Reduction In Wall Shear Stress Research Articles

Prognostic significance of pre-procedure wall shear stress in the ascending aorta in patients with aortic stenosis undergoing transcatheter aortic valve replacement

Abstract Background Accurate prognostication before the procedure is challenging in patients with aortic stenosis (AS) undergoing transcatheter aortic valve replacement (TAVR). Recently, blood flow dynamics assessed by four-dimensional flow cardiovascular magnetic resonance imaging (4D-flow CMR) showed a significant reduction in average wall shear stress (WSS) in the ascending aorta (AAo) after TAVR. However, the prognostic value of pre-procedure average WSS in the AAo is controversial. Purpose We aimed to investigate whether pre-procedure average WSS was associated with clinical outcomes in patients with AS undergoing TAVR. Methods We prospectively examined 159 consecutive AS patients who underwent TAVR (57 males, mean age 83.6 ± 4.7 years, median left ventricular ejection fraction 63.4%) between May 2018 and May 2022. We assessed average WSS in the AAo before TAVR using 4D-flow CMR. We divided the patients into high WSS (≥6.60 Pa, n = 79) and low WSS (<6.60 Pa, n = 80) groups according to the median value of pre-TAVR WSS. The primary outcome of interest was a composite of all-cause death and hospitalization for worsening heart failure. Results Patients with high WSS had lower age compared to those with low WSS. There were no significant differences on sex, body mass index, systolic blood pressure, hemoglobin, serum albumin, N-terminal pro b-type natriuretic peptide, left ventricular ejection fraction, mean transaortic valvular gradient, annulus area in aortic valve and AAo diameter between the groups. During a median follow-up period of 19.8 months (interquartile range 6.0-37.1), the primary outcome more frequently occurred in patients with low WSS than in those with high WSS (28% vs 5%, P <0.001) (Figure 1). A multivariable Cox regression showed that lower WSS independently associated with increased risk of the primary outcome (hazard ratio 0.69, 95% confidence interval [CI] 0.53-0.91, P <0.001), even after adjustment for significant prognostic covariates including age, sex, serum albumin, history of chronic obstructive pulmonary disease, peripheral artery disease and malignancy, and estimated glomerular filtration rate. Predictive performance for the primary outcome was improved when adding the value of WSS in the AAo to representative prognostic factors (c-index 0.81 95%CI 0.72-0.90 vs. 0.77 95%CI 0.67-0.88) (Figure 2). Conclusions In patients with AS who underwent TAVR, lower average WSS in the AAo before procedure was associated with higher risk of worse clinical outcomes, suggesting that pre-TAVR average WSS in the AAo would be a surrogate marker and useful for the risk stratification.Survival analysis for primary outcomePredictive performance

Numerical study of hemodynamic flow in the aortic vessel of Williams syndrome patient with congenital heart disease

Congenital arterial stenosis such as supravalvar aortic stenosis (SVAS) are highly prevalent in Williams syndrome (WS) and other arteriopathies pose a substantial health risk. Conventional tools for severity assessment, including clinical findings and pressure gradient estimations, often fall short due to their susceptibility to transient physiological changes and disease stage influences. Moreover, in the pediatric population, the severity of these and other congenital heart defects (CHDs) often restricts the applicability of invasive techniques for obtaining crucial physiological data. Conversely, evaluating CHDs and their progression requires a comprehensive understanding of intracardiac blood flow. Current imaging modalities, such as blood speckle imaging (BSI) and four-dimensional magnetic resonance imaging (4D MRI) face limitations in resolving flow data, especially in cases of elevated flow velocities. To address these challenges, we devised a computational framework employing zero-dimensional (0D) lumped parameter models coupled with patient-specific reconstructed geometries pre- and post-surgical intervention to execute computational fluid dynamic (CFD) simulations. This framework facilitates the analysis and visualization of intricate blood flow patterns, offering insights into geometry and flow dynamics alterations impacting cardiac function. In this study, we aim to assess the efficacy of surgical intervention in correcting an extreme aortic defect in a patient with WS, leading to reductions in wall shear stress (WSS), maximum velocity magnitude, pressure drop, and ultimately a decrease in cardiac workload.

