Wall Stress Values Research Articles

Cattaneo-Christov double diffusion based heat transport analysis for nanofluid flows induced by a moving plate

Hybrid nanofluids have gained much attention due to their better stability, enhanced thermal conductivity, and physical strength. This manuscript investigates nanofluid flow, induced due to an infinite plate, which is moving (at a constant velocity) toward/receding from normal stagnation point flow. The surface of the plate is suspended and immersed with water as a base fluid and nanoparticles, namely Copper and Alumina. The flow is governed by Reynolds number (Re), which is proportional to the constant velocity of the moving plate and is an exact reduction of the Navier–Stokes equations. Moreover, Hiemenz’s planar and Homann’s axisymmetric flows normal to the stagnation point are considered. Heat transport analysis is carried out by using Cattaneo–Christov theory with Ohmic heating and heat source/sink effects. The governing equations are solved by bvp4c in MATLAB. The behavior of skin friction, flow, and energy distribution is perceived by variation of pertinent parameters. The numerical and asymptotic solutions are computed for the wall shear stress parameter. It is seen that the numerical solution matches its asymptotic behaviors over an intermediate range of small and large valued Reynolds number. The asymptotic values (small-Re) of wall stress reduces for plate advancing toward the stagnation-point flow and receding from the stagnation-point flow for Hiemenz and Homann flow. The increment in magnetic parameter reduced the fluid flow by generating a resistive force against it; however, the energy of the system is enhanced because of it. The augmentation of nanofluid volume fraction initiates the random motion among the nanoparticles which raises the temperature field. Thermal relaxation time parameter causes the particles to require supplementary time for the conduction of heat toward the adjacent particles, which declines the thermal transport. In general, the thermal transport is enhanced for the hybrid nanofluids rather than mono nanofluids.

Multiple solutions for non-linear radiative mixed convective hybrid nanofluid flow over an exponentially shrinking surface

Hybrid nanofluids have gained too much attention due to their enhanced thermophysical properties and practical applications. In comparison to conventional nanofluids, their capacity to enhance heat transport is impressive. The simultaneous numerical calculations of hybrid and mono nanofluids across an exponentially shrinking surface in a porous medium are taken into consideration here. The analysis of the thermal energy distribution is carried out by using the convective boundary conditions. Shrinking, permeability, and magnetohydrodynamic controlled the motion of the flow. The objective of this research is to conduct stability analysis and identify the existence of dual solutions in the presence of heat source/sink and nonlinear Roseland thermal radiation. The technique, bvp4c, a collocation method is used to achieve numerical results. It is noted that the energy transport is enhanced immensely due to the presence of a mixture of nanoparticles (hybrid) in comparison to mono nanofluids. The stability analysis shows that the solutions for the upper branch were stable, while the solutions for the lower branch were unstable. Moreover, shrinking parameter contributes significantly to exhibit the dual nature of the solutions. Due to the increment in the heat generation/absorption and temperature ratio, the kinetic energy is inclined, which causes the temperature distribution to rise for both branches. For stable branches, an increase in wall stress values is evident as a result of permeability and stretching of sheet, whereas unstable branches show the opposite trend.

Ventricular wall stress and wall shear stress homeostasis predicts cardiac remodeling during pregnancy: A modeling study.

Pregnancy is a unique and dynamic process characterized by significant changes in the maternal cardiovascular system that are required to satisfy the increased maternal and fetal metabolic demands. Profound structural and hemodynamic adaptations occur during healthy pregnancy that allows the mother to maintain healthy hemodynamics and provide an adequate uteroplacental blood circulation to ensure physiological fetal development. Investigating these adaptations is crucial for understanding the physiology of pregnancy and may provide important insights for the management of high‐risk pregnancies. However, no previous modeling studies have investigated the maternal cardiac structural changes that occur during gestation. This study, therefore, had two aims. The first was to develop a lumped parameter model of the whole maternal circulation that is suitable for studying global hemodynamics and cardiac function at different stages of gestation. The second was to test the hypothesis that myofiber stress and wall shear stress homeostasis principles can be used to predict cardiac remodeling that occurs during normal pregnancy. Hemodynamics and cardiac variables predicted from simulations with and without controlled cardiac remodeling algorithms were compared and evaluated with reference clinical data. While both models reproduced the hemodynamic variations that arise in pregnancy, importantly, we show that the structural changes that occur with pregnancy could be predicted by assuming invariant homeostatic “target” values of myocardial wall stress and chamber wall shear stress.

