Mechanical properties of the aneurysmal aorta

Neurologic complications of diseases of the aorta

Neurologic complications of diseases of the aorta are common, as the brain and spinal cord function is highly dependent on the aorta and its branches for blood supply. Any disease impacting the aorta may have significant impact on the ability to deliver oxygenated blood to the central nervous system, resulting in ischemia-and if prolonged-cerebral and spinal infarct. The breadth of pathology affecting the aorta is diverse and neurologic complications can vary dramatically based on the location, severity, and underlying etiology. This chapter outlines the major pathology of the aorta while highlighting the associated neurologic complications. This chapter covers the entire spectrum of neurologic complications associated with aortic disease by beginning with a detailed overview of the spinal cord vascular anatomy followed by a discussion of the most common aortic pathologies affecting the nervous system, including aortic aneurysm, aortic dissection, aortic atherosclerosis, inflammatory and infectious aortopathies, congenital abnormalities, and aortic surgery.

The role of tissue remodeling in mechanics and pathogenesis of abdominal aortic aneurysms

Arterial walls can be regarded as composite materials consisting of collagen fibers embedded in an elastic matrix and smooth muscle cells. Remodeling of the structural proteins has been shown to play a significant role in the mechanical behavior of walls during pathogenesis of abdominal aortic aneurysms (AAA). In this study, we systematically studied the change in the microstructure, histology and mechanics to link them to AAA disease progression. We performed biaxial extension tests, second-harmonic generation imaging and histology on 15 samples from the anterior part of AAA walls harvested during open aneurysm surgery. Structural data were gained by fitting to a bivariate von Mises distribution and yielded the mean fiber direction and in- and out-of-plane fiber dispersions of collagen. Mechanical and structural data were fitted to a recently proposed material model. Additionally, the mechanical data were used to derive collagen recruitment points in the obtained stress-stretch curves. We derived 14 parameters from histology such as smooth muscle cell-, elastin-, and abluminal adipocyte content. In total, 22 parameters were obtained and statistically evaluated. Based on the collagen recruitment points we were able to define three different stages of disease progression. Significant differences in elastin content, collagen orientation and adipocyte contents were discovered. Nerves entrapped inside AAA walls pointed towards a significant deposition of newly formed collagen abluminally, which we propose as neo-adventitia formation. We were able to discriminate two types of remodeled walls with a high collagen content - potentially safe and possibly vulnerable walls with a high adipocyte content inside the wall and significant amounts of inflammation. The study yielded a hypothesis for disease progression, derived from the systematic comparison of mechanical, microstructural and histological changes in AAAs. STATEMENT OF SIGNIFICANCE: Remodeling of the structural proteins plays an important role in the mechanical behavior of walls during pathogenesis of abdominal aortic aneurysms (AAA). We analyzed changes in the microstructure, histology and biomechanics of 15 samples from the anterior part of AAA walls and, for the first time, linked the results to three different stages of disease progression. We identified significant differences in elastin content, collagen orientation, adipocyte contents, and also a deposition of newly formed collagen forming a neoadventitia. We could discriminate two types of remodeled walls: (i) potentially safe and (ii) possibly vulnerable associated with inflammation and a high amount of adipocytes.

Association of carotid atherosclerosis and stiffness with abdominal aortic aneurysm: The atherosclerosis risk in communities (ARIC) study

BACKGROUND AND AIMS

Individuals with atherosclerosis and stiffness often have increased abdominal aortic diameters, but prospective evidence linking them to the risk of abdominal aortic aneurysm (AAA) is limited.

METHODS

We prospectively examined the relationship of carotid atherosclerosis and stiffness with future risk of AAA in ARIC. At Visits 1 (1987-89) or 2 (1990-1992), we assessed carotid atherosclerosis (represented by greater carotid intima-media thickness [cIMT] or presence of atherosclerotic plaque) and lower carotid distensibility (reflected by a higher carotid Beta Index). We identified incident, clinical AAAs during follow-up through 2011 using hospital discharge codes, Medicare outpatient diagnoses, or death certificates.

RESULTS

Participants' mean age at baseline was 54.2 years (SD 5.8), 45% were male and 73% white. During a median of 22.5 years of follow-up, 542 clinical AAAs were ascertained. After multivariable adjustment, the presence of carotid atherosclerotic plaque at baseline was associated with 1.31 (95% CI: 1.10-1.57; p = 0.003) times higher risk of clinical AAA. Greater cIMT and Beta Index were also associated with clinical AAA with a dose-response across quartiles (p trend for both: 0.006; hazard ratios [95% CI] for the highest vs. lowest quartiles: 1.55 [1.13-2.11] and 1.68 [1.16-2.43], respectively). The associations of cIMT and Beta Index with AAA were independent of each other.

