Influence of viscosity and pressure on prosthetic valve regurgitation.

Blood viscosity varies during the course of artificial heart implants and is affected by pathological conditions. To gauge the potential effect of changing viscosity on valve performance, leakage rates were measured across a closed Medtronic-Hall valve with water, water/glycerol and fresh whole bovine blood for aortic and pulmonary pressure ranges. As might be expected from the low Reynolds numbers (< 140), losses across the valve were found to be primarily viscous. For the two Newtonian fluids, leakage was slightly less than linearly proportional to pressure. This is comparable with empirical data for orifice flow, which predicts three fifths power dependence on pressure. For blood, however, the greater than linear dependence on pressure found suggests that the pseudoplasticity (shear-thinning behavior) of blood is important. These data provide evidence that the viscous and non-Newtonian properties of blood must be taken into account in modelling prosthetic valve performance and may affect the test methods and flow regulation strategies for prosthetic blood pumps.