Wall Shear Stress Decrease Research Articles

Wall shear stress in intracranial aneurysms and adjacent arteries.

Hemodynamic parameters play an important role in aneurysm formation and growth. However, it is difficult to directly observe a rapidly growing de novo aneurysm in a patient. To investigate possible associations between hemodynamic parameters and the formation and growth of intracranial aneurysms, the present study constructed a computational model of a case with an internal carotid artery aneurysm and an anterior communicating artery aneurysm, based on the CT angiography findings of a patient. To simulate the formation of the anterior communicating artery aneurysm and the growth of the internal carotid artery aneurysm, we then constructed a model that virtually removed the anterior communicating artery aneurysm, and a further two models that also progressively decreased the size of the internal carotid artery aneurysm. Computational simulations of the fluid dynamics of the four models were performed under pulsatile flow conditions, and wall shear stress was compared among the different models. In the three aneurysm growth models, increasing size of the aneurysm was associated with an increased area of low wall shear stress, a significant decrease in wall shear stress at the dome of the aneurysm, and a significant change in the wall shear stress of the parent artery. The wall shear stress of the anterior communicating artery remained low, and was significantly lower than the wall shear stress at the bifurcation of the internal carotid artery or the bifurcation of the middle cerebral artery. After formation of the anterior communicating artery aneurysm, the wall shear stress at the dome of the internal carotid artery aneurysm increased significantly, and the wall shear stress in the upstream arteries also changed significantly. These findings indicate that low wall shear stress may be associated with the initiation and growth of aneurysms, and that aneurysm formation and growth may influence hemodynamic parameters in the local and adjacent arteries.

Improvement of Two-Stage Centrifugal Blood Pump for Cardiopulmonary Support System and Evaluation of Anti-Hemolysis Performance

Abstract In cardiopulmonary support systems with a membrane oxygenation such as a percutaneous cardiopulmonary support (PCPS) or an extracorporeal membrane oxygenation (ECMO), blood pumps need to generate the pressure rise of approximately 200mmHg or higher, due to the high hydraulic resistances of the membrane oxygenation and of the cannula tubing. In order to realize the blood pump with higher pressure rise, higher anti-hemolysis and thrombosis performances, the development of novel centrifugal blood pump composed of two-stage has been conducted by the authors. In the present paper, effective attempts to decrease the wall shear stress and to suppress the stagnation are introduced for the prevention of hemolysis and thrombosis in blood pumps. The hemolysis test was also carried out and it was clarified that the decrease of wall shear stress is effective as a guideline of design of blood pumps for improving the anti-hemolysis performance. Keywords: Centrifugal Pump, Artificial Heart, Blood, Hemolysis, Thrombosis

Influence of Temporal Variation in Wall Shear Stress on Intima‐Media Thickening in Arteriovenous Fistulae

Arteriovenous fistula (AVF) failure is mainly due to venous stenosis characterized by significant amount of intima-media thickening (IMT), probably in the presence of negative (inward) remodeling. Our hypothesis is that the longitudinal changes in wall shear stress (WSS) within different configurations of AVF can influence remodeling factors (changes in luminal diameter (ΔDh ) and IMT) during its maturation process. Dh is an equivalent diameter for a noncircular conduit. A total of six AVFs with curved (C-AVF; n = 3) and straight (S-AVF; n = 3) configurations were created between the femoral artery and vein of three pigs, bilaterally. CT scans and ultrasounds were utilized to calculate local WSS at 2D (D: days), 7D, and 28D postsurgery. For each AVF, IMT was measured at four regions along the vein using morphometric analyses. At these regions, repeated measurements of WSS and luminal diameter of each AVF were obtained over time. The ΔD(h) between 7D and 28D was significantly larger for C-AVF than for S-AVF (2.27 ± 0.67 mm vs. 0.02 ± 0.55 mm; p < 0.05). Also, at 28D the amount of IMT in C-AVF (77.46 ± 7.10 units) was significantly greater (p < 0.05) when compared with S-AVF (53.71 ± 8.23 units). These structural changes were accompanied by significantly different gradients of WSS over time (τ') for C-AVF (-0.56 ± 0.60 dyne/cm(2)/day) in comparison with S-AVF (0.71 ± 0.39 dyne/cm(2)/day). Negative τ' for C-AVF corresponded to reduction in WSS level over time resulting in a physiological level of WSS at 28D (4.08 ± 5.08 dyne/cm(2)). In contrast, a positive τ' for S-AVF was associated with the increase in WSS levels over time causing high levels of WSS at 28D (36.68 ± 5.32 dyne/cm(2)). The decrease in WSS levels for the C-AVF over time was associated with outward remodeling of the venous wall (favorable to maturation). In contrast, for S-AVF, the increase in WSS levels over time was associated with inward remodeling and subsequently, venous stenosis. Thus, temporal gradients of WSS, which could be altered by the surgical configuration of AVF, may provide important information on the remodeling behavior of AVFs. Identification of an optimal AVF configuration, which results in a temporal decrease in WSS and an outward remodeling of the venous wall, may reduce AVF maturation failure.

