Local Flow Patterns Research Articles

Differential wall shear stress patterns in cerebral circulation of prediabetic adults: Possible evidence for increased stroke risk?

Proper endothelial function is crucial to cerebral vascular health and is affected by wall shear stress (WSS) patterns. Metabolic syndrome (MetSyn) is a prediabetic state that contributes to endothelial dysfunction which can lead to clinical conditions, such as, stroke. The aim of this study was to quantify and compare resting WSS in multiple conduit cerebral arteries in healthy and MetSyn subjects. We hypothesized adults with Metsyn will display reduced WSS in cerebral arteries when compared to healthy controls. 7 healthy controls (29±2y) and 7 MetSyn (37±3y) underwent a single four‐dimensional (4D) flow MRI scan during quiet rest in the supine position. Data acquisition was based on a radial undersampling scheme named Phase Contrast Vastly‐undersampled Isotropic Projection Reconstruction (PC VIPR) used to generate a phase contrast (PC) angiogram, as well as, measure cerebral blood flow (CBF), and WSS non‐invasively without use of a contrast agent (three directional velocity encoding at a spatial and temporal resolution of 0.68 mm and 20 cardiac phases, respectively). Average scan time was ~ 5 minutes and produced large imaging volumes. Nine arteries were analyzed: Middle cerebral (MCA) ×2, anterior cerebral (ACA) ×2, internal carotid (ICA) ×2, vertebral (VA) ×2, and basilar. WSS was measured by segmenting the vascular tree from the PC angiograms and subsequently calculating the velocity gradient at the vessel wall from the 4D Flow MRI data over ~ 3 voxels. Total CBF was calculated as the sum of the left and right ICA and basilar artery. Resting total CBF was 577±45 ml· min−1 in healthy controls and 499±46 ml ·min−1 in MetSyn (p=0.25). Metsyn exhibited lower WSS (Pa) in the left MCA (3.7±0.2 vs. 3.0±0.2; p=0.01), right ACA (3.4±0.2 vs. 2.9±0.2; p=0.05), right ICA (2.7±0.3 vs. 1.8±0.1; p=0.01) and left ICA (2.8±0.3 vs. 1.5±0.1; p<0.01). This non‐invasive MRI approach provides new insight into local flow and shear patterns that might have a large impact on cerebral endothelial function. Younger adults with MetSyn exhibit lower WSS in anterior portions of the brain. Furthermore, these findings are consistent with the observation that stroke occurs most commonly in the anterior cerebral circulation. Given the young age (< 45 y) of these subjects, we propose chronically reduced WSS leads to endothelial dysfunction in middle/old age, thereby contributing to increased risk and severity of stroke in MetSyn.Support or Funding InformationFunded by the American Diabetes Association and an American Physiological Society Undergraduate Research Summer Fellowship.

Experimental study of effect of ambient flow condition on the performance of a passive autocatalytic recombiner

This paper examines the effect of ambient flow conditions on the function of a passive autocatalytic recombiner (PAR), including external flows blowing horizontally at the PAR inlet or outlet. A full size PAR unit was tested at a large-scale test facility. The PAR self-start hydrogen concentration threshold, capacity and ignition limit were determined based on global hydrogen concentration, and the ignition events were monitored using an infrared camera. The test results show that the PAR self-started at 1–2.1vol.% global H2 for all the test conditions and it tended to self-start at slightly higher global H2 concentration under higher ambient flow speed. The overall hydrogen removal rate (apparent capacity) by a given PAR calculated on the basis of global hydrogen concentration could be significantly affected by the local flow pattern. The ignition could occur at higher global H2 concentrations if there is a flow, comparable in size to the PAR self-sustained buoyancy flow, blowing at the PAR outlet. These experiments provide a more detailed understanding of the effect of local flow conditions on PAR effectiveness and indicate the importance of 3-D modeling to capture the local effects.

