Regions Of High Wall Shear Stress Research Articles

Computational fluid dynamic simulations informed by CT and 4D flow MRI for post-surgery aortic dissection – A case study

Aortic dissection is a serious pathological condition where tears in the aortic wall allow blood to flow between layers of the aortic wall, separating tissue and creating a false lumen, with type A dissection affecting the ascending aorta. The considerable change in post-surgical type A dissection morphology, with residual dissection within the descending aorta, significantly alters blood flow dynamics. Knowledge of haemodynamics in this patient population may provide greater insight into disease progression. Patient-specific computational fluid dynamics (CFD) can provide this haemodynamic information to assist clinicians in assessing timing and appropriateness of intervention to prevent long-term complications. In this work, an approach integrating anatomic data from CT and personalised haemodynamics derived from 4D flow magnetic resonance imaging (4D flow MRI) with CFD in qualitative and quantitative analyses of intra-aortic flow is presented for a patient with residual aortic dissection after ascending aortic repair for type A dissection. Results show good agreement between simulated flow and 4D flow MRI measurements in the aorta, but with discrepancies observed in the false lumen. Multiple recirculation zones are observed in the thoracic false lumen, which is likely to beassociated with thrombus deposition. Regions of lower time-averaged wall shear stress (WSS) and high pressure differences between true and false lumens were found, identified as risk factors for false lumen dilation in the literature. Regions of high time-averaged WSS are detected in the aorta adjacent to entrances and exits of the false lumen. WSS is regarded as a critical biomechanical factor in disease progression, and detailed analysis of computer derived WSS could in future provide evidence-based recommendations for clinical management.

Wall shear stress gradient is independently associated with middle cerebral artery aneurysm development: a case-control CFD patient-specific study based on 77 patients

BackgroundPreviously published computational fluid dynamics (CFD) studies regarding intracranial aneurysm (IA) formation present conflicting results. Our study analysed the involvement of the combination of high wall shear stress (WSS) and a positive WSS gradient (WSSG) in IA formation.MethodsWe designed a case-control study with a selection of 38 patients with an unruptured middle cerebral artery (MCA) aneurysm and 39 non-aneurysmal controls to determine the involvement of WSS, oscillatory shear index (OSI), the WSSG and its absolute value (absWSSG) in aneurysm formation based on patient-specific CFD simulations using velocity profiles obtained from transcranial colour-coded sonography.ResultsAmong the analysed parameters, only the WSSG had significantly higher values compared to the controls (11.05 vs − 14.76 [Pa/mm], P = 0.020). The WSS, absWSSG and OSI values were not significantly different between the analysed groups. Logistic regression analysis identified WSS and WSSG as significant co-predictors for MCA aneurysm formation, but only the WSSG turned out to be a significant independent prognosticator (OR: 1.009; 95% CI: 1.001–1.017; P = 0.025). Significantly more patients (23/38) in the case group had haemodynamic regions of high WSS combined with a positive WSSG near the bifurcation apex, while in the control group, high WSS was usually accompanied by a negative WSSG (14/39). From the analysis of the ROC curve for WSSG, the area under the curve (AUC) was 0.654, with the optimal cut-off value −0.37 Pa/mm. The largest AUC was recognised for combined WSS and WSSG (AUC = 0.671). Our data confirmed that aneurysms tend to form near the bifurcation apices in regions of high WSS values accompanied by positive WSSG.ConclusionsThe development of IAs is determined by an independent effect of haemodynamic factors. High WSS impacts MCA aneurysm formation, while a positive WSSG mainly promotes this process.

Hemodynamic Assessment of Hollow-Fiber Membrane Oxygenators Using Computational Fluid Dynamics in Heterogeneous Membrane Models.