Wall shear stress quantification in the human conjunctival pre-capillary arterioles in vivo

Blood volume flow (Q), wall shear rate (WSR) and wall shear stress (WSS) were quantified, for the first time, in the conjunctival pre-capillary arterioles of normal human volunteers with diameters (D) between 6 and 12μm. The variation of the blood velocity throughout the cardiac cycle was taken into account using high speed video microcinematography. The dual effect of arteriolar diameter, firstly on the WSR and secondly on the dynamic viscosity of blood, was taken into account in the estimation of WSS. The average Q, WSR and WSS, throughout the cardiac cycle ranged from 13 to 202 pl/s, 587 to 3515s−1 and 1.7 to 21.1N/m2 respectively. The best fit power law equations, giving the increase of Q and the decrease of WSR and WSS with diameter, are presented for the systolic and diastolic phase as well as for the averages throughout the cardiac cycle. According to the WSS best fit equation, the average WSS decreases from 10.5N/m2 at D=6μm down to 2.1N/m2 at D=12μm.

Common Carotid Wall Shear Stress and Carotid Atherosclerosis in End-Stage Renal Disease Patients

Decrease of arterial wall shear stress (WSS) is associated with higher probability of atherosclerotic plaque development in many disease conditions. End-stage renal diseases (ESRD) patients suffer from vascular disease frequently, but its nature differs from general population. This study was aimed at proving an association between common carotid wall shear stress and the presence of carotid bifurcation plaques in a group of ESRD patients. ESRD subjects, planned for the creation of a dialysis access and therapy were included. Wall shear rate (WSR) was used as a surrogate of WSS and was analyzed in the common carotid arteries by duplex ultrasonography. Intima media thickness (IMT) was measured at the same site. The presence/absence of carotid bifurcation plaques was recorded. The endothelial function was estimated by the levels of von Willebrand factor (vWf). 35 ESRD patients were included (19 females, 17 diabetics). Atherosclerotic plaque was present in 53 % of bifurcations. Wall shear rate was lower in arteries with plaques (349+/-148 vs. 506+/-206 s(-1), p=0.005) and was directly related to the height of IMT and inversely to the activity of vWf (r= -0.65, p=0.016). Lower wall shear rate in the common carotid arteries is linked to the endothelial dysfunction and to the presence of atherosclerotic plaques in carotid bifurcations in ESRD subjects. Faster arterial dilatation may facilitate this process in ESRD subjects.

The Interaction of Leukocytes and Adhesion Molecules in Mesenteric Microvessel Endothelial Cells after Internal Capsule Hemorrhage

