Effects of longitudinal stretch on VSM tone and distensibility of muscular conduit arteries

New Hemodynamic Parameters in Peri-Operative and Critical Care-Challenges in Translation

Hemodynamic monitoring technologies are evolving continuously-a large number of bedside monitoring options are becoming available in the clinic. Methods such as echocardiography, electrical bioimpedance, and calibrated/uncalibrated analysis of pulse contours are becoming increasingly common. This is leading to a decline in the use of highly invasive monitoring and allowing for safer, more accurate, and continuous measurements. The new devices mainly aim to monitor the well-known hemodynamic variables (e.g., novel pulse contour, bioreactance methods are aimed at measuring widely-used variables such as blood pressure, cardiac output). Even though hemodynamic monitoring is now safer and more accurate, a number of issues remain due to the limited amount of information available for diagnosis and treatment. Extensive work is being carried out in order to allow for more hemodynamic parameters to be measured in the clinic. In this review, we identify and discuss the main sensing strategies aimed at obtaining a more complete picture of the hemodynamic status of a patient, namely: (i) measurement of the circulatory system response to a defined stimulus; (ii) measurement of the microcirculation; (iii) technologies for assessing dynamic vascular mechanisms; and (iv) machine learning methods. By analyzing these four main research strategies, we aim to convey the key aspects, challenges, and clinical value of measuring novel hemodynamic parameters in critical care.

Fundamental Roles of Axial Stretch in Isometric and Isobaric Evaluations of Vascular Contractility

Vascular smooth muscle cells (VSMCs) can regulate arterial mechanics via contractile activity in response to changing mechanical and chemical signals. Contractility is traditionally evaluated via uniaxial isometric testing of isolated rings despite the in vivo environment being very different. Most blood vessels maintain a locally preferred value of in vivo axial stretch while subjected to changes in distending pressure, but both of these phenomena are obscured in uniaxial isometric testing. Few studies have rigorously analyzed the role of in vivo loading conditions in smooth muscle function. Thus, we evaluated effects of uniaxial versus biaxial deformations on smooth muscle contractility by stimulating two regions of the mouse aorta with different vasoconstrictors using one of three testing protocols: (i) uniaxial isometric testing, (ii) biaxial isometric testing, and (iii) axially isometric plus isobaric testing. Comparison of methods (i) and (ii) revealed increased sensitivity and contractile capacity to potassium chloride and phenylephrine (PE) with biaxial isometric testing, and comparison of methods (ii) and (iii) revealed a further increase in contractile capacity with isometric plus isobaric testing. Importantly, regional differences in estimated in vivo axial stretch suggest locally distinct optimal biaxial configurations for achieving maximal smooth muscle contraction, which can only be revealed with biaxial testing. Such differences highlight the importance of considering in vivo loading and geometric configurations when evaluating smooth muscle function. Given the physiologic relevance of axial extension and luminal pressurization, we submit that, when possible, axially isometric plus isobaric testing should be employed to evaluate vascular smooth muscle contractile function.

Aristoteline, an Indole-Alkaloid, Induces Relaxation by Activating Potassium Channels and Blocking Calcium Channels in Isolated Rat Aorta

Alkaloids derived from plants have shown great medicinal benefits, and are often reported for their use in cardiovascular disease management. Aristotelia chilensis (Molina) Stuntz (Maqui) has shown important medicinal properties in traditional useage. In this study, we evaluated the effect of the indole-alkaloid aristoteline (ARI), isolated from leaves of Maqui, on vascular reactivity of isolated aortic rings from normotensive rats. ARI induced relaxation (100%) in a concentration-dependent manner in intact or denuded-endothelium aortic rings pre-contracted with phenylephrine (PE; 1 μM). However, a specific soluble guanylyl cyclase inhibitor (ODQ; 1 μM) significantly reduced the relaxation to ARI in aortic rings pre-contracted with PE. In the presence of ARI, the contraction induced by KCl or PE was significantly (p < 0.05) decreased. Interestingly, the potassium channel blockade with 10 μM BaCl₂ (Kir), 10 μM glibenclamide (K_ATP), 1 mM tetraethylammonium (TEA; KCa1.1), or 1 mM 4-aminopyridine (4-AP; Kv) significantly (p < 0.05) reduced the ARI-induced relaxation. ARI significantly (p < 0.05) reduced the contractile response to agonist of Ca_V1.2 channels (Bay K8644; 10 nM), likely reducing the influx of extracellular calcium through plasma membrane. The mechanisms associated with this process suggest an activation of the potassium channels, a calcium-induced antagonism and endothelium independent vasodilation that possibly involves the nitric oxide-independent soluble guanylate cyclase pathway.