Extracorporeal membrane oxygenation (ECMO) has been used clinically for more than 40 years as a bridge to transplantation, with hollow-fiber membrane (HFM) oxygenators gaining in popularity due to their high gas transfer and low flow resistance. In spite of the technological advances in ECMO devices, the inevitable contact of the perfused blood with the polymer hollow-fiber gas-exchange membrane, and the subsequent thrombus formation, limits their clinical usage to only 2-4 weeks. In addition, the inhomogeneous flow in the device can further enhance thrombus formation and limit gas-transport efficiency. Endothelialization of the blood contacting surfaces of ECMO devices offers a potential solution to their inherent thrombogenicity. However, abnormal shear stresses and inhomogeneous blood flow might affect the function and activation status of the seeded endothelial cells (ECs). In this study, the blood flow through two HFM oxygenators, including the commercially available iLA® MiniLung Petite Novalung (Xenios AG, Germany) and an experimental one for the rat animal model, was modeled using computational fluid dynamics (CFD), with a view to assessing the magnitude and distribution of the wall shear stress (WSS) on the hollow fibers and flow fields in the oxygenators. This work demonstrated significant inhomogeneity in the flow dynamics of both oxygenators, with regions of high hollow-fiber WSS and regions of stagnant flow, implying a variable flow-induced stimulation on seeded ECs and possible EC activation and damage in a biohybrid oxygenator setting.

Revisiting Raupach’s Flow-Sheltering Paradigm

In this commentary, we revisit Raupach’s flow-sheltering paradigm that asserts reduced wall-shear stress behind a surface roughness element (MR Raupach in Boundary-Layer Meteorol, 60(4):375–395, 1992). Direct numerical simulations of a turbulent boundary layer over a wall-mounted rectangular roughness are conducted we consider roughness with three different aspect ratios and flows at two Reynolds numbers. A large computational domain is used to study the behaviours of the wall-shear stress in both the near-wake and the far-wake regions. Aside from a low wall-shear stress region in the near-wake as one would expect from the flow-sheltering paradigm, a high-stress region is found in the far-wake. The presence of such a high-stress region challenges the well-established flow sheltering paradigm and is also counter-intuitive. Detailed analysis of the vortical structures shows that the high wall-shear stress region is a consequence of the horse-shoe-vortex-induced downwash motion in the far-wake.

Coronary arteries hemodynamics: effect of arterial geometry on hemodynamic parameters causing atherosclerosis.

Coronary arteries hemodynamics: effect of arterial geometry on hemodynamic parameters causing atherosclerosis.

Dedifferentiation of smooth muscle cells in intracranial aneurysms and its potential contribution to the pathogenesis

Smooth muscle cells (SMCs) are the major type of cells constituting arterial walls and play a role to maintain stiffness via producing extracellular matrix. Here, the loss and degenerative changes of SMCs become the major histopathological features of an intracranial aneurysm (IA), a major cause of subarachnoid hemorrhage. Considering the important role of SMCs and the loss of this type of cells in IA lesions, we in the present study subjected rats to IA models and examined how SMCs behave during disease progression. We found that, at the neck portion of IAs, SMCs accumulated underneath the internal elastic lamina according to disease progression and formed the intimal hyperplasia. As these SMCs were positive for a dedifferentiation marker, myosin heavy chain 10, and contained abundant mitochondria and rough endoplasmic reticulum, SMCs at the intimal hyperplasia were dedifferentiated and activated. Furthermore, dedifferentiated SMCs expressed some pro-inflammatory factors, suggesting the role in the formation of inflammatory microenvironment to promote the disease. Intriguingly, some SMCs at the intimal hyperplasia were positive for CD68 and contained lipid depositions, indicating similarity with atherosclerosis. We next examined a potential factor mediating dedifferentiation and recruitment of SMCs. Platelet derived growth factor (PDGF)-BB was expressed in endothelial cells at the neck portion of lesions where high wall shear stress (WSS) was loaded. PDGF-BB facilitated migration of SMCs across matrigel-coated pores in a transwell system, promoted dedifferentiation of SMCs and induced expression of pro-inflammatory genes in these cells in vitro. Because, in a stenosis model of rats, PDGF-BB expression was expressed in endothelial cells loaded in high WSS regions, and SMCs present nearby were dedifferentiated, hence a correlation existed between high WSS, PDGFB and dedifferentiation in vivo. In conclusion, dedifferentiated SMCs presumably by PDGF-BB produced from high WSS-loaded endothelial cells accumulate in the intimal hyperplasia to form inflammatory microenvironment leading to the progression of the disease.

