The association between venous structural alterations and biomechanical weakness in patients with abdominal aortic aneurysms

Increased diameter and stiffness of elastic but not muscular arteries in men with abdominal aortic aneurysm

It has been proposed that formation of abdominal aortic aneurysm (AAA) is part of a systemic arterial dilatative disease. However, arteries in the upper extremities are scarcely studied and it remains unclear whether both muscular and elastic arteries are affected by the proposed systemic arterial dilatation. The aim of this study was to investigate the diameter and stiffness of muscular and elastic arteries in arterial branches originating from the aortic arch. Twenty-six men with AAA (69 ± 4 yr) and 57 men without AAA (70 ± 5 yr) were included in the study. Ultrasound was used to examine the distal and proximal brachial artery, axillary artery, and common carotid artery (CCA), and measurement of diameter and diameter change was performed with wall-tracking software. Blood pressure measurements were used to calculate local arterial wall stiffness indices. The AAA cohort presented larger arterial diameters in the CCA and axillary artery after adjustment for body surface area (P = 0.002, respectively), whereas the brachial artery diameters were unchanged. Indices of increased stiffness in CCA (e.g., lower distensibility, P = 0.003) were seen in subjects with AAA after adjustments for body mass index and mean arterial blood pressure. This study supports the theory of a systemic arterial dilating diathesis in peripheral elastic, but not in muscular, arteries. Peripheral elastic arteries also exhibited increased stiffness, in analogy with findings in the aorta in AAA.NEW & NOTEWORTHY We present data partially supporting the notion of abdominal aortic aneurysm being a systemic vascular disease with focal manifestation in the abdominal aorta, from two well-defined groups recruited from a regional screening program. We show that elastic arteries distal from the aorta exhibit vascular alterations without aneurysmal formation in subjects with AAA compared with controls while muscular arteries seem unaffected.

Assessment of Upper Extremity Venous Compliance in Patients With Abdominal Aortic Aneurysms

OBJECTIVE

Abdominal aortic aneurysm (AAA) is associated with morphological and functional changes in both aneurysmal and non-aneurysmal arteries. However, it remains uncertain whether similar changes also exist in the venous vasculature. The aim of this study was to evaluate global venous function in patients with AAA and controls.

METHODS

This experimental study comprised 31 men with AAA (mean ± standard deviation age 70.0 ± 2.8 years) and 29 male controls (aged 70.6 ± 3.4 years). Venous occlusion plethysmography (VOP) was used to evaluate arm venous compliance at venous pressures between 10 and 60 mmHg in steps of 5 mmHg. Compensatory mobilisation of venous capacitance blood (capacitance response) was measured with a volumetric technique during experimental hypovolaemia induced by lower body negative pressure (LBNP).

RESULTS

The VOP induced pressure-volume curve was significantly less steep in patients with AAA (interaction, p < .001), indicating lower venous compliance. Accordingly, the corresponding pressure-compliance curves displayed reduced venous compliance at lower venous pressures in patients with AAA vs. controls (interaction, p < .001; AAA vs. control, p = .018). After adjusting for arterial hypertension, diabetes mellitus, hyperlipidaemia, chronic obstructive pulmonary disease, and smoking, VOP detected differences in venous compliance remained significant at low venous pressures, that is, at 10 mmHg (p = .008), 15 mmHg (p = .013), and 20 mmHg (p = .026). Mean venous compliance was negatively correlated with aortic diameter (r = -.332, p = .010). Mobilisation of venous capacitance response during LBNP was reduced by approximately 25% in patients with AAA (p = .030), and the redistribution of venous blood during LBNP was negatively correlated with aortic diameter (r = -.417, p = .007).

CONCLUSION

Men with AAA demonstrated reduced venous compliance and, as a result, a lesser capacity to mobilise peripheral venous blood to the central circulation during hypovolaemic stress. These findings imply that the AAA disease may be accompanied by functional changes in the venous vascular wall.

MicroRNAs and Current Concepts on the Pathogenesis of Abdominal Aortic Aneurysm

OBJECTIVE

Abdominal aortic aneurysm is an important cause of morbidity and mortality in the elderly. Currently, the only way to prevent rupture and death related to abdominal aortic aneurysms is through surgical intervention. Endovascular treatment is associated with less morbidity than conventional treatment. The formation of an aneurysm is a complex multifactorial process, involving destructive remodeling of the connective tissue around the affected segment of the aorta wall. MicroRNAs are small sequences of non-coding RNAs that control diverse cellular functions by promoting degradation or inhibition of translation of specific mRNAs. A profile aberrant expression of miRNAs has been linked to human diseases, including cardiovascular dysfunction.

