A quantitative comparison of mechanical blood damage parameters in rotary ventricular assist devices: shear stress, exposure time and hemolysis index

Ventricular assist devices (VADs) have already helped many patients with heart failure but have the potential to assist more patients if current problems with blood damage (hemolysis, platelet activation, thrombosis and emboli, and destruction of the von Willebrand factor (vWf)) can be eliminated. A step towards this goal is better understanding of the relationships between shear stress, exposure time, and blood damage and, from there, the development of numerical models for the different types of blood damage to enable the design of improved VADs. In this study, computational fluid dynamics (CFD) was used to calculate the hemodynamics in three clinical VADs and two investigational VADs and the shear stress, residence time, and hemolysis were investigated. A new scalar transport model for hemolysis was developed. The results were compared with in vitro measurements of the pressure head in each VAD and the hemolysis index in two VADs. A comparative analysis of the blood damage related fluid dynamic parameters and hemolysis index was performed among the VADs. Compared to the centrifugal VADs, the axial VADs had: higher mean scalar shear stress (sss); a wider range of sss, with larger maxima and larger percentage volumes at both low and high sss; and longer residence times at very high sss. The hemolysis predictions were in agreement with the experiments and showed that the axial VADs had a higher hemolysis index. The increased hemolysis in axial VADs compared to centrifugal VADs is a direct result of their higher shear stresses and longer residence times. Since platelet activation and destruction of the vWf also require high shear stresses, the flow conditions inside axial VADs are likely to result in more of these types of blood damage compared with centrifugal VADs.