Abnormal Wall Shear Stress Research Articles

Optical coherence tomography-based patient-specific coronary artery reconstruction and fluid-structure interaction simulation.

Plaque rupture is related to the mechanical stress it suffered. The value and distribution of the mechanical stress in plaque could help on assessing plaque vulnerability. To look into the stress conditions in the coronary artery, a patient-specific coronary model was created by using optical coherence tomography (OCT) and angiography imaging data. The reconstructed coronary model consisted of the structure of the lumen, the arterial wall and plaque components. Benefited by the high resolution of OCT, detailed structures such as the thin fibrous cap could be observed and built into the geometry. On this reconstructed coronary model, a fully coupled fluid-structure interaction (FSI) simulation was performed. The principle stress in coronary plaque and the wall shear stress (WSS) were analyzed. The FSI simulation results show that the cap thickness had a significant effect on the stress, and the principle stress at the thin cap area was more than double of those at the locations with a larger thickness. WSS is thought as an important parameter to assess the potentially dangerous areas of the atherosclerosis-prone (caused by low WSS) and the plaque rupture (high WSS). From the WSS plots of our FSI model, the area with abnormal WSS value was detected around the position where a lipid core existed. The FSI simulation results were compared with the results from the conventional structure-only and the computational fluid dynamics (CFD)-only computational models to quantify the difference between the three models. We found little difference in the principle stress results between the FSI and the structure-only model, but a significant difference between the FSI and the CFD-only model when looking into the WSS. The WSS values at the two observation spots from the CFD-only model were higher than the values from the FSI model by 17.95% and 22.66% in average, respectively. Furthermore, the FSI model detected more areas of low WSS, because the fluid domain could expand circumferentially when pressure loaded on the flexible arterial. This study suggests that OCT-based FSI model may be useful for plaque vulnerability assessment and it may be critical to perform the FSI simulation if an accurate WSS value is required.

Hemodynamic Differences Between Recurrent and Nonrecurrent Intracranial Aneurysms: Fluid Dynamics Simulations Based on MR Angiography.

Although the role of wall shear stress (WSS) in the initiation, growth, and rupture of intracranial aneurysms has been well studied, its influence on aneurysm recurrence after endovascular treatment requires further investigation. We aimed to compare WSS at necks of recurrent and nonrecurrent aneurysms. Nine recurrent coil-embolized aneurysms were identified and matched with nine nonrecurrent aneurysms. Patient-specific vessel geometries reconstructed from follow-up 3-D time-of-flight magnetic resonance angiography were analyzed using computational fluid dynamics (CFD) simulations. Absolute WSS and the percentage of abnormally low and high WSS at the aneurysm neck compared to the near artery were measured. The median percentage of abnormal WSS at the aneurysm neck was 49.3% for recurrent and 34.7% for nonrecurrent aneurysms (P = .011). The area under the receiver-operating-characteristic curve for distinguishing these aneurysms according to the percentage of abnormal WSS was .86 (95% CI .62 to .98). The optimal cut-off value of 45.1% resulted in a sensitivity and a specificity of 88.89% (95% CI 51.8% to 99.7%). Our findings indicate that necks of recurrent aneurysms are exposed to abnormal WSS to a larger extent. Abnormal WSS may serve as a metric to distinguish them from nonrecurrent aneurysms with CFD simulations a priori.

Decreased rotational flow and circumferential wall shear stress as early markers of descending aorta dilation in Marfan syndrome: a 4D flow CMR study