CONCLUSIONS

This prospective population-based study found that indices of greater carotid atherosclerosis and lower carotid distensibility are markers of increased AAA risk.

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.

Empirically Determined Vascular Smooth Muscle Cell Mechano-Adaptation Law

Cardiovascular disease can alter the mechanical environment of the vascular system, leading to mechano-adaptive growth and remodeling. Predictive models of arterial mechano-adaptation could improve patient treatments and outcomes in cardiovascular disease. Vessel-scale mechano-adaptation includes remodeling of both the cells and extracellular matrix. Here, we aimed to experimentally measure and characterize a phenomenological mechano-adaptation law for vascular smooth muscle cells (VSMCs) within an artery. To do this, we developed a highly controlled and reproducible system for applying a chronic step-change in strain to individual VSMCs with in vivo like architecture and tracked the temporal cellular stress evolution. We found that a simple linear growth law was able to capture the dynamic stress evolution of VSMCs in response to this mechanical perturbation. These results provide an initial framework for development of clinically relevant models of vascular remodeling that include VSMC adaptation.

Abdominal aortic aneurysms and diabetes mellitus

There is accumulating evidence that risk profiles differ between coronary artery disease and abdominal aortic aneurysms (AAAs). However, diabetes mellitus (DM) appears to be negatively associated with AAA formation. The underlying mechanisms for this negative relationship are far from defined, but may include: increased arterial wall matrix formation via advanced glycation end products; suppression of plasmin and reduction of levels and activity of matrix metalloproteinases (MMP)-2 and 9; diminished aortic wall macrophage infiltration, elastolysis and neovascularization. In addition, the effect of pharmacological agents used for the treatment of patients with DM on AAA formation has been studied with rather controversial results. Statins, angiotensin converting enzyme inhibitors, angiotensin receptor blockers, fenofibrate, antibiotics and some hypoglycemic agents are beginning to be appreciated for a potential modest protection from AAAs, but further studies are needed.

The pathophysiologic basis of abdominal aortic aneurysm progression: a critical appraisal

An aneurysm of the abdominal aorta is a common pathology and a major cause of sudden death in the elderly. Currently, abdominal aortic aneurysms (AAAs) can only be treated by surgery and an effective medical therapy is urgently missing. The pathophysiology of AAAs is complex and is believed to be best described as a comprehensive inflammatory response with an accompanying proteolytic imbalance; the latter being held responsible for the progressive weakening of the aortic wall. Remarkably, while interference in inflammatory and/or proteolytic cascades proves highly effective in preclinical studies, emerging clinical studies consistently fail to show a benefit. In fact, some anti-inflammatory interventions appear to adversely influence the disease process. Altogether, recent clinical observations not only challenge the prevailing concepts of AAA progression, but also raise doubt on the translatability of findings from rodent models for growing AAA.

Segmental aortic stiffening contributes to experimental abdominal aortic aneurysm development

BACKGROUND

Stiffening of the aortic wall is a phenomenon consistently observed in age and in abdominal aortic aneurysm (AAA). However, its role in AAA pathophysiology is largely undefined.

METHODS AND RESULTS

Using an established murine elastase-induced AAA model, we demonstrate that segmental aortic stiffening precedes aneurysm growth. Finite-element analysis reveals that early stiffening of the aneurysm-prone aortic segment leads to axial (longitudinal) wall stress generated by cyclic (systolic) tethering of adjacent, more compliant wall segments. Interventional stiffening of AAA-adjacent aortic segments (via external application of surgical adhesive) significantly reduces aneurysm growth. These changes correlate with the reduced segmental stiffness of the AAA-prone aorta (attributable to equalized stiffness in adjacent segments), reduced axial wall stress, decreased production of reactive oxygen species, attenuated elastin breakdown, and decreased expression of inflammatory cytokines and macrophage infiltration, and attenuated apoptosis within the aortic wall, as well. Cyclic pressurization of segmentally stiffened aortic segments ex vivo increases the expression of genes related to inflammation and extracellular matrix remodeling. Finally, human ultrasound studies reveal that aging, a significant AAA risk factor, is accompanied by segmental infrarenal aortic stiffening.