ESTIMATION OF WALL SHEAR STRESS OF COMPLIANT THORACIC AORTA WITH ANEURYSM

In this paper, a numerical estimation of wall shear stress (WSS) in a compliant Thoracic Aorta (TA) with aneurysm is modeled and the hemodynamic pattern is studied using Computational Fluid Dynamics (CFD). Thoracic Aortic Aneurysm (TAA) is an excessively localized enlargement of TA caused by weakness in the arterial wall and it can rupture the inner wall intima and continue on to the outer wall adventitia. WSS is a tangential force exerted by blood flow on the vessel wall, and its estimation is clinically very important because any change in WSS is considered as a vital cue in the onset of aneurysm. In this work, a three-dimensional (3D) model of a TAA reconstructed from computed tomography (CT) images comprising of 600 slices with 1-mm resolution from neck to hip is considered and patient-specific simulations have been carried out in compliant TA under rest and exercise conditions. The findings show that the change in wall geometry was marginal due to variation in pressure forces inside and is not the primary source for expansion of an aneurysm. It was inferred that expansion was rather due to thinning of the wall, owing to damage caused to the inner lining of the tissues, at regions of high WSS. It was found that the geometry extraction is important as any change in length causes a corresponding variation in mass flow through it. Although mass conservation is maintained irrespective of the length, it does affect the rate of flow due to shifting in the pressure boundary conditions with the length as it varies the pressure inside the system. Modeling of the geometry is very important as the change in mass flow will affect the outlet velocity and strength of vortices. Surprisingly, the split-up of flow is consistent but the geometric change in the model has no effect on WSS values and flow pattern. The results of this study provide important information such as blood flow pattern and pressure drops in the compliant TA on WSS estimations with TAA diseases.

Low and Oscillatory Wall Shear Stress Is Not Related to Aortic Dilation in Patients With Bicuspid Aortic Valve: A Time-Resolved 3-Dimensional Phase-Contrast Magnetic Resonance Imaging Study.

To assess the relationship between regional wall shear stress (WSS) and oscillatory shear index (OSI) and aortic dilation in patients with bicuspid aortic valve (BAV). Approach and Results: Forty-six consecutive patients with BAV (63% with right-left-coronary-cusp fusion, aortic diameter ≤ 45 mm and no severe valvular disease) and 44 healthy volunteers were studied by time-resolved 3-dimensional phase-contrast magnetic resonance imaging. WSS and OSI were quantified at different levels of the ascending aorta and the aortic arch, and regional WSS and OSI maps were obtained. Seventy percent of BAV had ascending aorta dilation. Compared with healthy volunteers, patients with BAV had increased WSS and decreased OSI in most of the ascending aorta and the aortic arch. In both BAV and healthy volunteers, regions of high WSS matched regions of low OSI and vice versa. No regions of both low WSS and high OSI were identified in BAV compared with healthy volunteers. Patients with BAV with dilated compared with nondilated aorta presented low and oscillatory WSS in the aortic arch, but not in the ascending aorta where dilation is more prevalent. Furthermore, no regions of concomitant low WSS and high OSI were identified when BAV were compared according to leaflet fusion pattern, despite the well-known differences in regional dilation prevalence. Regions with low WSS and high OSI do not match those with the highest prevalence of dilation in patients with BAV, thus providing no evidence to support the low and oscillatory shear stress theory in the pathogenesis of proximal aorta dilation in the presence of BAV.

Homeobox B9 integrates bone morphogenic protein 4 with inflammation at atheroprone sites.

