A SQUEEZE FLOW PHENOMENON AT THE CLOSING OF A BILEAFLET MECHANICAL HEART-VALVE PROSTHESIS

In vivo cavitation in cardiovascular flow fields may occur under very unusual circumstances as a localized transient phenomenon which are co nfined to very small legions in the vicinity of the valve body or leaf let surface. The violent collapse of cavitation bubbles induces local erosion that may lead to structural damage. The fluid mechanical facto rs that may cause in vivo cavitation inception in mechanical heart val ve (MHV) prostheses are investigated. It is established that the closi ng velocity of the leaflet holds the key to MHV cavitation. During the final phase of valve closing, the fluid mass in the gap space between the closing occluder and the valve's body is squeezed into motion by the rapidly approaching boundaries. The flow pattern created by this m otion (termed 'squeeze flow'), is found to be related to the valve geo metry, and the impact velocity of the closing leaflet. Given the closi ng velocity of the leaflet and the geometry of the MHV, computational flow dynamics (CFD) are made to determine the velocity distributions i n the gap flow field of a bileaflet MHV in the mitral position. A two dimensional, time dependent model of the gap space show that flow velo city in the gap space can reach values as high as 30 m s(-1) in region s near the edge of the inflow surface of the Edwards Duromedics (ED) M HV leaflet. This high speed stream ejected from the gap channel can cr eate the conditions that characterize cavitation. The location of the isolated high speed region corresponds to the surface erosion that was observed in a number of damaged ED-MHV explants.