|摘要: ||本研究採用＃27的St. Jude Medical（SJM）雙葉片機械人工心瓣進行主動脈體外循環，實驗主要模擬人體左心循環運作，其參數設定為符合人體的血壓及流量波形，壓力的設定主動脈壓為80~120mmHg；左心室壓為0~120mmHg；心跳頻率為每分鐘70下、90下、120下，其每分鐘心臟輸出量為5升、6升、7.5升。利用質點流速儀（Digital Particle Image Velocimetry, DPIV）於主動脈瓣上游端進行心瓣關閉瞬間流場量測。量測位置位於心臟關閉時能產生回流量的位置，即主動脈瓣上游端葉片與葉片之間和葉片與心瓣環座之間。DPIV受PCB壓力計的觸發量測心瓣關閉瞬間流場，利用延遲產生器觸發0.3ms、2ms、5ms、10ms四個時間相位。利用Rankine vortex模式進行渦漩的定量分析，即可求出渦漩中心半徑、切向速度、環流強度與壓降值。|
The SJM 25 mm bileaflet test valve was positioned in the aorta position of a pulsatile mock circulatory loop system. The main experimental simulation human the left cardiac cycle. Its parameters set to comply with the body''s blood pressure and flow waveforms, the pressures in the aortic, left ventricle and left atrium were maintained at 80-120 mmHg, 0-120 mmHg and 5-7 mmHg. The pulse rates were set at 70, 90, and 120 bpm, with respective to the cardiac outputs of 5 L/min, 6 L/min, and 7.5 L/min. The heart valves shut down instantly, the flow field measurements were made with a Digital Particle Image velocimeter (DPIV) at the upstream end of aortic heart valves. The measurement points were primarily located at gaps between the closed leaflets during closure, between the leaflets and housing ring. The DPIV system was triggered by a PCB, it was also used to further trigger a pulse delay generator for instantaneous measurement of the flow field at 0.3, 2, 5, and 10 ms after the impact. And we use Rankine vortex model to quantitatively analyze the role of vortex, that contains information about vortex core redius, tangential velocity, circulation strength and maximum pressure drop.
In recent years, many scholars have found that the backflow velocity is relatively large than the forward flow in the valve closure. Results show that the peak values of backflow flow velocity during closure are concentrate at the center of the gap between the leaflets, that bring about the peak velocity. So the heart valves shut down instantly generate high flow rate, may damage blood cells and heart valves.
It’s noted that instantaneous valve closure,occluder rebound and high-speed leakage flow generate vortices. The PIV measurements confirmed the formation of these large-scale vortices. The maximum pressure drop in the vortex center is roughly 14.856 mmHg. Since cavitation formation requires the local pressure to drop below vapor pressure (about -740 mmHg),Our results clearly showed that vortex formation with a pressure drop of this order of magnitude cannot provide significant contribution to mechanical heart valve cavitation.