A Review of Biological Factors Implicated in Abdominal Aortic Aneurysm Rupture

Potential biomechanical roles of risk factors in the evolution of thrombus-laden abdominal aortic aneurysms

Abdominal aortic aneurysms (AAAs) typically harbour an intraluminal thrombus (ILT), yet most prior computational models neglect biochemomechanical effects of thrombus on lesion evolution. We recently proposed a growth and remodelling model of thrombus-laden AAAs that introduced a number of new constitutive relations and associated model parameters. Because values of several of these parameters have yet to be elucidated by clinical data and could vary significantly from patient to patient, the aim of this study was to investigate the possible extent to which these parameters influence AAA evolution. Given that some of these parameters model potential effects of factors that influence the risk of rupture, this study also provides insight into possible roles of common risk factors on the natural history of AAAs. Despite geometrical limitations of a cylindrical domain, findings support current thought that smoking, hypertension, and female sex likely increase the risk of rupture. Although thrombus thickness is not a reliable risk factor for rupture, the model suggests that the presence of ILT may have a destabilizing effect on AAA evolution, consistent with histological findings from human samples. Finally, simulations support two hypotheses that should be tested on patient-specific geometries in the future. First, ILT is a potential source of the staccato enlargement observed in many AAAs. Second, ILT can influence rupture risk, positively or negatively, via competing biomechanical (eg, stress shielding) and biochemical (ie, proteolytic) effects. Although further computational and experimental studies are needed, the present findings highlight the importance of considering ILT when predicting aneurysmal enlargement and rupture risk.

Novel Methodology for Characterizing Regional Variations in the Material Properties of Murine Aortas

Many vascular disorders, including aortic aneurysms and dissections, are characterized by localized changes in wall composition and structure. Notwithstanding the importance of histopathologic changes that occur at the microstructural level, macroscopic manifestations ultimately dictate the mechanical functionality and structural integrity of the aortic wall. Understanding structure-function relationships locally is thus critical for gaining increased insight into conditions that render a vessel susceptible to disease or failure. Given the scarcity of human data, mouse models are increasingly useful in this regard. In this paper, we present a novel inverse characterization of regional, nonlinear, anisotropic properties of the murine aorta. Full-field biaxial data are collected using a panoramic-digital image correlation (p-DIC) system. An inverse method, based on the principle of virtual power (PVP), is used to estimate values of material parameters regionally for a microstructurally motivated constitutive relation. We validate our experimental-computational approach by comparing results to those from standard biaxial testing. The results for the nondiseased suprarenal abdominal aorta from apolipoprotein-E null mice reveal material heterogeneities, with significant differences between dorsal and ventral as well as between proximal and distal locations, which may arise in part due to differential perivascular support and localized branches. Overall results were validated for both a membrane and a thick-wall model that delineated medial and adventitial properties. Whereas full-field characterization can be useful in the study of normal arteries, we submit that it will be particularly useful for studying complex lesions such as aneurysms, which can now be pursued with confidence given the present validation.

Biomechanical changes during abdominal aortic aneurysm growth

The biomechanics-based Abdominal Aortic Aneurysm (AAA) rupture risk assessment has gained considerable scientific and clinical momentum. However, such studies have mainly focused on information at a single time point, and little is known about how AAA properties change over time. Consequently, the present study explored how geometry, wall stress-related and blood flow-related biomechanical properties change during AAA expansion. Four patients with a total of 23 Computed Tomography-Angiography (CT-A) scans at different time points were analyzed. At each time point, patient-specific properties were extracted from (i) the reconstructed geometry, (ii) the computed wall stress at Mean Arterial Pressure (MAP), and (iii) the computed blood flow velocity at standardized inflow and outflow conditions. Testing correlations between these parameters identified several nonintuitive dependencies. Most interestingly, the Peak Wall Rupture Index (PWRI) and the maximum Wall Shear Stress (WSS) independently predicted AAA volume growth. Similarly, Intra-luminal Thrombus (ILT) volume growth depended on both the maximum WSS and the ILT volume itself. In addition, ILT volume, ILT volume growth, and maximum ILT layer thickness correlated with PWRI as well as AAA volume growth. Consequently, a large ILT volume as well as fast increase of ILT volume over time may be a risk factor for AAA rupture. However, tailored clinical studies would be required to test this hypothesis and to clarify whether monitoring ILT development has any clinical benefit.

Microstructure and mechanics of healthy and aneurysmatic abdominal aortas: experimental analysis and modelling

Soft biological tissues such as aortic walls can be viewed as fibrous composites assembled by a ground matrix and embedded families of collagen fibres. Changes in the structural components of aortic walls such as the ground matrix and the embedded families of collagen fibres have been shown to play a significant role in the pathogenesis of aortic degeneration. Hence, there is a need to develop a deeper understanding of the microstructure and the related mechanics of aortic walls. In this study, tissue samples from 17 human abdominal aortas (AA) and from 11 abdominal aortic aneurysms (AAA) are systematically analysed and compared with respect to their structural and mechanical differences. The collagen microstructure is examined by analysing data from second-harmonic generation imaging after optical clearing. Samples from the intact AA wall, their individual layers and the AAA wall are mechanically investigated using biaxial stretching tests. A bivariate von Mises distribution was used to represent the continuous fibre dispersion throughout the entire thickness, and to provide two independent dispersion parameters to be used in a recently proposed material model. Remarkable differences were found between healthy and diseased tissues. The out-of-plane dispersion was significantly higher in AAA when compared with AA tissues, and with the exception of one AAA sample, the characteristic wall structure, as visible in healthy AAs with three distinct layers, could not be identified in AAA samples. The collagen fibres in the abluminal layer of AAAs lost their waviness and exhibited rather straight and thick struts of collagen. A novel set of three structural and three material parameters is provided. With the structural parameters fixed, the material model was fitted to the mechanical experimental data, giving a very satisfying fit although there are only three material parameters involved. The results highlight the need to incorporate the structural differences into finite-element simulations as otherwise simulations of AAA tissues might not be good predictors for the actual in vivo stress state.

A machine learning approach to investigate the relationship between shape features and numerically predicted risk of ascending aortic aneurysm

Geometric features of the aorta are linked to patient risk of rupture in the clinical decision to electively repair an ascending aortic aneurysm (AsAA). Previous approaches have focused on relationship between intuitive geometric features (e.g., diameter and curvature) and wall stress. This work investigates the feasibility of a machine learning approach to establish the linkages between shape features and FEA-predicted AsAA rupture risk, and it may serve as a faster surrogate for FEA associated with long simulation time and numerical convergence issues. This method consists of four main steps: (1) constructing a statistical shape model (SSM) from clinical 3D CT images of AsAA patients; (2) generating a dataset of representative aneurysm shapes and obtaining FEA-predicted risk scores defined as systolic pressure divided by rupture pressure (rupture is determined by a threshold criterion); (3) establishing relationship between shape features and risk by using classifiers and regressors; and (4) evaluating such relationship in cross-validation. The results show that SSM parameters can be used as strong shape features to make predictions of risk scores consistent with FEA, which lead to an average risk classification accuracy of 95.58% by using support vector machine and an average regression error of 0.0332 by using support vector regression, while intuitive geometric features have relatively weak performance. Compared to FEA, this machine learning approach is magnitudes faster. In our future studies, material properties and inhomogeneous thickness will be incorporated into the models and learning algorithms, which may lead to a practical system for clinical applications.

