Evaluation of Hemodynamic Properties After Chimney and Fenestrated Endovascular Aneurysm Repair.

BACKGROUND: Fenestrated (FEVAR) and chimney (ChEVAR) endovascular aortic repair have been applied in anatomically suitable complex aortic aneurysms. However, local hemodynamic changes may occur after repair. This study aimed to compare FEVAR's and ChEVAR's hemodynamic properties, focusing on visceral arteries. METHODS: Preoperative and postoperative computed tomography angiographies have been used to reconstruct patient-based models. Data of 3 patients, for each modality, were analyzed. Following geometric reconstruction, computational fluid dynamics simulations were used to extract near-wall and intravascular hemodynamic indicators, such as pressure drops, velocity, wall shear stress, time averaged wall shear stress, oscillatory shear index, relative residence time, and local normalized helicity. RESULTS: An overall improvement in hemodynamics was detected after repair, with either technique. Preoperatively, a disturbed prothrombotic wall shear stress profile was recorded in several zones of the sac. The local normalized helicity results showed a better organization of the helical structures at postoperative setting, decreasing thrombus formation, with both modalities. Similarly, time averaged wall shear stress increased and oscillatory shear index decreased postoperatively, signaling nondisturbed blood flow. The relative residence time was locally reduced. The flow in visceral arteries tended to be more streamlined in ChEVAR, compared to evident recirculation regions at renal and superior mesenteric artery fenestrations (P = 0.06). CONCLUSIONS: ChEVAR and FEVAR seem to improve hemodynamics toward normal values with a reduction of recirculation zones in the main graft and aortic branches. Visceral artery flow comparison revealed that ChEVAR tended to present lower recirculation regions at parallel grafts' entries while FEVAR showed less intense flow regurgitation in visceral stents.

Abdominal Aortic Aneurysm Endovascular Repair: Profiling Postimplantation Morphometry and Hemodynamics With Image-Based Computational Fluid Dynamics.

Endovascular aneurysm repair (EVAR) has disseminated rapidly as an alternative to open surgical repair for the treatment of abdominal aortic aneurysms (AAAs), because of its reduced invasiveness, low mortality, and morbidity rate. The effectiveness of the endovascular devices used in EVAR is always at question as postoperative adverse events can lead to re-intervention or to a possible fatal scenario for the circulatory system. Motivated by the assessment of the risks related to thrombus formation, here the impact of two different commercial endovascular grafts on local hemodynamics is explored through 20 image-based computational hemodynamic models of EVAR-treated patients (N = 10 per each endograft model). Hemodynamic features, susceptible to promote thrombus formation, such as flow separation and recirculation, are quantitatively assessed and compared with the local hemodynamics established in image-based infrarenal abdominal aortic models of healthy subjects (N = 10). Moreover, the durability of endovascular devices is investigated analyzing the displacement forces (DFs) acting on them. The hemodynamic analysis is complemented by a geometrical characterization of the EVAR-induced reshaping of the infrarenal abdominal aortic vascular region. The findings of this study indicate that (1) the clinically observed propensity to thrombus formation in devices used in EVAR strategies can be explained in terms of local hemodynamics by means of image-based computational hemodynamics approach; (2) reportedly prothrombotic hemodynamic structures are strongly associated with the geometry of the aortoiliac tract postoperatively; and (3) DFs are associated with cross-sectional area of the aortoiliac tract postoperatively. In perspective, our study suggests that future clinical followup studies could include a geometric analysis of the region of the implant, monitoring shape variations that can lead to hemodynamic disturbances of clinical significance.

Remodeling effects of carotid artery stenting versus endarterectomy with patch angioplasty in terms of morphology and hemodynamics.

BACKGROUND: Carotid endarterectomy (CEA) remains the first-line treatment option of symptomatic and asymptomatic carotid stenosis, while stenting (CAS) is reserved for selected patients at high surgical risk. Here, we compare the vascular remodeling process in CEA- and CAS-treated patients with respect to morphological and hemodynamic features, because of their possible engagement in carotid atherosclerosis. METHODS: Twelve (12) patients were included, half with patched CEA and half with CAS. Pre- and post-operative 3D image-based models of the carotid bifurcation were anatomically characterized in terms of flare, tortuosity, and curvature. Individual computational fluid dynamics simulations allowed to quantify the postoperative hemodynamic milieu in terms of (1) wall shear stress and (2) helical flow. RESULTS: Carotid flare increased in all cases, but a more marked increase emerged after CEA compared to CAS. Tortuosity and curvature increased after CEA but decreased after CAS. CEA patients presented with significantly higher postoperative tortuosity than CAS patients. CEA was associated with a worse (non-statistically significant) score in all flow disturbance indicators vs. CAS. CONCLUSION: The increased flare and tortuosity of the carotid bifurcation after CEA vs. CAS is a marked difference in the vascular remodeling process between the two modalities. CAS seems to induce a less pro-restenosis hemodynamic environment compared to CEA. The emerged differences stimulate further analysis on a larger cohort with long-term outcomes, to shed light on the clinical impact of the observations.

Deciphering ascending thoracic aortic aneurysm hemodynamics in relation to biomechanical properties.

