Mechanical heart valves (MHVS) are widely used for the replacement of natural valves as well as in ventricular assist devices and artificial hearts. Valves can cause blood damage which may lead to hemolysis and thromboembolism. Hemodynamic stresses imposed on blood elements as they pass through the valve play a major role in blood damage. In addition, the formation and collapse of cavitation bubbles near mechanical heart valves at closure have been implicated in both blood element and valve material damage. A related problem is the generation of stable gas bubbles by MHVs which show up as emboli in the cranial circulation and are detected as high intensity transient signals (HITS) in transcranial Doppler diagnostics. In the next grant period we will focus on determining the detailed mechanisms which lead to stable gas bubble formation on current MHVs and measuring the fluid stresses and flow structures very close to the valve housings. This study will provide the basic science required to support the development of a new generation of MHVs with reduced thromboembolic potential.
The specific aims of the proposed research are: 1. To observe the formation of stable gas bubbles on MHVs using C02-supplemented test fluids and HSV. Valves with observation windows in their housings will be used to record the bubble formation process at framing rates up to 10000 I sec. An ultrasound Doppler system will be used to quantify stable bubble formation rates. Modified valves (occluder material and gap width) will also be studied and the bubble formation process will be related to the fluid mechanical structures observed under specific aim 2. In this way we will be able to test the hypothesis that vortex structures sustain bubble growth from nuclei generated by cavitation. 2. To determine the near-valve flow characteristics during and shortly after valve closure that are associated with the generation of Stable gas bubbles and elevated turbulent stresses. We will modify existing valves by cutting windows into the metal valve rings (housings) which will allow us to observe and quantify fluid flow structures and turbulence levels very close to the housing where cavitation and stable gas bubble formation are initiated. By retaining most of the valve housing intact, we will not alter the normal valve closing dynamics and energy transfer. We will use LDV and high resolution PIV to assess turbulent stresses and focal flow structures. To further enhance our understanding of mechanisms, we will alter closing dynamics and flow structures by using different disk materials (Delrin and pyrolytic carbon), which are known to alter cavitation potential, as well as valves of the same materials but with different gaps between the occluder housing which are expected to generate different vorticity structures.
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