Mechanical Properties Of Arterial Wall Research Articles

Study on methods measuring mechanical properties of arterial wall by macroscopic indentation

Accurately evaluating the local biomechanics of arterial wall is crucial for diagnosing and treating arterial diseases. Indentation measurement can be used to evaluate the local mechanical properties of the artery. However, the effects of the indenter's geometric structure and the analysis theory on measurement results remain uncertain. In this paper, four kinds of indenters were used to measure the pulmonary aorta, the proximal thoracic aorta and the distal thoracic aorta in pigs, and the arterial elastic modulus was calculated by Sneddon and Sirghi theory to explore the influence of the indenter geometry and analysis theory on the measured elastic modulus. The results showed that the arterial elastic modulus measured by cylindrical indenter was lower than that measured by spherical indenter. In addition, compared with the calculated results of Sirghi theory, the Sneddon theory, which does not take adhesion forces in account, resulted in slightly larger elastic modulus values. In conclusion, this study provides parametric support for effective measurement of arterial local mechanical properties by millimeter indentation technique.

Impact of disturbed flow and arterial stiffening on mechanotransduction in endothelial cells.

Disturbed flow promotes progression of atherosclerosis at particular regions of arteries where the recent studies show the arterial wall becomes stiffer. Objective of this study is to show how mechanotransduction in subcellular organelles of endothelial cells (ECs) will alter with changes in blood flow profiles applied on ECs surface and mechanical properties of arterial wall where ECs are attached to. We will examine the exposure of ECs to atherogenic flow profiles (disturbed flow) and non-atherogenic flow profiles (purely forward flow), while stiffness and viscoelasticity of arterial wall will change. A multicomponent model of endothelial cell monolayer was applied to quantify the response of subcellular organelles to the changes in their microenvironment. Our results show that arterial stiffening alters mechanotransduction in intra/inter-cellular organelles of ECs by slight increase in the transmitted stresses, particularly over central stress fibers (SFs). We also observed that degradation of glycocalyx and exposure to non-atherogenic flow profiles result in significantly higher stresses in subcellular organelles, while degradation of glycocalyx and exposure to atherogenic flow profiles result in dramatically lower stresses in the organelles. Moreover, we show that increasing the arterial wall viscoelasticity leads to slight increase in the stresses transmitted to subcellular organelles. FAs are particularly influenced with the changes in the arterial wall properties and viscoelasticity. Our study suggests that changes in viscoelasticity of arterial wall and degradation state of glycocalyx have to be considered along with arterial stiffening in designing more efficient treatment strategies for atherosclerosis. Our study provides insight into significant role of mechanotransduction in the localization of atherosclerosis by quantifying the role of ECs mechanosensors and suggests that mechanotransduction may play a key role in design of more efficient and precision therapeutics to slow down or block the progression of atherosclerosis.

3D-bioprinted, phototunable hydrogel models for studying adventitial fibroblast activation in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease of the lung vasculature, characterized by elevated pulmonary blood pressure, remodeling of the pulmonary arteries, and ultimately right ventricular failure. Therapeutic interventions for PAH are limited in part by the lack of in vitro screening platforms that accurately reproduce dynamic arterial wall mechanical properties. Here we present a 3D-bioprinted model of the pulmonary arterial adventitia comprised of a phototunable poly(ethylene glycol) alpha methacrylate (PEG-αMA)-based hydrogel and primary human pulmonary artery adventitia fibroblasts (HPAAFs). This unique biomaterial emulates PAH pathogenesis in vitro through a two-step polymerization reaction. First, PEG-αMA macromer was crosslinked off-stoichiometry by 3D bioprinting an acidic bioink solution into a basic gelatin support bath initiating a base-catalyzed thiol-ene reaction with synthetic and biodegradable crosslinkers. Then, matrix stiffening was induced by photoinitiated homopolymerization of unreacted αMA end groups. A design of experiments approach produced a hydrogel platform that exhibited an initial elastic modulus (E) within the range of healthy pulmonary arterial tissue (E = 4.7 ± 0.09 kPa) that was stiffened to the pathologic range of hypertensive tissue (E = 12.8 ± 0.47 kPa) and supported cellular proliferation over time. A higher percentage of HPAAFs cultured in stiffened hydrogels expressed the fibrotic marker alpha-smooth muscle actin than cells in soft hydrogels (88 ± 2% versus 65 ± 4%). Likewise, a greater percentage of HPAAFs were positive for the proliferation marker 5-ethynyl-2ʹ-deoxyuridine (EdU) in stiffened models (66 ± 6%) compared to soft (39 ± 6%). These results demonstrate that 3D-bioprinted, phototunable models of pulmonary artery adventitia are a tool that enable investigation of fibrotic pathogenesis in vitro.