Biomechanics and Mechanobiology of Saphenous Vein Grafts

Within several weeks of use as coronary artery bypass grafts (CABG), saphenous veins (SV) exhibit significant intimal hyperplasia (IH). IH predisposes vessels to thrombosis and atherosclerosis, the two major modes of vein graft failure. The fact that SV do not develop significant IH in their native venous environment coupled with the rapidity with which they develop IH following grafting into the arterial circulation suggests that factors associated with the isolation and preparation of SV and/or differences between the venous and arterial environments contribute to disease progression. There is strong evidence suggesting that mechanical trauma associated with traditional techniques of SV preparation can significantly damage the vessel and might potentially reduce graft patency though modern surgical techniques reduces these injuries. In contrast, it seems possible that modern surgical technique, specifically endoscopic vein harvest, might introduce other mechanical trauma that could subtly injure the vein and perhaps contribute to the reduced patency observed in veins harvested using endoscopic techniques. Aspects of the arterial mechanical environment influence remodeling of SV grafted into the arterial circulation. Increased pressure likely leads to thickening of the medial wall but its role in IH is less clear. Changes in fluid flow, including increased average wall shear stress, may reduce IH while disturbed flow likely increase IH. Nonmechanical stimuli, such as exposure to arterial levels of oxygen, may also have a significant but not widely recognized role in IH. Several potentially promising approaches to alter the mechanical environment to improve graft patency are including extravascular supports or altered graft geometries are covered.

Noradrenaline has opposing effects on the hydraulic conductance of arterial intima and media

The uptake of circulating macromolecules by the arterial intima is thought to be a key step in atherogenesis. Such transport is dominantly advective, so elucidating the mechanisms of water transport is important. The relation between vasoactive agents and water transport in the arterial wall is incompletely understood. Here we applied our recently-developed combination of computational and experimental methods to investigate the effects of noradrenaline (NA) on hydraulic conductance of the wall (Lp), medial extracellular matrix volume fraction (ϕECM) and medial permeability (K11) in the rat abdominal aorta. Experimentally, we found that physiological NA concentrations were sufficient to induce SMC contraction and produced significant decreases in Lp and increases in ϕECM. Simulation results based on 3D confocal images of the extracellular volume showed a corresponding increase in K11, attributed to the opening of the ECM. Conversion of permeabilities to layer-specific resistances revealed that although the total wall resistance increased, medial resistance decreased, suggesting an increase in intimal resistance upon application of NA.

Reduced Biaxial Contractility in the Descending Thoracic Aorta of Fibulin-5 Deficient Mice

The precise role of smooth muscle cell contractility in elastic arteries remains unclear, but accumulating evidence suggests that smooth muscle dysfunction plays an important role in the development of thoracic aortic aneurysms and dissections (TAADs). Given the increasing availability of mouse models of these conditions, there is a special opportunity to study roles of contractility ex vivo in intact vessels subjected to different mechanical loads. In parallel, of course, there is a similar need to study smooth muscle contractility in models that do not predispose to TAADs, particularly in cases where disease might be expected. Multiple mouse models having compromised glycoproteins that normally associate with elastin to form medial elastic fibers present with TAADs, yet those with fibulin-5 deficiency do not. In this paper, we show that deletion of the fibulin-5 gene results in a significantly diminished contractility of the thoracic aorta in response to potassium loading despite otherwise preserved characteristic active behaviors, including axial force generation and rates of contraction and relaxation. Interestingly, this diminished response manifests around an altered passive state that is defined primarily by a reduced in vivo axial stretch. Given this significant coupling between passive and active properties, a lack of significant changes in passive material stiffness may help to offset the diminished contractility and thereby protect the wall from detrimental mechanosensing and its sequelae.