To explore the correlation between hemorheological variations and the expression of cell adhesion molecules in mesenteric microvessel endothelial cells after internal capsule hemorrhage. We established an internal capsule hemorrhage model. Then leukocyte-endothelium interaction was observed and hemorheological variations in mesenteric microvessels were evaluated in the following aspects: blood flow volume, diameter of microvessels, blood flow rate, and shear rate. We also measured the expression of vascular cell adhesion molecule-l and intercellular adhesion molecule-1 (ICAM-1) in mesenteric microvessel endothelial cells with immunohistochemistry stain. Leukocyte-endothelium interaction intensified after internal capsule hemorrhage. Besides, blood flow volume and velocity decreased, diameter narrowed, and shear rate reduced. Immunohistochemical staining of vascular cell adhesion molecule-l and ICAM-1in mesenteric microvessel endothelial cells was stronger. VCAM-1 and ICAM-1 expression in mesenteric microvessels increased as a result of decreased wall shear stress in stress state following internal capsule hemorrhage, and then further shear stress change from interaction of enhanced production of CAMs and leukocytes created a vicious cycle of leukocytes margination, adhesion, and transmigration that could ultimately result in stress gastrointestinal ulcer.

Vertebrobasilar angulation and its association with sudden sensorineural hearing loss

The pathogenesis of sudden sensorineural hearing loss (SSNHL) is unclear, though some researchers postulate the major mechanism of onset to be via circulatory disturbance or cochlear inflammation. SSNHL can represent the sole manifestation of anterior inferior cerebellar artery infarction, and patients with a SSNHL may have higher than normal risk of future stoke. According to a vascular remodeling theory, vertebral arteries (VAs) are typically asymmetric with the basilar artery (BA) gradually curving in the opposite direction of the larger VA. Decreased wall shear stress on the inner surface of the curvature (weaker side of the vertebral artery) gives rise to an atherothrombogenic environment. It is hypothesized that angulation of the BA could contribute to the decline of anterior inferior cerebellar artery (AICA) flow or to the development of atheroma formation in the AICA orifice. Vertebrobasilar junction angulation could represent a simple and useful marker of SSNHL caused by a vascular compromise of the cochlea.

Boundary Layer Flow Due to a Moving Flat Plate in Micropolar Fluid

Pemodelan matematik bagi aliran lapisan sempadan terhadap plat rata bergerak telah dibincangkan. Satu plat bergerak berterusan dalam arah positif paksi–x dengan halaju tetap. Persamaan lapisan sempadan yang dihasilkan telah diselesaikan secara berangka dengan menggunakan skema beza terhingga tersirat. Keputusan berangka telah diberikan, ini termasuk profil halaju, profile komponen legaran dan perubahan tegasan ricih permukaan. Kajian ini menunjukkan bahawa keputusan bagi masalah dalam bendalir mikropolar berbanding dengan bendalir likat adalah sangat memuaskan apabila parameter bahan K = 0 (bendalir Newtonan). Seterusoya tegasan ricih permukaan meningkat dengan peningkatan nilai K. Untuk nilai K yang ditetapkan, didapati tegasan ricih permukaan menyusut dan kompenan legaran meningkat dengan peningkatan nilai n, dalam selang O

Contribution of the Hemodynamics of A1 Dysplasia or Hypoplasia to Anterior Communicating Artery Aneurysms

To explore the association between the hemodynamics and formation, growth, and rupture of aneurysms in anterior communicating arteries (ACoA) with A1 dysplasia or hypoplasia. A series of 3-dimensional numerical simulation models of the anterior communicating artery complex (ACoAC) were designed geometrically. The diameter of A1 was fixed on one side and decreased gradually on the other side. Three groups of ACoA aneurysm model growth were constructed with different positions to the dominant bifurcation. Blood flow was modeled as an incompressible Newtonian fluid described by the unsteady Navier-Stokes equations. Vessel walls were assumed to be rigid; no slip boundary conditions were applied at the walls. Wall shear stress (WSS), flow velocity, and pressure were influenced by the dynamic variations of A1 diameter. When the diameter of the nondominant A1 gradually decreased, WSS and flow velocity dynamically increased in the dominant bifurcation and pressure decreased. Wall shear stress differences were significant between the dominant and nondominant bifurcations (t = 6.543; P < 0.05). With aneurysm growth, WSS and flow velocity gradually decreased, and turbulence appeared. Wall shear stress was lower at the bifurcation than that 0.02 mm and 0.1 mm to the bifurcation, whereas flow velocity and turbulent flow were more obvious. A1 dysplasia/hypoplasia is a potential risk factor in the formation of ACoA aneurysms. Wall shear stress increase may contribute to aneurysm formation. Wall shear stress decrease and turbulent flow may be responsible for the growth and rupture of ACoA aneurysms. The hemodynamic mechanism in the growth and rupture of aneurysms in different locations might be different.

