Bluestein D, Einav S
Department of Mechanical Engineering, Florida International University, Miami 33139.
J Biomech Eng. 1994 Nov;116(4):477-87. doi: 10.1115/1.2895799.
The presence of turbulence in the cardiovascular system is generally an indication of some type of abnormality. Most cardiologists agree that turbulence near a valve indicates either valvular stenosis or regurgitation, depending on the phase of its occurrence during the cardiac cycle. As no satisfying analytical solutions of the stability of turbulent pulsatile flow exist, accurate, unbiased flow stability criteria are needed for the identification of turbulence initiation. The traditional approach uses a stability diagram based upon the stability of a plane Stokes layer where alpha (the Womersley parameter) is defined by the fundamental heart rate. We suggest a modified approach that involves the decomposition of alpha into its frequency components, where alpha is derived from the preferred modes induced on the flow by interaction between flow pulsation and the valve. Transition to turbulence in pulsatile flow through heart values was investigated in a pulse duplicator system using three polymer aortic valve models representing a normal aortic valve, a 65 percent stenosed valve and a 90 percent severely stenosed valve, and two mitral valve models representing a normal mitral valve and a 65 percent stenosed valve. Valve characteristics were closely simulated as to mimic the conditions that alter flow stability and initiate turbulent flow conditions. Valvular velocity waveforms were measured by laser Doppler anemometry (LDA). Spectral analysis was performed on velocity signals at selected spatial and temporal points to produce the power density spectra, in which the preferred frequency modes were identified. The spectra obtained during the rapid closure stage of the valves were found to be governed by the stenosis geometry. A shift toward higher dominant frequencies was correlated with the severity of the stenosis. According to the modified approach, stability of the flow is represented by a cluster of points, each corresponding to a specific dominant mode apparent in the flow. In order to compare our results with those obtained by the traditional approach, the cluster of points was averaged to collapse into a single point that represents the flow stability. The comparison demonstrates the bias of the traditional stability diagram that leads to unreliable stability criteria. Our approach derives the stability information from measured flow phenomena known to initiate flow instabilities. It differentiates between stabilizing and destabilizing modes and depicts an unbiased and explicit stability diagram of the flow, thus offering a more reliable stability criteria.
心血管系统中湍流的存在通常表明存在某种类型的异常。大多数心脏病专家认为，瓣膜附近的湍流表明存在瓣膜狭窄或反流，这取决于其在心动周期中出现的阶段。由于不存在令人满意的湍流脉动流稳定性分析解决方案，因此需要准确、无偏的流动稳定性标准来识别湍流的起始。传统方法使用基于平面斯托克斯层稳定性的稳定性图，其中α（沃默斯利参数）由基本心率定义。我们建议一种改进的方法，该方法涉及将α分解为其频率分量，其中α源自流动脉动与瓣膜相互作用在流动上诱导的优选模式。在脉冲复制器系统中，使用三个代表正常主动脉瓣、65%狭窄瓣膜和90%严重狭窄瓣膜的聚合物主动脉瓣模型以及两个代表正常二尖瓣和65%狭窄二尖瓣的二尖瓣模型，研究了通过心脏瓣膜的脉动流中的湍流转变。瓣膜特性被紧密模拟，以模仿改变流动稳定性并引发湍流条件的情况。通过激光多普勒测速仪（LDA）测量瓣膜速度波形。在选定的空间和时间点对速度信号进行频谱分析，以产生功率密度谱，从中识别出优选频率模式。发现在瓣膜快速关闭阶段获得的频谱受狭窄几何形状的支配。向更高主导频率的转变与狭窄的严重程度相关。根据改进的方法，流动的稳定性由一组点表示，每个点对应于流动中明显的特定主导模式。为了将我们的结果与传统方法获得的结果进行比较，将这组点进行平均以合并为一个代表流动稳定性的单点。比较表明传统稳定性图存在偏差，导致稳定性标准不可靠。我们的方法从已知会引发流动不稳定性的测量流动现象中获取稳定性信息。它区分稳定模式和不稳定模式，并描绘出流动的无偏且明确的稳定性图，从而提供更可靠的稳定性标准。
