Local Wall Stress Research Articles

Determinants of vascular structure and function in at-risk children born to mothers managed for pre-eclampsia (FINNCARE study).

Pre-eclampsia (PE) is related to elevated blood pressure (BP) in children. The study aims to investigate if elevated BP is reflected in child arterial health and how anthropometrics, body composition, and gestational and perinatal factors influenced this. In this prospective cohort study, we assessed the arteries of 182 children exposed (46 had an early onset, with a diagnosis before 34 gestational weeks, and 136 had a late onset) and 85 children unexposed (non-PE) to PE at 8-12 years from delivery using ultra-high-frequency ultrasound in addition to ambulatory and central BPs, body composition and anthropometrics, and tonometry-derived pulse wave velocity (PWV). No differences were found in intima-media thickness (IMT), adventitia thickness (AT), lumen diameter (LD), local carotid artery stiffness, distensibility, or wall stress between PE-exposed and non-PE-exposed children. All children's brachial, radial, and femoral artery IMTs were associated with 24-h systolic BP (SBP) and pulse pressure, carotid-femoral PWV, and anthropometric measures. The 24-h SBP and anthropometrics, notably lean body mass, were independent predictors of peripheral artery IMTs (brachial R2 = 0.217, radial R2 = 0.208, femoral R2 = 0.214; p < 0.001). Head circumference predicted carotid artery IMT and LD (β = 0.163, p = 0.009; β = 0.417, p < 0.001, respectively), but carotid artery IMT was not associated with BP. No independent associations were found for peripheral artery ATs. Local carotid artery stiffness, distensibility, and wall stress were independently associated with adiposity. No significant associations were found between gestational or perinatal factors and child vascular health parameters. The peripheral artery IMT of PE-exposed children is identical to that of non-PE-exposed children, but associated with BP. Adiposity is related to local carotid artery stiffness. These adverse associations in arterial health may reflect the early progression of cardiovascular disease in PE-exposed children.

Energy Harvesting Performance of Thick Oscillating Airfoils Using a Discrete Vortex Model

Abstract The energy harvesting performance of thick oscillating airfoils is predicted using an inviscid discrete vortex model (DVM). National Advisory Committee for Aeronautics (NACA) airfoils with different leading-edge geometries are modeled that undergo sinusoidal heaving and pitching with reduced frequencies, k=fc/U∞, in the range 0.06−0.14, where f is the heaving frequency of the foil, c is the chord length, and U∞ is the freestream velocity. The airfoil pitches about the midchord with heaving and pitching amplitudes of h0=0.5c and θ0=70 deg, respectively, known to be in the range of peak energy harvesting efficiencies. A vortex shedding initiation criteria is proposed based on the transient local wall stress distribution determined from computational fluid dynamics (CFD) simulations and incorporates both timing and location of leading-edge separation. The scaled shedding times are shown to be predicted over the range of reduced frequencies using a timescale based on the leading-edge shear velocity and radius of curvature. The convection velocity of the shed vortices is also modeled based on the reduced frequency to better capture the dynamics of the leading-edge vortex. An empirical trailing-edge separation correction is applied to the transient force results using the effective angle of attack modified to include the pitching component. Impulse theory is applied to the DVM to calculate the transient lift force and compares well with the CFD simulations. Results show that the power output increases with increasing airfoil thickness and is most notable at higher reduced frequencies where the power output efficiency is highest.

Heterogeneous myocyte remodelling and spatial heterogeneity of repolarization within the myocardial infarction border zone

Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Fund for Scientific Research-Flanders (FWO) Background Sudden cardiac death due to ventricular arrhythmias is a major cause of mortality after myocardial infarction (MI). The border zone (BZ) surrounding the infarct is the dominant source of arrhythmias. Here a substrate of heterogeneous repolarization is implicated, which could be due to heterogeneous myocyte remodelling. Objective To examine myocyte remodelling within the BZ, in comparison to the remote myocardium, and evaluate the local profile of repolarization of these regions in vivo. Methods MI was induced by 120-minute occlusion of the left anterior descending coronary artery followed by reperfusion in 6 domestic pigs. After 4 weeks, magnetic resonance imaging was performed to assess infarct remodelling and local wall thickness. Within 3 days, electro-anatomical mapping was performed. A non-contact recording of a 64-electrode array was translated to 2048 electrograms distributed over the LV and local activation-recovery-interval (ARI) determined by custom software. After recovery (2-4 days), the pigs were sacrificed, and samples collected from the BZ and remote region for RNA analysis and single cardiomyocyte isolation. Cell dimensions were measured and cellular AP duration (APD) was optically recorded using a fluorescent voltage dye, Di-8-Annepps (stimulation at 1Hz, 37°C). Expression and variability of cardiomyocyte hypertrophy biomarkers were extracted from single nuclear RNA sequencing data (10x Genomics). Results Cardiomyocyte APD in large population samples (&amp;gt; 100 cells per region in each pig) revealed higher heterogeneity in the BZ than the remote region, quantified as the standard deviation (SD) (BZ: 105.9 ± 17.0ms vs remote: 73.9 ± 8.6ms, P = 0.001). Cellular APD heterogeneity correlated strongly with in vivo local ARI heterogeneity, which demonstrated increased heterogeneity in the BZ (R2 = 0.67, P = 0.002). BZ myocytes were hypertrophied with greater increase in cell width than length, and cellular hypertrophy was more heterogeneous by SD in the BZ (BZ: 12.9 ± 2.4μm vs remote: 8.3 ± 1.1μm, P &amp;lt; 0.001). NPPB transcripts reporting on hypertrophic remodelling were higher in BZ than remote (mean lognorm gene expression, BZ: 0.431 ± 0.014 vs remote: 0.107 ± 0.004, P &amp;lt; 0.001), and showed greater heterogeneity in expression between cells by proportion of hypertrophic (NPPB +ve) cells (BZ: 30.86% vs remote: 8.37%, P &amp;lt; 0.001). Wall thickness variance was higher in the BZ compared to the remote region (anterior BZ: 0.15 ± 0.02mm, septal BZ: 0.16 ± 0.04mm vs remote: 0.04 ± 0.02mm, P &amp;lt; 0.001), contributing to increased heterogeneity of local wall stress in BZ. Conclusion Cardiomyocyte remodelling in the BZ is heterogeneous, possibly related to differences in local wall stress, which may contribute to heterogeneous repolarization in vivo and underlie arrhythmia vulnerability within the BZ.

A leading-edge vortex initiation criteria for large amplitude foil oscillations using a discrete vortex model

A leading-edge vortex initiation criterion of an oscillating airfoil is shown to provide a means to predict the onset of leading-edge separation. This result is found to collapse the occurrence of separation during the oscillation cycle when scaled using the leading-edge shear velocity determined from the foil oscillating kinematic motion. This is of importance in developing low order models for predicting energy harvesting performance, as is shown in this study using an inviscid discrete vortex model (DVM). Results are obtained for a thin flat airfoil undergoing sinusoidal heaving and pitching motions with reduced frequencies of k=fc/U∞ in the range of 0.06–0.16, where f is the heaving frequency of the foil, c is the chord length, and U∞ is the freestream velocity. The airfoil pitches about the mid-chord, and the heaving and pitching amplitudes of the airfoil are ho=0.5c and θ0=70°, respectively, illustrating results for conditions near peak efficiency for energy harvesting. The DVM uses a panel method, applicable to a wide range of foil geometries. An empirical trailing-edge separation correction is also applied to the transient force results. The vortex shedding criterion is based on the transient local wall stress distribution determined using a computational fluid dynamics (CFD) simulation, indicating the time and location of zero stress at the foil surface. In addition, the local pressure gradient minimum is also used as a local indicator. The effects of a wide range of Reynolds numbers on separation are shown for the given range of reduced frequencies. The use of the effective angle of attack, when modified to include the pitching component, is also shown to correlate the leading-edge vortex initiation time. The advantage of the proposed separation criteria is that it can be fully determined from the motion kinematics and then applied to a wide range of low order models. Model results are given for the transient lift force and compare well with the CFD simulations. It is noted that at higher reduced frequencies the DVM overpredicts portions of the transient loads possibly caused by the reversed viscous flow from the trailing edge.

