Experimental measurements of elastic properties of media and adventitia of bovine carotid arteries

An experimental procedure is described which has been developed to measure the elastic properties of the outer (adventitial) and inner (medial) layers of excised bovine carotid arteries. The data analysis is based on a two-layered arterial wall model and the theory of large elastic deformations. The energy density functions for both layers are exponentials. The results show that the media and the adventitia are anisotropic; that the media is stiffer, more non-linear, and subjected to higher stresses than commonly assumed; and that both layers are stiffer in the axial direction than in the tangential direction.     

Localization of atherosclerotic lesions in the curving sites of human internal carotid arteries

We studied the distribution of the early atherosclerotic lesions in the curving sites of the human internal carotid arteries composed of the carotid siphon portion (part I) and carotid canal portion (part II). These early atherosclerotic lesions included a localized cloudy thickening with pallor, slight elevation, a non-fibrotic lesion and gray-white or yellowish-white, firm, elevated fibrous plaques. These lesions had the same pattern-distribution in each curving artery. Both were located in the distal regions from the middle of the inner curvature of parts I and II, where eddying fluid motions and directional change in the wall shear stress were considered to occur. In part I, there was a localized cloudy thickening in the younger subjects (average age: 22.8 years) rather than fibrous plaques (average age: 63.3 years). A positive correlation between the extent of the surface areas involved with fibrous plaques and the age of subjects was found in parts I and II. The extent of the surface areas involved with fibrous plaques was significantly greater in part I (26.9%) than in part II (7.85%). The radius of curvature was shorter in the former than in the latter. These results suggest that hemodynamic factors associated with flow in the curving sites of arteries may be important for the localization and progression of atherosclerotic lesions.     

A comparison of age-related changes in axial prestretch in human carotid arteries and in human abdominal aorta

Biomechanics and Modeling in Mechanobiology - It is known that large arteries in situ are subjected to significant axial prestretch. This prestretch plays an important physiological role in...     

Complex neuroradiological diagnosis in case of atherosclerotic lesion of the carotid arteries

The diagnostic arsenal for extracranial stenosis includes both ultrasound techniques, such as transcranial Doppler and duplex sonography, which are not only screening methods, but also are used at surgery, and high-tech neuroradiological techniques, such as spiral computed tomographic and magnetic resonance angiography, which can visualize structural changes in the vessel walls and qualitatively and functionally evaluate blood flow in the vascular bed. The purpose of this investigation was to determine the efficacy of non-invasive diagnostic methods, such as magnetic resonance angiography, including phase-contrast magnetic resonance angiography and spiral computed tomographic angiography, in the detection and comprehensive evaluation of stenoses and to compare the feasibilities of noninvasive procedures with the gold standard--selective angiography.     

Histochemical evidence for inducible nitric oxide synthase in advanced but non-ruptured human atherosclerotic carotid arteries

In response to cytokine stimulation, the inducible isoform of the nitric oxide synthase (iNOS) produces large amounts of nitric oxide with potential consequences in the pathophysiology of atherosclerosis. Previous investigations have demonstrated the presence of iNOS in human atherosclerotic lesions. The goal of this study was to evaluate the occurrence of the expression of iNOS in ruptured versus non-ruptured human carotid atherosclerotic plaques. Using plastic-embedded sections, we performed in situ hybridization and immunohistochemistry on very advanced atherosclerotic lesions type V (non-ruptured) and type VI (ruptured) from 12 atheromatous carotid arteries from endarterectomy and six non-atherosclerotic internal mammary arteries from aorto-coronary bypass. Only one internal mammary artery expressed iNOS in the endothelium. In contrast, iNOS mRNA and protein were repeatedly expressed in advanced lesions type V in 5/7 cases, particularly in inflammatory regions. Specific cell markers identified iNOS-positive cells as macrophages and T-lymphocytes but also as smooth muscle cells and endothelial cells adjacent to these inflammatory regions. Nitration of protein tyrosines was not always associated to iNOS expression but more likely to the presence of inflammatory cells. In complicated lesions type VI, the occurrence of iNOS mRNA and protein expression diminished drastically (1/5 cases). Combined expression of iNOS mRNA and protein is frequently found in advanced but non-ruptured human atherosclerotic carotid lesions while it becomes rare after the plaque has ruptured. These findings suggest that iNOS could be an active participant in the plaque rupture event.     

