Normal fluid stresses are prevalent in rotary ventricular assist devices: a computational fluid dynamics analysis

Despite the evolution of Ventricular Assist Devices (VADs), VAD patients still suffer from complications due to the damage to blood by fluid dynamic stress. Since rotary VADs are assumed to exert mainly shear stress, studies of blood damage are based on shear flow experiments. However, measurements and simulations of cell and protein deformation show normal and shear stresses deform, and potentially damage, cells and proteins differently. The aim was to use computational fluid dynamics (CFD) to assess the prevalence of normal stress, in comparison with shear stress, in rotary VADs. Our calculations showed normal stresses do occur in rotary VADs: the fluid volumes experiencing normal stress above 10 Pa were 0.011 ml (0.0009%) and 0.027 ml (0.0039%) for the HeartWare HVAD and HeartMate II, and normal stresses over 100 Pa were present. However, the shear stress volumes were up to two orders of magnitude larger than the normal stress volumes. Considering thresholds for red blood
cell and von Willebrand factor deformation by normal and shear stresses, the fluid volumes causing deformation by normal stress were between 2.5 and 5 times the size of those causing deformation by shear stress. The results clearly show, for the first time, that while blood within rotary VADs experiences more shear stress at much higher magnitudes as compared with normal stress, there is sufficient normal stress exposure present to cause deformation of, and potentially damage to, the blood components. This study is the first to quantify the fluid stress components in real blood contacting devices.