Effects of Surfactant on Flow Boiling Heat Transfer of Ethylene Glycol/Water Mixtures in a Minitube

This study investigated effects of surfactant sodium dodecylbenzene sulfonate (SDBS) on flow boiling heat transfer of ethylene glycol/water mixtures in a vertical minitube. Experiments were performed using solutions containing 300 ppm by weight of surfactant and the results were compared with those of pure mixture. Local heat transfer coefficient was measured and found to be dependent on the mass quality. Although the surfactant intensifies the vaporization process, it doesn't necessarily enhance the heat transfer coefficient. Heat transfer coefficients were compared at two different mass fluxes, and the results might be explained based on the local flow pattern and the heat transfer mechanism. After a critical quality, higher quality tends to deteriorate heat transfer due to intermittent dryout, and therefore adding a surfactant to generate more vapor may be of negative effect on the flow boiling heat transfer in a minitube, which is contrary to the experience of enhancing nucleate pool boiling heat transfer with trace surfactants.

Virtual reconstruction and performance assessment of an eroded centrifugal pump impeller

Emphasis is placed on the performance of a two-stage pump equipped with an eroded impeller. Non-contact reverse engineering technique is utilized to acquire the point data constituting the three-dimensional geometrical model of the eroded impeller. With computational fluid dynamics, both the operation performance of the pump and flow patterns adjacent to the eroded blade surface are obtained. For comparison, the corresponding undamaged impeller is investigated as well. Preferable fidelity of the approach of constructing geometrical model of the damaged impeller is substantiated. The mechanism underlying performance degradation of the pump with the eroded impeller is evidenced. Local flow patterns near the eroded area are featured by explicit irregularity. Furthermore, between the defective and the undamaged pumps, disparity in positions where cavitation occurs is negligible.

Formation mechanism of channel segregation in carbon steels by inclusion flotation: X-ray microtomography characterization and multi-phase flow modeling

Recent experimental dissections of steel ingots and multi-scale simulations have led to the discovery of a potential driving force for channel segregation: the flotation of oxide-based inclusion (D. Li et al., Nat. Commun. 5:5572 (2014)). Further experimental analysis and numerical modeling are necessary to clarify this mechanism in detail. In this work, the inclusions in a carbon steel ingot that exhibits severe channel segregations were characterized by the 3D X-ray microtomography, which revealed a significant enrichment and growth of inclusions in the channels. Based on above microtomography characterization, a 2D macrosegregation model encompassing the inclusion flotation was established. In the model, the motions of solid inclusions and liquid were described using the multi-phase flow scheme within the Euler-Lagrange framework. The benchmark simulations showed that sufficient inclusion populations with appropriate sizes are capable of altering the local flow patterns and destabilize the mushy zone, initiating the subsequent channel segregation. The continuous interplay between melt convection, inclusion flotation and solidification eventually causes the formation of macroscale channel. The predicted sizes and volume fraction of inclusions that are able to trigger the channel segregation effectively are consistent with the data obtained via microtomography characterization. The macrosegregation model was then applied to predict the channel segregations in an industrial carbon steel ingot. A rather good agreement of A-segregates was achieved between the simulation and the dissected ingot.

Activation of integrin α5 mediated by flow requires its translocation to membrane lipid rafts in vascular endothelial cells

Local flow patterns determine the uneven distribution of atherosclerotic lesions. Membrane lipid rafts and integrins are crucial for shear stress-regulated endothelial function. In this study, we investigate the role of lipid rafts and integrin α5 in regulating the inflammatory response in endothelial cells (ECs) under atheroprone versus atheroprotective flow. Lipid raft proteins were isolated from ECs exposed to oscillatory shear stress (OS) or pulsatile shear stress, and then analyzed by quantitative proteomics. Among 396 proteins redistributed in lipid rafts, integrin α5 was the most significantly elevated in lipid rafts under OS. In addition, OS increased the level of activated integrin α5 in lipid rafts through the regulation of membrane cholesterol and fluidity. Disruption of F-actin-based cytoskeleton and knockdown of caveolin-1 prevented the OS-induced integrin α5 translocation and activation. In vivo, integrin α5 activation and EC dysfunction were observed in the atheroprone areas of low-density lipoprotein receptor-deficient (Ldlr(-/-)) mice, and knockdown of integrin α5 markedly attenuated EC dysfunction in partially ligated carotid arteries. Consistent with these findings, mice with haploinsufficency of integrin α5 exhibited a reduction of atherosclerotic lesions in the regions under atheroprone flow. The present study has revealed an integrin- and membrane lipid raft-dependent mechanotransduction mechanism by which atheroprone flow causes endothelial dysfunction.