Hemodynamic factors of spontaneous vertebral artery dissecting aneurysms assessed with numerical and deep learning algorithms: Role of blood pressure and asymmetry

Background and ObjectivesThe pathophysiology of spontaneous vertebral artery dissecting aneurysms (SVADA) is poorly understood. Our goal is to investigate the hemodynamic factors contributing to their formation using computational fluid dynamics (CFD) and deep learning algorithms. MethodsWe have developed software that can use patient imagery as input to recreate the vertebrobasilar arterial system, both with and without SVADA, which we used in a series of three patients. To obtain the kinematic blood flow data before and after the aneurysm forms, we utilized numerical methods to solve the complex Navier-Stokes partial differential equations. This was accomplished through the application of a finite volume solver (OpenFoam/Helyx OS). Additionally, we trained a neural ordinary differential equation (NODE) to learn and replicate the dynamical streamlines obtained from the computational fluid dynamics (CFD) simulations. ResultsIn all three cases, we observed that the equilibrium of blood pressure distributions across the VAs, at a specific vertical level, accurately predicted the future SVADA location. In the two cases where there was a dominant VA, the dissection occurred on the dominant artery where blood pressure was lower compared to the contralateral side. The SVADA sac was characterized by reduced wall shear stress (WSS) and decreased velocity magnitude related to increased turbulence. The presence of a high WSS gradient at the boundary of the SVADA may explain its extension. Streamlines generated by CFD were learned with a neural ordinary differential equation (NODE) capable of capturing the system’s dynamics to output meaningful predictions of the flow vector field upon aneurysm formation. ConclusionIn our series, asymmetry in the vertebrobasilar blood pressure distributions at and proximal to the site of the future SVADA accurately predicted its location in all patients. Deep learning algorithms can be trained to model blood flow patterns within biological systems, offering an alternative to the computationally intensive CFD. This technology has the potential to find practical applications in clinical settings.

Experimental investigation of a grid plasma jet array in a turbulent boundary layer for skin-friction drag reduction

Toward turbulent skin-friction drag reduction, a novel layout of plasma actuator with a grid anode is devised (grid edge length: L), capable of producing an array of wall-normal plasma jets. The characteristics of this plasma jet array and its interaction with a turbulent boundary layer are investigated experimentally with a particle imaging velocimetry. Results show that the quiescent flow field of the plasma actuator is dominated by a standing vortex ring attached at the inner sides of the square grid, a wall-normal jet flow issued from the grid center, and a downwash flow between adjacent grids. When a tandem array of 11 plasma jets are issued into the cross flow, an equilibrium stage is reached after the third jet. In this stage, the main body of the wall-normal jet bends noticeably to the cross flow, and its leeward side hosts a reverse flow zone, extending downstream to form a slender low-speed wedge (LSW). Two vortical structures are prominent: the streamwise counter-rotating vortex pair residing in the two sides of the jet body, and the arch-shape negative spanwise vortex situated on the LSW root. In the grid-middle plane, the production of turbulence is enhanced across the entire boundary layer, and a second production peak is identified at y+=26. Reduction of the spanwise-averaged wall shear stress is achieved downstream of plasma actuation at x/L≥19.4, and part of the drag reduction fruit earned by ejection is offset by the spanwise transportation of high-velocity fluids toward the middle plane.

