Time-averaged Wall Shear Stress Research Articles

Hemodynamics analysis and computational fluid dynamics simulations to identify potential location of rupture in abdominal aortic aneurysm

Abstract Background Abdominal Aortic Aneurysm (AAA) rupture is the most severe consequence of aneurysmal disease, with an overall death rate of 80%. Early diagnosis and appropriate follow-up and management of AAA will significantly reduce the risk of rupture and mortality. Rupture risk assessment of AAA has a critical role for the early warning. Purpose To Predict of the course of an AAA and determine the rupture risk and location from a biomechanical point of view. AAA hemodynamics were investigated to clarify the relation between the hemodynamic parameters and the AAA rupture. Methods Computed tomography (CT) images of one healthy aorta and 23 patients’ aorta with aneurysms were analyzed. WSS-related hemodynamic parameters of time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), endothelial cell activation potential (ECAP), and relative residence time (RRT) are investigated in patient-specific AAA geometries by performing computational fluid dynamics (CFD) simulations (Figure 1). Patient-specific AAA geometries are identified using medical imaging techniques and realistic boundary conditions are determined using Doppler ultrasonography. Results AAA rupture locations predicted by CFD simulations were in good agreement with the actual rupture locations of retrospective data. The rupture locations had relatively low TAWSS, high OSI, high ECAP, and high RTT levels. High risk of rupture is expected in regions with a local minimum of TAWSS, and a local maximum of OSI, ECAP, and RRT. Conclusions Examining the spatial distribution of the hemodynamic parameters was a powerful approach for rupture risk assessment of AAA. Predicting the rupture location using a single hemodynamic parameter is not feasible, and the accuracy of AAA rupture risk assessment can be improved by using multiple WSS-related parameters by identifying the common intersection regions of low TAWSS, high OSI, high ECAP, and high RRT.Figure 1

Plaque progression is dependent on the local hemodynamics (time-averaged wall shear stress and TSVI) in combination with the plaque phenotype

Abstract Objective To investigate the predictive power of TSVI for plaque growth in human coronary arteries in plaques with different phenotypes. Methods 38 non-stented, non-culprit coronary arteries (MPACT study) were imaged with near-infrared spectroscopy intravascular ultrasound (NIRS-IVUS) and optical coherence tomography (OCT) at the baseline and after 12 months [2]. Coronary reconstructions were based on fusion of the multimodality imaging and used as input for computational fluid dynamics. Vessel-specific flow was measured using Doppler wire and used as input to calculate TAWSS and TSVI [2]. The coronary arteries were subdivided into 1.5 mmx45º sectors and the TAWSS and TSVI were classified as low, mid and high based on the range per vessel. Per sector the plaque growth was quantified as plaque burden (Plaque area/total vessel area*100%) change over time (ΔPAV). Based on the NIRS and OCT data, the sectors were characterized as lipid rich plaque (wall thickness>0.5 mm & NIRS+, lipid rich(OCT)) or plaque without lipid (wall thickness >0.5 mm & NIRS-, fibrous (OCT)) or plaque free wall (wall thickness <0.5 mm &NIRS-, PFW (OCT)). The effect of TAWSS, TSVI and the combination with plaque phenotype on plaque progression was evaluated using Linear Mixed Model in SPSS. Results TSVI at baseline was significantly associated with PAV increase, with high PAV for high TSVI (p<0.001). A clear trend emerged for TAWSS, with high PAV for low TAWSS at baseline (Figure 1B). The highest plaque progression at 1-year follow-up was observed for lipid rich plaques (both based on NIRS and OCT) exposed to either low TAWSS or high TSVI (figure 2). Also in the PFW sectors clear plaque progression was observed dependent on either TAWSS or TSVI. Conclusion Lipid rich regions exposed to either low TAWSS or high TSVI are at the highest risk for plaque progression. These data suggest that although TAWSS and TSVI have a different mechanism of action on the endothelium, but both actions are involved in plaque progression.Figure 1Figure 2

Assessing turbulent effects in ascending aorta in presence of bicuspid aortic valve