Modeling Pulse Wave Propagation Through a Stenotic Artery With Fluid Structure Interaction: A Validation Study Using Ultrasound Pulse Wave Imaging.

Pulse wave imaging (PWI) is an ultrasound-based method that allows spatiotemporal mapping of the arterial pulse wave propagation, from which the local pulse wave velocity (PWV) can be derived. Recent reports indicate that PWI can help the assessment of atherosclerotic plaque composition and mechanical properties. However, the effect of the atherosclerotic plaque's geometry and mechanics on the arterial wall distension and local PWV remains unclear. In this study, we investigated the accuracy of a finite element (FE) fluid-structure interaction (FSI) approach to predict the velocity of a pulse wave propagating through a stenotic artery with an asymmetrical plaque, as quantified with PWI method. Experiments were designed to compare FE-FSI modeling of the pulse wave propagation through a stenotic artery against PWI obtained with manufactured phantom arteries made of polyvinyl alcohol (PVA) material. FSI-generated spatiotemporal maps were used to estimate PWV at the plaque region and compared it to the experimental results. Velocity of the pulse wave propagation and magnitude of the wall distension were correctly predicted with the FE analysis. In addition, findings indicate that a plaque with a high degree of stenosis (>70%) attenuates the propagation of the pulse pressure wave. Results of this study support the validity of the FE-FSI methods to investigate the effect of arterial wall structural and mechanical properties on the pulse wave propagation. This modeling method can help to guide the optimization of PWI to characterize plaque properties and substantiate clinical findings.

Towards the non-invasive determination of arterial wall distensible properties: New approach using old formulae

Arterial function and wall mechanical properties are important determinants of hemodynamics in the circulation. However, their non-invasive determination is not widely available. Therefore, the aim of this work is to present a novel approach for the non-invasive determination of vessel’s distensibility and elastic modulus.Simultaneous measurements of vessel’s Diameter (D) and flow velocity (U) were recorded to determine local wave speed (nC) in flexible tubes and calf aortas non-invasively using the lnDU-loop method, which was used to calculate the Distensibility (nDs) and Elastic Modulus (nE), also non-invasively. To validate the new approach, the non-invasive results were compared to traditionally invasive measurements of Dynamic Distensibility (Dsd) and Tangential Elastic Modulus (Em).In flexible tubes, the average nDs was higher and nE was lower than Dsd and Em by 1.6% and 6.9%, respectively. In calf aortas, the results of nDs and nE agreed well with those of Dsd and Em, as demonstrated by Bland-Altman technique.The results of nDs and nE are comparable to those determined using traditional techniques. Our results suggest that nDs and nE could be measured in-vivo non-invasively, given the possibility of measuring D and U to obtain nC. Further studies are warranted to establish the clinical usefulness of the new approach.

Multiscale Modeling of Endothelium Derived Wall Shear Stress Regulation in Common Carotid Artery

ABSTRACTShear induced autoregulation is the natural ability of organs to maintain the local hemodynamic stresses in a stable condition in spite of altering perfusion rate. Endothelium cells are shear sensitive mechanoreceptors that are responsible for regulating the arterial wall architecture and mechanical properties in order to maintain homeostasis. This occurs by means of vasoactive mediators, which cause vasodilation or vasoconstriction. In this paper we presented a multiscale model of local flow regulation. First, a lumped parameter model of the whole cardiovascular system was implemented. Then a 3D numerical model of human common carotid artery was constructed considering fluid-structure interaction. The CCA inflow waveform obtained from the extended 0D model was applied to the 3D model as the boundary condition. After applying the Head-Up Tilt test, the local hemodynamics were disturbed. By considering the wall shear stress as the regulation criterion, then altering the arterial mechanical properties and the following vasodilation, shear forces exerted on the inner lining of the vessel were regulated and returned to the normal range. The resulting 0D/3D model can be considered as a plat-form for a more complete model containing local and systemic cardiovascular control mechanisms and patient-specific geometries which can be used for clinical purposes.