Influence of deformed parent vessel on rupture risk of micro cerebral aneurysm: Numerical study

One of significant impact of stent usage for the treatment of the saccular aneurysm is deformation of the parent vessel. Present study demonstrates the influence of the aneurysm deformation caused by the stent on the risk of aneurysm rupture. Computational fluid dynamic is applied for the modeling of the blood flow inside three internal carotid artery (ICA) aneurysms with different neck angles. One-way fluid–structure interaction model is applied for the interaction of the blood and vessel. Two stage of deformation is applied on the parent vessel to analyze the influence of deformation on the hemodynamic factors of wall shear stress and oscillatory shear index. Achieved results indicate that the deformation of the aneurysm by the stent significantly reduce wall shear stress is on the sac wall and decrease the risk of internal carotid artery ICA aneurysm rupture. Our finding confirm that the main effect of aneurysm deformation is reduction of the blood velocity near ostium region.

Blood Speckle Imaging: An Emerging Method for Perioperative Evaluation of Subaortic and Aortic Valvar Repair.

This article presents the use of blood speckle Imaging (BSI) as an echocardiographic approach for the pre- and post-operative evaluation of subaortic membrane resection and aortic valve repair. BSI, employing block-matching algorithms, provided detailed visualization of flow patterns and quantification of parameters from ultrasound data. The 9-year-old patient underwent subaortic membrane resection and peeling extensions of the membrane from under the ventricular-facing surface of all three aortic valve leaflets. Post-operatively, BSI demonstrated improvements in hemodynamic patterns, where quantified changes in flow velocities showed no signs of stenosis and trivial regurgitation. The asymmetric jet with a shear layer and flow reversal on the posterior aspect of the aorta was corrected resulting in reduced wall shear stress on the anterior aspect and reduced oscillatory shear index, which is considered a contributing element in cellular alterations in the structure of the aortic wall. This proof-of-concept study demonstrates the potential of BSI as an emerging echocardiographic approach for evaluating subaortic and aortic valvar repair. BSI enhances the quantitative evaluation of the left ventricular outflow tract of immediate surgical outcomes beyond traditional echocardiographic parameters and aids in post-operative decision-making. However, larger studies are needed to validate these findings and establish standardized protocols for clinical implementation.

Pulse wave and vector flow Imaging for atherosclerotic disease progression in hypercholesterolemic swine

Non-invasive monitoring of atherosclerosis remains challenging. Pulse Wave Imaging (PWI) is a non-invasive technique to measure the local stiffness at diastolic and end-systolic pressures and quantify the hemodynamics. The objective of this study is twofold, namely (1) to investigate the capability of (adaptive) PWI to assess progressive change in local stiffness and homogeneity of the carotid in a high-cholesterol swine model and (2) to assess the ability of PWI to monitor the change in hemodynamics and a corresponding change in stiffness. Nine (n=9) hypercholesterolemic swine were included in this study and followed for up to 9 months. A ligation in the left carotid was used to cause a hemodynamic disturbance. The carotids with detectable hemodynamic disturbance showed a reduction in wall shear stress immediately after ligation (2.12 ± 0.49 to 0.98 ± 0.47 Pa for 40–90% ligation (Group B) and 1.82 ± 0.25 to 0.49 ± 0.46 Pa for >90% ligation (Group C)). Histology revealed subsequent lesion formation after 8–9 months, and the type of lesion formation was dependent on the type of the induced ligation, with more complex plaques observed in the carotids with a more significant ligation (C: >90%). The compliance progression appears differed for groups B and C, with an increase in compliance to 2.09 ± 2.90×10−10 m2 Pa−1 for group C whereas the compliance of group B remained low at 8 months (0.95 ± 0.94×10−10 m2 Pa−1). In summary, PWI appeared capable of monitoring a change in wall shear stress and separating two distinct progression pathways resulting in distinct compliances.