Aortic valves with bicuspids have two rather than three leaflets, which is a congenital heart condition. About 0.5–2% of people have a bicuspid aortic valve. Blood flow through the aorta is commonly believed to be laminar, although aortic valve disorders can cause turbulent transitions. Understanding the impact of turbulence is crucial for foreseeing how the disease will progress. The study’s objective was use large eddy simulation to provide a thorough analysis of the turbulence in bicuspid aortic valve dysfunction. Using a large eddy simulation, the blood flow patterns of the bicuspid and tricuspid aortic valves were compared, and significant discrepancies were found. The velocity field in flow in bicuspid configurations was asymmetrically distributed toward the ascending aorta. In tricuspid aortic valve (TAV) the flow, on the other hand, was symmetrical within the same aortic segment. Moreover, we looked into standard deviation, Q-criterion, viscosity ratio and wall shear stresses for each cases to understand transition to turbulence. Our findings indicate that in the bicuspid aortic valve (BAV) case, the fluid-dynamic abnormalities increase. The global turbulent kinetic energy and time-averaged wall shear stress for the TAV and BAV scenarios were also examined. We discovered that the global turbulent kinetic energy was higher in the BAV case compared to TAV, in addition to the increased wall shear stress induced by the BAV in the ascending aorta.

Numerical investigation of unruptured middle cerebral artery bifurcation aneurysms: influence of aspect ratio

An aneurysm is a disease condition, which is due to the pathological weakening of an arterial wall. These aneurysms are often found in various branch points and bifurcations of an artery in the cerebral circulation. Most aneurysms come to medical attention, either due to brain hemorrhages caused by rupture or found unruptured. To consider surgically invasive treatment modalities, clinicians need scientific methods such as, hemodynamic analysis to assess rupture risk. The arterial wall loses its structural integrity when wall shear stress (WSS) and other hemodynamic parameters exceed a certain threshold. In the present study, numerical simulations are carried out for unruptured middle cerebral artery (MCA) aneurysms. Three distinct representative sizes are chosen from a larger patient pool of 26 MCA aneurysms. Logically, these aneurysms represent three growth stages of any patient with similar anatomical structure. Simulations are performed to compare the three growth phases (with different aspect ratios) of an aneurysm and correlate their hemodynamic parameters. Simulations with patient specific boundary conditions reveal that, aneurysms with a higher aspect ratio (AR) correspond to an attendant decrease in both time-averaged wall shear stress (TAWSS) and spatial wall shear stress gradients (WSSG). Smaller MCAs were observed to have higher positive wall shear stress divergence (WSSD), exemplifying the tensile nature of arterial wall stretching. Present study identifies positive wall shear stress divergence (PWSSD) to be a potential biomarker for evaluating the growth of an aneurysm.

The influence of flow distribution strategy for the quantification of pressure- and wall shear stress-derived parameters in the coronary artery: A CTA-based computational fluid dynamics analysis

For image-based computational fluid dynamics (CFD) analysis to characterize the local coronary hemodynamic environment, the accuracy depends on the flow rate which is in turn associated with outlet branches’ morphology. A good flow distribution strategy is important to mitigate the effect when certain branches cannot be considered. In this study, stenotic coronary arteries from 13 patients were used to analyze the effect of missing branches and different flow distribution strategies. Pressure- and wall shear stress (WSS)-derived parameters around the stenotic region (ROI) were compared, including fractional flow reserve (CT-FFR), instantaneous wave-free ratio (CT-iFR), resting distal to aortic coronary pressure (CT-Pd/Pa), time-averaged WSS, oscillatory shear index (OSI) and relative residence time (RRT). Three flow distribution strategies were the Huo-Kassab model at distal outlets (Type I), flow distribution based on outlet resistances (Type II), and a developed algorithm distributing flow at each bifurcation until the final outlets (Type III). Results showed that Type III strategy for models with truncated branch(es) had a good agreement in both pressure- and WSS-related results (interquatile range less than 0.12% and 4.02%, respectively) with the baseline model around the ROI. The relative difference of pressure- and WSS-related results were correlated with the flow differences in the ROI to the baseline mode. Type III strategy had the best performance in maintaining the flow in intermediate branches. It is recommended for CFD analysis. Removal of branches distal to a stenosis can be undertaken with an improved performance and maintained accuracy, while those proximal to the ROI should be kept.