BackgroundDiseases of the descending aorta have emerged as a clinical issue in Marfan syndrome following improvements in proximal aorta surgical treatment and the consequent increase in life expectancy. Although a role for hemodynamic alterations in the etiology of descending aorta disease in Marfan patients has been suggested, whether flow characteristics may be useful as early markers remains to be determined.MethodsSeventy-five Marfan patients and 48 healthy subjects were prospectively enrolled. In- and through-plane vortexes were computed by 4D flow cardiovascular magnetic resonance (CMR) in the thoracic aorta through the quantification of in-plane rotational flow and systolic flow reversal ratio, respectively. Regional pulse wave velocity and axial and circumferential wall shear stress maps were also computed.ResultsIn-plane rotational flow and circumferential wall shear stress were reduced in Marfan patients in the distal ascending aorta and in proximal descending aorta, even in the 20 patients free of aortic dilation. Multivariate analysis showed reduced in-plane rotational flow to be independently related to descending aorta pulse wave velocity. Conversely, systolic flow reversal ratio and axial wall shear stress were altered in unselected Marfan patients but not in the subgroup without dilation. In multivariate regression analysis proximal descending aorta axial (p = 0.014) and circumferential (p = 0.034) wall shear stress were independently related to local diameter.ConclusionsReduced rotational flow is present in the aorta of Marfan patients even in the absence of dilation, is related to aortic stiffness and drives abnormal circumferential wall shear stress. Axial and circumferential wall shear stress are independently related to proximal descending aorta dilation beyond clinical factors. In-plane rotational flow and circumferential wall shear stress may be considered as an early marker of descending aorta dilation in Marfan patients.

Variational data assimilation for transient blood flow simulations: Cerebral aneurysms as an illustrative example.

Several cardiovascular diseases are caused from localised abnormal blood flow such as in the case of stenosis or aneurysms. Prevailing theories propose that the development is caused by abnormal wall shear stress in focused areas. Computational fluid mechanics have arisen as a promising tool for a more precise and quantitative analysis, in particular because the anatomy is often readily available even by standard imaging techniques such as magnetic resonance and computed tomography angiography. However, computational fluid mechanics rely on accurate initial and boundary conditions, which are difficult to obtain. In this paper, we address the problem of recovering high-resolution information from noisy and low-resolution physical measurements of blood flow (for example, from phase-contrast magnetic resonance imaging [PC-MRI]) using variational data assimilation based on a transient Navier-Stokes model. Numerical experiments are performed in both 3D (2D space and time) and 4D (3D space and time) and with pulsatile flow relevant for physiological flow in cerebral aneurysms. The results demonstrate that, with suitable regularisation, the model accurately reconstructs flow, even in the presence of significant noise.

Aortic flow after valve sparing root replacement with or without neosinuses reconstruction

ObjectivesThis study applied advanced 4-dimensional flow magnetic resonance imaging processing to assess differences in aortic flow dynamics after valve sparing root replacement, with and without reconstruction of the Valsalva sinuses. MethodsWe enrolled patients after valve sparing root replacement with a straight tubular prosthesis (n = 10) or with a prosthesis with Valsalva neosinuses (n = 10); age-matched subjects without cardiovascular diseases served as controls (n = 10). 4-Dimensional flow magnetic resonance imaging acquisitions were performed on a 3.0T magnetic resonance imaging unit. In-house processing was used to segment the aortic lumen and extract the volumetric 4-dimensional flow velocity field. Velocity flow streamlines were computed to compare the amount of rotational flow and wall shear stress. Occurrence of abnormal wall shear stress (WSS) was estimated within the descending aorta of each surgical group. ResultsPhysiologic-like sinus vortices were visible in the aortic root when using the prosthesis with neosinuses, whereas straight tubular graft revealed localized intrados malrotations (P = .003 for organized vortical structures vs neosinuses graft and P < .001 vs control). In the ascending aorta, recreation of the sinuses resulted in significantly lower velocity and WSS than in the straight tubular graft (P < .001) and controls (P < .001), these alterations were attenuated in the mid-descending aorta. Incidence of abnormal WSS was markedly higher in the straight tube grafts than neosinus of Valsalva grafts. ConclusionsRe-creation of the sinuses of Valsalva during valve-sparing root replacement is associated with more physiologic flow and significantly lower WSS in the aortic root. Lower WSSs in the distal thoracic aorta is a novel finding with potential implications on distal aortic remodeling.

Identification of vortex structures in a cohort of 204 intracranial aneurysms.

