Causes and formation of cavitation in mechanical heart valves.

Cavitation may develop on mechanical valvular prostheses in the mitral position; it causes blood damage and, under particularly adverse conditions, it may result in sudden failure of the prosthesis. Therefore, with regard to future development of mechanical heart valves, the pattern of cavitation and its predisposing factors in different types of prostheses were investigated in in vitro studies, which focused on the analysis of valve closure dynamics and the influence of design parameters on the cavitation-inducing pressure drop at the artificial valve. It was found that cavitation is produced primarily by the deceleration of the closing body of the valve. At 900g, the measured deceleration of the closing bodies falls in the range of the decelerations determined in oscillation experiments for investigating cavitation-induced material erosion. The pressure drop produced thereby is overlapped by the pressure drop in accelerated or turbulent flow regions produced by design characteristics at outlet struts, stop faces or sealing lips during backflow through the closing disc. These phenomena exist particularly in regions of high flow velocity, i.e. at the instant of closure at the maximum distance from the bearing axis of the closing body (12 o'clock position). The onset of cavitation is additionally promoted in this position by a tight joint between the closing body and the ring. Oscillations of the closing body generally have a negligible effect on the cavitation behavior. From these relationships one can infer that cavitation can be avoided in future in mechanical heart valves by locally limited design measures. Especially, unsteadiness in the backflow through the closing valve is to be avoided.