Effect of Oscillation Structures on Inertial-Range Intermittence and Topology in Turbulent Field

AbstractUsing the incompressible isotropic turbulent fields obtained from direct numerical simulation and large-eddy simulation, we studied the statistics of oscillation structures based on local zero-crossings and their relation with inertial-range intermittency for transverse velocity and passive scalar. Our results show that for both the velocity and passive scalar, the local oscillation structures are statistically scale-invariant at high Reynolds number, and the inertial-range intermittency of the overall flow region is determined by the most intermittent structures characterized by one local zero-crossing. Local flow patterns conditioned on the oscillation structures are characterized by the joint probability density function of the invariants of the filtered velocity gradient tensor at inertial range. We demonstrate that the most intermittent regions for longitudinal velocity tend to lay at the saddle area, while those for the transverse velocity tend to locate at the vortex-dominated area. The connection between the ramp-cliff structures in passive scalar field and the corresponding saddle regions in the velocity field is also verified by the approach of oscillation structure classification.

Towards a multi-physics modelling framework for thrombolysis under the influence of blood flow.

Thrombolytic therapy is an effective means of treating thromboembolic diseases but can also give rise to life-threatening side effects. The infusion of a high drug concentration can provoke internal bleeding while an insufficient dose can lead to artery reocclusion. It is hoped that mathematical modelling of the process of clot lysis can lead to a better understanding and improvement of thrombolytic therapy. To this end, a multi-physics continuum model has been developed to simulate the dissolution of clot over time upon the addition of tissue plasminogen activator (tPA). The transport of tPA and other lytic proteins is modelled by a set of reaction–diffusion–convection equations, while blood flow is described by volume-averaged continuity and momentum equations. The clot is modelled as a fibrous porous medium with its properties being determined as a function of the fibrin fibre radius and voidage of the clot. A unique feature of the model is that it is capable of simulating the entire lytic process from the initial phase of lysis of an occlusive thrombus (diffusion-limited transport), the process of recanalization, to post-canalization thrombolysis under the influence of convective blood flow. The model has been used to examine the dissolution of a fully occluding clot in a simplified artery at different pressure drops. Our predicted lytic front velocities during the initial stage of lysis agree well with experimental and computational results reported by others. Following canalization, clot lysis patterns are strongly influenced by local flow patterns, which are symmetric at low pressure drops, but asymmetric at higher pressure drops, which give rise to larger recirculation regions and extended areas of intense drug accumulation.

Sound wall barriers: Near roadway dispersion under neutrally stratified boundary layer

With the passage of California Senate Bill 375, which motivates infill development near transit hubs, there is the potential to increase vehicle congestion in residential communities and increase in human exposure to toxic mobile source pollutants. Among all the mitigation strategies that protect near roadway residents from health-affecting vehicular emissions (e.g. separating sensitive receptors from high traffic roadways), this paper discusses the impact of sound wall barriers (SBs) in reducing the air pollution exposure of nearby residents. To date, there have been some studies done to understand the impact of these structures on dispersion of vehicular emissions; however, no definitive conclusion has been drawn yet. The main objective of this paper is to provide more information and details on flow and dispersion affected by barriers through a systematic laboratory simulation of plume dispersion using a water channel. Three sets of experiments were conducted: (1) plume visualizations, (2) plume concentration measurements, and (3) flow velocity measurements. Results from this study shows that the deployment of sound barriers induces a recirculating flow over the roadway which transports the surface released emissions to the upwind side of the roadway, and then shifts the plume upward through an induced updraft motion. Plume visualizations clearly demonstrate that the presence of SBs induce significant vertical mixing and updraft motion on the roadway which increases the initial plume dilution and plume height and consequently results in reduced downwind ground level concentrations. Although different SB configurations result in different localized flow patterns, the dispersion pattern does not change significantly after several SB heights downwind of the roadway.