CONCLUSIONS

The present study introduces the novel concept of segmental aortic stiffening as an early pathomechanism generating aortic wall stress and triggering aneurysmal growth, thereby delineating potential underlying molecular mechanisms and therapeutic targets. In addition, monitoring segmental aortic stiffening may aid the identification of patients at risk for AAA.

Age-related vascular stiffening: causes and consequences

Arterial stiffening occurs with age and is closely associated with the progression of cardiovascular disease. Stiffening is most often studied at the level of the whole vessel because increased stiffness of the large arteries can impose increased strain on the heart leading to heart failure. Interestingly, however, recent evidence suggests that the impact of increased vessel stiffening extends beyond the tissue scale and can also have deleterious microscale effects on cellular function. Altered extracellular matrix (ECM) architecture has been recognized as a key component of the pre-atherogenic state. Here, the underlying causes of age-related vessel stiffening are discussed, focusing on age-related crosslinking of the ECM proteins as well as through increased matrix deposition. Methods to measure vessel stiffening at both the macro- and microscale are described, spanning from the pulse wave velocity measurements performed clinically to microscale measurements performed largely in research laboratories. Additionally, recent work investigating how arterial stiffness and the changes in the ECM associated with stiffening contributed to endothelial dysfunction will be reviewed. We will highlight how changes in ECM protein composition contribute to atherosclerosis in the vessel wall. Lastly, we will discuss very recent work that demonstrates endothelial cells (ECs) are mechano-sensitive to arterial stiffening, where changes in stiffness can directly impact EC health. Overall, recent studies suggest that stiffening is an important clinical target not only because of potential deleterious effects on the heart but also because it promotes cellular level dysfunction in the vessel wall, contributing to a pathological atherosclerotic state.

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.

Functional and molecular imaging techniques in aortic aneurysm disease

PURPOSE OF REVIEW

Functional and molecular aortic imaging has shown great promise for evaluation of aortic disease, and may soon augment conventional assessment of aortic dimensions for the clinical management of patients.

RECENT FINDINGS

A range of imaging techniques is available for evaluation of patients with aortic disease. Magnetic resonance blood flow imaging can identify atherosclerosis prone aortic regions and may be useful for predicting aneurysm growth. Computational modeling can demonstrate significant differences in wall stress between abdominal aortic aneurysms of similar size and may better predict rupture than diameter alone. Metabolic imaging with fluorodeoxyglucose-PET [(FDG)-PET] can identify focal aortic wall inflammation that may portend rapid progression of disease. Molecular imaging with probes that target collagen and elastin can directly exhibit changes in the vessel wall associated with disease.

SUMMARY

The complexity of aortic disease is more fully revealed with new functional imaging techniques than with conventional anatomic analysis alone. This may better inform surveillance imaging regimens, medical management and decisions regarding early intervention for aortic disease.

Hemodynamic regulation of reactive oxygen species: implications for vascular diseases

SIGNIFICANCE

Arterial blood vessels functionally and structurally adapt to altering hemodynamic forces in order to accommodate changing needs and to provide stress homeostasis. This ability is achieved at the cellular level by converting mechanical stimulation into biochemical signals (i.e., mechanotransduction). Physiological mechanical stress helps maintain vascular structure and function, whereas pathologic or aberrant stress may impair cellular mechano-signaling, and initiate or augment cellular processes that drive disease.

RECENT ADVANCES

Reactive oxygen species (ROS) may represent an intriguing class of mechanically regulated second messengers. Chronically enhanced ROS generation may be induced by adverse mechanical stresses, and is associated with a multitude of vascular diseases. Although a causal relationship has clearly been demonstrated in large numbers of animal studies, an effective ROS-modulating therapy still remains to be established by clinical studies.

CRITICAL ISSUES AND FUTURE DIRECTIONS

This review article focuses on the role of various mechanical forces (in the form of laminar shear stress, oscillatory shear stress, or cyclic stretch) as modulators of ROS-driven signaling, and their subsequent effects on vascular biology and homeostasis, as well as on specific diseases such as arteriosclerosis, hypertension, and abdominal aortic aneurysms. Specifically, it highlights the significance of the various NADPH oxidase (NOX) isoforms as critical ROS generators in the vasculature. Directed targeting of defined components in the complex network of ROS (mechano-)signaling may represent a key for successful translation of experimental findings into clinical practice.

