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Aortic insufficiency (AI) following left ventricular assist device (LVAD) implantation is common. Speed augmentation to overcome the regurgitant flow (RF) is used but this leads to progressive AI and may have deleterious effects on shear stress (SS) and the right ventricle (RV). We employed a closed-loop mathematical model of the cardiovascular system to generate boundary conditions for blood flow simulations performed in a three-dimensional (3D) model of the aortic arch. AI was introduced and the impact of speed augmentation and blood pressure control on SS and RV oxygen consumption (MVO2) were determined in a model with a coupled and uncoupled RV. Addition of severe AI to the coupled RV at 5500 RPM led to a reduction in net flow from 5.4 L/min (no AI) to 2.1 L/min (severe). Increasing speed to 6400 RPM in the severe AI and coupled RV, led to a 42% increase in net flow and a 16% increase in RF with a nominal decrease of 0.9% in RV MVO2. Blood pressure control with the coupled RV with severe AI at 5500 RPM led to an 81% increase in net flow with a 15% reduction RF and an 8% reduction in RV MVO2 (Fig A). With an uncoupled RV, addition of severe AI at 5500 RPM led to a reduction in net flow from 5.0 L/min (no AI) to 1.8 L/min (severe). Increasing speed to 6400 RPM with severe AI and uncoupled RV increased net flow by 45%, RF by 15% and RV MVO2 by 4.2%. For the uncoupled RV with severe AI, blood pressure control alone led to a 22% increase in net flow, 4.2% reduction in RF and 6.1% reduction in RV MVO2; whereas combined BP control and pulmonary vasodilation led to a 113% increase in net flow, 20% reduction in RF and 41% reduction in RV MVO2 (Fig B). Compared to speed augmentation, blood pressure control consistently resulted in a reduction in SS (Fig C). Speed augmentation to overcome AI in patients supported by CF-LVAD will augment flow but at the expense of RV MVO2, RF and SS. Aggressive blood pressure control and pulmonary vasodilation can improve net flow with more advantageous effects on the RV and aortic valve integrity. [ABSTRACT FROM AUTHOR]

Invasive cardiopulmonary exercise testing (iCPET) is helpful in evaluating dyspnea but is infrequently utilized in patient supported by left ventricular assist devices (LVAD). Computational fluid dynamics (CFD) allows for a more thorough analysis of fluid mechanics including shear stress (SS) and shear rate (SR). The additive information from iCPET and CFD may allow for optimal understanding of the pump-patient interaction during exercise. Hemodynamic and ventilatory gas measurements were recorded as part of an iCPET study on a patient with a HeartMate 3 (HM3). Exercise using a bicycle ergometer by increments of 20 watts every 2 minutes was performed. An anatomical model of the vasculature was reconstructed from computed tomography (CT) images, and the LVAD outflow cannula was used as the inflow boundary. The LVAD outflow was calculated separately using a lumped-parameter-model (LPM) of the circulation. Ventricular volumes were reconstructed from CT and the patient's hemodynamic tracings. The contribution of flow from the HM3 was implemented in the LPM via published H-Q curves. CFD simulations were then carried out over the stages of exercise. Blood velocities, SS and SR were quantified in the vascular beds. Compared to rest, blood volume exposed to SR >1000 s−1 increased 2.8 fold (0.47 to 1.31 cm3), while average SS increased 1.45 fold (2.61 to 3.79 Pa) in the cannula, 1.48 fold (0.89 to 1.31 Pa) in the ascending aorta, and 1.61 fold (0.67 to 1.08 Pa) in the aortic arch and cerebral circulation. Compared to rest, peak velocities increased nearly 5 fold in the aortic arch. CFD applied to iCPET provides valuable information about variations in flow to the vasculature with exercise as well as shear rate and shear stress augmentation. This patient-specific information may help with optimization of LVAD speeds to meet the physiological needs of exercise. [ABSTRACT FROM AUTHOR]

