Left Ventricular Assist Device Shear Stress Quantification Utilizing Computational Fluid Dynamics: Using Physics and Physiology to Optimize Programmable Flow Algorithms

Abstract

Our current understanding of left ventricular assist device (LVAD) flow algorithms and their impact on shear stress and platelet activation remains incompletely understood. Computational fluid dynamics (CFD) allows for analysis of flow in the cardiovascular system. CFD simulations were carried out on a patient with a HeartMate II. Simulations were also done substituting for HVAD with or without Lavare cycle, HeartMate 3 in continuous mode or with Artificial Pulse as well as novel flow algorithms including copulsation and counterpulsation modes. LVAD flow was calculated using a lumped-parameter-model (LPM) of the systemic circulation, which was calibrated with patient-specific ventricular volume changes from 4 dimensional CT and hemodynamic tracings. The LVADs were implemented in the LPM via published H-Q curves. Shear stress and platelet activation were quantified for each flow algorithm and pump type. Wall shear stress was greater with the HVAD when compared to the HMII or HM3 (Figure Top). In the aortic arch and carotid arteries, a 42% increase in shear stress was noted with the HVAD compared to the HM3 in continuous mode (1.86 vs. 1.31 Pa) with little impact from the addition of the Lavare Cycle (Figure Bottom). Platelet activation was similarly greatest with the HVAD in the carotid arteries. The addition of the artificial pulse to the HM3 flow algorithm led to a 168% increase in shear stress (0.363 vs 0.139 Pa) throughout the ascending aorta (Figure Bottom). CFD analysis reveals that the artificial pulse increases wall shear stress in the ascending aorta and may represent a plausible mechanism to explain the high observed rates of aortic insufficiency seen with the HM3. Similarly, the higher shear stress rates in the carotid arteries seen with the HVAD might partly explain the higher residual rates of stroke with this pump. These hypothesis generating questions need to be prospective explored.