Angiography-Based 4-Dimensional Superficial Wall Strain and Stress: A New Diagnostic Tool in the Catheterization Laboratory.

A novel method for four-dimensional superficial wall strain and stress (4D-SWS) is derived from the arterial motion as pictured by invasive coronary angiography. Compared with the conventional finite element analysis of cardiovascular biomechanics using the estimated pulsatile pressure, the 4D-SWS approach can calculate the dynamic mechanical state of the superficial wall in vivo, which could be directly linked with plaque rupture or stent fracture. The validation of this approach using in silico models showed that the distribution and maximum values of superficial wall stress were similar to those calculated by conventional finite element analysis. The in vivo deformation was validated on 16 coronary arteries, from the comparison of centerlines predicted by the 4D-SWS approach against the actual centerlines reconstructed from angiograms at a randomly selected time-point, which demonstrated a good agreement of the centerline morphology between both approaches (scaling: 0.995 ± 0.018 and dissimilarity: 0.007 ± 0.014). The in silico vessel models with softer plaque and larger plaque burden presented more variation in mean lumen diameter and resulted in higher superficial wall stress. In more than half of the patients (n = 16), the maximum superficial wall stress was found at the proximal lesion shoulder. Additionally, in three patients who later suffered from acute coronary syndrome, the culprit plaque rupture sites co-localized with the site of highest superficial wall stress on their baseline angiography. These representative cases suggest that angiography-based superficial wall dynamics have the potential to identify coronary segments at high-risk of plaque rupture and fracture sites of implanted stents. Ongoing studies are focusing on identifying weak spots in coronary bypass grafts, and on exploring the biomechanical mechanisms of coronary arterial remodeling and aneurysm formation. Future developments involve integration of fast computational techniques to allow online availability of superficial wall strain and stress in the catheterization laboratory.

Analysis of aortic wall stress and morphology in patients with type B aortic dissection

Risk assessment is critical in preventing aortic dissection (AD). This study aims to evaluate the wall stress (WS) distribution, especially at the locations of proximal tears in patient-specific type-B aortic dissection (TBAD) to explore the pathogenesis of dissection. In addition, the shape of the aortas were assessed and associated with TBAD risk. In this paper, 30 three-dimensional models were reconstructed based on patient-specific CT angiography images, and finite element analysis (FEA) was used to analyze the admission blood pressure. In addition, the anatomic variables including the head vessel angles, the aorta diameters, curvatures and torturosity ware measured. Tears occurred at either local high WS or low WS region, and these acute TBAD patients can be classified into three groups based on the location of initial intimal tears. In addition, the WS values at tear sites of each group showed significant difference (P＜0.001). Moreover, the angles of the left subclavian and brachiocephalic arteries were statistical different among three groups. Increased wall stress or decreased wall strength both contribute to the pathogenesis of aortic dissections. In addition, abnormal head vessel angles may be monitored as an important risk factor for aortic dissection, and its specific features may further help to determine the potential tear location.

Adventitial remodeling protects against aortic rupture following late smooth muscle-specific disruption of TGFβ signaling

Altered signaling through transforming growth factor-beta (TGFβ) increases the risk of aortic dissection in patients, which has been confirmed in mouse models. It is well known that altered TGFβ signaling affects matrix turnover, but there has not been a careful examination of associated changes in structure-function relations. In this paper, we present new findings on the rupture potential of the aortic wall following late postnatal smooth muscle cell (SMC)-specific disruption of type I and II TGFβ receptors in a mouse model with demonstrated dissection susceptibility. Using a combination of custom computer-controlled biaxial tests and quantitative histology and immunohistochemistry, we found that loss of TGFβ signaling in SMCs compromises medial properties but induces compensatory changes in the adventitia that preserve wall strength above that which is needed to resist in vivo values of wall stress. These findings emphasize the different structural defects that lead to aortic dissection and rupture – compromised medial integrity and insufficient adventitial strength, respectively. Relative differences in these two defects, in an individual subject at a particular time, likely reflects the considerable phenotypic diversity that is common in clinical presentations of thoracic aortic dissection and rupture. There is, therefore, a need to move beyond examinations of bulk biological assays and wall properties to cell- and layer-specific studies that delineate pathologic and compensatory changes in wall biology and composition, and thus the structural integrity of the aortic wall that can dictate differences between life and death.