A left lateral body position increases pulmonary vein stress in healthy humans.

Pulmonary vein (PV) stretch is proarrhythmic for atrial fibrillation (AF). AF patients often report that a left lateral (LL) body position can trigger arrhythmia symptoms. Because the PV myocardium is thought to trigger AF, we hypothesized that the LL compared to the supine body position increases PV wall stress. Functional cardiac magnetic resonance imaging was performed in supine and LL recumbent body position in awake condition in healthy human volunteers (n = 20). Following a change from supine to LL position, the heart moved in an anterior‐LL direction in the thorax. The right superior PV diameter was increased by 19% (24.6 ± 3.1 vs. 20.7 ± 3.2 mm, p = 0.009) and left atrial (LA) volume was larger by 17% (61.7[15.4] vs. 51.0[17.8] ml, p = 0.015) in LL than supine position, respectively. The passive LA conduit fraction (normalized difference between maximum and pre‐contraction LA volume) increased by 25% in LL compared to supine position (19.6 ± 9.0 vs. 15.7 ± 7.6%, respectively, p = 0.016). Local wall stress in the PV regions increased in LL compared to supine position (overall mean: 1.01 ± 0.12 vs. 1.10 ± 0.10 arb. unit, LL vs. supine, position effect p = 0.041), whereas this was not the case in the LA walls (overall mean: 1.18 ± 0.31 vs. 1.21 ± 0.21 arb. unit, LL vs. supine, position effect p = 0.381). In conclusion, a left lateral body position increases PV myocardial stress during the atrial relaxation phase of healthy volunteers. These results have implications for the mechanisms of posture‐triggered AF.

Hemodynamics in aneurysm blebs with different wall characteristics

BackgroundBlebs are important secondary structures of intracranial aneurysms associated with increased rupture risk and can affect local wall stress and hemodynamics. Mechanisms of bleb development and evolution are not clearly...

Is There a Relationship Between Stress in Walls of Abdominal Aortic Aneurysm and Symptoms?

BackgroundAbdominal aortic aneurysm (AAA) is a permanent and irreversible dilation of the lower region of the aorta. It is typically an asymptomatic condition that if left untreated can expand to the point of rupture. In simple mechanical terms, rupture of an artery occurs when the local wall stress exceeds the local wall strength. It is therefore understandable that numerous studies have attempted to estimate the AAA wall stress and investigate the relationship between the AAA wall stress and AAA symptoms. Materials and methodsWe conducted computational biomechanics analysis for 19 patients with AAA (a proportion of these patients were classified as symptomatic) to investigate whether the AAA wall stress fields (both the patterns and magnitude) correlate with the clinical definition of symptomatic and asymptomatic AAAs. For computation of AAA wall stress, we used a very efficient method recently presented by the Intelligent Systems for Medicine Laboratory. The Intelligent Systems for Medicine Laboratory's method uses geometry from computed tomography images and mean arterial pressure as the applied load. The method is embedded in the software platform BioPARR—Biomechanics based Prediction of Aneurysm Rupture Risk, freely available from http://bioparr.mech.uwa.edu.au/. The uniqueness of our stress computation approach is three-fold: i) the results are insensitive to unknown patient-specific mechanical properties of arterial wall tissue; ii) the residual stress is accounted for, according to Y.C. Fung's Uniform Stress Hypothesis; and iii) the analysis is automated and quick, making our approach compatible with clinical workflows. ResultsSymptomatic patients could not be identified from the plots (pattern) of AAA wall stress and stress magnitude. Although the largest stress was predicted for a patient who suffered from AAA symptoms, the three patients with the smallest stress were also symptomatic. ConclusionsThe results demonstrate, contrary to the common view, that neither the wall stress magnitude nor the stress distribution appears to be associated with the presence of clinical symptoms.