Characterization of arterial wall mechanical properties by a non–invasive method

AbstractSimulations of the behavior of blood vessels were performed, using ANSYS Inc. software, modeling fluid‐structure interactions with both strong and weak coupling. The objective was to determine the parietal deformations induced by thermodynamic conditions and pressure drops in the relevant vessels. The simulations involved hyperelastic and large deflection models to simulate the behavior of the wall. They allow the calculation of the numerical displacements and comparison with experimental displacements measured by magnetic resonance imaging, the aim is that the difference between numerical and experimental be as low as possible to deduce the adequate mechanical parameters for the artery. To identify the mechanical properties of the vessels, the optimization technique proposed in ANSYS based on genetic algorithms or gradient algorithms was used. One of the crucial points of identification involves the determination of the non‐stress state. If it is a known parameter for the elastic tube, it has to be determining for blood vessels. The challenge of this work is to determine from a “minimum” quantity of pressure and deformation information, the hyper‐elastic properties of blood vessels. The method based on a patient‐specific geometry deformation concluded that the tangent modulus in diastole is approximately 200 kPa while that in systole is in a range of 300 kPa to 1 MPa.

The advent of ultrafast ultrasound in vascular imaging: a review

The advent of ultrafast ultrasound in vascular imaging: a review Mathieu Couade SuperSonic Imagine, Aix en Provence, France Abstract: In the last 10 years the advent of computational power and the availability of fully programmable research ultrasound imaging systems have allowed the emergence of ultrafast ultrasound imaging (<1000 frames per second), which has become a central research topic in the ultrasound community. Ultrafast ultrasound imaging relies deeply on the capability of massive parallel beamforming/processing of a single transmit ultrasound event insonifying a large field of view. This ultrafast acquisition capability opens up new tradeoffs in terms of processing and observations of transient rapid phenomena. Firstly, it can be used to improve conventional imaging modes such as B-mode, Doppler modes, Contrast Enhanced Ultrasound in terms of sensitivity, contrast, spatial, and/or temporal resolutions. It can also enrich conventional color flow imaging modes by providing quantitative and synchronous spectral information on the whole image. Secondly it can be used to capture transient phenomena that where unseen with conventional frame rates (<50 Hz) enabling completely novel imaging modes such as real time shear wave elastography (SWE), a technique for mapping quantitatively soft tissue elasticities, or ultrafast PWV, a technique for measuring locally the pulse wave velocity (PWV). In this review, we first introduce the theoretical basis of ultrafast imaging. We then present a state of the art of ultrafast ultrasound imaging in the specific field of arterial wall mechanical properties characterization. Keywords: ultrafast imaging, SWE, PWV, arterial stiffness, ultrasound

Effects of the three-dimensional residual stresses on the mechanical properties of arterial walls

Effects of the three-dimensional residual stresses on the mechanical properties of arterial walls are analyzed in this paper, based on the model which considered the bending and stretching both in the circumferential and axial directions of the three distinct arterial layers. Moreover, different constitutive models are proposed to quantify the nonlinear mechanics of the three distinct layers and the important constituents, i.e. elastin, collagen fibers and smooth muscle cells (SMCs), are all taken into account. The stress distributions and pressure–radius curves of the arterial wall are given in details. Results demonstrate that the maximum values of the circumferential stress and the corresponding stress gradient in the media under the mean arterial pressure are reduced significantly as a consequence of the SMCs. The bending in the axial direction of the media and the opening angle of the intima have an obvious impact on the mechanical behaviors of arterial walls. This study may not only develop the understanding of effects of the three-dimensional residual stresses on the arterial wall response, but also can increase the accuracy of the analyses for patient-specific studies used for the treatments of arterial diseases.