Influence of Morphological Parameters on the Flow Development within Human Airways

Anatomical airways parameters, such as length, diameter and angles, have a strong effect on the flow dynamics. Aiming to explore the effect of variations of the bifurcation angle (BA) and carina rounding radius (CRR) of lower human airways on respiratory processes, numerical simulations of airflow during inhalation and exhalation were performed using synthetic bifurcation models. Geometries for the airways models were parameterized based on a set of different BA’s and several CRR’s. A range of Reynolds numbers (Re), relevant to the human breathing process, were selected to analyze airflow behavior. The numerical results showed a significant influence of BA and the CRR on the development of the airflow within the airways, and, therefore, affecting the following relevant features of the flow: the deformation of velocity profiles, alterations of pressure drop, flow patterns, and, finally, enhancement or attenuation of wall shear stresses (WSS) appearing during the regular respiratory process. The numerical results showed that increases in the bifurcation angle value were accompanied by pressure increases of about 20%, especially in the regions close to the bifurcation. Similarly, increases in the BA value led to a reduction in peak shear stresses of up to 70%. For the ranges of angles and radii explored, an increase in pressure of about 20% and a reduction in wall shear stress of more than 400% were obtained by increasing the carina rounding radius. Analysis of the coherent structures and secondary flow patterns also revealed a direct relationship between the location of the vortical structures, the local maxima of the velocity profiles and the local vorticity minima. This relationship was observed for all branches analyzed, for both the inhalation and exhalation processes of the respiratory cycle.

The site-specific distribution of atheromatous plaques in the coronary arteries.

The etiology of atherosclerosis is still unknown, but there are several hypotheses trying to explain this complex disease. Most consider atherosclerosis as a cholesterol storage disease. However, hypercholesterolemia is not a cause but a risk factor. Besides, like other well-known systemic risk factors, it does not explain the uneven distribution of atheromatous plaques in the vasculature. Atherosclerotic lesions develop mainly at vulnerable "risk points" of the arterial wall such as curvatures and near side branches, and predominantly in the left anterior descending (LAD), while the left circumflex (LCx) artery is relatively spared. Furthermore, atheromatous plaques are present mainly in the proximal segments in the LAD and LCx, in contrast to the right coronary artery (RCA), where plaques are more evenly distributed. The hemodynamic theory explains to some extent the distribution of atherosclerotic lesions and considers atherosclerosis as a reactive biological response of endothelial cells to wall shear stress. In this review, we discuss the interplay of concentration of low-density lipoproteins at the luminal surface and local hemodynamic forces (disturbed flow) that reduce wall shear stress in the process of plaque formation. Moreover, we present the distribution of atheromatous plaques in the coronary arteries in autopsy studies and imaging methods such as cardiac computed tomography angiography and invasive coronary angiography.

Cerebral hemodynamics during atrial fibrillation: Computational fluid dynamics analysis of lenticulostriate arteries using 7 T high-resolution magnetic resonance imaging.

Atrial fibrillation (AF) is the most common cardiac arrhythmia, inducing irregular and faster heart beating. Aside from disabling symptoms-such as palpitations, chest discomfort, and reduced exercise capacity-there is growing evidence that AF increases the risk of dementia and cognitive decline, even in the absence of clinical strokes. Among the possible mechanisms, the alteration of deep cerebral hemodynamics during AF is one of the most fascinating and least investigated hypotheses. Lenticulostriate arteries (LSAs)-small perforating arteries perpendicularly departing from the anterior and middle cerebral arteries and supplying blood flow to basal ganglia-are especially involved in silent strokes and cerebral small vessel diseases, which are considered among the main vascular drivers of dementia. We propose for the first time a computational fluid dynamics analysis to investigate the AF effects on the LSAs hemodynamics by using 7 T high-resolution magnetic resonance imaging (MRI). We explored different heart rates (HRs)-from 50 to 130 bpm-in sinus rhythm and AF, exploiting MRI data from a healthy young male and internal carotid artery data from validated 0D cardiovascular-cerebral modeling as inflow condition. Our results reveal that AF induces a marked reduction of wall shear stress and flow velocity fields. This study suggests that AF at higher HR leads to a more hazardous hemodynamic scenario by increasing the atheromatosis and thrombogenesis risks in the LSAs region.