Peripheral veins: influence of gender, body mass index, age and varicose veins on cross-sectional area

To investigate changes in the size of the deep and superficial venous systems associated with gender, age, body mass index and varicose veins, changes to the cross-sectional area of the femoral and the long saphenous veins were analysed in the Duesseldorf=Essen civil servant study population. Between December 1989 and July 1993 a total of 9935 employees were recruited; 9261 were then evaluated for this analysis. Diameters of the long saphenous and femoral veins were determined 2-3 cm distal to the confluence in lying (after 15 min rest) and standing (after 5 min) positions. Cross-sectional areas (CSA) were calculated. A total of 63% of all people were assigned to CEAP (clinical, etiological, anatomical pathophysiological) class 0, 27% to class 1, 8.5% to class 2, while 1.5% belonged to higher CEAP classes. In people without varicose veins (CEAP class 0) the CSA of the femoral and long saphenous veins were smaller in females than in males. In people with a normal body mass index (BMI) (20-25) the mean CSA of the femoral and long saphenous veins in a standing position was similar from the third up to the sixth decade of life. The volume increase due to a standing position expressed as the absolute increase in CSA of the femoral and long saphenous veins was not age-related, either. The relative volume increase expressed as a ratio remained unchanged with age. There was a strong relationship between the CSA of both veins and increasing BMI. In a lying position, the CSA of the femoral and long saphenous veins increased only slightly with increasing CEAP classes. In a standing position, the CSA of both veins increased even in CEAP class 1 (p < 0.001). In a stepwise multivariate regression analysis, the CSA of both veins in a standing position was not age-related but associated with BMI, CEAP classes and gender. The absolute increase in CSA was influenced by all four variables, but BMI and gender were most important. In Conclusion, this study shows that aging is not necessarily associated with an increase in venous CSA of the deep and superficial venous system. BMI is the most important determinant for an increase in CSA in standing position. Varicosity of the superficial venous system is always associated with similar changes in the deep venous system.

Pathophysiology and epidemiology of abdominal aortic aneurysms

Abdominal aortic aneurysms (AAAs) are found in up to 8% of men aged >65 years, yet usually remain asymptomatic until they rupture. Rupture of an AAA and its associated catastrophic physiological insult carries overall mortality in excess of 80%, and 2% of all deaths are AAA-related. Pathologically, AAAs are associated with inflammation, smooth muscle cell apoptosis, and matrix degradation. Once thought to be a consequence of advanced atherosclerosis, accruing evidence indicates that AAAs are a focal representation of a systemic disease of the vasculature. Risk factors for AAAs include increasing age, male sex, smoking, and low HDL-cholesterol levels. Familial associations exist and although susceptibility genes have been described on the basis of candidate-gene studies, robust genetic studies have failed to discover causative gene mutations. The surgical management of AAAs has been revolutionized by minimally invasive endovascular repair. Ongoing randomized trials will establish whether endovascular repair confers a survival advantage over open surgery for patients with a ruptured AAA. In many countries, centralization of vascular surgical services has largely been driven by the improved outcomes of elective aneurysm surgery in specialized centers, the widespread adoption of endovascular techniques, and the introduction of screening programs.

A zipper network model of the failure mechanics of extracellular matrices

Mechanical failure of soft tissues is characteristic of life-threatening diseases, including capillary stress failure, pulmonary emphysema, and vessel wall aneurysms. Failure occurs when mechanical forces are sufficiently high to rupture the enzymatically weakened extracellular matrix (ECM). Elastin, an important structural ECM protein, is known to stretch beyond 200% strain before failing. However, ECM constructs and native vessel walls composed primarily of elastin and proteoglycans (PGs) have been found to fail at much lower strains. In this study, we hypothesized that PGs significantly contribute to tissue failure. To test this, we developed a zipper network model (ZNM), in which springs representing elastin are organized into long wavy fibers in a zipper-like formation and placed within a network of springs mimicking PGs. Elastin and PG springs possessed distinct mechanical and failure properties. Simulations using the ZNM showed that the failure of PGs alone reduces the global failure strain of the ECM well below that of elastin, and hence, digestion of elastin does not influence the failure strain. Network analysis suggested that whereas PGs drive the failure process and define the failure strain, elastin determines the peak and failure stresses. Predictions of the ZNM were experimentally confirmed by measuring the failure properties of engineered elastin-rich ECM constructs before and after digestion with trypsin, which cleaves the core protein of PGs without affecting elastin. This study reveals a role for PGs in the failure properties of engineered and native ECM with implications for the design of engineered tissues.