An intracranial aneurysm (IA) is a cerebrovascular pathology that can lead to death or disability if ruptured. Abnormal wall shear stress (WSS) has been associated with IA growth and rupture, but little is known about the underlying flow physics related to rupture-prone IAs. Previous studies, based on analysis of a few aneurysms or partial views of three-dimensional vortex structures, suggest that rupture is associated with complex vortical flow inside IAs. To further elucidate the relevance of vortical flow in aneurysm pathophysiology, we studied 204 patient IAs (56 ruptured and 148 unruptured). Using objective quantities to identify three-dimensional vortex structures, we investigated the characteristics associated with aneurysm rupture and if these features correlate with previously proposed WSS and morphological characteristics indicative of IA rupture. Based on the Q-criterion definition of a vortex, we quantified the degree of the aneurysmal region occupied by vortex structures using the volume vortex fraction (vVF) and the surface vortex fraction (sVF). Computational fluid dynamics simulations showed that the sVF, but not the vVF, discriminated ruptured from unruptured aneurysms. Furthermore, we found that the near-wall vortex structures co-localized with regions of inflow jet breakdown, and significantly correlated to previously proposed haemodynamic and morphologic characteristics of ruptured IAs.

Evaluation of Aortic Blood Flow and Wall Shear Stress in Aortic Stenosis and Its Association With Left Ventricular Remodeling.

Aortic stenosis (AS) leads to variable stress for the left ventricle (LV) and consequently a broad range of LV remodeling. The aim of this study was to describe blood flow patterns in the ascending aorta of patients with AS and determine their association with remodeling. Thirty-seven patients with AS (14 mild, 8 moderate, 15 severe; age, 63±13 years) and 37 healthy controls (age, 60±10 years) underwent 4-dimensional-flow magnetic resonance imaging. Helical and vortical flow formations and flow eccentricity were assessed in the ascending aorta. Normalized flow displacement from the vessel center and peak systolic wall shear stress in the ascending aorta were quantified. LV remodeling was assessed based on LV mass index and the ratio of LV mass:end-diastolic volume (relative wall mass). Marked helical and vortical flow formation and eccentricity were more prevalent in patients with AS than in healthy subjects, and patients with AS exhibited an asymmetrical and elevated distribution of peak systolic wall shear stress. In AS, aortic orifice area was strongly negatively associated with vortical flow formation (P=0.0274), eccentricity (P=0.0070), and flow displacement (P=0.0021). Bicuspid aortic valve was associated with more intense helical (P=0.0098) and vortical flow formation (P=0.0536), higher flow displacement (P=0.11), and higher peak systolic wall shear stress (P=0.0926). LV mass index and relative wall mass were significantly associated with aortic orifice area (P=0.0611, P=0.0058) and flow displacement (P=0.0058, P=0.0283). In this pilot study, AS leads to abnormal blood flow pattern and peak systolic wall shear stress in the ascending aorta. In addition to aortic orifice area, normalized flow displacement was significantly associated with LV remodeling.

Abstract 13: Wall Shear Stress Associated to Intracranial Aneurysm Wall Inflammation

Introduction: Inflammation has been proposed as a possible mechanism involved in the degradation and weakening of the walls of intracranial aneurysms. Hypothesis: Abnormal wall shear stress (WSS) levels induce wall inflammation which then affects the wall structure and mechanics Methods: A total of 20 aneurysms which underwent surgical clipping were studied. Patient-specific computational fluid dynamics models were constructed from pre-surgical CTA images. Numerical simulations were carried out using pulsatile flows. After clipping the aneurysm, a tissue sample was resected from the dome and analyzed histologically with CD45 to search for evidence of wall inflammation. For analysis, the aneurysm series was divided in two different manners. First, aneurysms were classified into an “inflammation” group if the number of CD45+ cells was larger than the median of CD45+ cells in the entire sample of 20 aneurysms; otherwise they were classified as “no-inflammation”. Hemodynamic variables were then statistically compared between these two groups. Secondly, aneurysms were subdivided into three groups according to their mean WSS: 1) “low WSS” if WSS&lt;0.5*median(WSS), 2) “high WSS” if WSS&gt;2*median(WSS), and 3) “mid WSS” otherwise. The numbers of CD45+ cells in each group were then statistically compared. Results: Aneurysms in the “inflammation” group had significantly larger mean WSS (p=0.018), shear rate (p=0.015), vorticity (p=0.018), and viscous dissipation (p=0.015) than aneurysms in the “no-inflammation” group. Conversely, aneurysms in the “high WSS” group had significantly larger numbers of CD45+ cells (p=0.0046) than the “mid WSS” and “low WSS” groups. Interestingly, aneurysms with stable flow patterns also tended to have larger numbers of inflammatory cells (p=0.040) than aneurysms with unstable flows. Conclusion: These preliminary results suggest that there is a connection between intra-aneurysmal flow characteristics and wall inflammation in cerebral aneurysms. In particular, inflamed walls seem to be associated with higher levels of wall shear stress.