Shear stress and intravascular pressure effects on vascular dynamics: two-phase blood flow in elastic microvessels accounting for the passive stresses.

We study the steady hemodynamics in physiological elastic microvessels proposing an advanced fluid-structure interaction model. The arteriolar tissue is modeled as a two-layer fiber-reinforced hyperelastic material representing its Media and Adventitia layers. The constitutive model employed (Holzapfel et al. in J Elast 61:1-48, 2000) is parametrized via available data on stress-strain experiments for arterioles. The model is completed by simulating the blood/plasma flow in the lumen, using the thixotropic elasto-viscoplastic model in its core, and the linear Phan-Thien and Tanner viscoelastic model in its annular part. The Cell-Free Layer (CFL) and the Fåhraeus and Fåhraeus-Lindqvist effects are considered via analytical expressions based on experimental data (Giannokostas et al. in Materials (Basel) 14:367, 2021b). The coupling between tissue deformation and blood flow is achieved through the experimentally verified pressure-shear hypothesis (Pries et al. Circ Res 77:1017-1023, 1995). Our calculations confirm that the increase in the reference inner radius produces larger expansion. Also, by increasing the intraluminal pressure, the thinning of the walls is more pronounced and it may reach 40% of the initial thickness. Comparing our predictions with those in rigid-wall microtubes, we conclude that apart from the vital importance of vasodilation, there is an up to 25% reduction in wall shear stress. The passive vasodilation contributes to the decrease in the tissue stress fields and affects the hemodynamic features such as the CFL thickness, reducing the plasma layer when blood flows in vessels with elastic walls, in quantitative agreement with previous experiments. Our calculations verify the correctness of the pressure-shear hypothesis but not that of the Laplace law.

The Role of Multiple Re-Entry Tears in Type B Aortic Dissection Progression: A Longitudinal Study Using a Controlled Swine Model

Purpose: False lumen (FL) expansion often occurs in type B aortic dissection (TBAD) and has been associated with the presence of re-entry tears. This longitudinal study aims to elucidate the role of re-entry tears in the progression of TBAD using a controlled swine model, by assessing aortic hemodynamics through combined imaging and computational modeling. Materials and Methods: A TBAD swine model with a primary entry tear at 7 cm distal to the left subclavian artery was created in a previous study. In the current study, reintervention was carried out in this swine model to induce 2 additional re-entry tears of approximately 5 mm in diameter. Computed tomography (CT) and 4-dimensional (4D) flow magnetic resonance imaging (MRI) scans were taken at multiple follow-ups before and after reintervention. Changes in aortic volume were measured on CT scans, and hemodynamic parameters were evaluated based on dynamic data acquired with 4D-flow MRI and computational fluid dynamics simulations incorporating all available in vivo data. Results: Morphological analysis showed FL growth of 20% following the initial TBAD—growth stabilized after the creation of additional tears and eventually FL volume reduced by 6%. Increasing the number of re-entry tears from 1 to 2 caused flow redistribution, with the percentage of true lumen (TL) flow increasing from 56% to 78%; altered local velocities; reduced wall shear stress surrounding the tears; and led to a reduction in FL pressure and pressure difference between the 2 lumina. Conclusion: This study combined extensive in vivo imaging data with sophisticated computational methods to show that additional re-entry tears can alter dissection hemodynamics through redistribution of flow between the TL and FL. This helps to reduce FL pressure, which could potentially stabilize aortic growth and lead to reversal of FL expansion. This work provides a starting point for further study into the use of fenestration in controlling undesirable FL expansion. Clinical Impact Aortic growth and false lumen (FL) patency are associated with the presence of re-entry tears in type B aortic dissection (TBAD) patients. Guidelines on how to treat re-entry tears are lacking, especially with regards to the control and prevention of FL expansion. Through a combined imagining and computational hemodynamics study of a controlled swine model, we found that increasing the number of re-entry tears reduced FL pressure and cross lumen pressure difference, potentially stabilising aortic growth and leading to FL reduction. Our findings provide a starting point for further study into the use of fenestration in controlling undesirable FL expansion.