The impact of wall thickness and curvature on wall stress in patient-specific electromechanical models of the left atrium

The left atrium (LA) has a complex anatomy with heterogeneous wall thickness and curvature. The anatomy plays an important role in determining local wall stress; however, the relative contribution of wall thickness and curvature in determining wall stress in the LA is unknown. We have developed electromechanical finite element (FE) models of the LA using patient-specific anatomical FE meshes with rule-based myofiber directions. The models of the LA were passively inflated to 10mmHg followed by simulation of the contraction phase of the atrial cardiac cycle. The FE models predicted maximum LA volumes of 156.5 mL, 99.3 mL and 83.4 mL and ejection fractions of 36.9%, 32.0% and 25.2%. The median wall thickness in the 3 cases was calculated as 1.32, pm ,0.78 mm, 1.21, pm ,0.85 mm, and 0.74,pm ,0.34 mm. The median curvature was determined as 0.159,pm ,0.080 hbox {mm}^{-1}, 0.165,pm ,0.079,hbox {mm}^{-1}, and 0.166,pm ,0.077,hbox {mm}^{-1}. Following passive inflation, the correlation of wall stress with the inverse of wall thickness and curvature was 0.55–0.62 and 0.20–0.25, respectively. At peak contraction, the correlation of wall stress with the inverse of wall thickness and curvature was 0.38–0.44 and 0.16–0.34, respectively. In the LA, the 1st principal Cauchy stress is more dependent on wall thickness than curvature during passive inflation and both correlations decrease during active contraction. This emphasizes the importance of including the heterogeneous wall thickness in electromechanical FE simulations of the LA. Overall, simulation results and sensitivity analyses show that in complex atrial anatomy it is unlikely that a simple anatomical-based law can be used to estimate local wall stress, demonstrating the importance of FE analyses.

Correlation of Intraluminal Thrombus Deposition, Biomechanics, and Hemodynamics with Surface Growth and Rupture in Abdominal Aortic Aneurysm—Application in a Clinical Paradigm

The natural history of abdominal aortic aneurysm (AAA) can be investigated through longitudinal evaluation of localized aneurysm characteristics exploiting clinical images. The major challenge is to identify corresponding regions between follow-ups. We have recently developed an algorithm (VascForm) based on nonrigid registration that can obtain surface correspondence and quantify surface growth distribution. A ruptured AAA with an initial computed tomography scan 2years ago was studied. Following 3-dimensional reconstruction of outer wall and luminal surfaces, the wall/thrombus thickness was obtained. Wall stress distribution was computed with finite element analysis, and computational fluid dynamics simulation was performed. VascForm was applied and allowed for the ruptured wall site to be traced back to the initial wall surface and be correlated with local initial intraluminal thrombus thickness, wall stress, and hemodynamic parameters. It also allowed for the quantification of wall surface growth based on surface element growth. Rupture occurred at the posterolateral side. Initial wall surface growth was in most regions 40%. However, a large section of the posterior wall presented 110% growth. Initial thrombus deposition was more prevalent anteriorly, and a posterior thrombus-free isle was present. Peak wall stress (initial and follow-up) occurred at AAA neck. Nonrigid registration revealed that rupture originated from the vicinity of the initial thrombus-free isle. Furthermore, rupture occurred at the wall region with the largest growth (110%). No clear correlation between hemodynamics and rupture site could be identified. High local surface growth correlates with rupture site and could therefore potentially become a marker of rupture risk. The ongoing application of this methodology to a large cohort of AAA patients will focus on identifying characteristic features of AAA regions that present high surface growth in follow-up evaluations, to assist in improved rupture risk estimation.

Local Diameter, Wall Stress, and Thrombus Thickness Influence the Local Growth of Abdominal Aortic Aneurysms

