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heart valves occur within a few milliseconds after valve closure and have been observed on the upstream side of various valve types in vitro using stroboscopic videogra-phy on our group.19 Vaporous cavitation can damage valve materials, and there is evidence of surface pitting on valves explanted from animals and humans which indicates that cavitation does indeed occur in vivo.7 Us-ing a high fidelity pressure transducer mounted on the inlet cannula of a Penn State electric ventricular assist On the Medtronic–Hall valve, the major site of va-porous cavitation bubble formation is around the valve stop on the major orifice side of the disk.9,10,19 It has been suggested that this location is favored because at the instant of impact between the high velocity occluder tip and the stop, the fluid is ‘‘squeezed’ ’ between the stop and the occluder, and a local flow field with high velocity ~and implicitly, low pressure! is created which favors cavitation.9,10 Vaporous cavitation is also ob-served on the minor orifice side of the Bjork–Shiley Monostrut ~BSM! valve in the region where the occluderFlow Visualization in