Biancucci B A, Deutsch S, Geselowitz D B, Tarbell J M
Pennsylvania State University, Bioengineering Department, University Park, Pennsylvania 16802, USA.
J Heart Valve Dis. 1999 Mar;8(2):186-96.
Recent clinical research using transcranial Doppler ultrasonography has shown the presence of emboli in the cranial circulation of some mechanical heart valve patients. Due to the high-intensity signals produced by these emboli, it has been suggested that they are small gas bubbles. Meanwhile, transesophageal echocardiography of mechanical heart valve patients has shown images of bright, mobile particles (also considered to be gas bubbles) near the valve. Motivated by these reports, a series of in vitro studies was performed to investigate the relationship between dissolved gas concentration and the incidence of bubble formation after valve closure.
A mock circulatory loop was used to study a Medtronic Hall tilting disc valve in the mitral position of the Penn State Electrical Ventricular Assist Device (EVAD). The valve was videotaped as it operated in saline with various levels of dissolved CO2. A Doppler ultrasound probe served as a bubble detector on the outflow side of the EVAD. Measurements of vaporous cavitation intensity with a high-fidelity pressure transducer were also made. Similar experiments were then performed in porcine blood, using an imaging ultrasound transducer to detect bubbles.
Bubbles were seen moving off the valve in the retrograde direction just after closure. The ultrasound probe detected these bubbles downstream, indicating a bubble lifetime on the order of seconds. It was observed with high-speed video that bubble formation and cavitation are separate events and occur at different times during valve closure. Bubbles were more likely to be observed when CO2 levels were higher. Experiments in blood provided images of bubbles near the valve, predominantly at higher CO2 levels and high valve loading conditions.
These results show that stable gas bubbles can form during mechanical heart valve operation. The bubbles likely form from the combined effects of gaseous nuclei formed by cavitation, low-pressure regions associated with regurgitant flow, and the presence of CO2, a highly soluble gas.
近期使用经颅多普勒超声的临床研究显示，部分机械心脏瓣膜患者的颅内循环中存在栓子。由于这些栓子产生高强度信号，有人认为它们是小气泡。同时，对机械心脏瓣膜患者的经食管超声心动图检查显示，瓣膜附近有明亮、可移动颗粒（也被认为是气泡）的图像。受这些报告的启发，进行了一系列体外研究，以探讨溶解气体浓度与瓣膜关闭后气泡形成发生率之间的关系。
使用模拟循环回路，在宾夕法尼亚州立大学电动心室辅助装置（EVAD）的二尖瓣位置研究美敦力霍尔倾斜盘式瓣膜。在含有不同水平溶解二氧化碳的盐水中操作瓣膜时对其进行录像。在EVAD流出侧使用多普勒超声探头作为气泡探测器。还使用高保真压力传感器测量蒸汽空化强度。然后在猪血液中进行类似实验，使用成像超声换能器检测气泡。
瓣膜关闭后立即可见气泡沿逆行方向从瓣膜处移动。超声探头在下游检测到这些气泡，表明气泡寿命约为几秒。通过高速视频观察到气泡形成和空化是不同的事件，且在瓣膜关闭过程中的不同时间发生。当二氧化碳水平较高时，更有可能观察到气泡。在血液中进行的实验提供了瓣膜附近气泡的图像，主要出现在较高二氧化碳水平和高瓣膜负荷条件下。
这些结果表明，在机械心脏瓣膜操作过程中可形成稳定的气泡。这些气泡可能是由空化形成的气态核、与反流相关的低压区域以及高溶解性气体二氧化碳的共同作用形成的。