Abstract 008: Morphomechanical Analysis of Saccular Aneurysms

Introduction An irregular morphology is considered a high‐risk feature of unruptured intracranial aneurysms. Some of these aneurysms may have blebs where there is a high likelihood of rupture. The process of aneurysm rupture is dynamic and involves factors such as aneurysm geometry, flow dynamics, and aneurysmal wall enhancement. In our study, we aimed to conduct a comprehensive analysis of brain aneurysms, specifically focusing on irregular and regular aneurysms. This analysis included the examination of aneurysmal wall enhancement (AWE), computational fluid dynamics (CFD), and finite element analysis (FEA). Methods The study focused exclusively on unruptured aneurysms and employed high‐resolution vessel wall imaging using 3T MRI. Aneurysms were classified as irregular if they displayed a bleb or multiple lobes. A 3D segmentation of the aneurysm and vessel was generated using 3D Slicer software. Orthogonal spokes were extended from the segmentation into the vessel wall, enabling the creation of intensity (SI) heatmaps. Aneurysmal wall enhancement (AWE) was calculated based on the post‐contrast SI. Furthermore, wall tension (WT) was determined using FEA, and time‐averaged wall shear stress (TAWSS) was computed through CFD utilizing the 3D segmentation. To identify regions of notable activity within the aneurysm, an analysis was conducted by examining values exceeding the 50th percentile threshold. This approach facilitated the inclusion of areas demonstrating heightened activity within the aneurysm. The study comprised two primary analyses: first, a comparison between regular and irregular aneurysms, and second, a secondary analysis that focused specifically on aneurysms with blebs. During this analysis, the bleb, body, and neck regions were isolated for comparative evaluation among different compartments within the aneurysms. Results The study included a total of forty‐seven unruptured saccular aneurysms, with 38% (18/47) classified as having irregular morphology. Comparing aneurysms with irregular morphology to those with regular morphology, it was observed that irregular aneurysms exhibited higher WT (p=0.05) and lower TAWSS (p=0.04). Among the irregular aneurysms, 61% (11/18) were found to have blebs. Within aneurysms containing blebs, significant differences were observed in terms of WT and AWE between the bleb, neck, and aneurysm body (p<0.001 and p=0.03, respectively). Specifically, the blebs exhibited the highest AWE within the aneurysm, followed by the aneurysm body and the neck. The neck exhibited the highest value of WT, followed by the aneurysm body and the bleb. Finally, the neck also demonstrated the highest value of TAWSS, followed by the aneurysm body and the bleb. Conclusion Irregular aneurysms had a higher WT and lower TAWSS. Furthermore, the morphomechanical characteristics of irregular aneurysms varied between the aneurysm neck and body, indicating distinct properties in these regions. Unstable regions of the aneurysm, such as blebs, exhibited elevated AWE along with low TAWSS and WT.

Hemodynamic differences caused by left atrial appendage modeling contours

Comparing the hemodynamic parameters of thrombus-positive and thrombus-negative patients in the early stages of the disease (before thrombus formation occurs) can help predict atrial fibrillation-related thrombosis. However, most clinical images of thrombus-positive are of existing thrombus, and the presence of thrombi blurs the outline of the atrial appendage intima. Therefore, using the left atrial appendage (LAA) epicardial geometry for hemodynamic analysis has become a last resort. This study compares hemodynamic differences using the modeling contour of the inner and outer membranes of the LAA. The research results show the velocity and shear strain rate of the endocardial and epicardial geometries exhibit relative consistency. As for the parameters related to wall shear stress, the difference in time-averaged wall shear stress mainly occurs at the LAA entrance and does not affect the determination of thrombosis risk factors. The difference in the oscillatory shear index mainly occurs at the tip of LAA and the parts with larger curvature, which are seriously affected by geometry. The differences between endothelial cell activation potential (ECAP) and relative residence time (RRT) are concentrated at the tip of the LAA, but the maximum and minimum values are significantly different. After we exclude the top and bottom 5% of values, we believe that ECAP and RRT are reliable parameters. This investigation conducted both qualitative and quantitative assessments of the hemodynamic disparities between the endocardial and epicardial geometries. The findings offer valuable data reference for related research.