Analysis of high gradient magnetic field effects on distribution of nanoparticles injected into pulsatile blood stream

Magnetic nanoparticles are widely used in a wide range of applications including data storage materials, pharmaceutical industries as magnetic separation tools, anti-cancer drug carriers and micro valve applications. The purpose of the current study is to investigate the effect of a non-uniform magnetic field on bio-fluid (blood) with magnetic nanoparticles. The effect of particles as well as mass fraction on flow field and volume concentration is investigated. The governing non-linear differential equations, concentration and Navier–stokes are coupled with the magnetic field. To solve these equations, a finite volume based code is developed and utilized. A real pulsatile velocity is utilized as inlet boundary condition. This velocity is extracted from an actual experimental data. Three percent nanoparticles volume concentration, as drug carrier, is steadily injected in an unsteady, pulsatile and non-Newtonian flow. A power law model is considered for the blood viscosity. The results show that during the systole section of the heartbeat when the blood velocity increases, the magnetic nanoparticles near the magnetic source are washed away. This is due to the sudden increase of the hydrodynamic force, which overcomes the magnetic force. The probability of vein blockage increases when the blood velocity reduces during the diastole time. As nanoparticles velocity injection decreases (longer injection time) the wall shear stress (especially near the injection area) decreases and the retention time of the magnetic nanoparticles in the blood flow increases.

Impact of main branch stenting on endothelial shear stress: role of side branch diameter, angle and lesion

In-stent restenosis and stent thrombosis remain clinically significant problems for bifurcation lesions. The objective of this study is to determine the haemodynamic effect of the side branch (SB) on main branch (MB) stenting. We hypothesize that the presence of a SB has a negative effect on MB wall shear stress (WSS), wall shear stress gradient (WSSG) and oscillatory shear index (OSI); and that the bifurcation diameter ratio (SB diameter/MB diameter) and angle are important contributors. We further hypothesized that stent undersizing exaggerates the negative effects on WSS, WSSG and OSI. To test these hypotheses, we developed computational models of stents and non-Newtonian blood. The models were then interfaced, meshed and solved in a validated finite-element package. Stents at bifurcation models were created with 30° and 70° bifurcation angles and bifurcations with diameter ratios of SB/MB = 1/2 and 3/4. It was found that stents placed in the MB at a bifurcation lowered WSS dramatically, while elevating WSSG and OSI. Undersizing the stent exaggerated the decrease in WSS, increase in WSSG and OSI, and disturbed the flow between the struts and the vessel wall. Stenting the MB at bifurcations with larger SB/MB ratios or smaller SB angles (30°) resulted in lower WSS, higher WSSG and OSI. Stenosis at the SB lowered WSS and elevated WSSG and OSI. These findings highlight the effects of major biomechanical factors in MB stenting on endothelial WSS, WSSG, OSI and suggests potential mechanisms for the potentially higher adverse clinical events associated with bifurcation stenting.

Reattached Turbulent Submerged Offset Jets on Rough Beds with Shallow Tailwater

This study investigates the characteristics of reattached plane turbulent offset jets in channels with rough beds and shallow tailwater depths. The flow consists of a deflecting free jet caused by the Coanda effect and the evolving wall jet past reattachment. In limited tailwater conditions, the jet flow is affected by the relative tailwater depth or the submergence parameter, which is defined with respect to the maximum B-jump at a negative step. The results show that for an offset height larger than the jet thickness, the forward-flow momentum, local maximum velocity, and wall shear stress decrease faster in the longitudinal direction in the offset jet than in plane turbulent wall jets. The influence of roughness on reattached offset jets appears to be less than that in submerged wall jets on a similar rough bed. The presented results and the comparative analysis with respect to wall jets in hydraulic jumps are significant for controlling and implementing similar flows on rough beds with variable downst...

