在瓣叶前缘和后缘附近出现高涡度。在完全关闭期间，首先在瓣叶后缘附近形成一个短暂的（<3毫秒）“停止涡”，随后在瓣叶与外壳的交界处形成壁面射流。最大瓣叶关闭速度为1.4米/秒。
在体外模型MHV表面附近观察到了局部射流、陡峭的速度梯度、高涡度和涡旋再循环。在体内，这些流动现象中的每一种，当在瓣膜表面附近发生时，都可能促进局部血小板聚集。对于丙烯酸瓣叶，最大速度与热解碳瓣叶报道的结果相当。完全透明模型的PIV是评估腔内流动的一种有前景的方法。