Abstract 16806: Changes in Wall Stress in Ascending Thoracic Aortic Aneurysms From Systole to Diastole

Introduction: Aortic rupture or dissection of ascending thoracic aortic aneurysms (aTAA) is a life-threatening pathology that can occur when wall stress exceeds wall strength. Surgical guidelines for repair is when aneurysm max diameter ≥5.5 cm. However, rupture has been documented at various sizes, including 60% <4.5cm. Wall stress may be a better predictor of dissection than diameter, using finite element analysis (FEA) of patient-specific aTAA models to assess patient-specific risk of dissection. The purpose of this study was to determine changes in aneurysm FEA peak wall stresses in patients during the cardiac cycle from systole to diastole. Methods: Preoperative aTAA patients (n = 20) underwent ECG-gated computed tomography (CT) scans at systolic and diastolic phases. Patient-specific 3D aneurysm geometries were reconstructed from the imaging data at both phases and after accounting for pre-stress, underwent FEA using LS-DYNA solver. Wall stress distribution and magnitude were determined using user-defined fiber-embedded material model at systole (120mmHg) and diastole (80mmHg). Aortic volume changes during the cardiac cycle were determined using computer-aided design tools. Results: From the diastolic model, the 99 th percentile circumferential stresses were 348.8 ±105.6 kPa and 186.6 ±31.4 kPa for the sinus and ascending aorta respectively, while for the systolic model, they were 500.7 ±183.7 kPa (p=0.00072) and 296.5 ±64.4 kPa (p < .00001), respectively. The 99th percentile longitudinal stresses from the diastolic model were 223.2 ±75.4 kPa and 126.1 ±20.4 kPa for the sinus and ascending aorta, respectively, while for the systolic model, they were 336.3 ±78.4 kPa (p < .00001), and 235.2 ±48.0 kPa (p < .00001), respectively. Mean volume change of the ascending aorta in percentage between systolic and diastolic phases was 6.34%. Conclusions: Peak circumferential and longitudinal stresses changed significantly from systole to diastole. For patients with significant hypertension, these differences in wall stress may be magnified such that sudden or significant hypertension may result in wall stress values that exceed tissue strength leading to dissection.

Complementing sparse vascular imaging data by physiological adaptation rules.

Mathematical modeling of pressure and flow waveforms in blood vessels using pulse wave propagation (PWP) models has tremendous potential to support clinical decision making. For a personalized model outcome, measurements of all modeled vessel radii and wall thicknesses are required. In clinical practice, however, data sets are often incomplete. To overcome this problem, we hypothesized that the adaptive capacity of vessels in response to mechanical load could be utilized to fill in the gaps of incomplete patient-specific data sets. We implemented homeostatic feedback loops in a validated PWP model to allow adaptation of vessel geometry to maintain physiological values of wall stress and wall shear stress. To evaluate our approach, we gathered vascular MRI and ultrasound data sets of wall thicknesses and radii of central and arm arterial segments of 10 healthy subjects. Reference models (i.e., termed RefModel, n = 10) were simulated using complete data, whereas adapted models (AdaptModel, n = 10) used data of one carotid artery segment only, and the remaining geometries in this model were estimated using adaptation. We evaluated agreement between RefModel and AdaptModel geometries, as well as that between pressure and flow waveforms of both models. Limits of agreement (bias ± 2 SD of difference) between AdaptModel and RefModel radii and wall thicknesses were 0.2 ± 2.6 mm and -140 ± 557 µm, respectively. Pressure and flow waveform characteristics of the AdaptModel better resembled those of the RefModels as compared with the model in which the vessels were not adapted. Our adaptation-based PWP model enables personalization of vascular geometries even when not all required data are available.NEW & NOTEWORTHY To benefit personalized pulse wave propagation (PWP) modeling, we propose a novel method that, instead of relying on extensive data sets on vascular geometries, incorporates physiological adaptation rules. The developed vascular adaptation model adequately predicted arterial radius and wall thickness compared with ultrasound and MRI estimates, obtained in humans. Our approach could be used as a tool to facilitate personalized modeling, notably in case of missing data, as routinely found in clinical settings.