Purpose: To investigate the influence of the local diameter, the intraluminal thrombus (ILT) thickness, and wall stress on the local growth rate of abdominal aortic aneurysms. Methods: The infrarenal aortas of 90 asymptomatic abdominal aortic aneurysm (AAA) patients (mean age 70 years; 77 men) were retrospectively reconstructed from at least 2 computed tomography angiography scans (median follow-up of 1 year) and biomechanically analyzed with the finite element method. Each individual AAA model was automatically sliced orthogonally to the lumen centerline and represented by 100 cross sections with corresponding diameters, ILT thicknesses, and wall stresses. The data were grouped according to these parameters for comparison of differences among the variables. Results: Diameter growth was continuously distributed over the entire aneurysm sac, reaching absolute and relative median peaks of 3.06 mm/y and 7.3%/y, respectively. The local growth rate was dependent on the local baseline diameter, the local ILT thickness, and for wall segments not covered by ILT, also on the local wall stress level (all p<0.001). For wall segments that were covered by a thick ILT layer, wall stress did not affect the growth rate (p=0.08). Conclusion: Diameter is not only a strong global predictor but also a local predictor of aneurysm growth. In addition, and independent of the diameter, the ILT thickness and wall stress (for the ILT-free wall) also influence the local growth rate. The high stress sensitivity of nondilated aortic walls suggests that wall stress peaks could initiate AAA formation. In contrast, local diameters and ILT thicknesses determine AAA growth for dilated and ILT-covered aortic walls.

Elevated ventricular wall stress disrupts cardiomyocyte t-tubule structure and calcium homeostasis.

AimsInvaginations of the cellular membrane called t-tubules are essential for maintaining efficient excitation–contraction coupling in ventricular cardiomyocytes. Disruption of t-tubule structure during heart failure has been linked to dyssynchronous, slowed Ca2+ release and reduced power of the heartbeat. The underlying mechanism is, however, unknown. We presently investigated whether elevated ventricular wall stress triggers remodelling of t-tubule structure and function.Methods and resultsMRI and blood pressure measurements were employed to examine regional wall stress across the left ventricle of sham-operated and failing, post-infarction rat hearts. In failing hearts, elevated left ventricular diastolic pressure and ventricular dilation resulted in markedly increased wall stress, particularly in the thin-walled region proximal to the infarct. High wall stress in this proximal zone was associated with reduced expression of the dyadic anchor junctophilin-2 and disrupted cardiomyocyte t-tubular structure. Indeed, local wall stress measurements predicted t-tubule density across sham and failing hearts. Elevated wall stress and disrupted cardiomyocyte structure in the proximal zone were also associated with desynchronized Ca2+ release in cardiomyocytes and markedly reduced local contractility in vivo. A causative role of wall stress in promoting t-tubule remodelling was established by applying stretch to papillary muscles ex vivo under culture conditions. Loads comparable to wall stress levels observed in vivo in the proximal zone reduced expression of junctophilin-2 and promoted t-tubule loss.ConclusionElevated wall stress reduces junctophilin-2 expression and disrupts t-tubule integrity, Ca2+ release, and contractile function. These findings provide new insight into the role of wall stress in promoting heart failure progression.

Endoleak Assessment Using Computational Fluid Dynamics and Image Processing Methods in Stented Abdominal Aortic Aneurysm Models.

Endovascular aortic aneurysm repair (EVAR) is a predominant surgical procedure to reduce the risk of aneurysm rupture in abdominal aortic aneurysm (AAA) patients. Endoleak formation, which eventually requires additional surgical reoperation, is a major EVAR complication. Understanding the etiology and evolution of endoleak from the hemodynamic perspective is crucial to advancing the current posttreatments for AAA patients who underwent EVAR. Therefore, a comprehensive flow assessment was performed to investigate the relationship between endoleak and its surrounding pathological flow fields through computational fluid dynamics and image processing. Six patient-specific models were reconstructed, and the associated hemodynamics in these models was quantified three-dimensionally to calculate wall stress. To provide a high degree of clinical relevance, the mechanical stress distribution calculated from the models was compared with the endoleak positions identified from the computed tomography images of patients through a series of imaging processing methods. An endoleak possibly forms in a location with high local wall stress. An improved stent graft (SG) structure is conceived accordingly by increasing the mechanical strength of the SG at peak wall stress locations. The presented analytical paradigm, as well as numerical analysis using patient-specific models, may be extended to other common human cardiovascular surgeries.