Hydrodynamic study of a novel membrane aerated biofilm reactor (MABR): Tracer experiments and CFD simulation

The membrane-aerated biofilm reactor (MABR) is a promising technology for wastewater treatment, especially for simultaneous organic and nitrogen removal. The mass transfer phenomena induced by flow velocity and flow pattern is required in order to improve the reactor design and the removal pollutants efficiency. Tracer experiments and residence time distribution (RTD) theory were used to characterize the flow in a special MABR. The liquid phase flow patterns were investigated through tracer pulse stimulus-response technique using dextran blue as model tracer. RTD curves were analyzed by cold-model tests (axial dispersion model ADM, tanks in series model TIS and mixing cell model MCM). The detailed flow pattern of the reactor was obtained from computational fluid dynamic (CFD) simulation. According to experimental results of RTD studies and CFD simulation, the flow patterns were demonstrated to be analogous to completely mixed flow with deviations of the ideal hydrodynamic behavior; stagnant zones (low fluid velocity) in 85% of its volume, being the remainder a channeling trouble (high fluid velocity). These deviations were quantitatively described (macromixing level-global mixing) with a minimum quadratic error function value of (S) 0.01. The local mixing flow pattern (micromixing level) obtained by CFD allowed determining the location of each zone; the stagnant zone is situated in the area where membranes are located; therefore it is possible to assume that degradation reactions of pollutants would take place in this area.

Hemodynamic and thrombogenic analysis of a trileaflet polymeric valve using a fluid–structure interaction approach

Surgical valve replacement in patients with severe calcific aortic valve disease using either bioprosthetic or mechanical heart valves is still limited by structural valve deterioration for the former and thrombosis risk mandating anticoagulant therapy for the latter. Prosthetic polymeric heart valves have the potential to overcome the inherent material and design limitations of these valves, but their development is still ongoing. The aim of this study was to characterize the hemodynamics and thrombogenic potential of the Polynova polymeric trileaflet valve prototype using a fluid–structure interaction (FSI) approach. The FSI model replicated experimental conditions of the valve as tested in a left heart simulator. Hemodynamic parameters (transvalvular pressure gradient, flow rate, maximum velocity, and effective orifice area) were compared to assess the validity of the FSI model. The thrombogenic footprint of the polymeric valve was evaluated using a Lagrangian approach to calculate the stress accumulation (SA) values along multiple platelet trajectories and their statistical distribution. In the commissural regions, platelets were exposed to the highest SA values because of highest stress levels combined with local reverse flow patterns and vortices. Stress-loading waveforms from representative trajectories in regions of interest were emulated in our hemodynamic shearing device (HSD). Platelet activity was measured using our platelet activation state (PAS) assay and the results confirmed the higher thrombogenic potential of the commissural hotspots. In conclusion, the proposed method provides an in depth analysis of the hemodynamic and thrombogenic performance of the polymer valve prototype and identifies locations for further design optimization.

CFD Analysis of Convective Heat Transfer Coefficient on External Surfaces of Buildings

Convective heat transfer coefficients for external building surfaces are essential in building energy simulation (BES) to calculate convective heat gains and losses from building facades and roofs to the environment. These coefficients are complex functions of: building geometry, building surroundings, local air flow patterns and temperature differences. A microclimatic analysis in a typical urban configuration, has been carried out using Ansys Fluent Version 14.0, an urban street canyon, with a given H/W ratio, has been considered to simulate a three-dimensional flow field and to calculate the thermal fluid dynamics parameters that characterize the street canyon. In this paper, the convective heat transfer coefficient values on the windward external façade of the canyon and on the windward and leeward inner walls are analyzed and a comparison with values from experimental and numerical correlations is carried out.

Transthoracic Doppler for detection of stenoses in the three main coronary arteries by use of stenotic to prestenotic velocity ratio and aliased coronary flow.