Rheological properties of healthy and atherosclerotic human arteries

The aim of this study was to determine the comparative three-dimensional mechanical properties of healthy and atherosclerotic muscular human arteries. Using a previously developed experimental system, in vitro inflation tests were performed on twelve segments of arteries, in static conditions. Two different initial states were used to carry the mechanical study through (large deformation, thick-walled). Main significant differences between healthy and atherosclerotic tested segments are observed for axial traction force whatever the initial state and radial and circumferencial strains referenced to longitudinally pre-stretched state. We showed that strain energy allows to differentiate between both types of arteries only when absolute values of transversal components were considered. Differential values of energy were not discriminating. Our results also show the potential interest of studying arteries in vivo at low transmural pressure.     

Impact of obesity in intima media thickness of carotid arteries

Objective: To explore differences in intima media thickness (IMT) of the carotid arteries induced by differences in BMI. Research Methods and Procedures: Data from 3173 consecutive subjects, who were referred to our Hypertension Center from 1998 to 2004, were reviewed. Criteria for patients to be considered for further analysis included no past or concurrent antihypertensive medication, no concurrent medication with the potential to raise blood pressure (BP) (e.g., prednisone), and no clinical signs or laboratory evidence of secondary causes of hypertension. Our population was divided into four groups according to NIH criteria for obesity: underweight, normal weight, overweight, and obese. BMI, mean IMT of internal carotid arteries, and 24‐hour BP values were available for all subjects. Five hundred thirty six subjects of the four groups, matched for age, gender, and mean 24‐hour BP values, were included in the analysis. Results: Mean IMT of internal carotid arteries was increased with increasing BMI. Mean IMT was significantly higher in obese subjects compared with normal‐weight (p < 0.01) and underweight (p < 0.001) subjects. Mean IMT was significantly higher in overweight subjects compared with normal‐weight ones (p < 0.05). Furthermore, multivariate regression analysis in obese subjects revealed that fasting serum glucose was independently associated with IMT. Discussion: Obesity may be an important factor for carotid atherosclerosis, and at least some of the effects of obesity are independent of the BP levels. Fasting serum glucose levels in obese subjects may play an important role in carotid atherosclerosis.     

Methodology to study intimal failure mechanics in human internal carotid arteries

While the incidence of blunt carotid artery injuries is low, the mortality rate is extremely high (40%). Clinical evidence indicates that the intimal region of the artery often sustains failure, while maintaining the integrity of the outer layers. This condition may lead to delayed ischemic symptoms, commonly reported in clinical literature. To date, the mechanical properties of the intima relative to the outer vessel layers have not been quantified in the human carotid artery. The purpose of the present study was to develop a methodology to determine the longitudinal mechanical properties of the human internal carotid artery in tension, with an emphasis on intimal failure. This was accomplished by opening the vessel at the mid-diameter level, creating an ‘I’-shaped testing specimen, subjecting the specimen to failure loading, documenting the stretch characteristics of the intimal and adventitial sides in the temporal domain, and correlating the synchronized videography with mechanical loading. Intimal failure data were quantified using stress and strain parameters in conjunction with digital videography of the intimal and adventitial sides. The present methodology can be used to determine the mechanical properties of the intima relative to ultimate carotid artery failure. These data will assist in the understanding of blunt carotid artery injuries, its diagnosis and treatment.     

Neutrophil infiltration and oxidant-production in human atherosclerotic carotid plaques

To clarify the clinical implications of neutrophils in vulnerable plaques we evaluated the function and activity of infiltrated neutrophils in an atherosclerotic plaque, focusing on oxidant production. A histopathological investigation was performed using carotid arterial samples obtained from seven patients. The atherosclerotic plaques were examined cytochemically for naphthol-ASD-chloroacetate esterase activity and oxidant-production, and immunohistochemically using N-formyl peptide receptor-like 1 (fPRL1)-, CD66b-, CD68- or p22phox-specific antibodies. The cytoplasmic fPRL1 intensity value of the neutrophils in the plaque was estimated using an activity index. Naphthol-ASD-chloroacetate esterase activity was found in cells located in the atherosclerotic plaque, indicating that the cells were neutrophils. The cytoplasmic fPRL1 intensity value of the neutrophils in the plaque decreased to approximately 60% of the intensity observed in the capillary vessels. Oxidant-production was also detected in the plaques, and both neutrophils and macrophages were observed at the corresponding oxidant-production sites. p22phox expression was also located in the same areas in which oxidant-production was observed in these plaques. We could not directly evaluate how much ROS generated from the infiltrated neutrophils contributed the plaque vulnerability followed by its rupture. However, the infiltrated neutrophils in the atherosclerotic plaques morphologically appeared activated and were actively generating oxidant, implying that neutrophils, together with macrophages, infiltrate into atherosclerotic plaques and contribute to plaque vulnerability.     