Evaluation of arterial stiffness in patients with schizophrenia

The aim of this study was to evaluate arterial stiffness in schizophrenia patients. 28 male patients were included. The intima-media thickness (IMT) of the vessels were taken using high-resolution ultrasonography system. The mean carotid IMT and the mean femoral IMT values of the study group were found to be statistically significantly higher than the values of the control group. As the duration of the disease increased, there was an increase in the carotid IMT, carotid elastic modulus and femoral IMT, whereas there was a decrease in carotid and femoral diastolic wall stress in patients. No statistically significant differences were observed between the groups’ carotid and femoral compliance, distensibility and elastic modulus values. The mean systolic arterial blood pressure in the patient group was determined to be lower than that of the control group. The mean diastolic wall stress values in the carotid and femoral arteries were determined to be lower than those of the control group. There was no relationship between antipsychotic dose, blood pressure and arterial stiffness parameters. Schizophrenia patients are more prone to develop arterial stiffness by atherosclerosis either with the effect of the nature of the disease itself or antipsychotic treatment. But evaluation with more parameters (carotid and femoral compliance, distensibility and elastic modulus) did not indicate any difference from the control group in respect of arterial stiffness. Antipsychotic treatment may play a protective role in terms of arterial stiffness by causing a decrease in systolic arterial pressure and carotid and femoral diastolic wall stress.

Comparative finite element modelling of aneurysm formation and physiologic inflation in the descending aorta

Despite the general interest in aneurysm rupture prediction, the aneurysm formation has received limited attention. The goal of this study is to assess whether an aneurysm may be instigated in a healthy model of an aorta inflated by a supra-physiological pressure. The effect of two main aspects on numerical predictions has been explored: i) the geometric design and ii) the constitutive law adopted to represent the material properties. Firstly, higher values of wall stress and displacement magnitude were generated in the physiologic model compared to the cylindrical one when assigning the same material properties. Secondly, greater deformations are observed in the anisotropic model compared to the isotropic one.

Cardiovascular risk factors in children with type 1 diabetes mellitus.

Objective Cardiovascular disease is a major complication among children with type 1 diabetes mellitus (T1DM). This prospective study aimed at examining the presence of cardiovascular risk factors in children with T1DM. Methods We evaluated several cardiovascular risk factors, including atherosclerosis, artery intima-media thickness (IMT) and metabolic responses, in 175 children with T1DM, with 150 non-diabetic children as normal controls. Results The diabetic children had significantly higher carotid IMT (cIMT) and aortic IMT (aIMT), higher values for diastolic wall stress (DWS), incremental elastic modulus (IEM), and flow-mediated dilatation (FMD) than the controls. The levels of tumor necrosis factor-α (TNF-α), interleukin-4 (IL4), high-sensitivity C-reactive protein (hs-CRP) and leptin were significantly higher in T1DM patients. In T1DM children, the cIMT and aIMT were correlated with several risk factors, including age, weight, body mass index (BMI), duration of diabetes, waist/hip ratio, as well as levels of total cholesterol, triglycerides and apolipoprotein B (apoB). In addition to common risk factors, cIMT was also associated with systolic blood pressure (BP). Other risk factors, such as height, diastolic BP, low-density lipoprotein (LDL)/high-density lipoprotein (HDL)-cholesterol ratio, apolipoprotein A1 (apoA1) and S-creatinine levels, were not all independent risk factors of cardiovascular disease in T1DM children. Conclusions T1DM is associated with early impairment of the common carotid and aortic artery structure and function, and the diabetic state may be the main risk factor for arterial wall stiffening and thickening.

