RESUMO
The velocity fields downstream of four prosthetic heart valves were mapped in vitro over the entire cross-section of a model aortic root using laser Doppler anemometry. THe Björk-Shiley 60 degrees convexo-concave tilting disc valve, the Smeloff-Cutter caged ball valve, the St. Jude Medical bileaflet valve, and the Ionescu-Shiley standard bioprosthesis were examined under both steady and pulsatile flows. Velocity profiles under steady flow conditions were a good approximation for pulsatile profiles only during midsystole. The pulsatile flow characteristics of the four valves showed variation in large scale flow structures. Comparison of the valves according to pressure drop, shear stress and maximum velocities are also provided.
Assuntos
Valva Aórtica , Próteses Valvulares Cardíacas , Modelos Cardiovasculares , Bioprótese , Velocidade do Fluxo Sanguíneo , Pressão Sanguínea , Humanos , Desenho de Prótese , Fluxo Sanguíneo Regional , Reologia , Estresse MecânicoRESUMO
Thrombus formation on the aortic face of the disc and tissue overgrowth along the sewing ring adjacent to minor outflow region are major clinical pathologic findings observed with the present model Björk-Shiley aortic prosthesis. These pathologic findings may be attributed to the stagnation zone observed near the aortic face of the disc, and the low flow and shear in the minor outflow region of the prosthesis. In hope of reducing these pathologic conditions the new convexo-concave Björk-Shiley prosthesis was developed. In vitro velocity measurements made with a laser-Doppler anemometer in the immediate downstream vicinity of the convexo-concave Björk-Shiley aortic valve indicate that the design changes have decreased the size of the stagnation zone and have increased flow and shear in the minor outflow region. We conclude that the problems of thrombus formation and tissue overgrowth may be reduced in the new convexo-concave Björk-Shiley aortic valve.
Assuntos
Valva Aórtica/fisiopatologia , Próteses Valvulares Cardíacas , Valva Aórtica/cirurgia , Fenômenos Biomecânicos , Velocidade do Fluxo Sanguíneo , Próteses Valvulares Cardíacas/efeitos adversos , Modelos Biológicos , Desenho de Prótese , Trombose/etiologiaRESUMO
Thrombus formation and tissue overgrowth were observed in nine Björk-Shiley aortic prostheses recovered six months or longer after implantation. These pathologic findings may be attributed to the flow characteristics of the prosthesis. The open disc of the valve separates the flow into two unequal regions. Varying degrees of thrombus formation were observed in the minor outflow region, including the depression in the aortic face of the disc and the metal strut bridging this area. Tissue overgrowth was noted along the perimeter of the prosthesis adjacent to the minor outflow region. That overgrowth further reduced the available cross section for flow in this already constrained area. In vitro velocity measurements with a laser-Doppler anemometer identified a zone of stagnation about 20 mm wide near the aortic face of the disc. The average velocities in the major and minor outflow regions were around 100 and 25 cm/sec, respectively, and the corresponding peak-shear stresses were approximately 700 and 150 dynes/cm2. There is reason, then, to attribute the thrombus formation and tissue overgrowth to the stagnation zone and the low shear in the minor outflow region.
