A numerical analysis of the backflow between the leaflets of a St Jude Medical cardiac valve prosthesis

Clinically significant, unexplained hemolysis has been reported to occur in St Jude Medical (SJM) cardiac valve prostheses. The leakage phase of backflow is identified as having the most hemolytic potential, when compared with the other phases of the cardiac cycle. A two-dimensional, laminar, constant fluid property finite element analysis (FEA) is used to calculate the peak shear stress obtained in the flow through the narrow slit formed by the closed leaflets of an idealized size 29 mm (tissue annulus diameter) SJM cardiac valve prosthesis, during the leakage phase of backflow. The flow geometry was such that the simple laminar flow theory (SLFT) could be used to model the flow through the region of peak shear stresses with reasonable accuracy. The SLFT predicts that the maximum shear stress depends upon the thickness of the clearance space and the average velocity through that space. These results suggest that the magnitude of the peak shear stress is of the order of 700-1000 Pa for a duration of the order of 0.5-0.4 ms for pressure drops across the valve of 150-300 torr. This suggests that hemolysis is possible for certain unfortunate combinations of clearances and pressure conditions. However, further research is needed before this flow phenomenon can be associated with the reported clinical hemolysis.