How arterial pressures affect the consideration of internal carotid artery angle as a risk factor for carotid artherosclerotic disease

Patient–specific geometric factors together with traditional risk factors may aid the early identification of patients at high risk of developing carotid artery disease requiring surgical intervention. Recent studies have linked aspects of carotid geometry to the pathogenesis of internal carotid artery (ICA) stenosis. Abnormal wall shear stress (WSS) is found for large ICA angles. Low WSS is believed to correspond to plaque formation whereas high WSS may result in plaque rupture and clotting. Here, the meshless method, smoothed particle hydrodynamics, is used to simulate Newtonian flow through a clinical, rigid walled, carotid bifurcation. The resulting flow field and WSS are reported for a range of different ICA angles. Varying the angle without changing boundary pressure conditions produces minimal change in flow and WSS. Greater ICA downstream pressures appear important for maintaining well–behaved flow through the bifurcation by suppressing flow separation downstream of the stenosis resulting in more uniform wall stress.

Computation of hemodynamics in tortuous left coronary artery: a morphological parametric study.

Coronary tortuosity (CT) would alter the local wall shear stress (WSS) and may become a risk factor for atherosclerosis. Here we performed a systematic computational study to relate CT morphological parameters to abnormal WSS, which is a predisposing factor to the formation of atherosclerotic lesions. Several idealized left coronary artery (LCA) models were created to conduct a series of morphological parametric studies, in which we concentrate on three specific morphological parameters, the center line radius (CLR), the bend angle (BA), and the length between two adjust bends (LBB). The time averaged WSS (TAWSS), the oscillatory shear index (OSI), and the time averaged WSS gradient (WSSGnd) were explored by using the computational fluid dynamics (CFD) method, in order to determine susceptible sites for the onset of early atherosclerosis. In addition, two realistic LCA models were reconstructed to further validate the finding's credibility. The CLR and LBB had great impact on the distributions of WSS-derived parameters, while the BA had minor impact on the hemodynamic of the tortuous arteries. Abnormal regions with low TAWSS (TAWSS &lt; 0.5 Pa), high OSI (OSI &gt; 0.1) and high WSSGnd (WSSGnd &gt; 8) were observed at the inner wall of bend sections in the models with small CLR or small LBB. These findings were also confirmed in the realistic models. Severe CT with small CLR or LBB would lead to the formation of abnormal WSS regions at the bend sections and providing these regions with favorable conditions for the onset and/or progression of atherosclerosis.

A methodology to detect abnormal relative wall shear stress on the full surface of the thoracic aorta using four-dimensional flow MRI.

To compute cohort-averaged wall shear stress (WSS) maps in the thoracic aorta of patients with aortic dilatation or valvular stenosis and to detect abnormal regional WSS. Systolic WSS vectors, estimated from four-dimensional flow MRI data, were calculated along the thoracic aorta lumen in 10 controls, 10 patients with dilated aortas, and 10 patients with aortic valve stenosis. Three-dimensional segmentations of each aorta were coregistered by group and used to create a cohort-specific aortic geometry. The WSS vectors of each subject were interpolated onto the corresponding cohort-specific geometry to create cohort-averaged WSS maps. A Wilcoxon rank sum test was used to generate aortic P-value maps (P<0.05) representing regional relative WSS differences between groups. Cohort-averaged systolic WSS maps and P-value maps were successfully created for all cohorts and comparisons. The dilation cohort showed significantly lower WSS on 7% of the ascending aorta surface, whereas the stenosis cohort showed significantly higher WSS on 34% of the ascending aorta surface. The findings of this study demonstrated the feasibility of generating cohort-averaged WSS maps for the visualization and identification of regionally altered WSS in the presence of disease, compared with healthy controls.