The degeneration of the arterial wall at the basis of the ascending thoracic aortic aneurysm (ATAA) is a complex multifactorial process, which may lead to clinical complications and, ultimately, death. Individual genetic, biological or hemodynamic factors are inadequate to explain the heterogeneity of ATAA development/progression mechanisms, thus stimulating the analysis of their complex interplay. Here the disruption of the hemodynamic environment in the ATAA is investigated integrating patient-specific computational hemodynamics, CT-based in vivo estimation of local aortic stiffness and advanced fluid mechanics methods of analysis. The final aims are (1) deciphering the ATAA spatiotemporal hemodynamic complexity and its link to near-wall topological features, and (2) identifying the existing links between arterial wall degeneration and hemodynamic insult. Technically, two methodologies are applied to computational hemodynamics data, the wall shear stress (WSS) topological skeleton analysis, and the Complex Networks theory. The same analysis was extended to the healthy aorta. As main findings of the study, we report that: (1) different spatiotemporal heterogeneity characterizes the ATAA and healthy hemodynamics, that markedly reflect on their WSS topological skeleton features; (2) a link (stronger than canonical WSS-based descriptors) emerges between the variation of contraction/expansion action exerted by WSS on the endothelium along the cardiac cycle, and ATAA wall stiffness. The findings of the study suggest the use of advanced methods for a deeper understanding of the hemodynamics disruption in ATAA, and candidate WSS topological skeleton features as promising indicators of local wall degeneration.

Hemodialysis arterio-venous graft design reducing the hemodynamic risk of vascular access dysfunction.

Although arterio-venous grafts (AVGs) represent the second choice as permanent vascular access for hemodialysis, this solution is still affected by a relevant failure rate due to graft thrombosis, and development of neointimal hyperplasia (IH) at the distal vein. As a key role in these processes has been attributed to the abnormal hemodynamics establishing in the distal vein, the optimization of AVGs design aimed at minimizing flow disturbances would reduce AVG hemodynamic-related risks. In this study we used computational fluid dynamics to investigate the impact of alternative AVG designs on the reduction of IH and thrombosis risk at the distal venous anastomosis. The performance of the newly designed AVGs was compared to that of commercially available devices. In detail, a total of eight AVG models in closed-loop configuration were constructed: two models resemble the commercially available straight conventional and helical-shaped AVGs; six models are characterized by the insertion of a flow divider (FD), straight or helical shaped, differently positioned inside the graft. Unfavorable hemodynamic conditions were analyzed by assessing the exposure to disturbed shear at the distal vein. Bulk flow was investigated in terms of helical blood flow features, potential thrombosis risk, and pressure drop over the graft. Findings from this study clearly show that using a helically-shaped FD located at the venous side of the graft could induce beneficial helical flow patterns that, minimizing flow disturbances, reduce the IH-related risk of failure at the distal vein, with a clinically irrelevant increase in thrombosis risk and pressure drop over the graft.

In-stent graft helical flow intensity reduces the risk of migration after endovascular aortic repair.

During the last years endovascular aneurysm repair (EVAR) became the elective treatment for abdominal aortic aneurysms (AAAs) thanks to lower mortality and morbidity rates than open surgery. In face of these advantages, stent-graft performances are still clinically suboptimal. In particular, post-surgical complications derive from device migration as a consequence of the hemodynamic forces acting on the endograft. In this regard, while the importance of hemodynamic surface forces is well recognized, the role of the in-stent flow is still unclear. Here we hypothesize that in-stent helical blood flow patterns might influence the distribution of the displacement forces (DFs) acting on the stent-graft and, ultimately, the risk of stent migration. To test this hypothesis, the hemodynamics of 20 post-EVAR models of patients treated with two different commercial endografts was analyzed using computational hemodynamics. The main findings of the study indicate that: (1) helical flow intensity decreases the risk of endograft migration, as given by an inverse correlation between helicity intensity (h2) and time-averaged displacement forces (TADFs) (p < 0.05); (2) unbalanced counter-rotating helical structures in the legs of the device contribute, in particular along the systole, to significantly suppress TADFs (p < 0.01); (3) as expected, helical flow intensity is positively correlated with pressure drop and resistance to flow (p < 0.001). The findings of this study suggest that a design strategy promoting in-stent helical flow structures could contribute to minimize the risk of migration of implanted EVAR devices.

Healthy and diseased coronary bifurcation geometries influence near-wall and intravascular flow: A computational exploration of the hemodynamic risk.

Local hemodynamics has been identified as one main determinant in the onset and progression of atherosclerotic lesions at coronary bifurcations. Starting from the observation that atherosensitive hemodynamic conditions in arterial bifurcation are majorly determined by the underlying anatomy, the aim of the present study is to investigate how peculiar coronary bifurcation anatomical features influence near-wall and intravascular flow patterns. Different bifurcation angles and cardiac curvatures were varied in population-based, idealized models of both stenosed and unstenosed bifurcations, representing the left anterior descending (LAD) coronary artery with its diagonal branch. Local hemodynamics was analyzed in terms of helical flow and exposure to low/oscillatory shear stress by performing computational fluid dynamics simulations. Results show that bifurcation angle impacts lowly hemodynamics in both stenosed and unstenosed cases. Instead, curvature radius influences the generation and transport of helical flow structures, with smaller cardiac curvature radius associated to higher helicity intensity. Stenosed bifurcation models exhibit helicity intensity values one order of magnitude higher than the corresponding unstenosed cases. Cardiac curvature radius moderately affects near-wall hemodynamics of the stenosed cases, with smaller curvature radius leading to higher exposure to low shear stress and lower exposure to oscillatory shear stress. In conclusion, the proposed controlled benchmark allows investigating the effect of various geometrical features on local hemodynamics at the LAD/diagonal bifurcation, highlighting that cardiac curvature influences near wall and intravascular hemodynamics, while bifurcation angle has a minor effect.