Pressure field in the vicinity of mechanical valve occluders at the instant of valve closure: correlation with cavitation initiation.

In order to analyse cavitation dynamics during the closing phase of mechanical heart valve prostheses, an experimental study was performed to simulate a single closing event of the occluder with pressure measurements close to the occluder surface on the inflow side at the instant of valve closure. Cavitation bubble visualization was also performed using a stroboscopic lighting technique and photographic recording with tilting disc and bileaflet valve geometries. Large negative pressure transients were measured in the major orifice region of tilting disc valves and in the peripheral edges of bileaflet valves. The intensity of cavitation increased with increasing loading rate (transvalvular pressure rise rate) during the closing phase and with increasing valve size. The squeezing of fluid between the occluder and the seat stop or seating lip in the region where the occluder velocity at closure is at its maximum, and the resulting local flow dynamics are suggested as mechanisms for initiation of cavitation. The fluid forced through the clearance region due to the instantaneous large pressure difference across the occluder may be another source of large negative pressures and cavitation. The present in vitro study also suggests that cavitation may be the cause of platelet and red blood cell destruction, and ensuing thrombus formation associated with mechanical heart valves.