AimsAtherosclerosis develops near branches and bends of arteries that are exposed to disturbed blood flow which exerts low wall shear stress (WSS). These mechanical conditions alter endothelial cells (EC) by priming them for inflammation and by inducing turnover. Homeobox (Hox) genes are developmental genes involved in the patterning of embryos along their anterior–posterior and proximal–distal axes. Here we identified Hox genes that are regulated by WSS and investigated their functions in adult arteries.Methods and resultsEC were isolated from inner (low WSS) and outer (high WSS) regions of the porcine aorta and the expression of Hox genes was analysed by quantitative real-time PCR. Several Hox genes (HoxA10, HoxB4, HoxB7, HoxB9, HoxD8, HoxD9) were significantly enriched at the low WSS compared to the high WSS region. Similarly, studies of cultured human umbilical vein EC (HUVEC) or porcine aortic EC revealed that the expression of multiple Hox genes (HoxA10, HoxB9, HoxD8, HoxD9) was enhanced under low (4 dyn/cm2) compared to high (13 dyn/cm2) WSS conditions. Gene silencing studies identified Hox genes (HoxB9, HoxD8, HoxD9) that are positive regulators of inflammatory molecule expression in EC exposed to low WSS, and others (HoxB9, HoxB7, HoxB4) that regulated EC turnover. We subsequently focused on HoxB9 because it was strongly up-regulated by low WSS and, uniquely, was a driver of both inflammation and proliferation. At a mechanistic level, we demonstrate using cultured EC and murine models that bone morphogenic protein 4 (BMP4) is an upstream regulator of HoxB9 which elicits inflammation via induction of numerous inflammatory mediators including TNF and downstream NF-κB activation. Moreover, the BMP4-HoxB9-TNF pathway was potentiated by hypercholesterolaemic conditions.ConclusionsLow WSS induces multiple Hox genes that control the activation state and turnover of EC. Notably, low WSS activates a BMP4-HoxB9-TNF signalling pathway to initiate focal arterial inflammation, thereby demonstrating integration of the BMP and Hox systems in vascular pathophysiology.

Abstract 736: Increased Cell Death in Regions of Elevated Aortic Wall Shear Stress in Bicuspid Valve Aortopathy

Bicuspid aortic valve (BAV) is a common congenital malformation associated with aortopathy and potential rupture. Identifying those patients at greatest risk for dissection is difficult at present. Abnormal hemodynamics in the ascending aorta creates regions of increased wall shear stress (WSS) which causes extracellular matrix dysregulation and may affect cell death. Dying cells release fragmented DNA into the blood and levels of aorta-specific cell-free DNA (cfDNA), identified by the presence of tissue-specific differentially methylated regions (DMRs), could be leveraged as a biomarker. Our objective was to determine if a relationship between elevated WSS and apoptosis in the ascending aorta existed and identify aorta-specific DMRs in the cfDNA of BAV patients. We hypothesized that areas of increased WSS would demonstrate increased cell death that would correlate with increases in aorta-specific cfDNA from patient blood. BAV patients undergoing 4D-flow cardiac magnetic resonance imaging (CMR) and surgery for ascending aorta dilation (range 36-63 mm) were recruited. Blood was collected from 23 patients at the time of CMR and used for the isolation of plasma cfDNA. Aortic wall samples (n = 30) corresponding to regions of high and low WSS were collected at surgery from 15 patients and stained for cell death using the TUNEL assay. Publicly-available methylomes and Metilene were used to identify aorta-specific DMRs in silico . Regions of elevated WSS were seen in all individuals regardless of absolute aortic dimension. These regions of elevated WSS by CMR showed significantly greater cell death when compared to the region of normal WSS in the ascending aorta (0.14 ± 0.05 vs. 0.08 ± 0.06, p=0.00006). Levels of cell death did not correlate with maximal aortic diameter. Twenty-three aorta-specific DMRs were identified. Of the four that were tested, three were found to be hypomethylated in genomic DNA from the aorta compared to 14 other tissues. Levels of aorta-specific cfDNA based on our DMRs did not correlate with maximal aortic diameter. Increased regional cell death corresponding to elevated WSS may implicate abnormal hemodynamic flow in the progression of aortopathy and provides a biological rationale for the use of cfDNA as a biomarker for aortopathy.

