Steady flow velocity field and turbulent stress mappings downstream of a porcine bioprosthetic aortic valve in vitro.

ABSTRACT

Velocity profiles and Reynolds stresses downstream of heart valve prostheses are vital parameters in the study of hemolysis and thrombus formation associated with these valves. These parameters have previously been evaluated using single-point measurement techniques such as laser Doppler anemometry (LDA). The purpose of this study is to map the velocity vector fields and Reynolds stresses downstream of a porcine bioprosthetic heart valve in the aortic root region with particle image velocimetry (PIV) techniques in vitro under steady flow conditions. PIV is essentially a multipoint measurement technique that allows full-field measurement of instantaneous velocity vectors in a flow field, thus allowing us to map the entire velocity or stress field over the aortic root (where single-point measurements are difficult). Coupled with flow visualization techniques, the hydrodynamic consequences of introducing a porcine bioprosthetic heart valve into the aortic root was examined, and compared with data obtained from an empty aortic root and an aortic root with the valve mounting ring alone. From our velocity and stress mappings, we found that the valve mounting ring effectively diminishes the central orifice area, giving rise to a higher central axial flow with strong recirculating regions and a corresponding large pressure drop. This in turn produces an intermixing zone between the central jet and recirculating region further downstream from the valve, which contributes to the high-stress zone measured. The development of the flow is further restricted by the valve stents, giving rise to stagnation regions and wakes. High-velocity gradients were also measured at the interface of the jet and recirculating region in the sinus cavity. The overall view of the velocity and stress mappings helps to identify regions of flow disturbances that otherwise may be lost with single-point measuring systems. Although the PIV measurements may lack the accuracy of single-point measuring systems, the overall view of the flow in the aortic root region compensates for the shortcoming.