Elastin and collagen fibre microstructure of the human aorta in ageing and disease: a review

Aortic disease is a significant cause of death in developed countries. The most common forms of aortic disease are aneurysm, dissection, atherosclerotic occlusion and ageing-induced stiffening. The microstructure of the aortic tissue has been studied with great interest, because alteration of the quantity and/or architecture of the connective fibres (elastin and collagen) within the aortic wall, which directly imparts elasticity and strength, can lead to the mechanical and functional changes associated with these conditions. This review article summarizes the state of the art with respect to characterization of connective fibre microstructure in the wall of the human aorta in ageing and disease, with emphasis on the ascending thoracic aorta and abdominal aorta where the most common forms of aortic disease tend to occur.

Spatial orientation of collagen fibers in the abdominal aortic aneurysm's wall and its relation to wall mechanics

Collagen is the most abundant protein in mammals and provides the abdominal aortic aneurysm (AAA) wall with mechanical strength, stiffness and toughness. Specifically, the spatial orientation of collagen fibers in the wall has a major impact on its mechanical properties. Apart from valuable microhistological information, this data can be integrated by histomechanical constitutive models thought to improve biomechanical simulations, i.e. to improve the biomechanical rupture risk assessment of AAAs. Tissue samples (n = 24) from the AAA wall were harvested during elective AAA repair, fixated, embedded, sectioned and investigated by polarized light microscopy. The birefringent properties of collagen were reinforced by picrosirius red staining and the three-dimensional collagen fiber orientations were identified with a universal rotary stage. Two constitutive models for collagen fibers were used to integrate the identified structural information in a macroscopic AAA wall model. The collagen fiber orientation in the AAA wall was widely dispersed and could be captured by a Bingham distribution function (κ(1) = 11.6, κ(2) = 9.7). The dispersion was much larger in the tangential plane than in the cross-sectional plane, and no significant difference between the medial and adventitial layers could be identified. The layered directional organization of collagen in normal aortas was not evident in the AAA. The collagen organization identified, combined with constitutive descriptions of collagen fibers that depend on its orientation, explain the anisotropic (orthotropic) mechanical properties of the AAA wall. The mechanical properties of collagen fibers depend largely on their undulation, which is an important structural parameter that requires further experimental investigation.

Influence of aneurysm wall stiffness and the presence of intraluminal thrombus on the wall movement of an aneurysm - an in vitro study

The purpose of this in vitro study was to investigate the influence of aneurysm wall stiffness and of the presence of intraluminal thrombus (ILT) on aneurysm wall movement. Three latex aneurysms were used with different wall stiffness. The aneurysms, equipped with 20 tantalum markers, were attached to an in vitro circulation model. Fluoroscopic roentgenographic stereo photogrammetric analysis was used to measure marker movement during six cardiac cycles at three different systemic pressures. To investigate the influence of ILT on wall movement, we repeated the same experiment with one of the aneurysms. The aneurysm sac was then filled with one of two E-moduli differing thrombus analogues (Novalyse 8 and 20) or with perfusate as a control. It was noted that the amplitude of the wall movement (mm) increased significantly (P < 0.05) as the compliance of the wall increased. The mean amplitude of the wall movement decreased (P < 0.05) as the stiffness (E-modulus) of the ILT increased. In conclusion, ILT has a 'cushioning effect'. Wall movement (and theoretically wall stress) diminishes when the stiffness of the ILT increases. Compliance of the aneurysm wall influences wall movement. When the stiffness of the wall increases, the wall movement diminishes.

Mechanics, mechanobiology, and modeling of human abdominal aorta and aneurysms

Biomechanical factors play fundamental roles in the natural history of abdominal aortic aneurysms (AAAs) and their responses to treatment. Advances during the past two decades have increased our understanding of the mechanics and biology of the human abdominal aorta and AAAs, yet there remains a pressing need for considerable new data and resulting patient-specific computational models that can better describe the current status of a lesion and better predict the evolution of lesion geometry, composition, and material properties and thereby improve interventional planning. In this paper, we briefly review data on the structure and function of the human abdominal aorta and aneurysmal wall, past models of the mechanics, and recent growth and remodeling models. We conclude by identifying open problems that we hope will motivate studies to improve our computational modeling and thus general understanding of AAAs.