A constitutive modeling interpretation of the relationship among carotid artery stiffness, blood pressure, and age in hypertensive subjects

Aging has a profound influence on arterial wall structure and function. We have previously reported the relationship among pulse wave velocity, age, and blood pressure in hypertensive subjects. In the present study, we aimed for a quantitative interpretation of the observed changes in wall behavior with age using a constitutive modeling approach. We implemented a model of arterial wall biomechanics and fitted this to the group-averaged pressure-area ( P-A) relationship of the “young” subgroup of our study population. Using this model as our take-off point, we assessed which parameters had to be changed to let the model describe the “old” subgroup’s P-A relationship. We allowed elastin stiffness and collagen recruitment parameters to vary and adjusted residual stress parameters according to published age-related changes. We required wall stress to be homogeneously distributed over the arterial wall and assumed wall stress normalization with age by keeping average “old” wall stress at the “young” level. Additionally, we required axial force to remain constant over the cardiac cycle. Our simulations showed an age-related shift in pressure-load bearing from elastin to collagen, caused by a decrease in elastin stiffness and a considerable increase in collagen recruitment. Correspondingly, simulated diameter and wall thickness increased by about 20 and 17%, respectively. The latter compared well with a measured thickness increase of 21%. We conclude that the physiologically realistic changes in constitutive properties we found under physiological constraints with respect to wall stress could well explain the influence of aging in the stiffness-pressure-age pattern observed.

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.

Structural and mechanical characterisation of bridging veins: A review

Bridging veins drain the venous blood from the cerebral cortex into the superior sagittal sinus (SSS) and doing so they bridge the subdural space. Despite their importance in head impact biomechanics, little is known about their properties with respect to histology, morphology and mechanical behaviour. Knowledge of these characteristics is essential for creating a biofidelic finite element model to study the biomechanics of head impact, ultimately leading to the improved design of protective devices by setting up tolerance criteria. This paper presents a comprehensive review of the state-of-the-art knowledge on bridging veins. Tolerance criteria to prevent head injury through impact have been set by a number of research groups, either directly through impact experiments or by means of finite element (FE) simulations. Current state-of-the-art FE head models still lack a biofidelic representation of the bridging veins. To achieve this, a thorough insight into their nature and behaviour is required. Therefore, an overview of the general morphology and histology is provided here, showing the clearly heterogeneous nature of the bridging vein complex, with its three different layers and distinct morphological and histological changes at the region of outflow into the superior sagittal sinus. Apart from a complex morphology, bridging veins also exhibit complex mechanical behaviour, being nonlinear, viscoelastic and prone to damage. Existing material models capable of capturing these properties, as well as methods for experimental characterisation, are discussed. Future work required in bridging vein research is firstly to achieve consensus on aspects regarding morphology and histology, especially in the outflow cuff segment. Secondly, the advised material models need to be populated with realistic parameters through biaxial mechanical experiments adapted to the dimensions of the bridging vein samples. Finally, updating the existing finite element head models with these parameters will render them truly biofidelic, allowing the establishment of accurate tolerance criteria and, ultimately, better head protection devices.

Altered endothelin receptor expression and affinity in spontaneously hypertensive rat cerebral and coronary arteries

Background

Hypertension is associated with arterial hyperreactivity, and endothelin (ET) receptors are involved in vascular pathogenesis. The present study was performed to examine the hypothesis that ET receptors were altered in cerebral and coronary arteries of spontaneously hypertensive rats (SHR).

