A theoretical approach to the prediction of haemolysis in centrifugal blood pumps
The successful use of centrifugal pumps as temporary cardiac assist devices strongly depends on the extent to which they damage blood. The development of a theoretical pump evaluation model was performed in this study to facilitate an effective pump optimisation. The optimisation process seeks to maximise flow performance and minimise blood trauma which is primarily caused by hydrodynamic stresses. A general mechanical blood damage theory was developed which comprises a combination of the information about mechanical loading of blood with the knowledge of its resistance properties. In this theory arbitrary loading-time functions are reduced to simple loading functions for which the damage behaviour is known. A linear damage accumulation theory contributes towards the determination of partial damages and their correlation in the overall damage process. The application of this novel blood damage prediction theory was demonstrated for haemolysis prediction in a commercial centrifugal blood pump. Particle loading-time functions were determined with a 3-dimensional numerical flow analysis of the entire pump domain by means of assigning scalar stress values to particle streaklines. Scalar stress values were obtained by a theory which enables the comparison of a six-component stress tensor with uniaxial stresses as applied in blood damage tests. It was shown that particles undergo a complex, irregularly fluctuating stress loading and that turbulent stresses and flow conditions in the outlet domain are the most critical factors. Haemolysis tests using an oscillating capillary tube setup were performed to investigate blood damage resistance properties under cyclic stress loading in hitherto unexplored amplitude and frequency ranges. A non-linear damage curve for stress amplitude-cycle number was derived which indicated the existence of an endurance strength for red blood cells. For the first time, detailed information about the mechanical loading of blood within a centrifugal pump has been obtained and linked to its traumatic effect. It offers the possibility for an effective, multi-parameter optimisation of blood pumps in the design phase.