Hemodynamics in a Middle Cerebral Artery Aneurysm Before Its Growth and Fatal Rupture: Case Study and Review of the Literature

Hemodynamics plays an important role in the natural history of intracranial aneurysms. However, it is difficult to obtain longitudinal aneurysm image data. In this study, we had a rare chance to follow the process of growth and rupture of a middle cerebral artery (MCA) aneurysm. The aim of this study was to investigate the role of hemodynamics for growth and rupture in this particular aneurysm patient. A 57-year-old woman presented with 2 unruptured MCA aneurysms and 1 ruptured anterior choroidal artery (AchA) aneurysm. The unruptured left MCA aneurysm grew in 7 months and ruptured at 11 months. The rupture region was confirmed during the clipping treatment. Computational fluid dynamic (CFD) simulations were conducted on the AchA and MCA aneurysms in 3 serial imaging procedures, and calculated hemodynamics was correlated with aneurysm growth and rupture. The ruptured right AchA aneurysm had much lower wall shear stress (WSS) and higher oscillatory shear index (OSI) than the unruptured left MCA aneurysm. Comparison of the aneurysm growth region with the WSS distribution showed that growth occurred in a high WSS area. During the progression of the MCA aneurysm, we observed that the high aneurysmal WSS region grew first and then the growing region changed into a low WSS level; eventually, rupture occurred in a low WSS area. This case suggests that aneurysm rupture might be associated with low WSS and high OSI, whereas aneurysm growth may be related to high WSS in this particular individual MCA aneurysm patient.

A Mathematical Model on the Feedback Between Wall Shear Stress and Intimal Hyperplasia

In this paper, we present a mathematical model linking blood flow, shear-dependent endothelium permeability and intimal thickening (hyperplasia) of the arterial wall, which is an initial stage in the development of atherosclerosis. The key concepts are that the intimal layer swells in response to the presence of excess oxidised LDL (OxLDL) in foam cells. The hyperplasia disturbs blood flow, affecting endothelial permeability via the wall shear stress (WSS). These changes produce a feedback mechanism. LDL is transported through the arterial wall by advection and diffusion, and the concentration of LDL at each time step is assumed to be quasi-steady since it equilibrates on a fast time scale. The process is controlled by the slow timescale of the increase in concentration of OxLDL. We consider a section of uniform axisymmetric artery, and impose an initial local injury or ‘hotspot’ of relatively high permeability that enhances the influx of LDL, triggering the development of a bump-shaped lesion. In the absence of further inflammatory processes, the lesion eventually decays back to the homeostatic state. The model is used to explore how the shape of the lesion changes over time, its effect on WSS, influx rates of LDL and the sensitivity of these processes to oxidation parameters. The lesion is shown to propagate downstream driven by regions of high and low WSS on either side of the bump, and it persists for some time after the hotspot has vanished, leaving ample time for further pro-atherogenic processes to develop.

The Effect of Spatial and Temporal Resolution of Cine Phase Contrast MRI on Wall Shear Stress and Oscillatory Shear Index Assessment.

IntroductionWall shear stress (WSS) and oscillatory shear index (OSI) are associated with atherosclerotic disease. Both parameters are derived from blood velocities, which can be measured with phase-contrast MRI (PC-MRI). Limitations in spatiotemporal resolution of PC-MRI are known to affect these measurements. Our aim was to investigate the effect of spatiotemporal resolution using a carotid artery phantom.MethodsA carotid artery phantom was connected to a flow set-up supplying pulsatile flow. MRI measurement planes were placed at the common carotid artery (CCA) and internal carotid artery (ICA). Two-dimensional PC-MRI measurements were performed with thirty different spatiotemporal resolution settings. The MRI flow measurement was validated with ultrasound probe measurements. Mean flow, peak flow, flow waveform, WSS and OSI were compared for these spatiotemporal resolutions using regression analysis. The slopes of the regression lines were reported in %/mm and %/100ms. The distribution of low and high WSS and OSI was compared between different spatiotemporal resolutions.ResultsThe mean PC-MRI CCA flow (2.5±0.2mL/s) agreed with the ultrasound probe measurements (2.7±0.02mL/s). Mean flow (mL/s) depended only on spatial resolution (CCA:-13%/mm, ICA:-49%/mm). Peak flow (mL/s) depended on both spatial (CCA:-13%/mm, ICA:-17%/mm) and temporal resolution (CCA:-19%/100ms, ICA:-24%/100ms). Mean WSS (Pa) was in inverse relationship only with spatial resolution (CCA:-19%/mm, ICA:-33%/mm). OSI was dependent on spatial resolution for CCA (-26%/mm) and temporal resolution for ICA (-16%/100ms). The regions of low and high WSS and OSI matched for most of the spatiotemporal resolutions (CCA:30/30, ICA:28/30 cases for WSS; CCA:23/30, ICA:29/30 cases for OSI).ConclusionWe show that both mean flow and mean WSS are independent of temporal resolution. Peak flow and OSI are dependent on both spatial and temporal resolution. However, the magnitude of mean and peak flow, WSS and OSI, and the spatial distribution of OSI and WSS did not exhibit a strong dependency on spatiotemporal resolution.

