In vitro testing of artificial heart valves: comparison between Newtonian and non-Newtonian fluids

The in vitro testing of artificial heart valves is often performed with simple fluids like glycerol solutions. Blood, however, is a non-Newtonian fluid with a complex viscoelastic behavior, and different flow fields in comparable geometries may result. Therefore, we used different polymer solutions (Polyacrylamid, Xanthan gum) with blood-like rheological properties as well as various Newtonian fluids (water, glycerol solutions) in our heart valve test device. Hydrodynamic parameters of Björk-Shiley heart valves with a tissue annulus diameter (TAD) of 21-29 mm were investigated under aortic flow conditions. Major results can be summarized as follows. The mean systolic pressure differences depend on the model fluids tested. Closing time and closing volume are not influenced by the rheological behavior of fluids. These parameters depend on TAD and the pressure differences across the valve. In contrast, rheological behavior has a pronounced influence upon leakage flow and leakage volume, respectively. Results show furthermore that the apparent viscosity data as a function of shear rate are not sufficient to characterize the rheological fluid behavior relevant to hydrodynamic parameters of the heart valves investigated. Therefore, similarity in the yield curves of non-Newtonian test fluids mimicing blood is only a pre-requisite for a suitable test fluid. More information about the viscous and elastic component of the fluid viscosity is required, especially in geometries where a complex flow field exists as in the case of leakage flow.