Exploring the Biological and Mechanical Properties of Abdominal Aortic Aneurysms Using USPIO MRI and Peak Tissue Stress: A Combined Clinical and Finite Element Study

Inflammation detected through the uptake of ultrasmall superparamagnetic particles of iron oxide (USPIO) on magnetic resonance imaging (MRI) and finite element (FE) modelling of tissue stress both hold potential in the assessment of abdominal aortic aneurysm (AAA) rupture risk. This study aimed to examine the spatial relationship between these two biomarkers. Patients (n = 50) > 40 years with AAA maximum diameters > = 40 mm underwent USPIO-enhanced MRI and computed tomography angiogram (CTA). USPIO uptake was compared with wall stress predictions from CTA-based patient-specific FE models of each aneurysm. Elevated stress was commonly observed in areas vulnerable to rupture (e.g. posterior wall and shoulder). Only 16% of aneurysms exhibited co-localisation of elevated stress and mural USPIO enhancement. Globally, no correlation was observed between stress and other measures of USPIO uptake (i.e. mean or peak). It is suggested that cellular inflammation and stress may represent different but complimentary aspects of AAA disease progression.

miRNA‑504 inhibits p53‑dependent vascular smooth muscle cell apoptosis and may prevent aneurysm formation

Abdominal aortic aneurysm (AAA) is a common disease that is associated with the proliferation and apoptosis of vascular smooth muscle cells (VSMCs). VSMCs are regulated by microRNAs (miRNA). The aim of the present study was to identify miRNA sequences that regulate aortic SMCs during AAA. miRNA-504 was identified using a miRNA PCR array and by reverse transcription-quantitative polymerase chain reaction analysis, and its expression levels were observed to be downregulated in the aortic cells derived from patients with AAA when compared with controls. Transfection of SMCs with pMSCV-miRNA-504 vector was performed, and cell proliferation and the expression levels of proliferating cell nuclear antigen (PCNA), replication factor C subunit 4 (RFC4), B-cell lymphoma-2 (Bcl-2) and caspase-3/9 were measured by western blotting. The mechanisms underlying the effects of miRNA-504 was then analyzed. The results demonstrated that overexpression of miRNA-504 significantly upregulated the expression levels of PCNA, RFC4 and Bcl-2, while caspase-3/9 expression was significantly inhibited when compared with non-targeting controls. In addition, miRNA-504 overexpression was observed to promote the proliferation of SMCs. The expression level of the tumor suppressor, p53, which is known to be a direct target of miRNA-504, was inhibited following transfection of SMCs with pMSCV-miRNA-504. In addition, the expression of the downstream targets of p53, p21 and Bcl-like protein-4, were significantly reduced following overexpression of miRNA-504. These results revealed the anti-apoptotic role of miRNA-504 in SMCs derived from patients with AAA via direct targeting of p53.

High serum thrombospondin-1 concentration is associated with slower abdominal aortic aneurysm growth and deficiency of thrombospondin-1 promotes angiotensin II induced aortic aneurysm in mice

Abdominal aortic aneurysm (AAA) is a common age-related vascular disease characterized by progressive weakening and dilatation of the aortic wall. Thrombospondin-1 (TSP-1; gene Thbs1) is a member of the matricellular protein family important in the control of extracellular matrix (ECM) remodelling. In the present study, the association of serum TSP-1 concentration with AAA progression was assessed in 276 men that underwent repeated ultrasound for a median 5.5 years. AAA growth was negatively correlated with serum TSP-1 concentration (Spearman's rho -0.129, P=0.033). Men with TSP-1 in the highest quartile had a reduced likelihood of AAA growth greater than median during follow-up (OR: 0.40; 95% confidence interval (CI): 0.19-0.84, P=0.016, adjusted for other risk factors). Immunohistochemical staining for TSP-1 was reduced in AAA body tissues compared with the relatively normal AAA neck. To further assess the role of TSP-1 in AAA initiation and progression, combined TSP-1 and apolipoprotein deficient (Thbs1-/-ApoE-/-, n=20) and control mice (ApoE-/-, n=20) were infused subcutaneously with angiotensin II (AngII) for 28 days. Following AngII infusion, Thbs1-/- ApoE-/- mice had larger AAAs by ultrasound (P=0.024) and ex vivo morphometry measurement (P=0.006). The Thbs1-/-ApoE-/- mice also showed increased elastin filament degradation along with elevated systemic levels and aortic expression of matrix metalloproteinase (MMP)-9. Suprarenal aortic segments and vascular smooth muscle cells (VSMCs) isolated from Thbs1-/-ApoE-/- mice showed reduced collagen 3A1 gene expression. Furthermore, Thbs1-/-ApoE-/- mice had reduced aortic expression of low-density lipoprotein (LDL) receptor-related protein 1. Collectively, findings from the present study suggest that TSP-1 deficiency promotes maladaptive remodelling of the ECM leading to accelerated AAA progression.

The role of Hippo/yes-associated protein signalling in vascular remodelling associated with cardiovascular disease

Vascular remodelling is a vital process of a wide range of cardiovascular diseases and represents the altered structure and arrangement of blood vessels. The Hippo pathway controls organ size by regulating cell survival, proliferation and apoptosis. Yes-associated protein (YAP), a transcription coactivator, is a downstream effector of the Hippo pathway. There is growing evidence for the importance of the Hippo/YAP pathway in vascular-remodelling and related cardiovascular diseases. The Hippo/YAP pathway alters extracellular matrix production or degradation and the growth, death and migration of vascular smooth muscle cells and endothelial cells, which contributes to vascular remodelling in cardiovascular diseases such as pulmonary hypertension, atherosclerosis, restenosis, aortic aneurysms and angiogenesis. In this review, we summarize and discuss recent findings about the roles and mechanisms of Hippo/YAP signalling in vascular remodelling and related conditions.

LINKED ARTICLES

This article is part of a themed section on Spotlight on Small Molecules in Cardiovascular Diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.8/issuetoc.