Performance improvement and thermodynamic assessment of microchannel heat sink with different types of ribs and cones

The present study aims to investigate the performance of microchannel heat sink via numerical simulations, based on the first and second law of thermodynamics. The heat transfer and flow characteristics of rectangular microchannel heat sinks have been improved by adding six different types of surface enhancers. The cross-sections include rectangular, triangular, and hexagonal-shaped ribs and cones. The cones have been created from the same cross-sections of ribs by drafting them at an angle of 45° orthogonal to the base, which is expected to decrease the pressure drop, dramatically. The performance of ribs and cones has been evaluated using different parameters such as friction factor, wall shear stress, entropy generation rate, augmentation entropy generation number, thermal resistance, and transport efficiency of thermal energy. The results of the present study revealed that the novel effect of coning at an angle of 45° reduces frictional losses (Maximum pressure drop reduced is 85%), however; a compromise on thermal behavior has been shown (Maximum Nusselt number reduced is 25%). Similarly, the application of coning has caused a significant reduction in wall shear stress and friction factor which can lead to reducing the pumping power requirements. Moreover, triangular ribs have more ability to transfer thermal energy than rectangular and hexagonal ribs. Furthermore, it has been examined in the present study that the trend of total entropy generation rate for triangular ribs decreases up to Re = 400 and then increases onwards which means that thermal losses are more significant than frictional losses at lower Reynolds number. However, frictional losses dominate over thermal losses at higher Reynolds numbers, where vortex generation takes place, especially in triangular ribs.

The passive control on flow and heat transfer with streamwise micro grooves

The flow behavior of smooth channel and channel with streamwise micro grooves are numerically compared at Reynolds number 3000–6000. The numerical prediction results show that the streamwise micro grooves can effectively promote the heat transport in boundary layer. In special cases, the streamwise micro grooves can suppress the ejection and sweep motions of turbulent coherent structure to a certain extent, which is conducive to the improvement of the stability of near-wall flow structure and the reduction in wall shear stress. In addition, 2.16% of drag reduction can be obtained. Furthermore, the streamwise micro grooves increase the number of large-scale vortices and destroy the thermal boundary layer. With the help of the streamwise micro grooves, Nusselt number increases by 8.2% ~ 10.7% in the range of considered Reynolds number.

A Finite Element-Based Analysis of a Hemodynamics Efficient Flow Stent Suitable for Different Abdominal Aneurysm Shapes.

This research aimed to examine the impact of a proposed flow stent (PFS) on different abdominal artery shapes. For that purpose, a finite element-based model using the computational fluid dynamics (CFD) method is developed. The effect of PFS intervention on the hemodynamic efficiency is estimated by all of the significant criteria used for the evaluation of aneurysm occlusion and possible rupture; the flow velocity, pressure, wall shear stress (WSS), and WSS-related indices. Results showed that PFS intervention preserves the effects of high flowrate and decreases irregular flow recirculation in the sac of the aneurysm. The flow velocity reduction inside the aneurysm sac is in the range of 55% to 80% and the time-averaged wall shear stress (TAWSS) reduction is in the range of 42% to 53% by PFS deployment. The simulation results implies that PFS could heal an aneurysm efficiently with a mechanism that causes the development of thrombus and ultimately leads to aneurysm resorption.