Innervation of the carotid body. An experimental quantitative study

The innervation of the carotid body in the cat was studied by means of light- and electron-microscopic techniques. Sinus nerve resection, glossopharyngeal resection, bilateral cervical sympathectomy, excisions of two nerves, and injection of 6-hydroxydopamine (6-OH-DA) were performed in different groups of animals. It was found that resection of the sinus nerve produces a rapid phase of degeneration of intralobular fibers and synaptic boutons, followed by a reinnervation with a progressive reappearance of these elements. This reinnervation is retarded by sympathectomy and prevented by 6-OH-DA. It is therefore concluded that reinnervation is due to collateral regeneration of nearby sympathetic fibers. Resection of the sinus nerve produces an increase in the number of argentaffin cells and dense-cored vesicles in the cytoplasm of principal cells. These findings suggest the existence of efferent synaptic contacts between this nerve and principal cells. Part of the intralobular fibers and synaptic boutons degenerate after bilateral sympathectomy demonstrating that sympathetic axons connect synaptically to the principal cells. Sympathetic fibers reach the carotid body, not only from branches of the cervical plexuses but also from fibers running in the adventitia of the common carotid artery, and via glossopharyngeal and sinus nerves. The vagus nerve contributes a few fibers to the parenchymal lobules of the carotid body.     

Effect of pressure on circumferential order of adventitial collagen in human brain arteries.

A key factor in the contribution of collagen fibres to tissue mechanics is the alignment of the fibres, which we studied in brain arteries, focussing on alignment changes with distending pressure. Arteries from autopsy were cannulated and fixed at different distending pressures from 0 to 200 mmHg (1 mmHg = 133.32 Pa), alcohol dehydrated, paraffin embedded, sectioned, and stained for birefringent enhancement. The polarized light microscope was set for extinction and fibre orientations were precisely determined at the rotational position of extinction for 200 positions around the artery wall. Results from 22 arterial cross sections revealed, with fixation pressure, a significant but incomplete straightening of collagen (even at 200 mmHg). The mean angular deviation of alignment of fibres was +/- 30 degrees for arteries fixed at zero transmural pressure, which in contrast was +/- 7 degrees for the inner and +/- 13 degrees for the mid-adventitia for arteries fixed at 200 mmHg transmural pressure. We verified on vessels fixed at low pressure, by using a full wave plate in conjunction with the specificity of the interference colours, that the measurements were correct and not confused with angles at 90 degrees to the morphological axis. Alternative tissue processing was done with two arteries fixed at 120 mmHg and processed for frozen sections; the results showed diminished variability in alignment but within the range of measurements for wax embedded tissue. We concluded that the collagen fabric could contribute to the mechanics of brain arteries but that it would be with sinusoidal rather than straightened fibres of collagen.     

Thermomechanical behavior of human carotid arteries in the passive state

Localized heating or cooling is expanding the clinical procedures used to treat cardiovascular diseases. Advantageous implementation and development of these methods are linked indissolubly to a deeper understanding of the arterial response to combined mechanical and thermal loads. Despite this, the basic thermomechanical behavior of human blood vessels still remains largely unknown, primarily due to the lack of appropriate experimental data. In this work, the influence of temperature on the passive behavior of human carotid arteries was studied in vitro by means of inflation tests. Eleven carotid segments were tested in the range 0-200 mmHg at four different temperatures of 17, 27, 37, and 42 degrees C. The results show that the combined change of temperature and stress has a dramatic effect on the dilatation coefficient of the arterial wall, which is shifted from negative to positive depending on the stress state, whereas the structural stiffness of the arterial wall does not change appreciably in the range of temperatures tested.     