Very high Reynolds number boundary layers over 3D sparse roughness and obstacles: the mean flow

Hot-wire and oil-film interferometry measurements are taken for 3D rough wall boundary layers at very high Reynolds numbers (61,000 < Re θ < 120,000) with low blockage ratios, 10 < δ/H < 135, and high roughness, 100 < H + < 4,900. The results cover flows over both rough walls and over obstacles and are compared with and provide extension to recent lower Reynolds number results. The validity of the Townsend ‘wall similarity hypothesis’ in relation to consistently increasing 3D roughness is interrogated. In agreement with recent work, Schultz and Flack (J Fluid Mech 580:381–405, 2007) and Castro (J Fluid Mech 585:469–485, 2007) found that, for relatively low roughness, Townsend’s hypothesis holds for the mean velocity field. With increasing roughness, the equilibrium layer diminishes and gradually vanishes. The viscous component of the wall shear stress decreases, while the turbulent component increases as the roughness effects extend across the boundary layer.

Extrusion of highly unsaturated wet powders: Stress fields in extruder barrels

Wet glass bead powders with solid-to-liquid volume ratios between 2 and 3 were extruded through cylindrical dies by means of a ram extruder. The vertical stress exerted by the ram and the wall normal stress above the die were recorded. A continuum mechanical model was developed to calculate stress fields in the extruder barrel as a function of the load superimposed by the ram. The model predicts that the vertical stress, the wall normal stress and the wall shear stress decrease exponentially above the die entry region. A sudden increase in wall normal and wall shear stresses is observed where the powder in the die vicinity starts to be vertically accelerated. The stresses then decrease to zero from this point to the die which is in agreement with experimental observations. Calculated wall normal stresses are consistent with measured values.

Critical Influence of Framing Coil Orientation on Intra-Aneurysmal and Neck Region Hemodynamics in a Sidewall Aneurysm Model

Although coiling of intracranial aneurysms is thought to rely on obstruction of blood flow into the aneurysm and induction of intra-aneurysmal thrombosis, little data exist regarding the effect of coil deployment on hemodynamics. To evaluate the effects of simulated coiling of a model aneurysm on flow and wall shear stress in the dome and neck regions using computational fluid dynamic analysis. A spherical sidewall aneurysm on a curved parent vessel underwent simulated embolization with 1 or more computer-designed helical coils. The coils' axes had parallel, orthogonal, or transverse orientation with respect to blood flow. Pulsatile laminar flow computational fluid dynamic analysis was performed on high-resolution conformal meshes of the aneurysm-coil complex using realistic non-Newtonian blood viscosity. Intra-aneurysmal flow and energy flux into the dome were significantly reduced by coil insertion, with little effect on pressure distribution. Coiling increased viscosity in the distal dome with progressive spread toward the neck with greater coil packing. Coiling also decreased wall shear stress and its gradient both in the inflow zone and the downstream parent vessel. These alterations were dependent on coil orientation, with effectiveness rank order of parallel>transverse>orthogonal. We successfully modeled the hemodynamic effects of aneurysm coil embolization and uncovered a framing coil orientation dependence of dome and parent vessel hemodynamics. In addition to suggesting a pathophysiological link among coil configuration, protection from rupture, and aneurysm regrowth, these results pave the way for the analysis of aneurysm-coil complex interactions on a patient lesion-specific basis.

A Mechanism for the Rapid Development of Intracranial Aneurysms: A Case Study

Despite technical and diagnostic progress there are still open questions in the understanding of the pathophysiology of intracranial aneurysms. Within 44 days we observed the de novo genesis and rupture of an aneurysm of the basilar artery in a patient. We performed computational fluid dynamics on 3-dimensional (3D) models of the inconspicuous vessel and the same vessel with aneurysm. Based on the simulations we propose a mechanism of genesis of fast-growing aneurysms. Three-dimensional mesh models were built using computed tomography-angiography slices. Flow was modeled as a non-Newtonian blood model with shear-dependent dynamic viscosity. We investigated flow velocity, wall pressure, impingement point, wall shear stress (WSS), and asymmetric flows in 3D models of the vessel tree of the basilar artery. Impingement point and wall pressure had no clear relation to the origin of the aneurysm. The impingement point faded away during aneurysm growth. Instead we found an area of permanently low WSS in the original basilar artery. This location corresponded to the origin of the later developing aneurysm. Aneurysm growth was facilitated by an increasing overall expansion of the basilar tip and a constant decrease of WSS. Assuming a preexisting reduced resistibility of the vessel wall to pressure changes and an area of permanently low WSS, an increase in pressure induces geometrical changes. These cause changes of intravascular flow distribution, lowering the already low WSS in specific locations. This leads to endothelial damage in this area and to a decreasing stability of the vessel wall, causing aneurysm development, growth, and rupture.