Abstract TMP6: Computational Hemodynamic Analysis Of Intra-Aneurysmal Effects Due To Flow Diverter Stent Deployment Conformation

Introduction: Inflow blockage of aneurysms treated with flow diverter stents is dependent on the deployed braid angle (ϕ)—the angle between the yarns which comprise the stent braids and the longitudinal axis of the braid. We utilized computational fluid dynamic (CFD) analysis to examine the effects of braid angle on intra-aneurysmal hemodynamic parameters associated with long-term treatment success. Methods: Two flow diverter stents measuring 3.5 mm in diameter, each containing 88 strands with ϕ = 10° and ϕ = 35° were constructed and deployed into a patient model of an internal carotid artery saccular aneurysm containing pulsatile blood flow. Intra-aneurysmal flow velocity, pulsatility index (PI), turbulence, vorticity, and wall shear stress (WSS) parameters were obtained and compared for the two braid angles. Results: Reducing ϕ from 35° to 10° resulted in -2.59%, -1.92%, and -1.41% changes in systolic bulk velocity, and -1.79%, +0.57%, and -1.65% changes in PI at the aneurysm neck, body, and dome respectively. Vorticity changes were insignificant. High turbulence extending through the aneurysm neck peaked approximately 0.04 s later in systole, and was located closer to the parent vessel outlet when ϕ = 10°. Elevated intra-aneurysmal WSS was concentrated at the neck and body on the side of the parent vessel outlet for both ϕ = 10° and ϕ = 35°. Regions of WSS in excess of 10 Pa stretched higher into the aneurysm body when ϕ = 35°. Conclusion: Stenting at a lower ϕ resulted in higher blockage of flow and better reduction of WSS for initiating and facilitating long term intra-aneurysmal thrombosis. CFD analysis incorporating precise reconstruction of the stent deployment conformation into patient models can provide detailed visualization of flow parameters for assessing potential treatment efficacy throughout the diagnostic and treatment phases of patient care.

Using a Human Circulation Mathematical Model to Simulate the Effects of Hemodialysis and Therapeutic Hypothermia

Background: We developed a hemodynamic mathematical model of human circulation coupled to a virtual hemodialyzer. The model was used to explore mechanisms underlying our clinical observations involving hemodialysis. Methods: The model consists of whole body human circulation, baroreflex feedback control, and a hemodialyzer. Four model populations encompassing baseline, dialysed, therapeutic hypothermia treated, and simultaneous dialysed with hypothermia were generated. In all populations atrial fibrillation and renal failure as co-morbidities, and exercise as a treatment were simulated. Clinically relevant measurables were used to quantify the effects of each in silico experiment. Sensitivity analysis was used to uncover the most relevant parameters. Results: Relative to baseline, the modelled dialysis increased the population mean diastolic blood pressure by 5%, large vessel wall shear stress by 6%, and heart rate by 20%. Therapeutic hypothermia increased systolic blood pressure by 3%, reduced large vessel shear stress by 15%, and did not affect heart rate. Therapeutic hypothermia reduced wall shear stress by 15% in the aorta and 6% in the kidneys, suggesting a potential anti-inflammatory benefit. Therapeutic hypothermia reduced cardiac output under atrial fibrillation by 12% and under renal failure by 20%. Therapeutic hypothermia and exercise did not affect dialyser function, but increased water removal by approximately 40%. Conclusions: This study illuminates some mechanisms of the action of therapeutic hypothermia. It also suggests clinical measurables that may be used as surrogates to diagnose underlying diseases such as atrial fibrillation.

Abstract 13545: Patient-Specific Computational Flow Simulation Reveals Differences in Aortic Hemodynamics Between Two versus Four Vessel Fenestrated Endovascular Aneurysm Repair