Pulse wave imaging of normal and aneurysmal abdominal aortas in vivo

The abdominal aortic aneurysm (AAA) is a common vascular disease. The current clinical criterion for treating AAAs is an increased diameter above a critical value. However, the maximum diameter does not correlate well with aortic rupture, the main cause of death from AAA disease. AAA disease leads to changes in the aortic wall mechanical properties. The pulse-wave velocity (PWV) may indicate such a change. Because of limitations in temporal and spatial resolution, the widely used foot-to-foot method measures the global, instead of regional, PWV between two points at a certain distance in the circulation. However, mechanical properties are nonuniform along the normal and pathological (e.g., the AAA and atherosclerosis) arteries; thus, such changes are typically regional. Pulse-wave imaging (PWI) has been developed by our group to map the pulse-wave propagation along the abdominal aorta in mice in vivo. By using a retrospective electrocardiogram (ECG) gating technique, the radio-frequency (RF) signals over one cardiac cycle were obtained in murine aortas at the extremely high frame rate of 8 kHz and with a field-of-view (FOV) of 12 x 12 mm(2). The velocities of the aortic wall were estimated using an RF-based speckle tracking method. An Angiotensin II (AngII) infusion-based AAA model was used to simulate the human AAA case. Sequences of wall velocity images can noninvasively and quantitatively map the propagation of the pulse wave along the aortic wall. In the normal and sham aortas, the propagation of the pulse wave was relatively uniform along the wall, while in the AngII-treated aortas, the propagation was shown to be nonuniform. There was no significant difference ( p > 0.05) in the PWV between sham (4.67 +/- 1.15 m/s, n=5) and AngII-treated (4.34 +/- 1.48 m/s, n=17) aortas. The correlation coefficient of the linear regression was significantly higher ( p < 0.005) in the sham aortas (0.89 +/- 0.03, n=5 ) than in the AngII-treated ones (0.61 +/- 0.15, n=17). The wall velocities induced by the pulse wave were lower and the pulse wave moved nonuniformly along the AngII-treated aorta ( p < 0.005), with the lowest velocities at the aneurysmal regions. The discrepancy in the regional wall velocity and the nonuniform pulse-wave propagation along the AngII-treated aorta indicated the inhomogeneities in the aortic wall properties, and the reduced wall velocities indicated stiffening of the aneurysmal wall. This novel technique may thus constitute an early detection tool of vascular degeneration as well as serve as a suitable predictor of AAA rupture, complementary to the current clinical screening practice.

Intracranial and abdominal aortic aneurysms: similarities, differences, and need for a new class of computational models

Intracranial saccular and abdominal aortic aneurysms (ISAs and AAAs, respectively) result from different underlying disease processes and exhibit different rupture potentials, yet they share many histopathological and biomechanical characteristics. Moreover, as in other vascular diseases, hemodynamics and wall mechanics play important roles in the natural history and possible treatment of these two types of lesions. The goals of this review are twofold: first, to contrast the biology and mechanics of intracranial and abdominal aortic aneurysms to emphasize that separate advances in our understanding of each disease can aid in our understanding of the other disease, and second, to suggest that research on the biomechanics of aneurysms must embrace a new paradigm for analysis. That is, past biomechanical studies have provided tremendous insight but have progressed along separate lines, focusing on either the hemodynamics or the wall mechanics. We submit that there is a pressing need to couple in a new way the separate advances in vascular biology, medical imaging, and computational biofluid and biosolid mechanics to understand better the mechanobiology, pathophysiology, and treatment of these lesions, which continue to be responsible for significant morbidity and mortality. We refer to this needed new class of computational tools as fluid-solid-growth (FSG) models.

A novel noninvasive technique for pulse-wave imaging and characterization of clinically-significant vascular mechanical properties in vivo

The pulse-wave velocity (PWV) has been used as an indicator of vascular stiffness, which can be an early predictor of cardiovascular mortality. A noninvasive, easily applicable method for detecting the regional pulse wave (PW) may contribute as a future modality for risk assessment. The purpose of this study was to demonstrate the feasibility and reproducibility of PW imaging (PWI) during propagation along the abdominal aortic wall by acquiring electrocardiography-gated (ECG-gated) radiofrequency (rf) signals noninvasively. An abdominal aortic aneurysm (AAA) was induced using a CaCl2 model in order to investigate the utility of this novel method for detecting disease. The abdominal aortas of twelve normal and five CaCl2 mice were scanned at 30 MHz and electrocardiography (ECG) was acquired simultaneously. The radial wall velocities were mapped with 8000 frames/s. Propagation of the PW was demonstrated in a color-coded ciné-loop format all cases. In the normal mice, the wave propagated in linear fashion from a proximal to a distal region. However, in CaCl2 mice, multiple waves were initiated from several regions (i.e., most likely initiated from various calcified regions within the aortic wall). The regional PWV in normal aortas was 2.70 +/- 0.54 m/s (r2 = 0.85 +/- 0.06, n = 12), which was in agreement with previous reports using conventional techniques. Although there was no statistical difference in the regional PWV between the normal and CaCl2-treated aortas (2.95 +/- 0.90 m/s (r2 = 0.51 +/- 0.22, n = 5)), the correlation coefficient was found to be significantly lower in the CaCl2-treated aortas (p < 0.01). This state-of-the-art technique allows noninvasive mapping of vascular disease in vivo. In future clinical applications, it may contribute to the detection of early stages of cardiovascular disease, which may decrease mortality among high-risk patients.