Analysis of Flow and Wall Deformation in a Stenotic Flexible Channel Containing a Soft Core, Simulating Atherosclerotic Arteries

A novel eccentric stenotic channel was manufactured to provide an in vivo physical environment simulating an atherosclerotic artery. It was composed of an elastic wall containing a soft semi-cylindrical volume that simulated a lipid core. Wall strain and fluid shear stress distributions were computed using a finite element method incorporating fluid structure interaction. The particle image velocity measurement method was extended to measure wall deformation and wall deformation and flow velocity were simultaneously measured to validate the computational method. The maximum deformation was found to occur in the distal shoulder, while the maximum flow wall shear stress was skewed to the proximal shoulder of the lipid core. Measured maximum wall deformations and flow velocities agreed with the computational results within a 10% margin of error. The wall shear stress on the stenotic wall varied from 0 to 1.2 Pa. The maximum shear strain in the longitudinal plane and the normal strain in the transverse plane for the Core 1 model (Young’s modulus 50 kPa) were 3.8% and 0.7%, respectively. The maximum shear and normal wall strains for the Core 2 model (Young’s modulus 200 kPa) were 34% and 14% lower. These wall stress and strain values corresponded with those found in human atherosclerotic arteries. The developed channel could provide spatial and temporal variations as well as magnitudes of wall shear stress and strain observed in atherosclerotic arteries.

Combining in vivo and in vitro biomechanical data reveals key roles of perivascular tethering in central artery function.

Considerable insight into effectors of cardiovascular function can be gleaned from controlled studies on mice, especially given the diverse models that are available. Toward this end, however, there is a need for consistent and complementary methods of in vivo and in vitro data analysis, synthesis, and interpretation. The overall objective of this study is twofold. First, we present new semi-automated methods to quantify in vivo measurements of vascular function in anesthetized mice as well as new approaches to synthesize these data with those from in vitro biaxial mechanical characterizations. Second, we contrast regional differences in biomechanical behaviors along the central vasculature by combining biaxial strains measured in vivo with data on the unloaded geometry and biaxial material properties measured in vitro. Results support the hypothesis that the healthy ascending aorta stores significant elastic energy during systole, which is available to work on the heart and blood during diastole, particularly during periods of physical exertion, and further suggest that perivascular tethering allows arteries to work at lower values of wall stress and material stiffness than often assumed. The numerous measurements of vascular function and properties provided herein can also serve as reference values for normal wild-type male and female mice, to which values for myriad genetic, surgical, and pharmacological models can be compared in future studies.

The effect of thickness measurement on numerical arterial models

Several optical-based techniques for measuring the sample thickness (ST) of soft tissues have been proposed in the literature to overcome the limits of hand-operated procedures. However, ST measurement still remains arbitrary. The stress calculated during an experimental procedure, usually based on a constant thickness value for all samples, cannot be considered representative of the actual stress experienced by the tissue. Therefore, a new optical methodology to measure ST is proposed and compared to four different thickness estimations. A simplified aortic geometry, under physiologic pulsatile conditions, is used to assess the impact of ST measurement on stress predictions. An additional computational model investigates the effect of such thickness values on critical pressure levels that may instigate aneurysm formation in a homogeneous or artificially modified geometry. Comparing the results obtained for the application of a pulsatile load, wall stress values associated to minimum ST are at least 24kPa inferior to maximum ST. Critical pressure values appear to be inversely proportional to ST estimation: simulations, associated to maximum ST, predict aneurysm formation for pressure levels at least 7kPa inferior to minimum ST outcomes. Finally, the role of the strain-energy function used to fit the experimental data is demonstrated to be fundamental for predictions of aneurysm formation.

Influence of Different Boundary Conditions in Finite Element Analysis on Pelvic Biomechanical Load Transmission.

