Multi-stage vs. single-stage rotodynamic blood pumps : which are more suitable for pediatric VADs?

Abstract

The treatment of advanced heart failure in pediatric patients represents unique challenges, which are augmented by the limited options for suitable implantable Ventricular Assist Devices (VADs). The Medical University of Vienna is developing a new VAD solution for small pediatric patients that utilizes two pumping stages to build up the required pressure while decreasing the rotation rate. Hence, the circumferential speed is decreased, which may lower the shear stresses and expectably increase the hemocompatibility. The aim of this thesis is to evaluate the potential of a two-stage design by comparison with similar single-stage pumps.Two different single-stage pump concepts were designed and simulated, which are comparable to the two-stage pump (same specific speed/same diameter). These pumps operate with the same flow (Q2S= 1.5 L/min) and build up a similar pressure head (H2S= 54 mmHg) as the two-stage pump. Computational Fluid Dynamics (CFD) was employed to simulate the flow within each pump, using the results to iteratively refine each pump design. The simulation results were subjected to a mesh and time-step sensitivity study. Subsequent detailed CFD simulations on these refined designs provided insights into the shear stresses, normalized index of hemolysis (NIH), washout, and stagnation zones. The outcomes were compared to the results of the two-stage pump.The single-stage pumps generate similar pressure heads as the two-stage pump (H1= 56 mmHg, H2= 55 mmHg), but at lower efficiencies (η1= 30%, η2= 27% vs. η2S= 32%). The occurring shear stresses reach higher magnitudes (up to 1600 Pa) than the two-stage pump. However, the volume of the two-stage pump is approximately 3 times larger than each single-stage pump. The NIH in the single-stage pumps are higher (6.4 and 8.3 mg/100L) compared to the two-stage pump (3.7 mg/100L). The 90% washout of the single-stage pumps was achieved 2-3 times faster compared to the two-stage pump due to the lower volume. However, the detailed washout of the bearing should be subject to further examination in the future. Stagnation zones were only occurring at wall-near regions in all of the presented pump concepts. Despite a larger volume, the two-stage pump may have advantages in terms of hemolysis, NIH and washout. The lower hemolysis risk and NIH are attributable to the reduced shear stress magnitude, which is mainly caused by the lower circumferential speed of the impeller. The washout of the bearing geometry in the two-stage pump is advantageous, since a pressure gradient is present between both pump stages, which contribute to the enhanced washout. Therefore, the influence of the specific bearing design of the single-stage pumps need to be analyzed in more detail.