Javier Alejandro Dottori

Effects on Aortoiliac Fluid Dynamics After Endovascular Sealing of Abdominal Aneurysms

Objectives: To evaluate the effects on aortoiliac fluid dynamics after the implantation of an endograft based on endovascular aneurysm sealing (EVAS) versus endovascular aneurysm repair (EVAR) strategy. Methods: An adaptive geometrical deformable model was used for aortic lumen segmentation in 8 patients before and after the surgery. Abdominal aneurysms were treated with an endograft based on the EVAS system (Nellix, n = 4) and with a device based on an anatomical fixation technology (n = 4). Pressure, blood velocity, and wall shear stress (WSS) were estimated at different aortic regions using computational fluid dynamics methods. Physiologic inlet/outlet flow values at the abdominal aorta, the celiac trunk, and the mesenteric and the renal arteries were set. Pressure references were set at iliac arteries outlet. Results: Maximum aneurysm sizes were similar for both groups in the preoperative scans. The lumen area was lower after EVAR (P < .05) and EVAS (P < .01) compared to preoperative aortic lumen sizes. Pressure increase was higher in the proximal abdominal aorta after EVAS compared to EVAR (2.3 ± 0.3 mm Hg vs 0.9 ± 0.3 mm Hg, P < .001). Peak blood velocities inside the endografts were 3-fold higher for EVAS compared to EVAR (54 ± 5 cm/s vs 17 ± 4 cm/s, P < .01). Velocities at the iliac arteries also remained higher for EVAS (38 ± 4 cm/s vs 24 ± 4 cm/s, P < .05). Peak WSS at the iliac arteries remained higher for EVAS compared to EVAR group (P < .05). Conclusion: The significant modification of the aortic bifurcation anatomy after EVAS alters aortoiliac fluid dynamics, showing a pressure impact at the renal arteries level and an acceleration of the blood velocity at the iliac region with a concomitant increase in peak WSS.

A comparative study of porous medium CFD models for flow diverter stents: Advantages and shortcomings

In computational fluid dynamics, there is a high interest in modeling flow diverter stents as porous media due to its reduced computational loads. One of the main difficulties of such models is proper parameter setup. Most authors assume flow diverters wire screen as an isotropic and homogeneous medium, while others proposes anisotropic configurations, yet very little is discussed about the effect of these assumptions on models accuracy. In this paper, we compare the effect of different models on hemodynamics in relation to their parameters. The fidelity and efficiency of the different models to capture wire screen effect on fluid flow are quantitatively analyzed and compared.

Flow diverter stents simulation with CFD: porous media modelling

Intracranial aneurysm treatment with flow diverters stent (FDs) is a minimally invasive approach for use in human patients. Because this treatment is strongly related to blood flow, flow simulation by CFD is an attractive method to study FDs. Such flow simulations generally define geometries of aneurysms and stents in the computation by creating calculation meshes in the fluid space. For the other hand, generating a mesh in porous media (PM) sometimes represents a smaller computational load than generating realistic stent geometries with CFD, particularly for the small gaps between stent struts. For this reason, PMs become attractive to simulate FDs. To find the proper parameters, we investigated Darcy-Forchheimer model for porous media. The model describes the relation between the pressure drop and flow velocity considering a viscous permeability (linear models term), and an inertial permeability (quadratic models term). Finally, two stage studies were performed. First, we verified flow model validity at different angles in known flow conditions. Second, model validation was checked for a channel with no-slip boundary conditions. Results indicate that resistance calculated according to model has a difference of less than 3.5 % which is appropriate to characterize the FDs.

Changes on abdominal aortic fluid dynamics after implantation of grafts based on endovascular aneurysm sealing system (EVAS)

An innovative approach to treat abdominal aortic aneurysms, based on an endovascular aneurysm sealing system, claims to reduce both endoleak and graft migration with respect to conventional devices with proximal fixation technologies. However, the aortic bifurcation anatomy is significantly modified with this novel proposal and the hemodynamic influences on blood flow have not been addressed until now. In this work we evaluated the aortic fluid dynamics changes introduced after the implantation of a sealing device with respect to a conventional endograft on four adults with abdominal aorta aneurysms. An adaptive Geometrical Deformable Model was used for aortic segmentation and Finite Volume mesh generation. Inlet boundary conditions were set to reproduce normal physiological conditions at the abdominal aorta, and maximum pressure drop and maximum peak velocity for the models were estimated at 3 sections (proximal, mid and distal) using Computational Fluid Dynamics simulations. We found a systematic pressure increase in the proximal abdominal aorta segment for patients treated with the sealing device with respect to the more conventional endograft. Pressure values at the level of the renal arteries averaged a ≈3 mmHg pressure increase for the sealing device, compared to the ≈1 mmHg for the conventional device. Velocities inside the endograft were 4-fold higher for the sealing device with respect to the conventional device, reaching 0.41 m/s vs 0.13 m/s, respectively. Distal velocity also remained higher: 0.45 m/s vs 0.24 m/s, respectively. Although these results should be analyzed carefully due to the small number of participants, the orders of magnitude and tendencies evidence the influence that the novel sealing device has on aortic blood flow.

Downstream-Conditioned Maximum Entropy Method for Exit Boundary Conditions in the Lattice Boltzmann Method

A method for modeling outflow boundary conditions in the lattice Boltzmann method (LBM) based on the maximization of the local entropy is presented. The maximization procedure is constrained by macroscopic values and downstream components. The method is applied to fully developed boundary conditions of the Navier-Stokes equations in rectangular channels. Comparisons are made with other alternative methods. In addition, the new downstream-conditioned entropy is studied and it was found that there is a correlation with the velocity gradient during the flow development.