Hemodynamic analysis of coil filled patient-specific middle cerebral artery aneurysm using porous medium approach

Cerebral aneurysms are bulges of an artery, which could be life-threatening when ruptured. Depending on their size, shape, and location, they need to be managed either through clipping or an endovascular coiling intervention. When coiled, reduced hemodynamic activity enables the coil to get thrombosed and achieve flow stasis. However, some coils delivered into the aneurysm tend to prolapse into the parent vessel and cause stroke due to obstruction and embolization. The recurrence of an aneurysm after endovascular coiling is of concern in the treatment of wide necked aneurysms. The initial packing density or improper coiling of the aneurysm and its relation to recurrence remains uncertain. This study investigates the influence of reduction in coil fill volume and packing density on the aneurysm recurrence using hemodynamic parameters by analyzing its flow features. Finite element method based commercial computational fluid dynamics solver is employed for performing patient-specific simulations for the coil filled aneurysm. The present approach uses porous medium based formulation. The numerical simulations show that any reduction below the optimal coil fill volume and packing density inside the aneurysm increases the velocity magnitude, wall shear stress, time-averaged wall shear stress, and spatial gradient of wall shear stress and reduces the relative residence time. The hemodynamic parameters and flow features suggest that a reduction in the coil packing density inside the aneurysm increases the chances of aneurysm recurrence. Hence, an assessment on how to achieve optimal coil fill volume and packing density is critical in reducing the risk of aneurysm recurrence.

Investigation of the Effects of an Inferior Vena Cava Filter and Captured Clot Size on the Hemodynamic Parameters in Non-Newtonian Turbulent Pulsatile Blood Flow

In this computational fluid dynamics (CFD)-based study, the effects of inferior vena cava (IVC) filter implantation on the risk of IVC thrombosis have been investigated using different hemodynamic parameters, including time-averaged wall shear stress (TAWSS), the oscillating shear index (OSI), and the relative residence time (RRT). The boundary conditions in this study have been based on physiological pulses. Additionally, the k–ω model and the Carreau model have been chosen to represent the turbulent flow regime and non-Newtonian blood, respectively. For this purpose, three blood clots with the largest cross-sectional diameters of 30%, 50%, and 70% of the filter diameter have been used. Capturing a small clot in the filter has the minimum effect on the hemodynamic parameters, while by increasing the size of the captured clot, OSI and RRT parameters increase in areas downstream of the filter on the wall. The presence of a filter and clot increases the risk of thrombosis. In the case of capturing large clots, there is the possibility of damage to endothelial cells or platelet activation. Captured clots lead to the formation of plaque and thrombus on the IVC wall. However, the possibility of thrombus growth on its surface is not negligible, particularly if larger clots are trapped in the filter.

Atherosclerosis risk assessment in human carotid artery with variation in sinus length: a numerical approach

The mortality rates due to cardiovascular diseases are on a rise globally. One of the major cardiovascular diseases is stroke which occurs due to atherosclerotic plaques build-up in the carotid artery. The common carotid artery (CCA) bifurcates into the internal carotid artery (ICA) and external carotid artery (ECA). Sinus present at ICA is an ellipsoidal-shaped dilated region acting as a pressure receptor and blood flow regulator. Dimensions of the sinus vary from person to person, affecting the hemodynamics of the carotid artery. The current numerical study manifests a 3D flow analysis by varying the sinus length to investigate its local and global effects on the hemodynamics of the carotid artery using various biomechanical risk analysis parameters of atherosclerosis. User-defined function (UDF) dictates the pulsatile flow velocity profile imposed at the inlet. Near the outer wall (OW) of the sinus, the blood flow velocities are lower and recirculation zones are more. Though the recirculation zones for shorter sinus will be close to the inner wall (IW), interestingly, with an increase in the sinus length, the recirculation zones shift toward the OW with higher strength. These significantly decrease the x-wall shear stress (x-WSS) and time-averaged wall shear stress (TAWSS) values on the OW of the longer sinus. The other risk analysis parameters, like oscillatory shear index (OSI) and relative residence time (RRT), support the described consequences. These results reveal that sinus of increased length is more prone to developing atherosclerotic plaque.