Our aim was to determine the feasibility and accuracy of diagnosing significant coronary artery stenoses using peak stenotic to prestenotic velocity ratio (pSPVR) measurements when compared with results from quantitative coronary angiography and coronary flow velocity reserve (CFVR) assessed by transthoracic echocardiography (TTE). One hundred and eight patients scheduled for coronary angiography were studied using transthoracic Doppler echocardiography. Stenoses were identified by local colour aliasing by colour flow Doppler, and further evaluated by pSPVR, using a pSPVR of ≥2.0 as a cut-off for significant stenosis. When pSPVR could not be measured, local mosaic coronary flow pattern at Nyquist limit ≥0.48 m/s was used. Sixty-five lesions suggestive of stenosis were found by TTE. Combining findings of pSPVR ≥2.0 and local mosaic flow at Nyquist limit ≥0.48 m/s, the sensitivity and specificity of demonstrating significant stenoses (diameter stenosis, 50-99%) in the left main coronary artery (LM), left anterior descending coronary (LAD), left circumflex coronary (Cx), and right coronary artery (RCA) were 75 and 98%, 74 and 95%, 40 and 87%, and 34 and 98%, respectively. The pSPVR did not differ significantly between arteries with reduced and normal CFVR, with a cut-off of CFVR <2.0. Findings of pSPVR ≥2.0 or localized colour flow aliasing are useful in the non-invasive diagnosis of significant coronary disease in the three main coronary arteries, with high specificity for detecting significant stenoses. These findings showed high sensitivity for identifying significant stenoses in the LM and LAD, but showed lower ability to detect those lesions in the Cx and RCA.

204 Disturbed Flow Promotes Atherogenesis through the Activation of Endothelial-Mesenchymal Transition

<h3>Introduction</h3> Atherosclerosis is influenced by local blood flow patterns, which exert wall shear stress (WSS) on endothelial cells (EC). Low, oscillatory WSS promotes atherosclerosis by inducing EC proliferation and permeability, while high WSS is athero-protective. We recently used microarray technology coupled to computational fluid dynamics to study the transcriptome of EC at regions of the porcine aorta exposed to low, oscillatory or high, unidirectional WSS. The study revealed differential expression of GATA4 and Twist1. These transcription factors can promote endothelial-mesenchymal transition (EndMT), a process that involves altered vascular endothelial (VE)- cadherin function and enhanced EC proliferation. Here we tested the hypothesis that GATA4 and Twist1 may promote atherogenesis at sites of disturbed flow by inducing EndMT. <h3>Methods and results</h3> Quantitative RT-PCR and <i>en face</i> staining confirmed elevated GATA4, Twist1 and EndMT effector gene (Snail, Slug and N cadherin) expression at the inner curvature (lower WSS) compared to the outer curvature (higher WSS) of porcine and murine aortae (all p &lt; 0.05). Snail expression at the inner curvature of the murine aorta was reduced by deletion of Twist1 in EC (Tie-2 Twist1^KO) compared to controls (Twist1 ^fl/fl; p &lt; 0.05), demonstrating that Twist1 promotes Snail expression at a low WSS site <i>in vivo</i>. To establish a causal link between flow and EndMT, WSS was modified in murine carotid arteries using a constrictive cuff, the study revealed elevated Twist1 expression at the low WSS site (proximal to stenosis) and enhanced GATA4 and Snail expression at the low, oscillatory WSS site (distal). Similarly, GATA4, Twist1 and EndMT effector genes (Snail, Slug and N cadherin) were induced in porcine aortic EC (PAEC) or human umbilical vein EC (HUVEC) exposed to low, oscillatory WSS for 72h using an orbital plate system (all p &lt; 0.05). Gene silencing demonstrated that GATA4 and Twist1 are required for Snail induction in EC exposed to low, oscillatory WSS, and chromatin immunoprecipitation revealed GATA4 interaction with promoter regions of Twist1 and Snail. Low, oscillatory WSS promoted several changes that are characteristic of EndMT including N-cadherin induction, VE-cadherin disorganisation and enhanced proliferation (all p &lt; 0.05). Silencing of GATA4, Twist1 and Snail significantly reduced these processes and limited EC permeability (all p &lt; 0.05) in EC exposed to low, oscillatory WSS. <h3>Conclusions</h3> We conclude that low WSS induces EndMT and subsequent EC proliferation and permeability through the induction of GATA4 and Twist1. Our observations illuminate for the first time, the role of EndMT in arterial biomechanics and injury. Future studies should define the role of EndMT in focal atherosclerosis.

