A physical model describing the mechanism for formation of gas microbubbles in patients with mitral mechanical heart valves

This study was aimed at developing a physical model, supported by experimen tal observations, to describe the formation and growth of microbubbles seen in patients with mitral mechanical heart valves (MHV). This phenomenon, of ten referred to as high intensity transient signals (HITS), appears as brig ht, intense, high-velocity and persistent echoes detected by Doppler ultras onography at the instant of closure. The long-term clinical implications of HITS has yet to be determined. However, there are reports of a certain deg ree of neurological disorder in patients with mitral MHV. The numerical ana lysis has shown the existence of a twofold process (1) nucleation and (2) m icrobubble growth as a result of cavitation. While mild growth of nuclei is governed by diffusion, explosive growth of microbubbles is controlled by p ressure drop on the atrial side of mitral MHV. It was demonstrated that the re exist limits on both microbubble size and regurgitant velocity, above wh ich microbubbles grow explosively, and below which growth is almost nonexis tent. Therefore, prevention of excessive pressure drops induced by high clo sing velocities related to the dynamics of closure of mitral MHV may offer design changes in the future generations of mechanical valves. (C) 1999 Bio medical Engineering Society. [S0090-6964(99)00706-7].