Numerical study of hemodynamic changes in the Circle of Willis after stenosis of the internal carotid artery

Background and objectivesIn clinical practice a large number of patients with ischemic stroke have internal carotid artery (ICA) stenosis accompanied by Circle of Willis (CoW) stenosis. In the presence of carotid artery stenosis, CoW atherosclerosis may cause cerebral blood flow decompensation and may promote the development of ischemic stroke. The reason for the concomitant stenosis at both sites is unknown. This study investigated the hemodynamic effects of ICA stenosis on the CoW. MethodsWe developed a three-dimensional/zero-dimensional (3D/0D) closed-loop geometric multiscale model of the cerebral artery to quantify the hemodynamic indicators, including time-averaged wall shear stress (TAWSS) and oscillatory shear index (OSI). Mild (<50 %), moderate (50–69 %) and severe (>69 %) ICA stenoses were established based on 3D models of cerebral arteries in two volunteers. Geometric multiscale computing models were numerically evaluated to obtain local hemodynamic changes in the CoW in order to assess the risk of stenosis in the CoW. ResultsModel calculations showed that for all 3D models the A1 segment of the anterior cerebral artery (ACA) or the posterior communicating artery (PCA) within the CoW exhibited a hemodynamic environment with high OSI (>0.2) and low TAWSS (<1 Pa) when the ICA had a moderate stenosis. While in the case of mild and severe stenosis in ICA, there is no such phenomenon. The proportion of the surface area possessing high OSI and low TAWSS in the A1 segment of the ACA or in the PCA was mostly greater than 60 %, which might potentially cause the formation and development of atherosclerosis in CoW and finally lead to CoW stenosis. ConclusionsTherefore, although moderate carotid artery stenosis may not cause ischemic stroke, it may cause hemodynamic changes in the CoW, which in turn may promote CoW stenosis and cause CoW decompensation. In clinical treatment attention should be paid not only to stenosis of the carotid arteries but also to changes in the hemodynamic environment within the CoW, in order to prevent the adverse effects of CoW stenosis.

In vitro and in silico assessment of flow modulation after deploying the Contour Neurovascular System in intracranial aneurysm models

BackgroundThe novel Contour Neurovascular System (Contour) has been reported to be efficient and safe for the treatment of intracranial, wide-necked bifurcation aneurysms. Flow in the aneurysm and posterior cerebral arteries...

Aneurysmal Inflow Rate Coefficient Predicts Ultra-early Rebleeding in Ruptured Intracranial Aneurysms: Preliminary Report of a Computational Fluid Dynamics Study.

Rebleeding from a ruptured intracranial aneurysm has poor outcomes. Although numerous factors are associated with rebleeding, studies on computational fluid dynamics (CFD) on hemodynamic parameters associated with early rebleeding are scarce. In particular, no report of rebleeding in ultra-early phase exists. We aimed to elucidate the specific hemodynamic parameters associated with ultra-early rebleeding using CFD. In this study, the rebleeding group included patients with aneurysmal subarachnoid hemorrhage (aSAH) that rebled within 6 h from the onset. The control group included patients without rebleeding, observed for >10 h following the initial rupture. Clinical images after initial rupture and before rebleeding were used to build 3D vessel models for hemodynamic analysis focusing on the following parameters: time-averaged wall shear stress (WSS), normalized WSS, low shear area, oscillatory shear index, relative residence time, pressure loss coefficient, and aneurysmal inflow rate coefficient (AIRC). Five and 15 patients in the rebleeding and control groups, respectively, met the inclusion criteria. The World Federation of Neurosurgical Surgeons grade was significantly higher in the rebleeding group (p = 0.0088). Hemodynamic analysis showed significantly higher AIRC in the rebleeding group (p = 0.042). The other parameters were not significantly different between groups. There were no significant differences or correlations between SAH severity and AIRC. AIRC was identified as a hemodynamic parameter associated with ultra-early rebleeding of ruptured intracranial aneurysms. Thus, AIRC calculation may enable the prediction of ultra-early rebleeding.