Atmospheric Flow Development and Associated Changes in Turbulent Sensible Heat Flux over a Patchy Mountain Snow Cover

AbstractIn this study, the small-scale boundary layer dynamics and the energy balance over a fractional snow cover are numerically investigated. The atmospheric boundary layer flows over a patchy snow cover were calculated with an atmospheric model (Advanced Regional Prediction System) on a very high spatial resolution of 5 m. The numerical results revealed that the development of local flow patterns and the relative importance of boundary layer processes depend on the snow patch size distribution and the synoptic wind forcing. Energy balance calculations for quiescent wind situations demonstrated that well-developed katabatic winds exerted a major control on the energy balance over the patchy snow cover, leading to a maximum in the mean downward sensible heat flux over snow for high snow-cover fractions. This implies that if katabatic winds develop, total melt of snow patches may decrease for low snow-cover fractions despite an increasing ambient air temperature, which would not be predicted by most hydrological models. In contrast, stronger synoptic winds increased the effect of heat advection on the catchment’s melt behavior by enhancing the mean sensible heat flux over snow for lower snow-cover fractions. A sensitivity analysis to grid resolution suggested that the grid size is a critical factor for modeling the energy balance of a patchy snow cover. The comparison of simulation results from coarse (50 m) and fine (5 m) horizontal resolutions revealed a difference in the spatially averaged turbulent heat flux over snow of 40%–70% for synoptic cases and 95% for quiescent cases.

Abstract 566: Experimental Investigation of Blood Flow in the Femoral Bifurcated Artery

Stent implantation is one of the most widely used interventional treatments for arterial stenosis which occurs predominantly due to atherosclerosis. Although stent placement can ensure very good patency of the lumen, stent-induced hemodynamic disturbance, which can lead to further stenosis, still remains a common clinical complication. This study investigates the degree of hemodynamic disturbance induced by stenting an idealized bifurcated popliteal artery, which branches into the anterior and posterior tibial arteries, and is known as a site prone to atherosclerosis. Both stent-free and stented bifurcated arteries were examined, and the local flow patterns analysed for the comparative disturbance through the use of Micro Particle Image Velocity (micro-PIV) system. A life-size model of the artery was reconstructed using dimensions obtained from a patient specific MRI scan. The experiments were conducted under steady flow conditions, and the flow rates across the bifurcation were visualized and measured using the micro-PIV system. It was shown that hemodynamic disturbances induced by the blood flow over the stent can further disrupt the arterial wall downstream of the stent causing further downstream vascular damage in addition to the in-stent restenosis. This downstream vascular disruption may require additional treatment depending on the type and severity of the damage. The results also support the hypothesis that links certain flow dynamic behaviour with the development of early intimal thickening, as the near wall low fluid momentum regions are found at locations where thickening was localized in bifurcated arteries in clinical studies.

Abstract 166: Activation of Integrin α5 Mediated by Flow Requires Its Translocation to Membrane Lipid Raft in Vascular Endothelial Cells

Rationale: Local flow patterns determine the uneven distribution of atherosclerotic lesions. Sensing mechanical forces, membrane lipid raft and integrin are crucial for shear stress-regulated endothelial function. Objective: In this study, we investigate the role of lipid raft and integrin α5 in regulating the inflammatory response in endothelial cells (ECs) under the atheroprone flow. Methods and Results: Lipid raft proteins were isolated from ECs exposed to pulsatile shear stress (PS) or oscillatory shear stress (OS) and then analyzed by quantitative proteomics. Among 396 proteins redistributed in lipid raft, integrin α5 was most significantly elevated in lipid raft under OS. Moreover, OS increased the level of activated integrin α5, which was exclusively located in lipid raft. Disruption of F-actin-based cytoskeleton and knockdown of caveolin-1 prevented the OS-induced integrin α5 translocation and activation, suggesting significant roles of the cytoskeleton and caveolin-1 in integrin α5 translocation. Furthermore, OS activation of integrin α5 induced the NLRP3 inflammasome, leading to increased inflammatory responses. In vivo, activation of integrin α5 and increased expression of adhesion molecules were seen in the atheroprone areas of LDLR-/- mice. Knockdown of integrin α5 greatly attenuated EC dysfunction in partially ligated carotid arteries in LDLR-/-mice. Consistently, genetic ablation of integrin α5 in α5-/- murine embryonic fibroblasts and α5+/- mice caused a reduction of inflammation under the atheroprone flow. Conclusions: Atheroprone flow activates integrin α5 through lipid raft anchoring with ensuing induction of NLRP3 inflammasome in ECs. These findings reveal a novel mechanotransduction mechanism by which atheroprone flow causes endothelial dysfunction.