Review of Current Theories for Abdominal Aortic Aneurysm Pathogenesis

Atherosclerotic plaques are a feature of abdominal aortic aneurysms (AAAs). Atherosclerosis and AAA appear to share similar risk factors. These observations have led to the conclusion that AAAs are a consequence of advanced atherosclerosis.

This review explores current theories regarding the pathogenesis of AAA and their implications for treatment.

A systematic literature search was conducted using the search terms abdominal aortic aneurysm, atherosclerosis, pathogenesis, and systemic disease. Articles were categorized according to the association of AAAs with atherosclerosis, arteriomegaly, peripheral aneurysm, systemic expression, genetics, autoimmunity, oxidative stress, and systemic disease. Twenty-nine articles reporting changes in the systemic vasculature associated with AAA and 12 articles examining the shared risk factor hypothesis were identified.

There is insufficient evidence to confirm that AAAs are the result of advanced atherosclerosis. The bulk of evidence points to AAA disease being a systemic disease of the vasculature, with a predetermined genetic susceptibility leading to a phenotype governed by environmental factors.

Mechanical and failure properties of extracellular matrix sheets as a function of structural protein composition

The goal of this study was to determine how alterations in protein composition of the extracellular matrix (ECM) affect its functional properties. To achieve this, we investigated the changes in the mechanical and failure properties of ECM sheets generated by neonatal rat aortic smooth muscle cells engineered to contain varying amounts of collagen and elastin. Samples underwent static and dynamic mechanical measurements before, during, and after 30 min of elastase digestion followed by a failure test. Microscopic imaging was used to measure thickness at two strain levels to estimate the true stress and moduli in the ECM sheets. We found that adding collagen to the ECM increased the stiffness. However, further increasing collagen content altered matrix organization with a subsequent decrease in the failure strain. We also introduced collagen-related percolation in a nonlinear elastic network model to interpret these results. Additionally, linear elastic moduli correlated with failure stress which may allow the in vivo estimation of the stress tolerance of ECM. We conclude that, in engineered replacement tissues, there is a tradeoff between improved mechanical properties and decreased extensibility, which can impact their effectiveness and how well they match the mechanical properties of native tissue.

Matrix metalloproteinase-2: the forgotten enzyme in aneurysm pathogenesis

The pathogenesis of abdominal aortic aneurysm (AAAs) involves progressive cycles of proteolysis and inflammation, the product of proteolysis driving subsequent inflammation. Little is yet known about the initiating events. We review the specific literature examining the possibility that MMP-2 may be the initial catalyst. Histologically, elastolysis is one of the earliest observable events in aneurysm genesis. Matrix metalloproteinase-2 (MMP-2), as the dominant gelatinase differentially expressed in aneurysmal tissue and cells derived from aneurysms, would be a good candidate. We report the results of in vivo and in vitro experiments, which lend support to the importance of MMP-2 as an aneurysmal initiator.

Effects of elastase on the mechanical and failure properties of engineered elastin-rich matrices

Pulmonary emphysema and vessel wall aneurysms are diseases characterized by elastolytic damage to elastin fibers that leads to mechanical failure. To model this, neonatal rat aortic smooth muscle cells were cultured, accumulating an extracellular matrix rich in elastin, and mechanical measurements were made before and during enzymatic digestion of elastin. Specifically, the cells in the cultures were killed with sodium azide, the cultures were lifted from the flask, cut into small strips, and fixed to a computer-controlled lever arm and a force transducer. The strips were subjected to a broadband displacement signal to study the dynamic mechanical properties of the samples. Also, quasi-static stress-strain curves were measured. The dynamic data were fit to a linear viscoelastic model to estimate the tissues' loss (G) and storage (H) modulus coefficients, which were evaluated before and during 30 min of elastase treatment, at which point a failure test was performed. G and H decreased significantly to 30% of their baseline values after 30 min. The failure stress of control samples was approximately 15 times higher than that of the digested samples. Understanding the structure-function relationship of elastin networks and the effects of elastolytic injury on their mechanical properties can lead to the elucidation of the mechanism of elastin fiber failure and evaluation of possible treatments to enhance repair in diseases involving elastolytic injury.