Hemodynamic Profile of Two Aortic Endografts Accounting for Their Postimplantation Position

Endovascular aneurysm repair (EVAR) is a clinically effective technique for treating anatomically eligible abdominal aortic aneurysms (AAAs), involving the deployment of an endograft (EG) that is designed to prevent blood leakage in the aneurysmal sac. While most EGs have equivalent operating principles, the hemodynamic environment established by different EGs is not necessarily the same. So, to unveil the post-EVAR hemodynamic properties, we need an EG-specific computational approach that currently lacks from the literature. Endurant and Excluder are two EGs with similar pre-installation designs. We assumed that the flow conditions in the particular EGs do not vary significantly. The hypothesis was tested combining image reconstructions, computational fluid dynamics (CFD), and statistics, taking into account the postimplantation position of the EGs. Ten patients with Endurant EGs and ten patients with Excluder EGs were included in this study. The two groups were matched with respect to the preoperative morphological characteristics of the AAAs. The EG models are derived from image reconstructions of postoperative computed tomography scans. Wall shear stress (WSS), displacement force, velocity, and helicity were calculated in regions of interest within the EG structures, i.e., the main body, the upper and lower part of the limbs. Excluder generated higher WSS compared to Endurant, especially on the lower part of the limbs (p = 0.001). Spatial fluctuations of WSS were observed on the upper part of the Excluder limbs. Higher blood velocity was induced by Excluder in all the regions of interest (p = 0.04, p = 0.01, and p = 0.004). Focal points of secondary flow were detected in the main body of Endurant and the limbs of Excluder. The displacement force acting on the lower part of the Excluder limbs was stronger compared to the Endurant one (p = 0.03). The results showed that two similar EGs implanted in similar AAAs can induce significantly different flow properties. The delineation of the hemodynamic features associated with the various commercially available EGs could further promote the personalization of treatment offered to aneurysmal patients and inspire ideas for the improvement of EG designs in the future.

Histology and Biaxial Mechanical Behavior of Abdominal Aortic Aneurysm Tissue Samples

Abdominal aortic aneurysms (AAAs) represent permanent, localized dilations of the abdominal aorta that can be life-threatening if progressing to rupture. Evaluation of risk of rupture depends on understanding the mechanical behavior of patient AAA walls. In this project, a series of patient AAA wall tissue samples have been evaluated through a combined anamnestic, mechanical, and histopathologic approach. Mechanical properties of the samples have been characterized using a novel, strain-controlled, planar biaxial testing protocol emulating the in vivo deformation of the aorta. Histologically, the tissue ultrastructure was highly disrupted. All samples showed pronounced mechanical stiffening with stretch and were notably anisotropic, with greater stiffness in the circumferential than the axial direction. However, there were significant intrapatient variations in wall stiffness and stress. In biaxial tests in which the longitudinal stretch was held constant at 1.1 as the circumferential stretch was extended to 1.1, the maximum average circumferential stress was 330 ± 70 kPa, while the maximum average axial stress was 190 ± 30 kPa. A constitutive model considering the wall as anisotropic with two preferred directions fit the measured data well. No statistically significant differences in tissue mechanical properties were found based on patient gender, age, maximum bulge diameter, height, weight, body mass index, or smoking history. Although a larger patient cohort is merited to confirm these conclusions, the project provides new insight into the relationships between patient natural history, histopathology, and mechanical behavior that may be useful in the development of accurate methods for rupture risk evaluation.

A homeostatic-driven turnover remodelling constitutive model for healing in soft tissues

Remodelling of soft biological tissue is characterized by interacting biochemical and biomechanical events, which change the tissue's microstructure, and, consequently, its macroscopic mechanical properties. Remodelling is a well-defined stage of the healing process, and aims at recovering or repairing the injured extracellular matrix. Like other physiological processes, remodelling is thought to be driven by homeostasis, i.e. it tends to re-establish the properties of the uninjured tissue. However, homeostasis may never be reached, such that remodelling may also appear as a continuous pathological transformation of diseased tissues during aneurysm expansion, for example. A simple constitutive model for soft biological tissues that regards remodelling as homeostatic-driven turnover is developed. Specifically, the recoverable effective tissue damage, whose rate is the sum of a mechanical damage rate and a healing rate, serves as a scalar internal thermodynamic variable. In order to integrate the biochemical and biomechanical aspects of remodelling, the healing rate is, on the one hand, driven by mechanical stimuli, but, on the other hand, subjected to simple metabolic constraints. The proposed model is formulated in accordance with continuum damage mechanics within an open-system thermodynamics framework. The numerical implementation in an in-house finite-element code is described, particularized for Ogden hyperelasticity. Numerical examples illustrate the basic constitutive characteristics of the model and demonstrate its potential in representing aspects of remodelling of soft tissues. Simulation results are verified for their plausibility, but also validated against reported experimental data.

Biomechanical rupture risk assessment of abdominal aortic aneurysms based on a novel probabilistic rupture risk index

A rupture risk assessment is critical to the clinical treatment of abdominal aortic aneurysm (AAA) patients. The biomechanical AAA rupture risk assessment quantitatively integrates many known AAA rupture risk factors but the variability of risk predictions due to model input uncertainties remains a challenging limitation. This study derives a probabilistic rupture risk index (PRRI). Specifically, the uncertainties in AAA wall thickness and wall strength were considered, and wall stress was predicted with a state-of-the-art deterministic biomechanical model. The discriminative power of PRRI was tested in a diameter-matched cohort of ruptured (n = 7) and intact (n = 7) AAAs and compared to alternative risk assessment methods. Computed PRRI at 1.5 mean arterial pressure was significantly (p = 0.041) higher in ruptured AAAs (20.21(s.d. 14.15%)) than in intact AAAs (3.71(s.d. 5.77)%). PRRI showed a high sensitivity and specificity (discriminative power of 0.837) to discriminate between ruptured and intact AAA cases. The underlying statistical representation of stochastic data of wall thickness, wall strength and peak wall stress had only negligible effects on PRRI computations. Uncertainties in AAA wall stress predictions, the wide range of reported wall strength and the stochastic nature of failure motivate a probabilistic rupture risk assessment. Advanced AAA biomechanical modelling paired with a probabilistic rupture index definition as known from engineering risk assessment seems to be superior to a purely deterministic approach.

FSI Simulations of Pulse Wave Propagation in Human Abdominal Aortic Aneurysm: The Effects of Sac Geometry and Stiffness

This study aims to quantify the effects of geometry and stiffness of aneurysms on the pulse wave velocity (PWV) and propagation in fluid–solid interaction (FSI) simulations of arterial pulsatile flow. Spatiotemporal maps of both the wall displacement and fluid velocity were generated in order to obtain the pulse wave propagation through fluid and solid media, and to examine the interactions between the two waves. The results indicate that the presence of abdominal aortic aneurysm (AAA) sac and variations in the sac modulus affect the propagation of the pulse waves both qualitatively (eg, patterns of change of forward and reflective waves) and quantitatively (eg, decreasing of PWV within the sac and its increase beyond the sac as the sac stiffness increases). The sac region is particularly identified on the spatiotemporal maps with a region of disruption in the wave propagation with multiple short-traveling forward/reflected waves, which is caused by the change in boundary conditions within the saccular region. The change in sac stiffness, however, is more pronounced on the wall displacement spatiotemporal maps compared to those of fluid velocity. We conclude that the existence of the sac can be identified based on the solid and fluid pulse waves, while the sac properties can also be estimated. This study demonstrates the initial findings in numerical simulations of FSI dynamics during arterial pulsations that can be used as reference for experimental and in vivo studies. Future studies are needed to demonstrate the feasibility of the method in identifying very mild sacs, which cannot be detected from medical imaging, where the material property degradation exists under early disease initiation.