The Impact of Left Ventricular Assist Device Outflow Graft Positioning on Aortic Hemodynamics: Improving Flow Dynamics to Mitigate Aortic Insufficiency.

Heart failure is a global health concern with significant implications for healthcare systems. Left ventricular assist devices (LVADs) provide mechanical support for patients with severe heart failure. However, the placement of the LVAD outflow graft within the aorta has substantial implications for hemodynamics and can lead to aortic insufficiency during long-term support. This study employs computational fluid dynamics (CFD) simulations to investigate the impact of different LVAD outflow graft locations on aortic hemodynamics. The introduction of valve morphology within the aorta geometry allows for a more detailed analysis of hemodynamics at the aortic root. The results demonstrate that the formation of vortex rings and subsequent vortices during the high-velocity jet flow from the graft interacted with the aortic wall. Time-averaged wall shear stress (TAWSS) and oscillatory shear index (OSI) indicate that modification of the outflow graft location changes mechanical states within the aortic wall and aortic valve. Among the studied geometric factors, both the height and inclination angle of the LVAD outflow graft are important in controlling retrograde flow to the aortic root, while the azimuthal angle primarily determines the rotational direction of blood flow in the aortic arch. Thus, precise positioning of the LVAD outflow graft emerges as a critical factor in optimizing patient outcomes by improving the hemodynamic environment.

Numerical analysis of hemodynamic parameters in stenosed arteries under pulsatile flow conditions

This research studies the changes in flow patterns and hemodynamic parameters of diverse shapes and sizes of stenosis. Six different shapes and sizes of stenosis are constructed to investigate the variations in hemodynamics as the morphology changes. Changes in shape (trapezoidal and bell-shaped) and sizes of stenosis change the stresses on the walls and their flow patterns. TAWSS and OSI results specify that trapezoidal stenosis exerts greater stress than bell-shaped stenosis. Also, as the length of the trapezoidal stenosis increases, the TAWSS increases, whereas the trend is the opposite for bell-shaped stenosis. Later, this paper also studies different degrees of stenosis extracted from real images. Changes in velocity flow patterns, wall shear stress (WSS), Time-averaged wall shear stress (TAWSS) and Oscillatory shear index (OSI) have been studied for these images. Results illustrate that the peak velocity rises drastically as the stenosis percentage increases. Negative velocity is seen close to the artery's walls, indicating flow separation. This flow separation region is seen throughout the cycle except in the accelerating flow region. An increase in stenosis also increases WSS and TAWSS drastically. Negative WSS is seen downstream of stenosis, indicating flow recirculation. Such negative WSS in the blood vessels also promotes endothelial dysfunction. OSI values greater than 0.2 are seen near the stenosis region, indicating atherosclerosis growth. Regions of high OSI and low TAWSS are also identified, indicating probable regions of plaque development.

Assessment of Complex Flow Patterns in Patients With Carotid Webs, Patients With Carotid Atherosclerosis, and Healthy Subjects Using 4D Flow MRI.