To observe the effects of boundary conditions and connect conditions on biomechanics predictions in finite element (FE) pelvic models. Three FE pelvic models were constructed to analyze the effect of boundary conditions and connect conditions in the hip joint: an intact pelvic model assumed contact of the hip joint on both sides (Model I); and a pelvic model assumed the hip joint connecting surfaces fused together with (Model II) or without proximal femurs (Model III). The model was validated by bone surface strains obtained from strain gauges in an in vitro pelvic experiment. Vertical load was applied to the pelvic specimen, and the same load was simulated in the FE model. There was a strong correlation between the FE analysis results of Model I and the experimental results (R 2 = 0.979); meanwhile, the correlation coefficient and the linear regression function increased slightly with increasing load force. Comparing the three models, the stress values in the point near the pubic symphysis in Model III were 48.52 and 39.1% lower, respectively, in comparison with Models I and II. Furthermore, the stress values on the dome region of the acetabulum in Models II and III were 103.61 and 390.53% less than those of Model I. Besides, the posterior acetabular wall stress values of Model II were 197.15 and 305.17% higher than those of Models I and III, respectively. These findings suggest that the effect of the connect condition in the hip joint should not be neglected, especially in studies related to clinical applications.

Te Comparison of the Effects of the Cilostazol on the Stiffness Parameters in Different Arterial Systems in the Patients with Peripheral Arterial Disease

Objective : Object of this study is to determine the effect of cilostazol on arterial compliance, distensibilty and vascular stiffness in carotid, femoral and brachial arteries through non-invasive methods in patients with stable IC/PAD. Material and Methods: Twenty-nine patients (22 male, 7 female) who had been treated with cilostazol 100 mg twice daily during six month were included in the study. Ultrasound measurements of carotid, femoral and brachial arteries were performed at baseline, at 3 and 6 months afer starting of treatment and the evaluation of the arterial stiffness measurements were performed by the pre-defned method. Results : Te mean age of the participants was 64 years. Pulsatility index, resistive index, systolic/diastolic velocity ratio, intima-media thickness and elastic modulus values were decreased while cross-sectional compliance, cross-sectional distensibility and diastolic wall stress values were increased with treatment in carotid, femoral and brachial arteries. Carotid, femoral and brachial arteries have similar rates of change. Conclusion : Cilostazol treatment reduced the progression of atherosclerosis without increasing the risk of bleeding in patients with peripheral arterial disease during a long-term follow-up

Simultaneous pulsatile flow and oscillating wall of a non-Newtonian liquid

In this work, analytical predictions of the rectilinear flow of a non-Newtonian liquid are given. The fluid is subjected to a combined flow: A pulsatile time-dependent pressure gradient and a random longitudinal vibration at the wall acting simultaneously. The fluctuating component of the combined pressure gradient and oscillating flow is assumed to be of small amplitude and can be adequately represented by a weakly stochastic process, for which a quasi-static perturbation solution scheme is suggested, in terms of a small parameter. This flow is analyzed with the Tanner constitutive equation model with the viscosity function represented by the Ellis model. According to the coupled Tanner-Ellis model, the flow enhancement can be separated in two contributions (pulsatile and oscillating mechanisms) and the power requirement is always positive and can be interpreted as the sum of a pulsatile, oscillating, and the coupled systems respectively. Both expressions depend on the amplitude of the oscillations, the perturbation parameter, the exponent of the Ellis model (associated to the shear thinning or thickening mechanisms), and the Reynolds and Deborah numbers. At small wall stress values, the flow enhancement is dominated by the axial wall oscillations whereas at high wall stress values, the system is governed by the pulsating noise perturbation. The flow transition is obtained for a critical shear stress which is a function of the Reynolds number, dimensionless frequency and the ratio of the two amplitudes associated with the pulsating and oscillating perturbations. In addition, the flow enhancement is compared with analytical and numerical predictions of the Reiner-Phillipoff and Carreau models. Finally, the flow enhancement and power requirement are predicted using biological rheometric data of blood with low cholesterol content.