A simple, effective and clinically applicable method to compute abdominal aortic aneurysm wall stress

Abdominal aortic aneurysm (AAA) is a permanent and irreversible dilation of the lower region of the aorta. It is a symptomless condition that if left untreated can expand to the point of rupture. Mechanically-speaking, rupture of an artery occurs when the local wall stress exceeds the local wall strength. It is therefore desirable to be able to non-invasively estimate the AAA wall stress for a given patient, quickly and reliably.In this paper we present an entirely new approach to computing the wall tension (i.e. the stress resultant equal to the integral of the stresses tangent to the wall over the wall thickness) within an AAA that relies on trivial linear elastic finite element computations, which can be performed instantaneously in the clinical environment on the simplest computing hardware. As an input to our calculations we only use information readily available in the clinic: the shape of the aneurysm in-vivo, as seen on a computed tomography (CT) scan, and blood pressure. We demonstrate that tension fields computed with the proposed approach agree well with those obtained using very sophisticated, state-of-the-art non-linear inverse procedures. Using magnetic resonance (MR) images of the same patient, we can approximately measure the local wall thickness and calculate the local wall stress. What is truly exciting about this simple approach is that one does not need any information on material parameters; this supports the development and use of patient-specific modelling (PSM), where uncertainty in material data is recognised as a key limitation.The methods demonstrated in this paper are applicable to other areas of biomechanics where the loads and loaded geometry of the system are known.

Interaction of Biomechanics with Extracellular Matrix Components in Abdominal Aortic Aneurysm Wall

Little is known about the interactions between extracellular matrix (ECM) proteins and locally acting mechanical conditions and material macroscopic properties in abdominal aortic aneurysm (AAA). In this study, ECM components were investigated with correlation to corresponding biomechanical properties and loads in aneurysmal arterial wall tissue. Fifty-four tissue samples from 31 AAA patients (30♂; max. diameter Dmax 5.98±1.42cm) were excised from the aneurysm sac. Samples were divided for corresponding immunohistological and mechanical analysis. Collagen I and III, total collagen, elastin, and proteoglycans were quantified by computational image analysis of histological staining. Pre-surgical CT data were used for 3D segmentation of the AAA and calculation of mechanical conditions by advanced finite element analysis. AAA wall stiffness and strength were assessed by repeated cyclical, sinusoidal and destructive tensile testing. Amounts of collagen I, III, and total collagen were increased with higher local wall stress (p=.002, .017, .030, respectively) and strain (p=.002, .012, .020, respectively). AAA wall failure tension exhibited a positive correlation with collagen I, total collagen, and proteoglycans (p=.037, .038, .022, respectively). α-Stiffness correlated with collagen I, III, and total collagen (p=.011, .038, and .008), while β-stiffness correlated only with proteoglycans (p=.028). In contrast, increased thrombus thickness was associated with decreased collagen I, III, and total collagen (p=.003, .020, .015, respectively), and AAA diameter was negatively associated with elastin (p=.006). The present results indicate that in AAA, increased locally acting biomechanical conditions (stress and strain) involve increased synthesis of collagen and proteoglycans with increased failure tension. These findings confirm the presence of adaptive biological processes to maintain the mechanical stability of AAA wall.

Typical Takotsubo syndrome and McConnell's phenomenon: What else lies beneath?

We read with great interest the paper by Shao et al. highlighting the possible shared mechanism underlying McConnell's phenomenon and reverse Takotsubo cardiomyopathy (TTC) [1]. The authors brilliantly relate ventricular wall overstretching induced by abrupt increase in afterload and transient ventricular dyskinesis observable in pulmonary embolism (McConnell's sign [2]) with basal dyskinesis characterizing reverse TTC [3]. The authors also remarked that myocardial ischemia possibly induced by increased local wall stress may further precipitate myocardial stunning observable in both conditions, usually featured by local dyskinesis and generally related to catecholamine toxicity in TTC.