Wall shear stress at the initiation site of cerebral aneurysms.

Hemodynamics are believed to play an important role in the initiation of cerebral aneurysms. In particular, studies have focused on wall shear stress (WSS), which is a key regulator of vascular biology and pathology. In line with the observation that aneurysms predominantly occur at regions of high WSS, such as bifurcation apices or outer walls of vascular bends, correlations have been found between the aneurysm initiation site and high WSS. The aim of our study was to analyze the WSS field at an aneurysm initiation site that was neither a bifurcation apex nor the outer wall of a vascular bend. Ten cases with aneurysms on the A1 segment of the anterior cerebral artery were analyzed and compared with ten controls. Aneurysms were virtually removed from the vascular models of the cases to mimic the pre-aneurysm geometry. Computational fluid dynamics (CFD) simulations were created to assess the magnitude, gradient, multidirectionality, and pulsatility of the WSS. To aid the inter-subject comparison of hemodynamic variables, we mapped the branch surfaces onto a two-dimensional parametric space. This approach made it possible to view the whole branch at once for qualitative evaluation. It also allowed us to empirically define a patch for quantitative analysis, which was consistent among subjects and encapsulated the aneurysm initiation sites in our dataset. To test the sensitivity of our results, CFD simulations were repeated with a second independent observer virtually removing the aneurysms and with a 20% higher flow rate at the inlet. We found that branches harboring aneurysms were characterized by high WSS and high WSS gradients. Among all assessed variables, the aneurysm initiation site most consistently coincided with peaks of temporal variation in the WSS magnitude.

Impact of bifurcation angle and other anatomical characteristics on blood flow – A computational study of non-stented and stented coronary arteries

The hemodynamic influence of vessel shape such as bifurcation angle is not fully understood with clinical and quantitative observations being equivocal. The aim of this study is to use computational modeling to study the hemodynamic effect of shape characteristics, in particular bifurcation angle (BA), for non-stented and stented coronary arteries.Nine bifurcations with angles of 40°, 60° and 80°, representative of ±1 SD of 101 asymptomatic computed tomography angiogram cases (average age 54±8 years; 57 females), were generated for (1) a non-stented idealized, (2) stented idealized, and (3) non-stented patient-specific geometry. Only the bifurcation angle was changed while the geometries were constant to eliminate flow effects induced by other vessel shape characteristics. The commercially available Biomatrix stent was used as a template and virtually inserted into each branch, simulating the T-stenting technique. Three patient-specific geometries with additional shape variation and ±2 SD BA variation (33°, 42° and 117°) were also computed. Computational fluid dynamics (CFD) analysis was performed for all 12 geometries to simulate physiological conditions, enabling the quantification of the hemodynamic stress distributions, including a threshold analysis of adversely low and high wall shear stress (WSS), low time-averaged WSS (TAWSS), high spatial WSS gradient (WSSG) and high Oscillatory Shear Index (OSI) area.The bifurcation angle had a minor impact on the areas of adverse hemodynamics in the idealized non-stented geometries, which fully disappeared once stented and was not apparent for patient geometries. High WSS regions were located close to the carina around peak-flow, and WSSG increased significantly after stenting for the idealized bifurcations. Additional shape variations affected the hemodynamic profiles, suggesting that BA alone has little effect on a patient׳s hemodynamic profile. Incoming flow angle, diameter and tortuosity appear to have stronger effects.This suggests that other bifurcation shape characteristics and stent placement/strategy may be more important than bifurcation angle in atherosclerotic disease development, progression, and stent outcome.