Collagen degradation in the abdominal aneurysm: a conspiracy of matrix metalloproteinase and cysteine collagenases

Growth and rupture of abdominal aortic aneurysms (AAAs) result from increased collagen turnover. Collagen turnover critically depends on specific collagenases that cleave the triple helical region of fibrillar collagen. As yet, the collagenases responsible for collagen degradation in AAAs have not been identified. Increased type I collagen degradation products confirmed collagen turnover in AAAs (median values: <1, 43, and 108 ng/mg protein in control, growing, and ruptured AAAs, respectively). mRNA and protein analysis identified neutrophil collagenase [matrix metalloproteinase (MMP)-8] and cysteine collagenases cathepsin K, L, and S as the principle collagenases in growing and ruptured AAAs. Except for modestly increased MMP-14 mRNA levels, collagenase expression was similar in growing and ruptured AAAs (anterior-lateral wall). Evaluation of posttranslational regulation of protease activity showed a threefold increase in MMP-8, a fivefold increase in cathepsins K and L, and a 30-fold increase in cathepsin S activation in growing and ruptured AAAs. The presence of the osteoclastic proton pump indicated optimal conditions for extracellular cysteine protease activity. Protease inhibitor mRNA expression was similar in AAAs and controls, but AAA protein levels of cystatin C, the principle cysteine protease inhibitor, were profoundly reduced (>80%). We found indications that this secondary deficiency relates to cystatin C degradation by (neutrophil-derived) proteases.

Biomechanics of abdominal aortic aneurysm

Abdominal aortic aneurysm (AAA) is a condition whereby the terminal aorta permanently dilates to dangerous proportions, risking rupture. The biomechanics of AAA has been studied with great interest since aneurysm rupture is a mechanical failure of the degenerated aortic wall and is a significant cause of death in developed countries. In this review article, the importance of considering the biomechanics of AAA is discussed, and then the history and the state-of-the-art of this field is reviewed--including investigations into the biomechanical behavior of AAA tissues, modeling AAA wall stress and factors which influence it, and the potential clinical utility of these estimates in predicting AAA rupture.

Analysis and computer program for rupture-risk prediction of abdominal aortic aneurysms

BACKGROUND

Ruptured abdominal aortic aneurysms (AAAs) are the 13th leading cause of death in the United States. While AAA rupture may occur without significant warning, its risk assessment is generally based on critical values of the maximum AAA diameter (> 5 cm) and AAA-growth rate (> 0.5 cm/year). These criteria may be insufficient for reliable AAA-rupture risk assessment especially when predicting possible rupture of smaller AAAs.

METHODS

Based on clinical evidence, eight biomechanical factors with associated weighting coefficients were determined and summed up in terms of a dimensionless, time-dependent severity parameter, SP(t). The most important factor is the maximum wall stress for which a semi-empirical correlation has been developed.

RESULTS

The patient-specific SP(t) indicates the risk level of AAA rupture and provides a threshold value when surgical intervention becomes necessary. The severity parameter was validated with four clinical cases and its application is demonstrated for two AAA cases.

CONCLUSION

As part of computational AAA-risk assessment and medical management, a patient-specific severity parameter 0 < SP(t) < 1.0 has been developed. The time-dependent, normalized SP(t) depends on eight biomechanical factors, to be obtained via a patient's pressure and AAA-geometry measurements. The resulting program is an easy-to-use tool which allows medical practitioners to make scientific diagnoses, which may save lives and should lead to an improved quality of life.