Low wall shear stress predominates at sites of abdominal aortic aneurysm rupture

Aortic diameter as the primary criterion in the decision to repair abdominal aortic aneurysms (AAAs) has drawbacks as some rupture below size thresholds, whereas others reach extreme size without rupture. Predictions of static aortic wall stress have also failed to reliably predict rupture potential. The objective of this study was to computationally assess blood flow characteristics at the site of infrarenal AAA rupture. On the basis of the finite element literature correlating rupture location with high static local wall stress, we hypothesized that a computational fluid dynamics approach would also demonstrate rupture at regions of high pressure and wall shear stress (WSS). Three-dimensional AAA geometry was generated from computed tomography angiography images of seven ruptured AAAs. Aortic blood flow velocity, pressure, and WSS were computationally determined. Flow characteristics at the site of rupture were determined and compared across all cases. AAA size at the time of rupture was 8.3± 0.9cm. Only three of the seven AAAs ruptured at the site of maximal diameter. Blood flow velocity in the aneurysmal aorta showed dominant flow channels with zones of recirculation, where low WSS predominated. Regardless of aneurysm size or configuration, rupture occurred in or near these flow recirculation zones in all cases. WSS was significantly lower and thrombus deposition was more abundant at the site of rupture. This computational study was the first to assess blood flow characteristics at the site of infrarenal AAA rupture in realistic aortic geometries. In contradiction to our initial hypothesis, rupture occurred not at sites of high pressure and WSS but rather at regions of predicted flow recirculation, where low WSS and thrombus deposition predominated. These findings raise the possibility that this flow pattern may lead to thrombus deposition, which may elaborate adventitial degeneration and eventual AAA rupture.

The biaxial biomechanical behavior of abdominal aortic aneurysm tissue.

Rupture of the abdominal aortic aneurysm (AAA) occurs when the local wall stress exceeds the local wall strength. Knowledge of AAA wall mechanics plays a fundamental role in the development and advancement of AAA rupture risk assessment tools. Therefore, the aim of this study is to evaluate the biaxial mechanical properties of AAA tissue. Multiple biaxial test protocols were performed on AAA samples harvested from 28 patients undergoing open surgical repair. Both the Tangential Modulus (TM) and stretch ratio (λ) were recorded and compared in both the circumferential (ϴ) and longitudinal (L) directions at physiologically relevant stress levels, the influence of patient specific factors such as sex, age AAA diameter and status were examined. The biomechanical response was also fit to a hyperplastic material model. The AAA tissue was found to be anisotropic with a greater tendency to stiffen in the circumferential direction compared to the longitudinal direction. An anisotropic hyperelastic constitutive model represented the data well and the properties were not influenced by the investigated patient specific factors however, a future study utilizing a larger cohort of patients is warranted to confirm these findings. This work provides further insights on the biomechanical behavior of AAA and may be useful in the development of more reliable rupture risk assessment tools.

Local Diameter, Wall Stress and Thrombus Thickness Influence the Local Growth of Abdominal Aortic Aneurysms

Introduction: Abdominal aortic aneurysms (AAA) size is related to growth rate, and thresholds for both are used to indicate AAA repair. Fast aneurysm growth could compromise the wall's structural integrity and increases risk for rupture. Similarly, an intraluminal thrombus (ILT) weakens the aortic wall and influences growth rate. High wall stress accelerates metabolism in the aortic aneurysm wall, which also could increase growth rate and rupture risk. The present study investigated the influence of local diameter, ILT layer thickness and wall stress on the growth rate. This information could improve elective repair indication and individualize patient follow-up intervals. Methods: Patients (n = 90) were retrospectively monitored between 6 and 24 months with contrast-enhanced Computer Tomography-Angiography (CT-A). Symptomatic patients were excluded and in total 200 CT-A scans were analyzed. The infrarenal aorta was reconstructed and biomechanically analyzed with A4clinics (VASCOPS GmbH, Graz, Austria). Each individual AAA Finite Element model was automatically sliced orthogonally to the lumen centerline, and represented by 100 cross-sections with corresponding diameter, ILT thickness and wall stress. An exponential growth model was used to extract the aneurysm expansion all along the centerline and significance was tested with Wilcoxon signed rank and Friedman's tests. Results: Both maximum diameter (43.9 mm, IQR: 39.5 to 48.6 mm) and maximum wall stress (118.6 kPa, IQR: 104.3 to 131 kPa) were localized in the central aneurysm region, while maximum ILT thickness (7.80 mm, IQR: 4.06 to 10.8 mm) appeared in the distal part. Median growth of 3.06 mm/year (IQR, 1.21 to 4.32 mm/year) was recorded and aneurysm growth was continuously distributed all over the aneurysm sac. The growth rate was depended on the local baseline diameter, the ILT thickness and, for wall segments not covered by ILT, also on the wall stress level (all P < .001). For wall segments that were covered by a thick ILT layer, wall stress did not affect the growth rate (P = .08). Conclusion: Diameter is not only a strong global but also local predictor of growth. In addition, and independently from the diameter, also the ILT thickness and wall stress (for the ILT-free wall) influence local growth rate. Interestingly not all portions of the AAA enlarge over time; some were stable or even shrunk over time. Such a behavior cannot be predicted be a single property (like the local diameter) but requires multidimensional approaches incorporating more that geometrical properties.

