Fluid Dynamics of a Pediatric Ventricular Assist Device

Abstract

The number of pediatric patients requiring some form of mechanical circulatory assistance is growing throughout the world because of new surgical procedures and the success of pediatric cardiac transplantation. However, the salvage rate for those patients requiring circulatory support may be as low as 25%. Despite the fact that Penn State's 70 cc pneumatic ventricular assist device has been used with a success rate of over 90% in more than 250 patients worldwide, efforts to scale down the pump have encountered difficulties. Animal experiments with a 15 cc version were unsuccessful, with explanted pumps showing extensive thrombus deposition within the pumping chamber. The materials used to fabricate the smaller pump as well as the basic operating principles are identical to the successful adult-sized version. It is therefore believed that reducing the size of the pump altered the internal flow field, and that fluid dynamic factors were responsible for the high degree of thrombus observed with the implanted devices. A dimensional analysis was conducted that revealed significant differences in both Reynolds (Re) and Strouhal (St) numbers between the successful and unsuccessful pumps. Two component laser Doppler velocimetry was then used to characterize the internal flow field quantitatively. Comparison with data from the 70 cc pump showed a reduction in wall shear stress and turbulence levels in the 15 cc pump that would yield an environment conducive to clot formation.

Models of Flow-Induced Loading on Blood Cells in Laminar and Turbulent Flow, with Application to Cardiovascular Device Flow

“…In the development of cardiovascular implants such as mechanical heart valves 7,12,37 and ventricular assist devices 2,6 , it is desirable to predict the occurrence of mechanically induced thrombosis and hemolysis as a function of flow field parameters. Usually, such blood damage is predicted as functions of viscous shear stress (for laminar flow) or Reynolds stress (for turbulent flow), on the basis of in vitro experiments on blood.…”

Steady and unsteady laminar flows of Newtonian and generalized Newtonian fluids in a planar T‐junction

“…However even in large blood vessels, flow unsteadiness leads to regions of low deformation rate where the viscoelasticity of blood becomes especially relevant and affecting the stress field. This has been shown by Vlastos et al [19], who measured blood rheology under combined oscillatory and steady shear, and recently by Bachmann et al [24], who measured the flows of Newtonian and non-Newtonian blood analogs in a pediatric ventricle with handmade ball and cage valves at normal physiologic conditions and showing the dramatic differences in wall shear stresses. Therefore, the assumption of Newtonian behaviour of blood should be taken with caution and there is the need to investigate in more detail the dynamics of non-Newtonian fluids under unsteady flow conditions of relevance to hemodynamics.…”

Validation of a CFD Methodology for Positive Displacement LVAD Analysis Using PIV Data

“…While the blood analog displays some non-Newtonian characteristics, we assume that these effects are negligible and therefore the fluid is treated as Newtonian in our CFD simulations. The device Reynolds number of 1865 and Strouhal number of 9.0 are computed based on the following equations, which are variations in those used by Bachmann [10] and Deutsch [13]:…”

“…A second program aims at a fully implantable electric 50 cc device for small adults and adolescents. Scaleddown devices have been prone to thrombus formation during animal testing [10][11][12][13]. It has been hypothesized that the increased level of thrombosis is related to changes in the flow field when scaling down from the 70 cc device.…”