Closure technique after carotid endarterectomy influences local hemodynamics

Meta-analysis supports patch angioplasty after carotid endarterectomy (CEA); however, studies indicate considerable variation in practice. The hemodynamic effect of a patch is unclear and this study attempted to elucidate this and guide patch width selection. Four groups were selected: healthy volunteers and patients undergoing CEA with primary closure, trimmed patch (5 mm), or 8-mm patch angioplasty. Computer-generated three-dimensional models of carotid bifurcations were produced from transverse ultrasound images recorded at 1-mm intervals. Rapid prototyping generated models for flow visualization studies. Computational fluid dynamic studies were performed for each model and validated by flow visualization. Mean wall shear stress (WSS) and oscillatory shear index (OSI) maps were created for each model using pulsatile inflow at 300 mL/min. WSS of <0.4 Pa and OSI >0.3 were considered pathological, predisposing to accretion of intimal hyperplasia. The resultant WSS and OSI maps were compared. The four groups comprised 8 normal carotid arteries, 6 primary closures, 6 trimmed patches, and seven 8-mm patches. Flow visualization identified flow separation and recirculation at the bifurcation increased with a patch and was related to the patch width. Computational fluid dynamic identified that primary closure had the fewest areas of low WSS or elevated OSI but did have mild common carotid artery stenoses at the proximal arteriotomy that caused turbulence. Trimmed patches had more regions of abnormal WSS and OSI at the bifurcation, but 8-mm patches had the largest areas of deleteriously low WSS and high OSI. Qualitative comparison among the four groups confirmed that incorporation of a patch increased areas of low WSS and high OSI at the bifurcation and that this was related to patch width. Closure technique after CEA influences the hemodynamic profile. Patching does not appear to generate favorable flow dynamics. However, a trimmed 5-mm patch may offer hemodynamic benefits over an 8-mm patch and may be the preferred option.

Abstract 18862: Wall Shear Stress in Pulmonary Hypertension

Pulmonary arterial hypertension (PAH) leads to morphometric remodeling of the pulmonary arteries. Computational fluid dynamics (CFD) modeling has the potential to reveal continuum metrics associated with the stress acting on the vascular endothelium in a non- invasive manner. We hypothesized that the PA endothelium of PAH patients will be subject to abnormal wall shear stress (WSS), and thus investigated the relationship of spatially averaged WSS (SAWSS) with invasive measurements of pulmonary vascular resistance (PVR) and compliance (C). Methods: Computed tomography angiography (CTA) and RHC were used to acquire images and hemodynamic data in 19 PH patients. Each patient’s pulmonary vascular tree was reconstructed from CT images up to 6 generations. The inlets and outlets were extended to 10 times their hydraulic diameters and a volumetric mesh created within element dimensions determined by a systematic mesh convergence study. Each CFD model was simulated using a structured tree outflow boundary condition. Constant flow rate simulations were performed using the numerical solver Fluent (ANSYS Inc., Lebanon, NH) after adjusting for patient-specific scaling parameters. Finally, SAWSS on the surface of the pulmonary arterial endothelium was obtained during post-processing. Correlations between SAWSS and hemodynamics were made. Results: Figure 1 shows a power-regression correlation between SAWSS and PVR, and linear-regression correlation between SAWSS and C. SAWSS decreases for increasing PVR and decreasing C. Conclusions: Pulmonary arterial WSS obtained by our CFD model can be used as a physiologically relevant quantitative metric of PAH severity, which likely precedes vascular and ventricular remodeling. Its application as a non-invasive surrogate for disease progression and evaluation of response to PH specific therapies needs further investigation.

Correlation of Hemodynamic Parameters to Endothelial Cell Proliferation in an End to Side Anastomosis

A strong correlation between the localization of atherosclerotic lesions with abnormal wall shear stress (WSS) has long been recognized at the distal anastomosis of a peripheral bypass where disturbed flow occurs. Identification of a WSS variable that significantly contributes to disease formation at this site has been elusive to date, as endothelial cell (EC) response to the abnormal hemodynamics at this anastomotic junction has yet to be quantitatively characterized. In vitro experiments were performed exposing human aortic ECs to pulsatile flow variables appropriate to a distal bypass graft junction using a novel in vitro device. Computational fluid dynamics was employed to detail the hemodynamic variables of this flow chamber. These variables were then correlated to EC proliferation which was used as an indicator of intimal hyperplasia (IH) development. Under pulsatile flow, maximum absolute temporal WSS gradient was found to be the parameter that most significantly correlated to EC proliferation (p = 0.0001, r = 0.8947). This metric can be utilized as an indicator to detect arterial segments prone to the initiation and localization of IH, it may also be used to help optimize the surgical construction of coronary artery bypass, peripheral bypass and arteriovenous anastomotic junctions to improve patient outcomes.

