This presentation was made at CAASE18, The Conference on Advancing Analysis & Simulation in Engineering. CAASE18 brought together the leading visionaries, developers, and practitioners of CAE-related technologies in an open forum, to share experiences, discuss relevant trends, discover common themes, and explore future issues.

Resource Abstract

Purpose
A series of four chronic animal studies was performed with Cleveland Clinic’s continuous-flow total artificial heart (CFTAH), without administration of postoperative anticoagulation. Upon explant, the blood flow paths for the pumps were clean except for varying amounts of thrombus attached to the right pump impeller. The current work focuses on using computational fluid dynamics (CFD) to identify an improved right impeller design that could reduce the potential for thrombus formation.

Methods
Transient, rotor-stator CFD (CFX, ANSYS Inc., Canonsburg, PA) analyses of the CFTAH were performed. The flow patterns near the left impeller surface, which was thrombus free, were used to gain insight into the desired flow patterns for different right impeller designs. Several right impeller design variations were evaluated including changes to the inlet design, impeller cone angle, and the blade shape. A non-Newtonian blood viscosity was used for the simulations. The key flow-related parameters evaluated included an Eulerian platelet stress accumulation (PSA) model, inward radial flow patterns, impeller shear stress, and flow residence time/local shear stress on surfaces just offset from the impeller were used to compare the different impeller designs.

Results
The CFD results for the right impeller designs modeled revealed similar trends in the flow patterns with flow recirculation commonly occurring on the suction side of the right impeller blades. These recirculation regions corresponded well with the thrombus seen along the suction side of the impeller blades for the explanted pumps. For one explanted pump, a star-shaped thrombus formed around the impellers which was consistent with the areas of increased residence time/local shear stress. Iso-volumes of relative inward radial velocity also revealed regions of flow recirculation near the trailing edges of the impeller blades.

Conclusions
The CFD simulations revealed flow separation off the leading edge of the impeller blades and flow wrapping over the trailing edges of the impeller blades. Both effects causing flow recirculation on the suction side of the impeller blades. These flow separation effects were not seen around the left pump impeller. The right pump impeller flow is constrained by a cylindrical housing and a narrow flow aperture through which the flow passes to reach the right pump outlet. The aperture is used to balance the left/right pump flows and pressures. In comparison, the flow from the left impeller passes directly into the left pump volute and outlet port. We have recently identified a potentially improved design that showed reduced regions of flow recirculation and is undergoing further evaluation.