Exploring The Dynamics of Steady Flow: An In-vitro and In-silico Study for LVAD Therapy

Introduction: Left Ventricular Assist Device (LVAD) therapy is the only viable alternative to heart transplantation for end-stage heart failure patients. Recent efforts have focused on evaluating the flow variability under different operating conditions including constant and variable LVAD speeds. However, the actual variation in the flow even at constant speeds is unknown – thus bringing up the question of how steady is steady flow? This study explores the dynamics of a ‘steady’ flow condition using both in vitro and in silico analyses.

Materials and

Methods: In vitro: A rigid aortic model with planar geometry was attached to a silicone model of the left ventricle and a HeartMate3 LVAD with an outflow graft angle of 50° and integrated into a mock circulatory loop. Pressure and flow were measured in the outflow graft (QOG), arch branches (Qb) and distal aorta (QDA) for constant LVAD speeds of 3.9 krpm and 5.8krpm. 3-D velocity fields were measured with particle image velocimetry (PIV) at a frame rate of 750 Hz for 3 seconds.
In silico: The same geometric model of the aorta and LVAD outflow graft anastomoses was also used to perform high-fidelity computational fluid dynamics (CFD) simulations. The experimentally measured inflow waveform including all fluctuations was used as an input to the model (with minimal smoothing). The CFD model including over 1.2 million elements and a timestep size of < 0.001 seconds for resolving the fluctuations in the flow patterns in the aortic arch. All CFD simulations were performed in ANSYS Fluent and most postprocessing was performed in MATLAB.

Results, Conclusions, and Discussions: The measured flows (mean ± std.dev.) during steady-state conditions were QOG=3.25±0.02, Qb=0.66±0.01 and QAD=2.71±0.05 L/min which produced a fluctuating velocity field characterized by a fluctuating velocity magnitude of 0.12±0.007 m/s. 3D PIV flow patterns illustrate the helical flow introduced by the LVAD and retrograde flow to the aortic root as shown in representative Fig. 1(a). The velocity fields from CFD simulations agreed very well with the in vitro measurements including the complex flow patterns in the aortic root, ascending aorta and great vessel branches. Flow exiting the outflow graft split into two main streams, forming a significant recirculation region at the aortic root (Fig. 1(b) and (c)). Helical flow was observed for the other branch as flow was redirected into the ascending aortic arch, followed by splitting into the three major branches. Due to the incorporation of fluctuating flow entering the outflow graft, CFD simulations captured the major fluctuations that were comparable to experimentally measured values mentioned above (Fig. 1(d)). These results indicate that the hemodynamic stimuli on blood particulates (RBCs, platelets) and the arterial walls is highly dynamic, even under perceived steady flow conditions. This may lead to abnormal stimuli on blood cells and platelets, potentially leading to activation and aggregation, along with increased damage to arterial walls. Thus, it is essential to investigate the dynamic and fluctuating nature of flow, especially for LVAD patients who are already at high risk of complications. Ongoing and future work include investigation of LVAD speed changes and their effects on hemodynamic flow patterns and associated hemodynamic stimuli in this highly dynamic environment.

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