Patient specific stress and rupture analysis of ascending thoracic aneurysms

An ascending thoracic aortic aneurysm (ATAA) is a serious medical condition which, more often than not, requires surgery. Aneurysm diameter is the primary clinical criterion for determining when surgical intervention is necessary but, biomechanical studies have suggested that the diameter criterion is insufficient. This manuscript presents a method for obtaining the patient specific wall stress distribution of the ATAA and the retrospective rupture risk for each patient. Five human ATAAs and the preoperative dynamic CT scans were obtained during elective surgeries to replace each patient's aneurysm with a synthetic graft. The material properties and rupture stress for each tissue sample were identified using bulge inflation tests. The dynamic CT scans were used to generate patient specific geometries for a finite element (FE) model of each patient's aneurysm. The material properties from the bulge inflation tests were implemented in the FE model and the wall stress distribution at four different pressures was estimated. Three different rupture risk assessments were compared: the maximum diameter, the rupture risk index, and the overpressure index. The peak wall stress values for the patients ranged from 28% to 94% of the ATAA's failure stress. The rupture risk and overpressure indices were both only weakly correlated with diameter (ρ=−0.29, both cases). In the future, we plan to conduct a large experimental and computational study that includes asymptomatic patients under surveillance, patients undergoing elective surgery, and patients who have experienced rupture or dissection to determine if the rupture risk index or maximum diameter can meaningfully differentiate between the groups.

Fluid-structure interaction in abdominal aortic aneurysms: Structural and geometrical considerations

Rupture of the abdominal aortic aneurysm (AAA) is the result of the relatively complex interaction of blood hemodynamics and material behavior of arterial walls. In the present study, the cumulative effects of physiological parameters such as the directional growth, arterial wall properties (isotropy and anisotropy), iliac bifurcation and arterial wall thickness on prediction of wall stress in fully coupled fluid-structure interaction (FSI) analysis of five idealized AAA models have been investigated. In particular, the numerical model considers the heterogeneity of arterial wall and the iliac bifurcation, which allows the study of the geometric asymmetry due to the growth of the aneurysm into different directions. Results demonstrate that the blood pulsatile nature is responsible for emerging a time-dependent recirculation zone inside the aneurysm, which directly affects the stress distribution in aneurismal wall. Therefore, aneurysm deviation from the arterial axis, especially, in the lateral direction increases the wall stress in a relatively nonlinear fashion. Among the models analyzed in this investigation, the anisotropic material model that considers the wall thickness variations, greatly affects the wall stress values, while the stress distributions are less affected as compared to the uniform wall thickness models. In this regard, it is confirmed that wall stress predictions are more influenced by the appropriate structural model than the geometrical considerations such as the level of asymmetry and its curvature, growth direction and its extent.

Regulation of Cardiomyocyte T-Tubule Organization and Density by Ventricular Wall Stress

During heart failure development, t-tubules become disorganized which disrupts Ca2+ homeostasis and weakens contraction of the heart. The mechanisms controlling t-tubular structure in the normal and failing heart remain unknown, but accumulating data suggest that ventricular workload may be an important regulator. In Wistar rats which had developed heart failure 6 weeks following myocardial infarction, we observed that marked t-tubule disruption occurred preferentially in regions of the heart that are proximal to the infarct site, while t-tubule density was normal in distal locations. In vivo imaging by MRI has revealed near-isometric contraction in this proximal zone, with dramatically elevated wall stress due to local thinning of the ventricular wall and elevated diastolic blood pressure. To directly investigate whether elevated wall stress triggers t-tubule disruption, we developed an in vitro model for culturing isolated rat left ventricle papillary muscles in a myobath system (0.5 Hz stimulation, 48 hours). Muscles were subjected to varying amounts of stretch to approximate wall stress values observed in vivo. Muscles exposed to low diastolic wall stress similar to that observed in sham-operated hearts (3.5-4.5 mN/mm2), exhibited well-maintained t-tubule organization during culture (t-tubule fraction of cross-sectional area = 0.174 ± SE 0.007). Exposure to high wall stress (10-15 mN/mm2) triggered marked t-tubule loss during culture (t-tubule fraction = 0.079 ± SE 0.012), and reduction in both peak and between-peak power in Fast-Fourier Transform analyses. In addition, cell size was observed to be markedly increased by elevated wall stress in comparison with muscles that were exposed to low wall stress (cross-sectional area = 1022.71 ± SE 48.00 µm2 vs 564.41 ± SE 25.34 µm2, P < 0.05). Thus, our data indicate that wall stress is an important regulator of both cellular geometry and t-tubular structure.