With the discontinuation of the HeartWare HVAD, patients who experience a device complication will require exchange to another pump. The HeartMate 3 (HM3) is now the only commercially available LVAD in the United States. The optimal surgical configuration for connecting the 10 mm HVAD outflow graft to the 14 mm HM3 outflow graft has yet to be defined. A lumped-parameter computational fluid dynamic (CFD) model of the circulatory system, including HQ curves of the HM3 was created. The model was coupled with 3D CFD model, in which outflow cannula flow served as the boundary conditions for blood flow simulations in the aorta. In total, 4 variations of the outflow cannula were tested: 1) Anastomosis near the pump (long 10 mm); 2) Anastomosis near the aorta (long 14 mm); 3) Anastomosis 5 cm from the aorta (mid transition); 4) The entire cannula 14 mm (complete exchange). The flow patterns were characterized based on the results of 3D CFD simulations. Maximum velocity was highest in the configurations with longer sections of 10 mm graft (long 10 mm, 4.20 m/s; long 14 mm, 4.12 m/s, mid transition, 4.18 m/s; complete exchange, 2.27 m/s). This resulted in a 176% larger blood volume exposed to very high shear rate (SR) >5000 s−1 for the long 10 mm vs the long 14 mm configuration (0.59 ml vs 0.21 ml) and a 805% increase compared to the complete exchange (0.065 ml). There was a 21% increase in right coronary flow (0.29 ml/s vs 0.24 ml/s) and a 22% increase in left coronary flow (1.41 ml/s vs 1.16 ml/s) with the long 10 mm compared to the 14 mm configuration. When the long 10 mm cannula was compared against complete exchange, the flow increase was 26% (0.29 ml/s vs 0.23 ml/s) for right coronary and 62% (1.41 ml/s vs 0.87 ml/s) for left coronary. When planning for a HVAD to HM3 exchange, the location of the anastomosis and relative length of the 10 and 14 mm grafts has important implications related to shear rate and coronary blood flow which should be accounted for during surgery. [ABSTRACT FROM AUTHOR]

Invasive hemodynamic ramp studies have been shown to decrease the rates of left ventricular assist device (LVAD) complications however hemocompatibility related adverse events (HRAE) still remain unacceptably high. Computational fluid dynamics (CFD) allows for a more thorough analysis of shear rate (SR) and shear stress (SS) which influences endothelial function and thrombotic potential. An invasive hemodynamic ramp study was performed on a patient with a HeartMate 3 (HM3). An anatomical model of the patient was reconstructed from computed tomography (CT) images, and the LVAD outflow cannula was used as the inflow boundary. The LVAD flow was calculated separately using a lumped-parameter-model (LPM) of the circulation. Ventricular volumes were reconstructed from CT and the patient's hemodynamic tracings. The contribution of flow from the HM3 was implemented in the LPM via published H-Q curves. CFD simulations were then carried out over the entire clinical operating condition of the HM3 from 4600 to 6400 RPM. SS and SR were quantified in the outflow cannula, ascending aorta, aortic arch and cerebral circulation. As speed was increased from 4600 to 5500, and then to 6400 RPM, blood volume exposed to a SR greater than 500 s−1 increased, respectively, by 1.46 fold and 2.18 fold (2.00 to 2.93 to 4.37 cm3). Increase of SS in the cannula was 1.37 fold and 1.74 fold (1.88 to 2.56 to 3.26 Pa), in the ascending aorta was 1.44 fold and 1.92 fold (0.60 to 0.86 to 1.14 Pa), and in the aortic arch and cerebral circulation was 1.40 fold and 1.88 fold (0.46 to 0.65 to 0.87 Pa). Hemodynamic optimization during ramp studies needs to be balanced with increased rates of SS and SR that occur with incremental speed increases. CFD when applied to a ramp study can provide additive patient-specific information that may further mitigate HRAE. [ABSTRACT FROM AUTHOR]