Biomechanical determinants of abdominal aortic aneurysm rupture

Rupture of abdominal aortic aneurysm (AAA) represents a significant clinical event, having a mortality rate of 90% and being currently ranked as the 13th leading cause of death in the US. The ability to reliably evaluate the susceptibility of a particular AAA to rupture on a case-specific basis could vastly improve the clinical management of these patients. Because AAA rupture represents a mechanical failure of the degenerated aortic wall, biomechanical considerations are important to understand this process and to improve our predictions of its occurrence. Presented here is an overview of research to date related to the biomechanics of AAA rupture. This includes a summary of results related to ex vivo and in vivo mechanical testing, noninvasive AAA wall stress estimations, and potential mechanisms of AAA wall weakening. We conclude with a demonstration of a biomechanics-based approach to predicting AAA rupture on a patient-specific basis, which may ultimately prove to be superior to the widely and currently used maximum diameter criterion.

Effect of intraluminal thrombus on pressure transmission in the abdominal aortic aneurysm

OBJECTIVE

To clarify the effect of intraluminal thrombus on pressure transmission.

METHODS

A saccular aneurysm was inserted into an artificial circulation system. Subsequently, the saccular aneurysm was filled with eight different human aortic aneurysm thrombus samples. Starch solution in an empty aneurysm was used as a control. A pressure sensor measured the pressure in the circulation, and a second piezoelectric sensor measured the pressure in the saccular aneurysm at 3, 2, and 1 cm from the endoluminal surface (23 locations). The influence of the elastic characteristics of the aneurysm wall on the extent of pressure reduction was evaluated by experiments performed with aneurysms made of rubber and paraffin.

RESULTS

The pressures measured in the empty aneurysm were identical to those measured in circulation (P > .05). The pressure measured in the thrombus was significantly lower than the pressure measured in the circulation (P < .05). The mean pressure ratio between the systolic thrombus pressure and systolic circulation pressure at 1, 2, and 3 cm was 0.90 +/- 0.09, 0.86 +/- 0.10, and 0.81 +/- 0.09, respectively. However, there was a clear correlation between the pressure in the circulation and in the thrombus (Pearson correlation coefficient: mean, r = 0.997; range, 0.975-0.999; P < .01). The change in circulatory pressure was followed by an almost identical change in thrombus pressure (regression coefficient: mean, beta = .997; range, .983-1.000; P < .01). In stiff aneurysms, the pressure reduction is less than in more compliant ones (P < .05).

CONCLUSIONS

In an in vitro model, pressure in the aneurysmal sac is reduced by fibrinous thrombus.

CLINICAL RELEVANCE

Endovascular aneurysm repair (EVAR) aims at reducing the pressure in the aneurysmal sac. Therefore, it seems attractive to use pressure monitoring in the aneurysmal sac as a follow-up after EVAR. This study contributes to the development of the rationale of pressure monitoring in the aneurysmal sac as a follow-up method after EVAR. The aneurysmal sac is filled with thrombus. To interpret pressure measurements in the thrombus, we have to learn about the effect of the thrombus on pressure transmission and on the pressure measurements themselves. Our study demonstrates that reduction of pressure occurs as it is transmitted through a human aortic thrombus.

A Review of Biological Factors Implicated in Abdominal Aortic Aneurysm Rupture

Abdominal aortic aneurysm (AAA) rupture is the 13th commonest cause of death in the Western World. Although considerable research has been applied to the aetiology and mechanism of aneurysm expansion, little is known about the mechanism of rupture. Aneurysm rupture was historically considered to be a simple physical process that occurred when the aortic wall could no longer contain the haemodynamic stress of the circulation. However, AAAs do not conform to the law of Laplace and there is growing evidence that aneurysm rupture involves a complex series of biological changes in the aortic wall. This paper reviews the available data on patient variables associated with aneurysm rupture and presents the evidence implicating biological factors in AAA rupture.

A mathematical model for the growth of the abdominal aortic aneurysm

We present the first mathematical model to account for the evolution of the abdominal aortic aneurysm. The artery is modelled as a two-layered, cylindrical membrane using nonlinear elasticity and a physiologically realistic constitutive model. It is subject to a constant systolic pressure and a physiological axial prestretch. The development of the aneurysm is assumed to be a consequence of the remodelling of its material constituents. Microstructural 'recruitment' and fibre density variables for the collagen are introduced into the strain energy density functions. This enables the remodelling of collagen to be addressed as the aneurysm enlarges. An axisymmetric aneurysm, with axisymmetric degradation of elastin and linear differential equations for the remodelling of the fibre variables, is simulated numerically. Using physiologically determined parameters to model the abdominal aorta and realistic remodelling rates for its constituents, the predicted dilations of the aneurysm are consistent with those observed in vivo. An asymmetric aneurysm with spinal contact is also modelled, and the stress distributions are consistent with previous studies.