Objectives: Fenestrated endovascular aneurysm repair (fEVAR) endografts with additional proximal fenestrations are the subject of upcoming pivotal trials in the United States. This study sought to use patient-specific flow simulation (CFS) to assess differences in hemodynamics associated with two (2v) versus four-vessel (4v) fEVAR after repair of complex aortic aneurysms. Methods: Patients from two institutions who underwent 2v fEVAR for juxatrenal, and 4v fEVAR for pararenal aortic aneurysms with the Cook Zenith Fenestrated platform were retrospectively selected. Device diameters were matched between 2v and 4v groups. Finite element volume meshes were created from pre- and post-operative imaging. Pulsatile flow simulations were performed using a non-compressible Navier-Stokes solver with equivalent patient-specific boundary conditions between pre- and post-operative states. Results: Patient-specific CFS was performed on 20 patients (10 2v, 10 4v fEVAR) with a total of 70 target vessels (40 renal, 10 celiac, 10 superior mesenteric (SMA) stents, and 10 SMA scallops). 4v fEVAR was associated with a decrease in SMA peak flow rate (-7.1 vs +1.1%, p=.042) compared to 2v fEVAR. 4v fEVAR was associated with decreased paravisceral aortic wall shear stress (WSS) (-31.3 vs +0.6%, p=.008), whereas SMA WSS was increased (+36.9 vs +15.5%, p=.049) compared to 2v fEVAR. No differences in celiac or renal perfusion or WSS was observed between groups (p=.14-.93). Streamline visualization revealed flow disturbances surrounding flared branch graft protrusions into the aortic lumen which may be associated with changes in SMA perfusion after 4v fEVAR ( Figure ). In conclusion, structural changes in paravisceral aortic geometry following 4v fEVAR may be associated with a decrease in SMA perfusion and large reduction in aortic wall shear stress compared to 2v fEVAR. Patient-specific CFS is useful for studying the unique hemodynamic effects of additional fenestrations.

Numerical modelling of blood rheology and platelet activation through a stenosed left coronary artery bifurcation.

Coronary bifurcations are prone to atherosclerotic plaque growth, experiencing regions of reduced wall shear stress (WSS) and increased platelet adhesion. This study compares effects across different rheological approaches on hemodynamics, combined with a shear stress exposure history model of platelets within a stenosed porcine bifurcation. Simulations used both single/multiphase blood models to determine which approach best predicts phenomena associated with atherosclerosis and atherothrombosis. A novel Lagrangian platelet tracking model was used to evaluate residence time and shear history of platelets indicating likely regions of thrombus formation. Results show a decrease in area of regions with pathologically low time-averaged WSS with the use of multiphase models, particularly in a stenotic bifurcation. Significant non-Newtonian effects were observed due to low-shear and varying hematocrit levels found on the outer walls of the bifurcation and distal to the stenosis. Platelet residence time increased 11% in the stenosed artery, with exposure times to low-shear sufficient for red blood cell aggregation (>1.5 s). increasing the risk of thrombosis. This shows stenotic artery hemodynamics are inherently non-Newtonian and multiphase, with variations in hematocrit (0.163–0.617) and elevated vorticity distal to stenosis (+15%) impairing the function of the endothelium via reduced time-averaged WSS regions, rheological properties and platelet activation/adhesion.

The Role of Elevated Wall Shear Stress in Progression of Pulmonary Vein Stenosis: Evidence from Two Case Studies

Pulmonary vein stenosis is a serious condition characterized by restriction or blockage due to fibrotic tissue ingrowth that develops in the pulmonary veins of infants or children. It is often progressive and can lead to severe pulmonary hypertension and death. Efforts to halt or reverse disease progression include surgery and catheter-based balloon dilation and stent implantation. Its cause and mechanism of progression are unknown. In this pilot study, we propose and explore the hypothesis that elevated wall shear stress at discrete pulmonary venous sites triggers stenosis. To assess this theory, we retrospectively analyzed cardiac catheterization, lung scan, and X-ray computed tomography data to estimate wall shear stress in the pulmonary veins at multiple time points during disease progression in two patients. Results are consistent with the existence of a level of elevated wall shear stress above which the disease is progressive and below which progression is halted. The analysis also suggests the possibility of predicting the target lumen size necessary in a given vein to reduce wall shear stress to normal levels and remove the trigger for stenosis progression.