Mechanisms of cavitation and the formation of stable bubbles on the Björk-Shiley Monostrut prosthetic heart valve.
Transcranial Doppler studies performed on patients with the Björk-Shiley Monostrut mechanical heart valve have detected signals typical of gaseous emboli. In this study, a high-speed digital imaging system was used to examine the closure event of the Björk-Shiley Monostrut valve in vitro. Observations support the hypothesis that cavitation occurs before the formation of stable gas bubbles. Bubble cavitation occurs at the instant of valve closure and lasts on the order of 0.3 ms. The rebounding motion of the occluder initiates the development of a vortex which induces vortex cavitation. Vortex cavitation begins approximately 0.5 ms after impact of the occluder and the valve housing (approximately 0.2 ms after bubble cavitation has subsided), and lasts for approximately 1 ms. The formation of stable bubbles occurs later, along the center of the vortex that persists throughout much of the first rebound. It is hypothesized that the low-pressure region at the center of the vortex contributes to the formation of stable bubbles by collecting expanded nuclei that arise from both bubble and vortex cavitation, and providing a low-pressure environment in which the nuclei combine and continue to grow.