Abstract 141: Bicuspid Aortic Valve Hemodynamic Abnormalities Promote Early Development of Calcific Aortic Valve Disease

INTRODUCTION: The bicuspid aortic valve (BAV) is the most common congenital cardiac anomaly and is frequently associated with calcific aortic valve disease (CAVD). Although CAVD also develops in the normal tricuspid aortic valve (TAV), its progression in the BAV is more rapid. While the accelerated calcification of BAV leaflets has been linked to genetic and atherogenic predispositions, hemodynamic abnormalities are increasingly pointed as potential pathogenic contributors. HYPOTHESIS: Supported by our previous work, which demonstrated the sensitivity of valve leaflets to the surrounding blood flow and associated wall-shear stress (WSS), we hypothesize that the abnormal WSS experienced by BAV leaflets contribute to CAVD development by promoting valvular inflammation, remodeling and osteogenic differentiation. OBJECTIVE: This study aims at comparing ex vivo the effects of TAV and BAV leaflet WSS on valvular pathogenesis. METHODS: The native, side-specific WSS experienced by TAV and type-I (i.e., fused and non-coronary) BAV leaflets were obtained computationally using fluid-structure interaction simulations. Fresh porcine leaflets were subjected for 48 hours to each of the three WSS conditions using a novel double-sided shear stress bioreactor. Tissue response was characterized via Western blot and immunohistochemistry in terms of markers of endothelial activation (VCAM-1, ICAM-1), paracrine expression (BMP-4), TGF-β/Wnt signaling pathways (TGF-β1, β-catenin), extracellular matrix remodeling (cathepsin L, MMP-2, MMP-9) and osteogenic differentiation (α-SMA, osteocalcin). RESULTS: No significant differences in VCAM-1 and ICAM-1 expressions were detected between tissue exposed to TAV and BAV WSS. While the native WSS experienced by the TAV and non-coronary BAV leaflets maintained tissue homeostasis, tissue exposure to the fused BAV leaflet WSS resulted in a significant pathological response marked by the upregulations of BMP-4, β-catenin, MMP-2 and osteocalcin expressions. CONCLUSION: This study demonstrates the pathological nature of the native BAV hemodynamics and confirms the higher susceptibility of the fused BAV leaflet to calcify. The results provide new insights into the hemodynamic theory of BAV calcification.

Shear stress and plaque development

Although traditional cardiovascular risk factors ‘prime the soil’ for atherogenesis systemically, atherosclerosis primarily occurs in a site-specific manner with a predilection towards the inner wall of curvatures and outer wall of bifurcations with sparing of flow-dividers. Wall shear stress is a frictional force exerted parallel to the vessel wall that leads to alteration of the endothelial phenotype, endothelial cell signaling, gene and protein expression leading to a proinflammatory phenotype, reduced nitric oxide availability and disruption of the extracellular matrix, which in turn leads to plaque development. Clinical and experimental data are emerging that suggest the pathobiology associated with abnormal wall shear stress results in atherosclerotic plaque development and progression.

Wall Shear Stress on Ruptured and Unruptured Intracranial Aneurysms at the Internal Carotid Artery

Hemodynamics is often recognized as one of the major factors in aneurysm rupture. Flow impingement, greater pressure, and abnormal wall shear stress are all indications for aneurysm rupture. Characterizing wall shear stress for intracranial aneurysms at similar anatomic locations may help in understanding its role. Twenty-six intracranial aneurysms at the paraclinoid and superclinoid segments of the internal carotid artery from 25 patients between July 2006 and July 2007 were studied retrospectively. Among them, 8 aneurysms were ruptured and 18 were unruptured. Computational fluid dynamics was used to determine the wall shear distribution. Morphologic and hemodynamic variables was analyzed by using the Mann-Whitney rank sum test. Wall shear stress was qualitatively the same throughout the cardiac cycle; thus, only wall shear stress at the end of diastole was compared. Both ruptured and unruptured aneurysms have similar maximal wall shear stress (26 versus 23 N/m(2)), and mean wall shear stress is shown to be a function of the aneurysm area. Ruptured aneurysms also have a greater portion of aneurysm under low wall shear stress (27% versus 11% for unruptured aneurysms, P = .03). For intracranial aneurysms at the internal carotid artery, an area of low wall shear is associated with aneurysm rupture.

