A Practical Application of Sonoluminescence to the Evaluation of the Cavitation Potential of the Mechanical Heart Valve

One of the causes of the breakdown of the mechanical heart valves (MHVs) is cavitation. A reliable in vitro test method to evaluate the cavitation potential has to be developed for the patients’ safety. It is the bubble implosion that damages the materials, thus observing the cavity implosion behavior is essential. According to sonoluminescence, the collapsing cavitation bubbles emit the faint light. Therefore, our group tried to analyze the cavitation on the MHVs by observing the faint light. Throughout the experiments, a 20-mm Björk-Shiley valve was used as a specimen, and this valve was submerged in the water tank of 10 L deionized water. The pressure difference of 120-150 mmHg was exerted on the valve at a rate of 60 bpm with a pulse duplicator, assuming the mitral valve prosthesis. All the system was settled in the lighttight box. First, the highly sensitive CCD camera with an image intensifier was tried to capture the spatial distribution of the faint light. After two-hour exposure, two-dimensional images of the light were obtained successfully. The light was detected around the positions where the bubble images were captured with the stroboscopic method. Second, the photon counting system was developed using a photomultiplier tube for the temporal analysis of the light emission. This system revealed that the light was emitted immediately after the bubbles disappeared on the stroboscopic photos. These spatial and temporal analyses of the light emission showed that the light came from the collapsing cavities. According to this study, the behavior of the cavity implosion on the MHV was successfully obtained based on the faint light detection. Our group will examine the reliability of this method and try to quantify the cavitation potential of the MHVs. INTRODUCTION The replacement of the diseased heart valves by prostheses has been an established medical treatment for more than 40 years. The introduction of pyrolytic carbon in the 1970s brought the excellent biocompatibility and reduced the durability problems of the mechanical heart valves (MHVs) [1]. In the late 1980s, however, it was reported that some MHVs were broken in in vivo application [2], and this kind of damage was also confirmed in in vitro tests [3,4]. The careful investigation of the damage concluded that these troubles could be induced by cavitation. Then the cavitation has been drawing great attention of the researchers, the manufacturers and the regulating agencies of MHVs. The history of the MHV cavitation research was summarized in reference [5]. Since the early 1990s, a simple and reliable in vitro test method of the cavitation has been tried to be developed to evaluate the cavitation potential of MHVs. The U.S. Food and Drug Administration (FDA) proposed its protocol and conducted interlaboratory comparison of the test results [6]. Other researchers also tried their own evaluation methods [4,7,8]. These methods could not lead to the consistent conclusion about the cavitation potential of MHVs. Thus, there is still no standard protocol for in vitro cavitation tests on MHVs. The damage due to cavitation occurs at the moment of the cavity collapse, so that the behavior of the bubble implosion must be observed in order to analyze the cavitation damage. Because most of the proposed methods were based on the visual detection of the cavitation bubbles using stroboscopes or high speed cameras, these methods couldn’t detect the cavity collapse with their insufficient spatial and temporal resolutions. Some researchers applied another method to detect the