A Characteristic-Based Constitutive Law for Dispersed Fibers

Biological tissues are typically constituted of dispersed fibers. Modeling the constitutive laws of such tissues remains a challenge. Direct integration over all fibers is considered to be accurate but requires very expensive numerical integration. A general structure tensor (GST) model was previously developed to bypass this costly numerical integration step, but there are concerns about the model's accuracy. Here we estimate the approximation error of the GST model. We further reveal that the GST model ignores strain energy induced by shearing motions. Subsequently, we propose a new characteristic-based constitutive law to better approximate the direct integration model. The new model is very cost-effective and closely approximates the "true" strain energy as calculated by the direct integration when stress-strain nonlinearity or fiber dispersion angle is small.

Prior Distributions of Material Parameters for Bayesian Calibration of Growth and Remodeling Computational Model of Abdominal Aortic Wall

For the accurate prediction of the vascular disease progression, there is a crucial need for developing a systematic tool aimed toward patient-specific modeling. Considering the interpatient variations, a prior distribution of model parameters has a strong influence on computational results for arterial mechanics. One crucial step toward patient-specific computational modeling is to identify parameters of prior distributions that reflect existing knowledge. In this paper, we present a new systematic method to estimate the prior distribution for the parameters of a constrained mixture model using previous biaxial tests of healthy abdominal aortas (AAs). We investigate the correlation between the estimated parameters for each constituent and the patient's age and gender; however, the results indicate that the parameters are correlated with age only. The parameters are classified into two groups: Group-I in which the parameters ce, ck1, ck2, cm2,Ghc, and ϕe are correlated with age, and Group-II in which the parameters cm1, Ghm, G1e, G2e, and α are not correlated with age. For the parameters in Group-I, we used regression associated with age via linear or inverse relations, in which their prior distributions provide conditional distributions with confidence intervals. For Group-II, the parameter estimated values were subjected to multiple transformations and chosen if the transformed data had a better fit to the normal distribution than the original. This information improves the prior distribution of a subject-specific model by specifying parameters that are correlated with age and their transformed distributions. Therefore, this study is a necessary first step in our group's approach toward a Bayesian calibration of an aortic model. The results from this study will be used as the prior information necessary for the initialization of Bayesian calibration of a computational model for future applications.

Ruptured hepatic artery aneurysm precipitated by gangrenous perforated appendicitis: a case report

Hepatic artery aneurysms are among the most common visceral artery aneurysms although still relatively rare. Repair of aneurysms >2 cm in diameter is important due to the high rate of rupture and associated mortality. Here, we present a case of a sudden rupture of a hepatic artery aneurysm after presentation with a perforated gangrenous appendicitis. There is increasing evidence that expansion and rupture of abdominal aortic aneurysm is related to degradation of elastin and collagen by matrix metalloproteinases (MMPs). Elastin degradation leads to expansion, while collagen degradation leads to rupture. The activity of MMPs has been shown to be upregulated by both sepsis and peritonitis. Here, we suggest that the inflammation from sepsis and peritonitis led to the activation and/or upregulation of MMPs, which precipitated aneurysm rupture via collagenase activity.

Multiple 18F-Fluorodeoxyglucose Positron Emission Tomography Scans Showing Progression of Abdominal Aortic Aneurysm: A Case Report

Although the precise mechanisms underlying the pathogenesis of abdominal aortic aneurysm (AAA) remain unclear, aortic wall inflammation has been implicated in AAA development. Several studies have reported the use of fluoro-deoxyglucose (F-FDG)/positron emission tomography (PET) to assess the nature of AAA.We present a case of 77-year-old Japanese male with juxta-anastomotic AAA who was followed up with multiple F-FDG-PET/CT scans over 7 years. The scans revealed chronological changes in aortic wall inflammation leading to progress and eventual rupture.This case supports a notion that aortic wall inflammation plays a role in AAA progression and rupture.

Stability analysis of a continuum-based constrained mixture model for vascular growth and remodeling

A stabilizing criterion is derived for equations governing vascular growth and remodeling. We start from the integral state equations of the continuum-based constrained mixture theory of vascular growth and remodeling and obtain a system of time-delayed differential equations describing vascular growth. By employing an exponential form of the constituent survival function, the delayed differential equations can be reduced to a nonlinear ODE system. We demonstrate the degeneracy of the linearized system about the homeostatic state, which is a fundamental cause of the neutral stability observations reported in prior studies. Due to this degeneracy, stability conclusions for the original nonlinear system cannot be directly inferred. To resolve this problem, a sub-system is constructed by recognizing a linear relation between two states. Subsequently, Lyapunov's indirect method is used to connect stability properties between the linearized system and the original nonlinear system, to rigorously establish the neutral stability properties of the original system. In particular, this analysis leads to a stability criterion for vascular expansion in terms of growth and remodeling kinetic parameters, geometric quantities and material properties. Numerical simulations were conducted to evaluate the theoretical stability criterion under broader conditions, as well as study the influence of key parameters and physical factors on growth properties. The theoretical results are also compared with prior numerical and experimental findings in the literature.

Biomechanical Rupture Risk Assessment: A Consistent and Objective Decision-Making Tool for Abdominal Aortic Aneurysm Patients

Abdominal aortic aneurysm (AAA) rupture is a local event in the aneurysm wall that naturally demands tools to assess the risk for local wall rupture. Consequently, global parameters like the maximum diameter and its expansion over time can only give very rough risk indications; therefore, they frequently fail to predict individual risk for AAA rupture. In contrast, the Biomechanical Rupture Risk Assessment (BRRA) method investigates the wall's risk for local rupture by quantitatively integrating many known AAA rupture risk factors like female sex, large relative expansion, intraluminal thrombus-related wall weakening, and high blood pressure. The BRRA method is almost 20 years old and has progressed considerably in recent years, it can now potentially enrich the diameter indication for AAA repair. The present paper reviews the current state of the BRRA method by summarizing its key underlying concepts (i.e., geometry modeling, biomechanical simulation, and result interpretation). Specifically, the validity of the underlying model assumptions is critically disused in relation to the intended simulation objective (i.e., a clinical AAA rupture risk assessment). Next, reported clinical BRRA validation studies are summarized, and their clinical relevance is reviewed. The BRRA method is a generic, biomechanics-based approach that provides several interfaces to incorporate information from different research disciplines. As an example, the final section of this review suggests integrating growth aspects to (potentially) further improve BRRA sensitivity and specificity. Despite the fact that no prospective validation studies are reported, a significant and still growing body of validation evidence suggests integrating the BRRA method into the clinical decision-making process (i.e., enriching diameter-based decision-making in AAA patient treatment).