Preliminary Results

PURPOSE

To assess aortic wall compliance as a portent of rupture risk in patients with abdominal aortic aneurysms.

METHODS

In this pilot study, 38 patients (32 men; median age 78 years, range 63-95) underwent an ultrasound scan: 20 pre-repair and 24 post-repair (18 endovascular [EVR] and 6 open). Six patients from the pre-repair group were included in a post repair study after EVR. Cine loop images were analyzed offsite using wall tracking software, which measured aortic diameter changes during cardiac cycles. Brachial blood pressure was measured, and elastic modulus (Ep) and stiffness (beta) were calculated. Preop Ep and beta were determined at the neck, inflection points (IP), and mid sac levels. Postop Ep and beta were calculated in mid sac only for technical reasons.

RESULTS

Preoperative Ep and beta were significantly higher at IP compared with neck (median Ep 24.22 versus 12.95 N/cm(2), p<0.003; median beta 16.27 versus 8.65, p<0.003). At the mid sac, Ep and beta were also significantly higher compared with neck: Ep 26.41 versus 12.95 N/cm(2), p=0.001; beta 17.94 versus 8.65, p=0.001. The values for IP and mid sac were Ep 24.22 versus 26.41 N/cm(2), p=0.64; beta 16.27 versus 17.94, p=0.64. In the postop cases (n=24), Ep and beta in successful endovascular repair (n=12) were significantly higher than in open repair, respectively: median Ep 34.31 versus 12.33 N/cm(2), p<0.001; median beta 23.18 versus 8.24, p<0.001. Patients with endoleaks or endotension (n=6) had significantly elevated Ep and beta compared with those without endoleaks (n=12): median Ep 79.79 versus 34.31 N/ cm(2), p=0.002; median beta 51.52 versus 23.18, p<0.002. Six patients scanned before and after EVR showed a decrease of Ep and beta in 3, no change in 1, and an increase in 2. An increase greater than 2 fold was noted in a patient with a gross type II endoleak.

CONCLUSIONS

This pilot study shows that estimates of aortic wall compliance agree well with known values for wall stress distribution. EVR leaves patients with greater wall stiffness than those undergoing open repair, a situation accentuated by endoleaks. Wall compliance and stiffness measurement promises to be useful for the evaluation of success of endovascular repair.

Matrix metalloproteinases: contribution to pathogenesis, diagnosis, surveillance and treatment of abdominal aortic aneurysms

BACKGROUND

Aortic abdominal aneurysm (AAA) represents a common chronic degenerative disease of the aortic wall. Chronic inflammation and enzymatic degradation of elastic lamellae and extracellular matrix (ECM) proteins constitute the most prominent characteristics of AAAs. There is mounting evidence that matrix metalloproteinases (MMPs) are the predominant proteinases in the AAA wall. These enzymes represent a potential target for therapeutic intervention to modify vascular pathology. This paper is an overview of matrix metalloproteinases and their role in the pathophysiology, diagnosis and treatment of AAA.

LITERATURE SEARCH

Comprehensive search of the MEDLINE, EMBASE and HEAL-Link databases from 1980 to 2003.

FINDINGS

Increased levels of MMPs expression and activity have been demonstrated within the aortic wall of AAA, associating with histological alterations. An imbalance between MMPs and their inhibitors (Tissue Inhibitors of Matrix Metalloproteinases - TIMPs), may tip the equilibrium towards matrix degradation. MMPs as systemic biochemical markers of AAAs may contribute to diagnosis of unsuspected AAAs or to the surveillance of patients with small AAAs. Evidence of variations in MMPs, TIMPs and their mediator genes promoting the increased inheritance susceptibility of AAAs is less well documented. However,a broad spectrum of pharmaceutical agents (e.g. doxycycline, statins etc.) is known to inhibit MMP activity and attenuate medial destruction.

CONCLUSION

Randomized clinical studies in patients in the early stages of AAA or in healthy individuals with great propensity to AAA development are required to demonstrate the causative relationship between MMPs and AAA. It still remains obscure whether long-term administration of MMP inhibitors can decelerate or even prevent the need for surgical repair.