Determination of the axial and circumferential mechanical properties of the skin tissue using experimental testing and constitutive modeling

The skin, being a multi-layered material, is responsible for protecting the human body from the mechanical, bacterial, and viral insults. The skin tissue may display different mechanical properties according to the anatomical locations of a body. However, these mechanical properties in different anatomical regions and at different loading directions (axial and circumferential) of the mice body to date have not been determined. In this study, the axial and circumferential loads were imposed on the mice skin samples. The elastic modulus and maximum stress of the skin tissues were measured before the failure occurred. The nonlinear mechanical behavior of the skin tissues was also computationally investigated through a suitable constitutive equation. Hyperelastic material model was calibrated using the experimental data. Regardless of the anatomic locations of the mice body, the results revealed significantly different mechanical properties in the axial and circumferential directions and, consequently, the mice skin tissue behaves like a pure anisotropic material. The highest elastic modulus was observed in the back skin under the circumferential direction (6.67 MPa), while the lowest one was seen in the abdomen skin under circumferential loading (0.80 MPa). The Ogden material model was narrowly captured the nonlinear mechanical response of the skin at different loading directions. The results help to understand the isotropic/anisotropic mechanical behavior of the skin tissue at different anatomical locations. They also have implications for a diversity of disciplines, i.e., dermatology, cosmetics industry, clinical decision making, and clinical intervention.     

Tensile and compressive properties of fresh human carotid atherosclerotic plaques

Accurate characterisation of the mechanical properties of human atherosclerotic plaque is important for our understanding of the role of vascular mechanics in the development and treatment of atherosclerosis. The majority of previous studies investigating the mechanical properties of human plaque are based on tests of plaque tissue removed following autopsy. This study aims to characterise the mechanical behaviour of fresh human carotid plaques removed during endarterectomy and tested within 2 h. A total of 50 radial compressive and 17 circumferential tensile uniaxial tests were performed on samples taken from 14 carotid plaques. The clinical classification of each plaque, as determined by duplex ultrasound is also reported. Plaques were classified as calcified, mixed or echolucent. Experimental data indicated that plaques were highly inhomogeneous; with variations seen in the mechanical properties of plaque obtained from individual donors and between donors. The mean behaviour of samples for each classification indicated that calcified plaques had the stiffest response, while echolucent plaques were the least stiff. Results also indicated that there may be a difference in behaviour of samples taken from different anatomical locations (common, internal and external carotid), however the large variability indicates that more testing is needed to reach significant conclusions. This work represents a step towards a better understanding of the in vivo mechanical behaviour of human atherosclerotic plaque.     

Dissection properties of the human aortic media: an experimental study

Aortic dissection occurs frequently and is clinically challenging; the underlying mechanics remain unclear. The present study investigates the dissection properties of the media of 15 human abdominal aortas (AAs) by means of direct tension tests (n=8) and peeling tests (n=12). The direct tension test demonstrates the strength of the media in the radial direction, while the peeling test allows a steady-state investigation of the dissection propagation. To explore the development of irreversible microscopic changes during medial dissection, histological images (n=8) from four AAs at different peeling stages are prepared and analyzed. Direct tension tests of coin-shaped medial specimens result in a radial failure stress of 140.1+/-15.9 kPa (mean+/-SD, n=8). Peeling tests of rectangular-shaped medial strips along the circumferential and axial directions provide peeling force/width ratios of 22.9+/-2.9 mN/mm (n=5) and 34.8+/-15.5 mN/mm (n=7); the related dissection energies per reference area are 5.1+/-0.6 mJ/cm(2) and 7.6+/-2.7 mJ/cm(2), respectively. Although student's t-tests indicate that force/width values of both experimental tests are not significantly different (alpha=0.05, p=0.125), the strikingly higher resisting force/width obtained for the axial peeling tests is perhaps indicative of anisotropic dissection properties of the human aortic media. Peeling in the axial direction of the aorta generates a remarkably "rougher" dissection surface with respect to the surface generated by peeling in the circumferential direction. Histological analysis of the stressed specimens reveals that tissue damage spreads over approximately six to seven elastic laminae, which is about 15-18% of the thickness of the abdominal aortic media, which forms a pronounced cohesive zone at the dissection front.     

LPS and oxLDL-induced S100A12 and RAGE expression in carotid arteries of atherosclerotic Yucatan microswine

Molecular Biology Reports - S100A12, also known as Calgranulin C, is a ligand for the receptor for advanced glycation end products (RAGE) and plays key roles in cardiovascular and other...