Matricellular protein CCN3 mitigates abdominal aortic aneurysm

Abdominal aortic aneurysm (AAA) is a major cause of morbidity and mortality; however, the mechanisms that are involved in disease initiation and progression are incompletely understood. Extracellular matrix proteins play an integral role in modulating vascular homeostasis in health and disease. Here, we determined that the expression of the matricellular protein CCN3 is strongly reduced in rodent AAA models, including angiotensin II-induced AAA and elastase perfusion-stimulated AAA. CCN3 levels were also reduced in human AAA biopsies compared with those in controls. In murine models of induced AAA, germline deletion of Ccn3 resulted in severe phenotypes characterized by elastin fragmentation, vessel dilation, vascular inflammation, dissection, heightened ROS generation, and smooth muscle cell loss. Conversely, overexpression of CCN3 mitigated both elastase- and angiotensin II-induced AAA formation in mice. BM transplantation experiments suggested that the AAA phenotype of CCN3-deficient mice is intrinsic to the vasculature, as AAA was not exacerbated in WT animals that received CCN3-deficient BM and WT BM did not reduce AAA severity in CCN3-deficient mice. Genetic and pharmacological approaches implicated the ERK1/2 pathway as a critical regulator of CCN3-dependent AAA development. Together, these results demonstrate that CCN3 is a nodal regulator in AAA biology and identify CCN3 as a potential therapeutic target for vascular disease.

MECHANICAL CHARACTERIZATION OF ABDOMINAL AORTIC ANEURYSM WALL IN RAT MODEL TREATED BY MESENCHYMAL STEM CELLS

In this work, we study the mechanical properties of abdominal aortic aneurysms (AAAs) treated by cell therapy. Based on the xenograft model in rats, we analyze the effects of the injection of bone marrow mesenchymal stem cells (MSCs) on the stiffness of the arterial wall. Uniaxial tests performed on control, treated and untreated samples, have led to the identification of a nonlinear behavior law, using a mechanical model based on a stress-stretch exponential relation. The comparison of the mechanical behavior shows the benefits of the proposed cell therapy which improves the mechanical strength of the aneurysmal vessel wall. A histological study has shown the favorable change expression of elastin and collagen which are involved in the mechanical behavior of repaired arterial tissue. Thus, this work is part of MSCs biological understanding and it contributes to evaluate the approaches used in cell therapy and regenerative medicine to treat AAAs.

Positron Emission Tomography and Magnetic Resonance Imaging of Cellular Inflammation in Patients with Abdominal Aortic Aneurysms

Objectives

Inflammation is critical in the pathogenesis of abdominal aortic aneurysm (AAA) disease. Combined ¹⁸F-fludeoxyglucose (¹⁸F-FDG) positron emission tomography with computed tomography (PET-CT) and ultrasmall superparamagnetic particles of iron oxide (USPIO)-enhanced magnetic resonance imaging (MRI) are non-invasive methods of assessing tissue inflammation. The aim of this study was to compare these techniques in patients with AAA.

Materials and methods

Fifteen patients with asymptomatic AAA with diameter 46 ± 7 mm underwent PET-CT with ¹⁸F-FDG, and T2*-weighted MRI before and 24 hours after administration of USPIO. The PET-CT and MRI data were then co-registered. Standardised uptake values (SUVs) were calculated to measure ¹⁸F-FDG activity, and USPIO uptake was determined using the change in R2*. Comparisons between the techniques were made using a quadrant analysis and a voxel-by-voxel evaluation.

Results

When all areas of the aneurysm were evaluated, there was a modest correlation between the SUV on PET-CT and the change in R2* on USPIO-enhanced MRI (n = 70,345 voxels; r = .30; p < .0001). Although regions of increased ¹⁸F-FDG and USPIO uptake co-localised on occasion, this was infrequent (kappa statistic 0.074; 95% CI 0.026–0.122). ¹⁸F-FDG activity was commonly focused in the shoulder region whereas USPIO uptake was more apparent in the main body of the aneurysm. Maximum SUV was lower in patients with mural USPIO uptake.

Conclusions

Both ¹⁸F-FDG PET-CT and USPIO-MRI uptake identify vascular inflammation associated with AAA. Although they demonstrate a modest correlation, there are distinct differences in the pattern and distribution of uptake, suggesting a differential detection of macrophage glycolytic and phagocytic activity respectively.

(18)F-FDG PET scanning of abdominal aortic aneurysms and correlation with molecular characteristics: a systematic review

Purpose

The purpose of this study is to give an overview of studies investigating the role of fludeoxyglucose F18 (¹⁸F-FDG) positron emission tomography (PET) scanning in patients with aortic aneurysms with a focus on molecular characteristics of the aneurysm wall.

Methods

MEDLINE, EMBASE, and the Cochrane database were searched for relevant articles. After inclusion and exclusion, we selected 18 relevant articles reporting on ¹⁸F-FDG PET scanning of aortic aneurysms.

Results

The sample size of studies is limited, and there are no standardized imaging protocols and quantification methods. ¹⁸F-FDG PET scanning was shown to display molecular characteristics of the aortic wall. Different studies showed contradictory findings of aortic ¹⁸F-FDG uptake in aneurysm patients compared to controls.

Conclusions

Non-invasively determining molecular characteristics of aortic wall weakening might lead to better rupture and growth prediction. This might influence the decision of the surgeon between conservative and surgical treatment of aneurysms. To date, there is conflicted evidence regarding the use of ¹⁸F-FDG PET scanning to predict aneurysm rupture and growth. The role of ¹⁸F-FDG PET scanning in rupture risk prediction needs to be further investigated, and standardized imaging protocols and quantification methods need to be implemented.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-015-0153-8) contains supplementary material, which is available to authorized users.

Gene silencing of TACE enhances plaque stability and improves vascular remodeling in a rabbit model of atherosclerosis

We aimed to test the hypothesis that gene silencing of tumor necrosis factor alpha converting enzyme (TACE) may attenuate lesion inflammation and positive vascular remodeling and enhance plaque stability in a rabbit model of atherosclerosis. Lentivirus-mediated TACE shRNA was injected into the abdominal aortic plaques of rabbits which effectively down-regulated TACE expression and activities from week 8 to week 16. TACE gene silencing reduced remodeling index and plaque burden, and diminished the content of macrophages and lipids while increased that of smooth muscle cells and collagen in the aortic plaques. In addition, TACE gene silencing attenuated the local expression of P65, iNOS, ICAM-1, VEGF and Flt-1 and activities of MMP9 and MMP2 while increased the local expression of TGF-β1 together with reduced number of neovessels in the aorta. TACE shRNA treatment resulted in down-regulated expression of TACE in macrophages and blunted ERK-P38 phosphorylation and tube formation of co-cultured mouse vascular smooth muscle cells or human umbilical vein endothelial cells. In conclusion, gene silencing of TACE enhanced plaque stability and improved vascular positive remodeling. The mechanisms may involve attenuated local inflammation, neovascularization and MMP activation, as well as enhanced collagen production probably via down-regulated ERK-NF-κB and up-regulated TGF-β1 signaling pathways.

