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Eur J Cardiothorac Surg 2003;24:364-370
© 2003 Elsevier Science NL

Mitral mechanical heart valves: in vitro studies of their closure, vortex and microbubble formation with possible medical implications

^a Faculty of Medicine, Department of Cardiac Surgery, Rambam Medical Center, Haifa 31096, Israel
^b Cardiovascular Fluid Dynamic Research Laboratory, California Institute of Technology, Pasadena, CA, USA
^c Faculty of Mechanical Engineering, Technion – Institute of Technology, Haifa, Israel

Received 13 December 2002; received in revised form 17 May 2003; accepted 26 May 2003.

^* Corresponding author. Tel.: +972-4-854-2631; fax: +972-4-854-2949
e-mail: smilo{at}rambam.health.gov.il

Objective: The goal of the present work was to create the closest possible in vitro fluid dynamic environment in which prosthetic mitral valves in the patients’ hearts function, in order to demonstrate whether microbubbles are generated, and if yes, under what conditions and at which stage of the cardiac cycle. Microbubbles were observed in the blood of patients with mitral mechanical heart valves (MHV) by means of echocardiography. The phenomenon, often referred to as high-intensity transient signals (HITS), appears as bright, intense, high-velocity and persistent echoes detected by Doppler echocardiography at the instant of valve closure. The question is no longer whether microbubbles are being formed in patients with MHV. as an inherent aspect of their design, but rather how they evolve and when. The answer to this question was the objective of the present paper. Methods: Hemodynamic conditions in which microbubbles were observed in patients with mitral MHV were simulated in our laboratory. We were able to describe the bubble formation process, as one consisting of nucleation and microbubble growth. While mild growth of nuclei is governed by diffusion, extensive growth of microbubbles is controlled by pressure drop during deceleration of the leaflets on the housing on the atrial side of the mitral MHV. Results: The present study has shown that bubbles form in a fluid at the instant of closure of mechanical valves. The formation of vortices after valve closure, although clinically not yet observed, was also demonstrated in the present in vitro studies. We believe that impact of such vortices on the endothelial layer of the left atrial wall may have clinical significance. These two phenomena were not observed in bioprosthetic valves. Conclusions: As demonstrated, there exist two distinct phenomena characteristic of mechanical heart valves, which take place during valve closure, namely, that of vortex formation and that of microbubble growth. Both phenomena may have far reaching clinical implications.

Key Words: Mitral valve • Mechanical heart valve • Microbubble formation • Valve closure • High intensity transient signal

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