Carotid webs (CaWs) are fibromuscular projections in the internal carotid artery (ICA) that cause mild luminal narrowing (<50%), but may be causative in up to one-third of seemingly cryptogenic strokes. Understanding hemodynamic alterations caused by CaWs is imperative to assessing stroke risk. Time-Average Wall Shear Stress (TAWSS) and Oscillatory Shear Index (OSI) are hemodynamic parameters linked to vascular dysfunction and thrombosis. To test the hypothesis: "CaWs are associated with lower TAWSS and higher OSI than mild atherosclerosis or healthy carotid bifurcation." Prospective study. A total of 35 subjects (N = 14 bifurcations with CaW, 11F, age: 49 ± 10, 10 mild atherosclerosis 6F, age: 72 ± 9, 11 healthy 9F, age: 42 ± 13). 4D flow/STAR-MATCH/3D TOF/3T MRI, CTA. 4D Flow velocity data were analyzed in two ways: 1) 3D ROI in the ICA bulbar segment (complex flow patterns are expected) was used to quantify the regions with low TAWSS and high OSI. 2) 2D planes were placed perpendicular to the centerline of the carotid bifurcation for detailed analysis of TAWSS and OSI. Independent-samples Kruskal-Wallis-H test with 0.05 used for statistical significance. The percent surface area where low TAWSS was present in the ICA bulb was 12.3 ± 8.0% (95% CI: 7.6-16.9) in CaW subjects, 1.6 ± 1.9% (95% CI: 0.2-2.9) in atherosclerosis, and 8.5 ± 7.7% (95% CI: 3.6-13.4) in healthy subjects, all differences were statistically significant (ƞ2 = 0.3 [95% CI: 0.05-0.5], P-value CaW vs. healthy = 0.2). OSI had similar values in the CCA between groups (ƞ2 = 0.07 [95% CI: 0.0-0.2], P-value = 0.5), but OSI was significantly higher downstream of the bifurcation in CaW subjects compared to atherosclerosis and normal subjects. OSI returned to similar values between groups 1.5 diameters distal to the bifurcation (ƞ2 = 0.03 [95% CI: 0.0-0.2], P-value = 0.7). Lower TAWSS and higher OSI are present in the ICA bulb in patients with CaW when compared to patients with atherosclerotic or healthy subjects. 2 TECHNICAL EFFICACY: Stage 2.

A hemodynamic analysis of fenestrated physician-modified endograft repair for complicated aortic dissections involving the visceral arteries

ObjectiveThe aim of this study is to perform patient-specific hemodynamic simulations of the patients with complicated aortic dissection underwent Physician-modified endograft (PMEG) and evaluate the treatment outcome. Method12 patient-specific models were reconstructed from computed tomography angiography (CTA) data of 6 patients with complicated aortic dissection before and after the PMEG. Hemodynamic simulations were conducted with the same time-varying volumetric flow rate extracted from the literature and 3-element Windkessel model (3 EWM) boundary conditions were applied at the aortic outlet. Hemodynamic indicators such as time-averaged wall shear stress (TAWSS), relative residence time (RRT) and endothelial cell activation potential (ECAP) were obtained to evaluate the postoperative effect of PMEG. ResultsComparing with the preoperative models, the flow rates of most visceral arteries were increased in the postoperative models (PSMA = 0.012, PRRA = 0.013, and PLRA = 0.005). Pressure and TAWSS in visceral regions were significantly reduced (PP = 0.003 and PTAWSS = 0.017). With the false lumens (FL) covered by the stent grafts, the average TAWSS level increased in the regions of postoperative abdominal aorta (P = 0.002), and the average RRT and ECAP values decreased significantly (PRRT = 0.02 and PECAP = 0.003). ConclusionThis study shows that PMEG, as a new technique for the treatment of complicated aortic dissection involving the distal tears in the visceral region, can effectively restore the abnormal blood supply of the visceral arteries, reduce the risk of aortic rupture, the formation of aortic dissection aneurysm (ADA), and thrombosis. This corresponds well with clinical retrospective studies and 1-year follow-up outcomes. The findings of this study are of great significance for the development of PMEG.