Endovascular grafts for abdominal aortic aneurysm

During the last two decades, endovascular technology has revolutionized the management of patients with abdominal aortic aneurysm (AAA). Today, endovascular aortic repair (EVAR) is the treatment of choice for the majority of patients with an AAA. Randomized controlled trials provide robust evidence for the indication of AAA repair and the rationale for the use of EVAR in selected patients. However, despite that, practice varies and several areas need further elucidation. Important future challenges and areas of research include the role of medical therapy in AAA, whether the indication for repair should be any different in women and in the elderly, and long-term follow-up of patients undergoing complex EVAR with adjuncts, both for elective treatment and for ruptured AAA. Continuous rapid technical and clinical development is to be expected. In this paper, we review the current practice and evidence of stenting in AAA.

Evidence of Cyclic Changes in the Metabolism of Abdominal Aortic Aneurysms During Growth Phases: ¹⁸F-FDG PET Sequential Observational Study

The rates of growth of medically treated abdominal aortic aneurysms (AAA) are difficult to determine, and relationships with parietal inflammation and with metabolic parameters from (18)F-FDG PET remain unclear. This (18)F-FDG PET sequential observational study was aimed at analyzing the metabolic changes accompanying the growth phases of medically treated AAA.

METHODS

Thirty-nine patients (37 men; age [mean ± SD], 71 ± 12 y) exhibiting small and medically treated AAA (maximal diameter, 46 ± 3 mm) underwent (18)F-FDG PET and CT angiography at baseline and 9 mo later. Clinical and imaging parameter correlates of the 9-mo increase in maximal diameter were investigated; these included (18)F-FDG maximal standardized uptake values (SUVmax) averaged for slices encompassing the AAA volume.

RESULTS

Of the 39 patients, 9 (23%) had a significant (≥2.5 mm) increase in maximal diameter at 9 mo, whereas the remaining 30 did not. The patients with an increase in maximal diameter at 9 mo exhibited lower SUVmax within the AAA at baseline than patients who did not have such an increase (1.80 ± 0.45 vs. 2.21 ± 0.52; P = 0.04); they also displayed a trend toward greater changes in SUVmax at 9 mo (difference between 9 mo and baseline: +0.40 ± 0.85 vs. -0.06 ± 0.57; P = 0.07). Similar levels were ultimately reached in both groups at 9 mo (2.20 ± 0.83 and 2.15 ± 0.66). SUVmax was a significant, yet modest, baseline predictor of the absolute change in maximal diameter during follow-up (P = 0.049).

CONCLUSION

The enhancement in the maximal diameter of small AAA was preceded by a stage with a low level of (18)F-FDG uptake, but this low level of uptake was no longer documented after the growth phases, suggesting a pattern of cyclic metabolic changes.

Structure-based constitutive model can accurately predict planar biaxial properties of aortic wall tissue

Structure-based constitutive models might help in exploring mechanisms by which arterial wall histology is linked to wall mechanics. This study aims to validate a recently proposed structure-based constitutive model. Specifically, the model's ability to predict mechanical biaxial response of porcine aortic tissue with predefined collagen structure was tested. Histological slices from porcine thoracic aorta wall (n=9) were automatically processed to quantify the collagen fiber organization, and mechanical testing identified the non-linear properties of the wall samples (n=18) over a wide range of biaxial stretches. Histological and mechanical experimental data were used to identify the model parameters of a recently proposed multi-scale constitutive description for arterial layers. The model predictive capability was tested with respect to interpolation and extrapolation. Collagen in the media was predominantly aligned in circumferential direction (planar von Mises distribution with concentration parameter bM=1.03 ± 0.23), and its coherence decreased gradually from the luminal to the abluminal tissue layers (inner media, b=1.54 ± 0.40; outer media, b=0.72 ± 0.20). In contrast, the collagen in the adventitia was aligned almost isotropically (bA=0.27 ± 0.11), and no features, such as families of coherent fibers, were identified. The applied constitutive model captured the aorta biaxial properties accurately (coefficient of determination R(2)=0.95 ± 0.03) over the entire range of biaxial deformations and with physically meaningful model parameters. Good predictive properties, well outside the parameter identification space, were observed (R(2)=0.92 ± 0.04). Multi-scale constitutive models equipped with realistic micro-histological data can predict macroscopic non-linear aorta wall properties. Collagen largely defines already low strain properties of media, which explains the origin of wall anisotropy seen at this strain level. The structure and mechanical properties of adventitia are well designed to protect the media from axial and circumferential overloads.

Association of polymorphisms of the receptor for advanced glycation end products gene and susceptibility to sporadic abdominal aortic aneurysm

Accumulating evidence has suggested that receptor for advanced glycation end products (RAGE) is involved in the development and progression of human abdominal aortic aneurysms (AAAs). However, the association between RAGE gene polymorphisms and AAA has not yet been determined. The present study was aimed at analyzing the potential association between the RAGE gene polymorphisms and AAAs. A cohort of 381 patients and 436 age-matched healthy controls were genotyped to detect the three RAGE polymorphisms (-374 T/A, -429 T/C, and G82S) using SNaPshot. Our study demonstrated a significant difference in the genotype and allele frequencies of the RAGE G82S polymorphism between the AAA patients and the controls. Further stratification by gender and smoking status revealed that the presence of the RAGE 82S allele confers a higher risk for developing AAA in men and smokers. Moreover, AAA patients with the variant 82S allele of RAGE presented with reduced serum soluble RAGE (sRAGE) production, and this decrease was more significant in men and smokers with AAA. Our study provides preliminary evidence that the 82S allele of RAGE is a risk factor for AAA. This new piece of knowledge regarding RAGE may be clinically important for the prevention and therapy of AAAs.

Vascular inflammation in a growing iliac artery aneurysm

We have used a multimodality imaging approach to assess the inflammatory component of a growing iliac artery aneurysm in a 53-year-old man who presented with related ureteral obstruction. Research suggests that episodic and heterogeneous inflammatory processes are important for the progression of aneurysms. The combined PET and MRI evaluation of inflammation that we present here is a novel approach to vascular imaging that is well suited for emerging hybrid PET/MRI systems.