Hemodynamic Differences Between Central ECMO and Peripheral ECMO: A Primary CFD Study.

BackgroundVeno-arterial extracorporeal membrane oxygenation (VAECMO), including central ECMO (cECMO) and peripheral ECMO (pECMO), is widely used in cardiopulmonary surgery. The outcomes and complications of both types of ECMO are quite different from each other. The hemodynamic differences among them are hypothesized as a key factor. Hence, a numerical study was conducted to test this hypothesis.Material/MethodsIdeal cardiovascular models with pECMO and cECMO were established. The aortic pressure and flow rate were chosen as boundary conditions. The flow pattern, blood flow distributions, flow junction, harmonic index (HI) of blood flow, wall shear stress (WSS), and the oscillatory shear index (OSI) were calculated to evaluate the hemodynamic states.ResultspECMO could achieve better upper limb and brain perfusion (0.05458 vs. 0.05062 kg/s), and worse lower limb perfusion (0.03067 vs. 0.03401 kg/s). There exist low WSS (<0.4 pa) regions at the inner and posterior wall of the aorta, and high WSS (>2 pa) region at the access of the femoral artery. These regions also have relatively high OSI value (reaching 0.45). In contrast, for cECMO, there exist high WSS at the posterior wall of the aortic arch.ConclusionsThe hemodynamic performances of various types of ECMO are different from each other, which maybe the key reasons for the differences in the outcomes and complications. Therefore, for pEMCO, the lower-extremity ischemia is a complication that must be considered. The type, support level, and duration of ECMO should also be carefully regulated according to the patients’ condition, as they are the important factors related to vascular complications.

Role of biomechanical forces in the natural history of coronary atherosclerosis.

Atherosclerosis remains a major cause of morbidity and mortality worldwide, and a thorough understanding of the underlying pathophysiological mechanisms is crucial for the development of new therapeutic strategies. Although atherosclerosis is a systemic inflammatory disease, coronary atherosclerotic plaques are not uniformly distributed in the vascular tree. Experimental and clinical data highlight that biomechanical forces, including wall shear stress (WSS) and plaque structural stress (PSS), have an important role in the natural history of coronary atherosclerosis. Endothelial cell function is heavily influenced by changes in WSS, and longitudinal animal and human studies have shown that coronary regions with low WSS undergo increased plaque growth compared with high WSS regions. Local alterations in WSS might also promote transformation of stable to unstable plaque subtypes. Plaque rupture is determined by the balance between PSS and material strength, with plaque composition having a profound effect on PSS. Prospective clinical studies are required to ascertain whether integrating mechanical parameters with medical imaging can improve our ability to identify patients at highest risk of rapid disease progression or sudden cardiac events.

Inhibition of corrosion in amine air cooled heat exchanger: Experimental and numerical study

The major challenges of CO2 capture by alkanolamines are associated with corrosion process. In this paper, the corrosion characteristics of carbon steel were analyzed in the air-cooled heat exchanger (ACHE) of a petrochemical plant in Kermanshah, Iran. The chemical analysis, weight loss method, electrochemical techniques, and eddy current measurements were applied to determine corrosion products and corrosion rate. Computational fluid dynamics was also applied to search for the links between the observed patterns of attack seen in the amine plant's heat exchanger tubes and the hydrodynamics inside the tubes. Validation of the computational fluid dynamics results was compared with the visualized model of the tubes in the first pass of ACHE header. It was found that the erosion–corrosion damage was a consequence of increased levels of turbulence at tubes' inlet, caused by a rapidly turning flow. An alternative flow correction was numerically investigated applying another nozzle in the first pass header of inlet flow and were recommended for implementation. As a result of the proposed configuration, the maximum wall shear stress was reduced from 90 Pa to 40 Pa in the inlet tube. Furthermore, the length of high wall shear stress region was decreased from 15 cm to 5 cm.