Insights Into Regional Adaptations in the Growing Pulmonary Artery Using a Meso-Scale Structural Model: Effects of Ascending Aorta Impingement

As the next step in our investigations into the structural adaptations of the main pulmonary artery (PA) during postnatal growth, we utilized the extensive experimental measurements of the growing ovine PA from our previous study (Fata et al., 2013, "Estimated in vivo Postnatal Surface Growth Patterns of the Ovine Main Pulmonary Artery and Ascending Aorta," J. Biomech. Eng., 135(7), pp. 71010-71012). to develop a structural constitutive model for the PA wall tissue. Novel to the present approach was the treatment of the elastin network as a distributed fiber network rather than a continuum phase. We then utilized this model to delineate structure-function differences in the PA wall at the juvenile and adult stages. Overall, the predicted elastin moduli exhibited minor differences remained largely unchanged with age and region (in the range of 150 to 200 kPa). Similarly, the predicted collagen moduli ranged from ∼1,600 to 2700 kPa in the four regions studied in the juvenile state. Interestingly, we found for the medial region that the elastin and collagen fiber splay underwent opposite changes (collagen standard deviation juvenile = 17 deg to adult = 28 deg, elastin standard deviation juvenile = 35 deg to adult = 27 deg), along with a trend towards more rapid collagen fiber strain recruitment with age, along with a drop in collagen fiber moduli, which went from 2700 kPa for the juvenile stage to 746 kPa in the adult. These changes were likely due to the previously observed impingement of the relatively stiff ascending aorta on the growing PA medial region. Intuitively, the effects of the local impingement would be to lower the local wall stress, consistent with the observed parallel decrease in collagen modulus. These results suggest that during the postnatal somatic growth period local stresses can substantially modulate regional tissue microstructure and mechanical behaviors in the PA. We further underscore that our previous studies indicated an increase in effective PA wall stress with postnatal maturation. When taken together with the fact that the observed changes in mechanical behavior and structure in the growing PA wall were modest in the other three regions studied, our collective results suggest that the majority of the growing PA wall is subjected to increasing stress levels with age without undergoing major structural adaptations. This observation is contrary to the accepted theory of maintenance of homeostatic stress levels in the regulation of vascular function, and suggests alternative mechanisms might regulate postnatal somatic growth. Understanding the underlying mechanisms will help to improve our understanding of congenital defects of the PA and lay the basis for functional duplication in their repair and replacement.

Validation of numerical flow simulations against in vitro phantom measurements in different type B aortic dissection scenarios

An aortic dissection (AD) is a serious condition defined by the splitting of the arterial wall, thus generating a secondary lumen [the false lumen (FL)]. Its management, treatment and follow-up are clinical challenges due to the progressive aortic dilatation and potentially severe complications during follow-up. It is well known that the direction and rate of dilatation of the artery wall depend on haemodynamic parameters such as the local velocity profiles, intra-luminal pressures and resultant wall stresses. These factors act on the FL and true lumen, triggering remodelling and clinical worsening. In this study, we aimed to validate a computational fluid dynamic (CFD) tool for the haemodynamic characterisation of chronic (type B) ADs. We validated the numerical results, for several dissection geometries, with experimental data obtained from a previous in vitro study performed on idealised dissected physical models. We found a good correlation between CFD simulations and experimental measurements as long as the tear size was large enough so that the effect of the wall compliance was negligible.