In vivo strain assessment of the abdominal aortic aneurysm

The only criteria currently used to inform surgical decision for abdominal aortic aneurysms are maximum diameter (>5.5 cm) and rate of growth, even though several studies have identified the need for more specific indicators of risk. Patient-specific biomechanical variables likely to affect rupture risk would be a valuable addition to the science of understanding rupture risk and prove to be a life saving benefit for patients. Local deformability of the aorta is related to the local mechanical properties of the wall and may provide indication on the state of weakening of the wall tissue. We propose a 3D image-based approach to compute aortic wall strain maps in vivo. The method is applicable to a variety of imaging modalities that provide sequential images at different phases in the cardiac cycle. We applied the method to a series of abdominal aneurysms imaged using cine-MRI obtaining strain maps at different phases in the cardiac cycle. These maps could be used to evaluate the distensibility of an aneurysm at baseline and at different follow-up times and provide an additional index to clinicians to facilitate decisions on the best course of action for a specific patient.

Strain analysis of wall motion in abdominal aortic aneurysms

OBJECTIVE

In this exploratory study, we used ultrasound speckle-tracking methods, originally used for analyzing cardiac wall motion, to evaluate aortic wall motion.

MATERIALS AND METHODS

We compared 19 abdominal aortic aneurysm (AAA) patients with 10 healthy volunteers (diameter, 48 mm vs. 15 mm). Motion pictures of the axial view of the aneurysm using ultrasonography were analyzed. Circumferential strain and strain rate at 6 equally divided segments of the aorta were semiautomatically calculated. We termed 'peak' strain and strain rate as the maximum of strain and strain rate in a cardiac cycle for each segment. We also evaluated the coefficient of variation of peak strain rate for the six segments.

RESULTS

In the aneurysm and control groups, the mean values of peak strain along the 6 segments were 1.5% ± 0.6% vs. 4.7% ± 1.6% (p <0.0001), respectively. The coefficient of variation of the peak strain rate was higher in the AAA group (0.74 ± 0.20) than in the control group (0.56 ± 0.12; p <0.05).

CONCLUSIONS

Aortic wall compliance decreased in the more atherosclerotic AAA group. The higher relative dispersion of strain rates in the AAA group is indicative of the inhomogeneous movement of the aortic wall.

Regional changes in elastic fiber organization and transforming growth factor β signaling in aortas from a mouse model of marfan syndrome

In Marfan Syndrome (MFS), development of thoracic aortic aneurysms (TAAs) is characterized by degeneration of the medial layer of the aorta, including fragmentation and loss of elastic fibers, phenotypic changes in the smooth muscle cells, and an increase in the active form of transforming growth factor-β (TGFβ), which is thought to play a major role in development and progression of the aneurysm. We hypothesized that regional difference in elastic fiber fragmentation contributes to TGFβ activation and hence the localization of aneurysm formation. The fibrillin-1-deficient mgR/mgR mouse model of MFS was used to investigate regional changes in elastin fiber fragmentation, TGFβ activation and changes in gene expression as compared to wild-type littermates. Knockdown of Smad 2 and Smad 3 with shRNA was used to determine the role of the specific transcription factors in gene regulation in aortic smooth muscle cells. We show increased elastin fiber fragmentation in the regions associated with aneurysm formation and altered TGFβ signaling in these regions. Differential effects of Smad 2 and Smad 3 were observed in cultured smooth muscle cells by shRNA-mediated knockdown of expression of these transcription factors. Differential signaling through Smad 2 and Smad 3 in regions of active vascular remodeling likely contribute to aneurysm formation in the mgR/mgR model of MFS. Increased elastin fiber fragmentation in these regions is associated with these changes as compared to other regions of the thoracic aorta and may contribute to the changes in TGFβ signaling in these regions.

Leukocyte mimetic polysaccharide microparticles tracked in vivo on activated endothelium and in abdominal aortic aneurysm

We have developed injectable microparticles functionalized with fucoidan, in which sulfated groups mimic the anchor sites of P-selectin glycoprotein ligand-1 (PSGL-1), one of the principal receptors supporting leukocyte adhesion. These targeted microparticles were combined with a fluorescent dye and a T2(∗) magnetic resonance imaging (MRI) contrast agent, and then tracked in vivo with small animal imaging methods. Microparticles of 2.5μm were obtained by a water-in-oil emulsification combined with a cross-linking process of polysaccharide dextran, fluorescein isothiocyanate dextran, pullulan and fucoidan mixed with ultrasmall superparamagnetic particles of iron oxide. Fluorescent intravital microscopy observation revealed dynamic adsorption and a leukocyte-like behaviour of fucoidan-functionalized microparticles on a calcium ionophore induced an activated endothelial layer of a mouse mesentery vessel. We observed 20times more adherent microparticles on the activated endothelium area after the injection of functionalized microparticles compared to non-functionalized microparticles (197±11 vs. 10±2). This imaging tool was then applied to rats presenting an elastase perfusion model of abdominal aortic aneurysm (AAA) and 7.4T in vivo MRI was performed. Visual analysis of T2(∗)-weighted MR images showed a significant contrast enhancement on the inner wall of the aneurysm from 30min to 2h after the injection. Histological analysis of AAA cryosections revealed microparticles localized inside the aneurysm wall, in the same areas in which immunostaining shows P-selectin expression. The developed leukocyte mimetic imaging tool could therefore be relevant for molecular imaging of vascular diseases and for monitoring biologically active areas prone to rupture in AAA.

Monocyte activity is linked with abdominal aortic aneurysm diameter

BACKGROUND

Systemic inflammation and increased matrix metalloproteinase (MMP) cause elastin degradation leading to abdominal aortic aneurysm (AAA) expansion. Several prospective studies report that statin therapy can reduce AAA expansion through anti-inflammation. We hypothesize that monocyte activity plays a pivotal role in this AAA development and this study examines patient peripheral blood monocyte cell adhesion, transendothelial migration, and MMP concentrations between AAA and non-AAA patients.

MATERIALS AND METHODS

Peripheral blood was collected and monocytes isolated from control (n=15) and AAA (n=13) patients. Monocyte adhesion, transmigration, and permeability assays were assessed. Luminex assays determined MMP-9 and tissue inhibitor of metalloproteinase-4 (TIMP-4) concentrations from cell culture supernatant and patient serum.

RESULTS

AAA patient monocytes showed increased adhesion to the endothelium relative fluorescence units (RFU, 0.33±0.17) versus controls (RFU, 0.13±0.04; P=0.005). Monocyte transmigration was also increased in AAA patients (RFU, 0.33±0.11) compared with controls (RFU, 0.25±0.04, P=0.01). Greater numbers of adhesive (R2=0.66) and transmigratory (R2=0.86) monocytes were directly proportional to the AAA diameter. Significantly higher serum levels of MMP-9 (2149.14±947 pg/mL) were found in AAA patients compared with controls (1189.2±293; P=0.01). TIMP-4 concentrations were significantly lower in AAA patients (826.7±100 pg/mL) compared with controls (1233±222 pg/mL; P=0.02). Cell culture supernatant concentrations of MMP and TIMP from cocultures were higher than monocyte-only cultures.