Experimental and constitutive modeling approaches for a study of biomechanical properties of human coronary arteries

The study concerns the determination of mechanical properties of human coronary arterial walls with both experimental and constitutive modeling approaches. The research material was harvested from 18 patients (range 50-84 years). On the basis of hospital records and visual observation, each tissue sample was classified according to the stage (0, I, II, III) of atherosclerosis development (SAD). Then, strip samples considered as a membrane with the shape of rectangular parallelepiped were preconditioned and subjected to uniaxial tensile tests in longitudinal (n=27) and circumferential (n=4) direction. With experimental data obtained, the stress-strain characteristics were prepared. Furthermore, tensile strengths and related strains, stiffness coefficients and tangent modules of elasticity were computed. For a constitutive model of passive mechanical behavior of coronary arteries, values of material parameters were computed. The studies led to the following conclusions. Most importantly, the atherosclerotic changes affect all the mechanical properties of arterial walls. A progress of arteriosclerosis contributes to an increase of vascular stiffness. The highest values of the stiffness coefficients are obtained for the tissues in the advanced stage of the disease. We were also able to observe that gradual calcification, progression of atherosclerosis and degradation of collagen in the tissue caused a decrease of tensile strengths and related strains. Finally, a comparison made for the tissues with the advanced SAD showed that the tensile strengths and strains were much higher in the case of the samples with the circumferential orientation rather than those with the longitudinal one.

Analysis of uniaxial Tension and circumferential Inflation on the Mechanical Property of Arterial Wall

Considering the incompressible strain hardening materials, a new strain energy density function is modified from Gent Model, which fulfills the requirement that the modified strain energy function will transform into Gent model with n=1 . When n=1 and Jm goes into infinity ,the modified strain energy density function can be transformed into neo-hooken model. Arterial wall was considered as incompressible hyper-elastic materials. Uniaxial tension and inflation of arterial wall was been researched by utilizing the modified strain energy function. Influence of constitutive parameters on the mechanical property of arterial wall has been researched. In order to verify the theoretical theory, non-linear finite element analysis of arterial wall was proposed. By utilizing the commercial finite element software MSC.Marc, numerical simulation on mechanical property of arterial wall was carried out based on the modified strain energy function, which showed the finite element results agree with the theoretical results properly. The discussions illustrate that new constitutive model describes the finite deformation property of arterial wall reasonably and the applied range has been broadened by using the new constitutive model.DOI: http://dx.doi.org/10.5755/j01.mech.21.2.8743

Studies on arterial stiffness and wave reflections in hypertension.

Patho-physiological and pharmacological studies have consistently noticed that, with the exception of subjects with end-stage renal disease, total intravascular blood volume is not increased in patients with chronic hypertension. Because the mean circulatory pressure is enhanced in such subjects, it was postulated that the compliance of the cardiovascular system could be abnormally low in this particular population. This simple observation has influenced a great part of our experimental and clinical research directed toward subjects with hypertension and their relationship with the compliance of the vascular system. These works started between 1970 and 1980 by methodological investigations and validations followed by analysis of clinical situations that showed that venous and mostly arterial stiffness were significantly increased in hypertensive patients independently of blood pressure level. During the same time, we assessed the role of endothelium on the large arterial wall mechanical properties in normotensive and hypertensive rats. Thereafter more specific directions have been developed, affecting large arteries structure and function and arterial wall remodeling, including their consequences on central and peripheral hemodynamics. In parallel, epidemiological studies identified the pulsatile hemodynamic parameters as major independent predictors of cardiovascular risks. The consequences of these alterations on clinical pharmacology and therapeutics in hypertension are analyzed in detail.

Attenuation of Systolic Blood Pressure and Pulse Transit Time Hysteresis During Exercise and Recovery in Cardiovascular Patients

