Mitral heart valve cavitation in an artificial heart environment

The formation and subsequent collapse of vaporous cavities in the fluid around mechanical heart valves at valve closure can create stresses large enough to damage both the valve itself and blood cells. Improved understanding of cavitation mechanisms should lead to a reduction in the cavitation potential of future valve designs.

This study compares eight mechanical mitral valves of two different geometries (Monostrut and Medtronic Hall), occluder housing gaps (tight, medium, and leaky), and occluder materials (Delrin and pyrolytic carbon). The valves were evaluated in a model ventricle of the Penn State Electric Ventricular Assist Device (EVAD) operating within a mock circulatory loop. The EVAD represents one half of a total artificial heart. The mock loop consisted of silicone tubing connected to elements designed to mimic the compliant and resistant properties of the natural circulation. Cavitation was controlled by varying the degree of filling of the ventricle: low filling caused higher valve closing velocities resulting in greater cavitation intensities than complete filling of the ventricle. The intensity of cavitation was quantified using a parameter derived from the high frequency fluctuations in the mitral pressure that occur around the valve during cavitation events. The shape of the cavitation pressure signature and that of the power spectrum of the cavitation pressure signature were used in addition to the cavitation intensity parameter to make comparisons between valves.

Of the three valve characteristics studied, occluder material showed the most significant influence on cavitation intensity: valves with pyrolytic carbon occluders demonstrated greater cavitation than did those with Delrin discs.

It is hypothesized that the dominant form of cavitation on the valves studied is related to vortex formation and that occluder material influences the intensity of cavitation through the strength of the tension wave generated at valve closure, while geometry and gap have only secondary effects. Future studies are planned to incorporate this technique in an in vivo environment.