CONCLUSIONS

Monocytes from AAA patients have greater adhesion and transmigration through the endothelium in vitro, leading to elevated MMP-9 levels and the appropriate decrease in TIMP-4 levels. The ability to modulate monocyte activity may lead to novel medical therapies to decrease AAA expansion.

Visceral adiposity is not associated with abdominal aortic aneurysm presence and growth

Previous studies in rodent models and patients suggest that visceral adipose could play a direct role in the development and progression of abdominal aortic aneurysm (AAA). This study aimed to assess the association of visceral adiposity with AAA presence and growth. This study was a case-control investigation of patients that did (n=196) and did not (n=181) have an AAA who presented to The Townsville Hospital vascular clinic between 2003 and 2012. Cases were patients with AAA (infra-renal aortic diameter >30 mm) and controls were patients with intermittent claudication but no AAA (infra-renal aortic diameter <30 mm). All patients underwent computed tomography angiography (CTA). The visceral to total abdominal adipose volume ratio was estimated from CTAs by assessing total and visceral adipose deposits using an imaging software program. Measurements were assessed for reproducibility by repeat assessments on 15 patients. AAA risk factors were recorded at entry. Forty-five cases underwent two CTAs more than 6 months apart to assess AAA expansion. The association of visceral adiposity with AAA presence and growth was examined using logistic regression. Visceral adipose assessment by CTA was highly reproducible (mean coefficient of variation 1.0%). AAA was positively associated with older age and negatively associated with diabetes. The visceral to total abdominal adipose volume ratio was not significantly associated with AAA after adjustment for other risk factors. Patients with a visceral to total abdominal adipose volume ratio in quartile four had a 1.63-fold increased risk of AAA but with wide confidence intervals (95% CI 0.71-3.70; p=0.248). Visceral adiposity was not associated with AAA growth. In conclusion, this study suggests that visceral adiposity is not specifically associated with AAA presence or growth although larger studies are required to confirm these findings.

Active smoking increases microsomal PGE2-synthase-1/PGE-receptor-4 axis in human abdominal aortic aneurysms

Background. The cyclooxygenase- (COX-) 2/microsomal PGE-synthase- (mPGES-) 1/PGE-receptor- (EP-) 4 axis could play a key role in the physiopathology of abdominal aortic aneurysm (AAA) in humans. In this study, we investigated the influence of cardiovascular risk factors on the expression of the PGE₂ pathway in human AAA. Methods. Aortic (n = 89) and plasma (n = 79) samples from patients who underwent AAA repair were collected. Patients were grouped according to risk factors. COX-isoenzymes, mPGES-1, EPs, α-actin, and CD45 and CD68 transcripts levels were quantified by QRT-PCR and plasma PGE₂ metabolites by EIA. Results. Current smoking (CS) patients compared to no-CS had significantly higher local levels of mPGES-1 (P = 0.009), EP-4 (P = 0.007), and PGE₂ metabolites plasma levels (P = 0.008). In the multiple linear regression analysis, these parameters remained significantly enhanced in CS after adding confounding factors. Results from association studies with cell type markers suggested that the increased mPGES-1/EP-4 levels were mainly associated with microvascular endothelial cells. Conclusions. This study shows that elements of the PGE₂ pathway, which play an important role in AAA development, are increased in CS. These results provide insight into the relevance of tobacco smoking in AAA development and reinforce the potential of mPGES-1 and EP-4 as targets for therapy in AAA patients.

Advancements in identifying biomechanical determinants for abdominal aortic aneurysm rupture

Abdominal aortic aneurysms are a common health problem and currently the need for surgical intervention is determined based on maximum diameter and growth rate criteria. Since these universal variables often fail to predict accurately every abdominal aortic aneurysms evolution, there is a considerable effort in the literature for other markers to be identified towards individualized rupture risk estimations and growth rate predictions. To this effort, biomechanical tools have been extensively used since abdominal aortic aneurysm rupture is in fact a material failure of the diseased arterial wall to compensate the stress acting on it. The peak wall stress, the role of the unique geometry of every individual abdominal aortic aneurysm as well as the mechanical properties and the local strength of the degenerated aneurysmal wall, all confer to rupture risk. In this review article, the assessment of these variables through mechanical testing, advanced imaging and computational modeling is reviewed and the clinical perspective is discussed.

Correlation of bacterial coinfection versus matrix metalloproteinase 9 and tissue inhibitor of metalloproteinase 1 expression in aortic aneurysm and atherosclerosis

BACKGROUND

We searched for any relationship between Chlamydophila pneumoniae, Mycoplasma pneumoniae, matrix metalloproteinase 9 (MMP-9), and tissue inhibitor of metalloproteinase 1 (TIMP-1) in aneurysmatic atherosclerotic lesions, and whether this relationship differed from that in atherosclerotic nonaneurysmatic lesions.

METHODS

Twenty-eight tissue samples paired by age and sex were grouped as follows: group 1 included 14 nonaneurysmal atherosclerotic fragments obtained from abdominal aortas collected from necropsies; group 2 included 14 aneurysmatic atherosclerotic aortic fragments obtained from patients during corrective surgery. Immunohistochemistry reactions were evaluated for C pneumoniae, M pneumoniae, MMP-9, and TIMP-1 antigens. Both groups were compared using the Mann-Whitney test, and the correlations among variables were obtained using the Spearman correlation test. P ≤ 0.05 was considered statistically significant.

RESULTS

C pneumoniae and M pneumoniae antigens were detected in 100% of cases. A higher amount of C pneumoniae (P = 0.005), M pneumoniae (P = 0.002), and MMP-9 (P = 0.021) was found in adventitia of group 2 with aneurysm. A positive correlation was found in the aneurysm group, as follows: intima C pneumoniae versus adventitia thickness (r = 0.70; P = 0.01), media C pneumoniae versus adventitia C pneumoniae (r = 0.75; P = 0.002), intima C pneumoniae versus media C pneumoniae (r = 0.8; P = 0.00), and adventitia C pneumoniae versus intima M pneumoniae (r = 0.54; P = 0.05); negative correlations were as follows: adventitia thickness and adventitia M pneumoniae (r = -0.65; P = 0.01), media MMP-9 and media thickness (r = -0.55; P = 0.04), TIMP-1 media versus adventitia C pneumoniae (r = -0.86; P = 0.00), and TIMP-1 media versus M pneumoniae intima (r = -0.67; P = 0.03). Nonaneurysmal atherosclerotic group 1 results are as follows: adventitia C pneumoniae versus TIMP-1 media (r = 0.75; P = 0.01) and media C pneumoniae and adventitia C pneumoniae (r = 0.59; P = 0.03).

CONCLUSIONS

The present work favors a role for coinfection of both M pneumoniae and C pneumoniae in the development of aortic atherosclerotic aneurysm, with increased adventitial inflammation, inhibition of TIMP-1 activity, and increased collagen degradation.