Pulse transit time (PTT) is a cardiovascular parameter of emerging interest due to its potential to estimate blood pressure (BP) continuously and without a cuff. Both linear and nonlinear equations have been used in the estimation of BP based on PTT. This study, however, demonstrates that there is a hysteresis phenomenon between BP and PTT during and after dynamic exercise. A total of 46 subjects including 16 healthy subjects, 13 subjects with one or more cardiovascular risk factors, and 17 patients with cardiovascular disease underwent graded exercise stress test. PTT was measured from electrocardiogram and photoplethysmogram of the left index finger of the subject, i.e., a pathway that includes predominately aorta, brachial, and radial arteries. The results of this study showed that, for the same systolic BP (SBP), PTT measured during exercise was significantly larger than PTT measured during recovery for all subject groups. This hysteresis was further quantified as both normalized area bounded by the SBP-PTT relationship (AreaN) and SBP difference at PTT during peak exercise plus 20 ms (ΔSBP20). Significant attenuation of both AreaN (p <; 0.05) and ΔSBP20 (p <; 0.01) is observed in cardiovascular patients compared with healthy subjects, independent of resting BP. Since the SBP-PTT relationship are determined by the mechanical properties of arterial wall, which is predominately mediated by the sympathetic nervous system through altered vascular smooth muscle (VSM) tone during exercise, results of this study are consistent with the previous findings of autonomic nervous dysfunction in cardiovascular patients. We further conclude that VSM tone has a nonnegligible influence on the BP-PTT relationship and thus should be considered in the PTT-based BP estimation.

Micromechanical modelling of the arterial wall: influence of mechanical heterogeneities on the wall stress distribution and the peak wall stress

Previous tensile tests have shown the high anisotropy and heterogeneity of the arterial wall mechanical properties as a function of age, pathology (e.g. abdominal aortic aneurysm, AAA) and location (Vande Geest et al. 2006). The issues of this anisotropic behaviour and the AAA geometrical shapes have been tackled in many numerical studies. However, the wall mechanical heterogeneity has been very sparsely considered (Tierney et al. 2012). This work aims to study the effects of such heterogeneity on the stress distribution and the peak wall stress during a static pressurisation. Thence, a micromechani- cal-based model was used for the wall, and finite element analyses (FEA) were carried out on idealised AAAs. Unlike many previous phenomenological models, the current constitutive model depends on five material parameters only, which also allow us to control the wall microarchitecture.

A 2D non-invasive ultrasonic method for simultaneous measurement of arterial strain and flow pattern

Many cardiovascular diseases are closely associated with the mechanical properties of arterial wall and hemodynamic parameters. Simultaneous measurements of the arterial strain and flow pattern may aid diagnosis of cardiovascular diseases and may be useful to study fluid-structure interaction between blood and vessel. This paper proposes a 2D non-invasive ultrasonic method to simultaneously measure arterial strain and flow pattern with sub-pixel accuracy. The method uses a multiple iterative algorithm to estimate the geometrical transformations of arterial wall and high-velocity gradient flows simultaneously. The accuracy of the method was validated by an in vitro arterial phantom and in vivo common carotid arteries (CCAs) of 12 mice using a Sonix RP (10 MHz) and a VisualSonics Vevo 2100 (30 MHz) ultrasound imaging system, respectively. For the arterial phantom, the calculated elasticity modulus from the strain profile shows good agreement with the mechanical testing value, deviating no more than 9.3%. The calculated flow velocity agrees well with the value obtained from the rotameter, deviating only 4.3%. For the CCAs of mice, good agreement is found between the calculated flow velocity and the measured value by ultrasound Doppler. The mean elasticity modulus of CCAs is 134.62 ± 54.3 kPa, which is in accordance with published data. The proposed method is capable of measuring the arterial wall strain and flow velocity pattern. This may be clinically useful for early detecting and monitoring cardiovascular diseases and may provide an essential tool in modelling the fluid-structure interaction between the blood and blood vessel.

Assessment of stiffness changes in the ex vivo porcine aortic wall using magnetic resonance elastography

Magnetic resonance elastography (MRE) is a noninvasive phase-contrast technique for estimating the mechanical properties of tissues by imaging propagating mechanical waves within the tissue. In this study, we hypothesize that changes in arterial wall stiffness, experimentally induced by formalin fixation, can be measured using MRE in ex vivo porcine aortas. In agreement with our hypothesis, the significant stiffness increase after sample fixation was clearly demonstrated by MRE and confirmed by mechanical testing. The results indicate that MRE can be used to examine the stiffness changes of the aorta. This study has provided evidence of the effectiveness of using MRE to directly assess the stiffness change in aortic wall. The results offer motivation to pursue MRE as a noninvasive method for the evaluation of arterial wall mechanical properties.