4D FLOW MRI IDENTIFIES REGIONS OF MORE SEVERE AORTOPATHY IN THE HUMAN BICUSPID AORTA

Patients with congenital bicuspid aortic valves (BAV) are prone to progressive enlargement of the ascending aorta (aortopathy), characterized by medial elastin fiber degradation. We previously demonstrated that non-invasive hemodynamic assessment of regional wall shear stress (WSS) using 4D flow MRI can identify tissue regions with more severe elastin degeneration. The specificity of these observations to regions of high WSS is unclear. In this study, we extend these observations and examined molecular mediators of ECM homeostasis and histopathology in aortic tissue from BAV patients in the context of differential regional hemodynamics. Aortic tissue from regions of normal and elevated WSS (from mean aortic diameter 4.6±0.3 cm; range: 4.0-5.2 cm) was collected from BAV patients (N=17) undergoing ascending aortic surgery and who received pre-operative 4D flow MRI. To determine regions of “normal” and “elevated” WSS, WSS heatmaps of each aorta were constructed relative to the 95% confidence interval for normal physiological WSS values of a tricuspid aortic valve population. Tissue was evaluated for elastin fiber content and architecture (fiber thickness and fiber-to-fiber distance). Mediators of ECM degradation (TGFβ-1, MMPs and TIMPs) were assessed in areas of normal and elevated WSS. MMP-2 concentration was positively correlated with TIMP-2 concentration in all tissues (r=0.744; p<0.0001) independent of regional WSS. However, in samples from regions of elevated WSS, TGFβ-1 concentrations negatively correlated with the amount of elastin fibers (r=-0.548; p=0.053) and concentration of MMP-2 positively correlated with elastin fiber-to-fiber distance (r=0.539; p=0.026). Importantly, no correlations between mediators of ECM degradation and metrics of elastin fiber degradation were observed in adjacent tissue regions subjected to normal WSS. ECM dysregulation in the BAV aorta correlated with aortopathy in areas of elevated WSS but not in adjacent areas of normal WSS. These data further implicate WSS and regional hemodynamics in the expression of human bicuspid aortopathy. 4D flow MRI may be a promising non-invasive imaging tool to guide patient-specific resection strategies given the observed specificity of elevated WSS to predict tissue regions with more severe aortopathy.

Valve-Related Hemodynamics Mediate Human Bicuspid Aortopathy: Insights From Wall Shear Stress Mapping

BackgroundSuspected genetic causes for extracellular matrix (ECM) dysregulation in the ascending aorta in patients with bicuspid aortic valves (BAV) have influenced strategies and thresholds for surgical resection of BAV aortopathy. Using 4-dimensional (4D) flow cardiac magnetic resonance imaging (CMR), we have documented increased regional wall shear stress (WSS) in the ascending aorta of BAV patients. ObjectivesThis study assessed the relationship between WSS and regional aortic tissue remodeling in BAV patients to determine the influence of regional WSS on the expression of ECM dysregulation. MethodsBAV patients (n = 20) undergoing ascending aortic resection underwent pre-operative 4D flow CMR to regionally map WSS. Paired aortic wall samples (i.e., within-patient samples obtained from regions of elevated and normal WSS) were collected and compared for medial elastin degeneration by histology and ECM regulation by protein expression. ResultsRegions of increased WSS showed greater medial elastin degradation compared to adjacent areas with normal WSS: decreased total elastin (p = 0.01) with thinner fibers (p = 0.00007) that were farther apart (p = 0.001). Multiplex protein analyses of ECM regulatory molecules revealed an increase in transforming growth factor β-1 (p = 0.04), matrix metalloproteinase (MMP)-1 (p = 0.03), MMP-2 (p = 0.06), MMP-3 (p = 0.02), and tissue inhibitor of metalloproteinase-1 (p = 0.04) in elevated WSS regions, indicating ECM dysregulation in regions of high WSS. ConclusionsRegions of increased WSS correspond with ECM dysregulation and elastic fiber degeneration in the ascending aorta of BAV patients, implicating valve-related hemodynamics as a contributing factor in the development of aortopathy. Further study to validate the use of 4D flow CMR as a noninvasive biomarker of disease progression and its ability to individualize resection strategies is warranted.