O-017 Effect of biomechanical environment in vessel wall on stent restenosis

IntroductionStenting has become a standard procedure to treat atherosclerotic vessels. However, restenosis is reported in approximately one-third of patients treated with intracranial stenting, and its avoidance is drawing attention. The...

Effect of Particle Migration on Flow and Convective Heat Transfer of Nanofluids Flowing Through a Circular Pipe

This paper aims to study the effect of particle migration on flow and heat transfer of nanofluids flowing through a circular pipe. To do this, a two-component model proposed by Buongiorno (2006, “Convective Transport in Nanofluids,” ASME J. Heat Transfer, 128, pp. 240–250) was used and a numerical study on laminar flow of alumina-water nanofluid through a constant wall temperature tube was performed. The effects of nonuniform distribution of particles on heat-transfer coefficient and wall shear stress are shown. Obtained results illustrate that by considering the particle migration, the heat-transfer coefficient increases while the wall shear stress decreases, compared with uniform volume fraction. Thus, it can be concluded that the enhancement of the convective heat transfer could not be solely attributed to the enhancement of the effective thermal conductivity, and beside other reasons, which may be listed as this higher enhancement, particle migration is proposed to be an important reason.

Flow simulation along a seal: the impact of an external device

An increasing number of marine mammal studies on physiology, behaviour and ecology rely on data, which have been collected from back-mounted devices, such as bio-logging tags and satellite transmitters. However, external devices may influence an animal’s hydrodynamics, behaviour and energy expenditure and, therefore, can impede the individual animal. To investigate the influence of external devices on seals, the water flow along a grey seal was simulated using computational fluid dynamics calculations. The simulations revealed several changes in forces and moments and thus balance, due to this device. The investigated satellite transmitter creates an average 12% increase of the drag coefficient. Additionally, there are significant relative transmitter-induced increases in pitching moment (32%) and lift (240%). The simulations also showed that the transmitter generates areas of decreased wall shear stress on the seal’s back. The results of this study demonstrate that external devices can change the hydrodynamics of the seal, which is expected to alter the seal’s physiology and behaviour and its use of the ecosystem. Long-term attachment may have adverse effects on the animal’s welfare. It is important to take these effects into consideration when studying tagged seals; otherwise, the value of the data obtained will be poor. Therefore, interpretations and extrapolations regarding ‘natural behaviour’ of animals in their ‘natural environment’ should only be made with great caution.

Two -fluid nonlinear mathematical model for pulsatile blood flow through catheterized arteries

The pulsatile flow of blood through a catheterized artery is analyzed, assuming the blood as a two-fluid model with the suspension of all the erythrocytes in the core region as a Herschel-Bulkley fluid and the peripheral region of plasma as a Newtonian fluid. The resulting system of the nonlinear implicit system of partial differential equations is solved by perturbation method. The expressions for shear stress, velocity, flow rate, wall shear stress and longitudinal impedance are obtained. The variations of these flow quantities with yield stress, catheter radius ratio, amplitude, pulsatile Reynolds number ratio and peripheral layer thickness are discussed. The velocity and flow rate are observed to decrease, and the wall shear stress and resistance to flow increase when the yield stress increases. The plug flow velocity and flow rate decrease, and the longitudinal impedance increases when the catheter radius ratio increases. The velocity and flow rate increase while the wall shear stress and longitudinal impedance decrease with the increase of the peripheral layer thickness. The estimates of the increase in the longitudinal impedance are significantly lower for the present two-fluid model than those of the single-fluid model.