Mechanical valve closing dynamics: relationship between velocity of closing, pressure transients, and cavitation initiation.

In this study, the closing dynamics of mechanical heart valves was experimentally analyzed with the valves mounted in the mitral position of an in vitro flow chamber simulating a single closing event. The average linear velocity of the edge of the leaflet during the final 2.065 degrees of the traverse before closing was measured using a laser sweeping technique, and the negative pressure transients at 2 mm from the leaflet inflow surface in the fully closed position was recorded at the instant of valve closure. The cavitation number was computed for the various mechanical valves at a range of load at valve closure. The data were correlated with cavitation bubble visualization previously obtained with the same experimental set up. Cavitation incipience with mechanical valves was found to be independent of the flexibility of the valve holder. For the same loading rate at valve closure, valves with flexible (polyethylene) leaflets were found to close with comparable velocity to those with rigid (pyrolytic carbon) leaflets, but the negative pressure transients did not reach magnitudes close to the vapor pressure for the fluid with flexible leaflets. For the same leaflet closing velocity (and hence the cavitation number), valves with a seat stop or a seating lip in the region of maximum leaflet velocity were observed to cavitate earlier, suggesting that the effect of "squeeze flow" may be an important factor in cavitation incipience.