Highlighting Hemodynamic Risks for Bioresorbable Stents in Coronary Arteries

A three-dimensional, transient computational fluid dynamics analysis was conducted on an idealised geometry of a coronary artery fitted with representative geometries of an Absorb bioresorbable vascular scaffold (BVS) or a Xience drug-eluting stent (DES) in order to identify and compare areas of disturbed flow and potential risk sites. A non-Newtonian viscosity model was used with a transient velocity boundary condition programmed with user-defined functions. At-risk areas were quantified in terms of several parameters linked to restenosis: wall shear stress, time-averaged wall shear stress, oscillatory shear index, particle residence time, and shear rate. Results indicated that 71% of the BVS stented surface area had time-averaged wall shear stress values under 0.4 Pa compared to 45% of the DES area. Additionally, high particle residence times were present in 23% and 8% of the BVS and DES areas, respectively, with risk areas identified as being more prominent in close proximity to crowns and link struts. These results suggest an increased risk for thrombosis and neointimal hyperplasia for the BVS compared to the DES, which is in agreement with the outcomes of clinical trials. It is intended that the results of this study may be used as a pre-clinical tool to aid in the design of bioresorbable coronary stents.

Haemodynamic changes in visceral hybrid repairs of type III and type V thoracoabdominal aortic aneurysms

The visceral hybrid procedure combining retrograde visceral bypass grafting and completion endovascular stent grafting is a feasible alternative to conventional open surgical or wholly endovascular repairs of thoracoabdominal aneurysms (TAAA). However, the wide variability in visceral hybrid configurations means that a priori prediction of surgical outcome based on haemodynamic flow profiles such as velocity pattern and wall shear stress post repair remain challenging. We sought to appraise the clinical relevance of computational fluid dynamics (CFD) analyses in the setting of visceral hybrid TAAA repairs. Two patients, one with a type III and the other with a type V TAAA, underwent successful elective and emergency visceral hybrid repairs, respectively. Flow patterns and haemodynamic parameters were analysed using reconstructed pre- and post-operative CT scans. Both type III and type V TAAAs showed highly disturbed flow patterns with varying helicity values preoperatively within their respective aneurysms. Low time-averaged wall shear stress (TAWSS) and high endothelial cell action potential (ECAP) and relative residence time (RRT) associated with thrombogenic susceptibility was observed in the posterior aspect of both TAAAs preoperatively. Despite differing bypass configurations in the elective and emergency repairs, both treatment options appear to improve haemodynamic performance compared to preoperative study. However, we observed reduced TAWSS in the right iliac artery (portending a theoretical risk of future graft and possibly limb thrombosis), after the elective type III visceral hybrid repair, but not the emergency type V repair. We surmise that this difference may be attributed to the higher neo-bifurcation of the aortic stent graft in the type III as compared to the type V repair. Our results demonstrate that CFD can be used in complicated visceral hybrid repair to yield potentially actionable predictive insights with implications on surveillance and enhanced post-operative management, even in patients with complicated geometrical bypass configurations.

Biomechanics of abdominal aortic aneurysm in the framework of Windkessel effect and fully-developed inflow velocity via two-way non-linear FSI

Abdominal aortic aneurysm (AAA) is a pathological condition characterised by localised dilation of the infrarenal aorta, which can be fatal if rupture occurs. Current diagnostic criteria for repair are not patient-specific, but computational modelling may help to predict the potential growth and risk of AAA rupture accurately. This paper highlights how important using the Windkessel effect and fully-developed inflow velocity framework for the biomechanical analysis of AAA is; in other words, this research investigates how different boundary conditions in modelling methods affect the ability to correctly represent the biomechanics of AAA with complex non-linear parameters using a two-way fluid–structure interaction (FSI) technique. Time-averaged wall shear stress (TAWSS) is highly sensitive to boundary conditions, so unrealistic or simplified boundary conditions can lead to very different TAWSS results. This study also revealed that the use of outflow 3-element Windkessel boundary conditions on pulsatile pressure affects the flow pattern and increases the flow recirculation period. By accounting for the Windkessel effect and preventing backflow in numerical studies of AAA, the maximum displacement found in the distal branches can also be reduced. Models assuming plug velocity profiles overestimate peak wall stress by approximately 20% compared with models using fully developed parabolic inlet velocity profiles. This study incorporated a three-layer anisotropic material model and non-Newtonian fluid properties into the simulation, providing a more realistic representation of the effect of boundary condition selection on AAA mechanics. The results summarised from the parametric study could contribute to better prediction of AAA growth and rupture risk, and have important implications for the development of improved treatment strategies for AAA patients.