An S-type bypass can improve the hemodynamics in the bypassed arteries and suppress intimal hyperplasia along the host artery floor

The development of distal end-to-side anastomotic intimal hyperplasia (IH) has been attributed to the flow disturbance and abnormal wall shear stress (WSS) distribution there. The geometry of the bypass has a strong influence on the flow pattern and WSS distribution. Using a canine model of end-to-side anastomosis, a 45° S-type bypass was compared with 60°, 45° and 30° conventional bypasses in the term of IH along the host artery floor. Numerical blood flow simulations were also carried out to characterize the flow patterns at the distal parts of the bypassed arteries for the 4 models. The results showed that the averaged intima thicknesses of the host artery floors for the 4 bypass models were 119.50±10.30 μm (60°), 65.56±6.53 μm (45°), 45.26±5.99 μm (30°) and 47.64±4.85 μm (S-type), respectively, vs. 9.81±1.88 μm in the control group (without bypass surgery). Compared with the control group, neointima thickness in all 4 bypass models was significantly increased, but the neointima thickness of the 45° S-type bypass was apparently much better than its 45° conventional counterpart, and was as good as the 30° conventional bypass. The numerical simulation revealed an apparent swirling flow pattern in the S-type bypass, which was very different than the flow patterns in the 3 conventional bypass models. This swirling flow altered the overall flow pattern in the distal part of the bypassed artery and eliminated the low WSS zone along the host artery floor. The improvement in the term of IH for the S-type bypass is most likely due to the alteration of the overall flow pattern and WSS distribution by the geometrical configuration of the S-type bypass.

Measurement of Hemodynamic and Anatomic Parameters in a Swine Arteriovenous Fistula Model

Although arteriovenous fistulae (AVFs) are currently the preferred mode of permanent hemodialysis access they do have significant problems due to initial non-maturation and a later venous stenosis. These problems appear to have been exacerbated following a push to increase AVF prevalence in the US. The reasons for both AVF non-maturation and the later venous stenoses are unclear but are thought to be related to abnormal hemodynamic wall shear stress (WSS) profiles. This technical note aims to describe the successful development of measurement techniques that can be used to establish a complete hemodynamic profile in a pig model with two different configurations of AVF. The curved and straight AVF configurations were created in an in vivo pig model. Flow and pressure in the AVFs were measured using the perivascular flow probes and Doppler flow wires while the pressure was recorded using a pressure transducer. The anatomical configuration was obtained using two different approaches: a) combination of intravascular ultrasound (IVUS) and angiograms, (b) 64 slice CT angiography. 3D models were reconstructed using image processing and computer modeling techniques. Numerical calculations were then performed by applying the measured flow and pressure data into the configurations to obtain the hemodynamic WSS profiles. The described methodologies will allow the calculation and optimization of WSS profiles in animal models. This information could then be translated to the clinical setting where it would have a positive impact on improving the early maturation rates of AVFs as well as reducing the late venous stenoses.

Fluid Dynamic Study on Budd-Chiari Syndrome: Sensitivity Study of Vessel Reconstruction on Image-Based Simulation

Budd-Chiari syndrome (BCS) is a symptomatic complete or partial occlusion of the hepatic vein and/or the inferior vena cava. The cause of BCS is not well known yet, however; abnormal vessel wall shear stress caused by blood flow is thought to increase the likelihood of developing BCS. We investigate the blood flow field in the vicinity of the junction of the inferior vena cava and hepatic veins by using numerical simulation based on medical imaging. However, there exist inaccuracies in computational vessel models. The sensitivities of flow characteristics to vessel shape have not been fully investigated. In this study, the characteristics of the blood flow in the vicinity of the junction of the hepatic vein and the inferior vena cava are discussed based on the numerical results obtained by several reconstructed vessel models which are slightly different in shapes and surface smoothness. It is also demonstrated that relatively small differences in the shapes and surface smoothness of the reconstructed vessel model cause noticeable effects on the flow characteristics in the vicinity of the junction of the inferior vena cava and large hepatic veins.