Impact of local flow haemodynamics on atherosclerosis in coronary artery bifurcations.

Coronary artery bifurcations are susceptible to atherosclerosis as a result of the unique local flow patterns and the subsequent endothelial shear stress (ESS) environment that are conducive to the development of plaques. Along the lateral walls of the main vessel and side branches, a distinct flow pattern is observed with local low and oscillatory ESS, while high ESS develops at the flow divider (carina). Histopathologic studies have shown that the distribution of plaque at bifurcation regions is related to the local ESS patterns. The local ESS profile also influences the outcome of percutaneous coronary interventions in bifurcation lesions. A variety of invasive and non-invasive imaging modalities have enabled 3D reconstruction of coronary bifurcations and thereby detailed local ESS assessment by computational fluid dynamics. Highly effective strategies for treatment and ultimately prevention of atherosclerosis in coronary bifurcations are anticipated with the use of advanced imaging and computational fluid dynamic techniques.

Local blood flow patterns in stented coronary bifurcations: an experimental and numerical study.

Despite the atheroprone environment of blood flow in coronary bifurcations, limited quantitative information is available on the hemodynamics occurring in these geometries, both before and after their treatment with endovascular stents. Previous studies have focused on computational fluid dynamics (CFD) analyses and have bypassed the challenges associated with experimentally representing the flow environment, providing no means for validation. This study analyzed steady flow conditions in 3 bifurcation angles and 4 different single- and double-stenting procedures, which are used clinically in coronary bifurcations. The numerical aspect of this study utilized geometries derived from CAD models (nonstented cases) and finite element simulations (stented cases). Digital particle image velocimetry (DPIV) testing was conducted within compliant bifurcating models for which an uncertainty analysis was performed at each measurement location for CFD validation purposes. Results were analyzed in terms of velocity magnitude contour maps and axial velocity profiles at several locations in the bifurcated vessels. Qualitatively, the 2 approaches showed agreement in the bulk flow patterns. However, the velocity computed with CFD was outside the DPIV uncertainty estimates, which can be attributed to the intrinsic difference and modeling assumptions of the 2 approaches. The findings reveal that wider bifurcation angles and double-stenting procedures are both characterized by increased areas of low flow and recirculation. Additionally, inferior performance in terms of viscous and wall shear stresses was observed in double-stented cases.

Adaptation of local muscle blood flow and surface electromyography to repeated bouts of eccentric exercise.

The aim of this randomized controlled crossover study was to investigate the effect of a bout of unaccustomed eccentric exercise (ECC) followed by a consecutive bout of the same intensity on local muscle blood flow, amplitude, and frequency of the electromyographic (EMG) signal from the exercised tibialis anterior muscle. Sixteen healthy male participants (age, 25.7 (0.6) years; body mass index 24.8 (1) kg·m(-2) participated in this study. Two identical bouts of high-intensity ECC were performed on the tibialis anterior muscle 7 days apart. Control sessions involving no exercise were performed 4 weeks either before or after the exercise sessions. Changes in local total blood flow [ΔtHb], EMG root mean square, and median power frequency were recorded during isometric maximum voluntary contraction of ankle dorsiflexion. Measurements were performed before, immediately after, and the day after both ECCs (ECC1 and ECC2). The participants rested quietly in a chair in the control session. Eccentric exercise 1 led to a significant decrease in [ΔtHb] on the day after (p ≤ 0.05), whereas ECC2 did not. Median power frequency decreased significantly in ECC2 compared with ECC1 (p < 0.01). Root mean square was unchanged in all the instants. The present study showed that adaptation is depicted in the local muscle blood flow and the frequency contents of the EMG after an unaccustomed ECC inducing muscle soreness. These alterations provide a potential mechanism for a rapid adaptation, which decreases susceptibility of the muscle to develop further soreness in the subsequent ECC bout.