A compensative model for the angle‐dependence of motion estimates in noninvasive vascular elastography

Atherosclerosis of peripheral cerebral arteries can lead to stroke either by stenosis formation or plaque rupture. This pathology is initiated by the alteration of arterial wall mechanical properties shown to be assessable by ultrasound elastography. Recently, noninvasive vascular elastography (NIVE) was introduced for noninvasive imaging of the mechanical properties of superficial arteries as markers of vulnerable plaques. However, NIVE motion estimates are angle-dependent, with optimal scanning angle being represented by the alignment of tissue motion with ultrasound beam orientation. The objective of this study was to introduce a model that compensates for such angle-dependence in order to reduce the bias on strain estimates, namely, when investigating longitudinal vessel segments. The model is based on the Lagrangian speckle model estimator (LSME) because the LSME assesses the 2D-deformation matrix required to compute the scanning angle. Experiments on vessel-mimicking phantoms indicated that such a model enables the estimation of scanning angle with less than 3-degrees error. The method was also validated in vivo in human carotid arteries where less than 4-degrees error was observed. In both cases, the compensative model estimated the inclination angles with low variability. Angle-dependence may be an important factor to consider in avoiding potentially distort clinical diagnoses. Results, reported in this article, suggest that the LSME-based compensative model might be considered as a very interesting and promising clinical tool for NIVE applications.

Noninvasive vascular ultrasound elastography of carotid arteries for stroke prevention

Carotid plaque rupture leading to stroke involves alteration of arterial wall mechanical properties. This paper addresses reproducibility and potential clinical impact of non‐invasive vascular elastography. Fifteen symptomatic and 15 asymptomatic patients with greater than 50% stenoses of their internal carotids, and 15 control subjects were scanned independently by two radiologists. Radio‐frequency cine‐loops were acquired on left and right common and proximal internal carotids. For control subjects, homogeneous strains with successive compression and dilatation of the vascular wall from systole to diastole and few shear patterns were observed. The axial strain did not differ between recording sites, sides and radiologists (ANOVA, p > 0.19). For patients, heterogeneous axial strain patterns with both compression and dilatation of tissues within plaques were noted along with shear concentrations at interfaces of tissue structures. Despite similar blood pressures, larger mean instantaneous axial strains were measured in symptomatic (1.5 ± 0.6%) versus asymptomatic (1.2 ± 0.5%) patients in systole for common carotids (t‐test, p < 0.02). To conclude, reproducible results were obtained for control subjects and in patients, larger axial strains in common carotids may be associated with plaque rupture. This new imaging method may become a unique approach to characterize vulnerable plaques for stroke prevention.

Arterial haemodynamics in patients after repair of tetralogy of Fallot: influence on left ventricular after load and aortic dilatation

Background:Recent histological studies of the aortic wall of patients with tetralogy of Fallot (TOF) have shown massive degeneration of the tunica media of the aorta. Such changes in arterial wall...

Adventitia‐dependent mechanical properties of brachiocephalic ovine arteries in in vivo and in vitro studies

An adventitia dependent regulation of the vascular smooth muscle tone has been described. However, if the adventitia plays an active role on arterial wall biomechanical behaviour and functions remains to be established. Our aim was to characterize the influence of adventitia on arterial wall mechanical properties and the arterial conduit and buffer functions. Ovine brachiocephalic arteries were studied in vivo (n = 8) and in vitro (with null tone) in a circulation mock (n = 8). Isobaric, isoflow and isofrequency studies were performed. In each segment, pressure and diameter waves were assessed before and after adventitia removal. From the arterial stress-strain relationship, we derived the elastic and the viscous modulus. The buffering and conduit functions were calculated using the Kelvin-Voigt's time constant and the inverse of the characteristic impedance, respectively. In in vivo studies arterial diameter decreased after adventitia removal (P < 0.05). Elastic and viscous modulus in in vivo studies were significantly higher in adventitia-removed arteries, compared with values in intact vessels (P < 0.05). This behaviour was not observed in in vitro experiments. An impairment of buffer and conduit functions was observed in vivo after adventitia removal (P < 0.05), while both functions remain unchanged in in vitro studies (P > 0.05). Arterial wall viscosity and elasticity were influenced by adventitia removal in in vivo studies, possibly by a smooth muscle-dependent mechanism, since it was not present in in vitro experiments. Adventitia would be involved in a physiological mechanism of arterial wall viscous and elastic properties regulation, that could influence arterial buffering and conduit functions.