Methodology/Principal Findings

Cerebral and coronary arteries were removed from SHR. Vascular contraction was recorded using a sensitive myograph system. Real-time PCR and Western blotting were used to quantify mRNA and protein expression of receptors and essential MAPK pathway molecules. The results demonstrated that both ET_A and ET_B receptor-mediated contractile responses in SHR cerebral arteries were shifted to the left in a nonparallel manner with increased maximum contraction compared with Wistar-Kyoto (WKY) rats. In SHR coronary arteries, the ET_A receptor-mediated contraction curve was shifted to the left in parallel with an increased pEC₅₀ compared with the arteries in WKY rats. There was no significant increase in ET_B receptor-mediated contraction in SHR coronary arteries. ET_A receptor mRNA and protein expression was increased in SHR cerebral arteries compared with the arteries in WKY rats. However, ET_A receptor mRNA and protein levels in coronary arteries and ET_B receptor protein levels in cerebral and coronary arteries remained unchanged in SHR compared with WKY rats. Meanwhile, phosphorylated ERK1/2 protein was significantly increased in SHR brain and heart vessels.

Conclusions/Significance

In SHR cerebral arteries, ET_A receptor expression was upregulated. ET_A receptor affinity was increased in coronary arteries, and ET_B receptor affinity was increased in cerebral arteries. The ERK1/2 activation may be involved in the receptor alterations.

Differences in transmural pressure and axial loading ex vivo affect arterial remodeling and material properties

Arterial axial strains, present in the in vivo environment, often become reduced due to either bypass grafting or the normal aging process. Since the prevalence of hypertension increases with aging, arteries are often exposed to both decreased axial stretch and increased transmural pressure. The combined effects of these mechanical stimuli on the mechanical properties of vessels have not previously been determined. Porcine carotid arteries were cultured for 9 days at normal and reduced axial stretch ratios in the presence of normotensive and hypertensive transmural pressures using ex vivo perfusion techniques. Measurements of the amount of axial stress were obtained through longitudinal tension tests while inflation-deflation test results were used to determine circumferential stresses and incremental moduli. Macroscopic changes in artery geometry and zero-stress state opening angles were measured. Arteries cultured ex vivo remodeled in response to the mechanical environment, resulting in changes in arterial dimensions of up to approximately 25% and changes in zero-stress opening angles of up to approximately 55 degrees . While pressure primarily affected circumferential remodeling and axial stretch primarily affected axial remodeling, there were clear examples of interactions between these mechanical stimuli. Culture with hypertensive pressure, especially when coupled with reduced axial loading, resulted in a rightward shift in the pressure-diameter relationship relative to arteries cultured with normotensive pressure. The observed differences in the pressure-diameter curves for cultured arteries were due to changes in artery geometry and, in some cases, changes in the arteries' intrinsic mechanical properties. Relative to freshly isolated arteries, arteries cultured under mechanical conditions similar to in vivo conditions were stiffer, suggesting that aspects of the ex vivo culture other than the mechanical environment also influenced changes in the arteries' mechanical properties. These results confirm the well-known importance of transmural pressure with regard to arterial wall mechanics while highlighting additional roles for axial stretch in determining mechanical behavior.

Arterial biomechanics after destruction of cytoskeleton by Cytochalasin D

Vascular smooth muscle is a major structural element of the arterial wall. We examined the effects of cytoskeleton destruction, after administration of Cytochalasin D, on the biomechanical properties of porcine common carotids. Compared to untreated, maximally dilated controls, Cytochalasin D-treated arteries have shown a marked increase in compliance in the elastin-dominated pressure range. After weakening the VSM stress-bearing cytoskeleton by Cytochalasin D the artery would expand, reaching a new equilibrium state. This study brings further evidence that VSM is under tension, even when it is under zero load and at maximal vasodilation. This residual tension was released upon partial destruction of the cytoskeleton with Cytochalasin D. From a biomechanical standpoint, this means that the zero stress states of the in-series and parallel elastic components are substantially different.

Transmural pressure and axial loading interactively regulate arterial remodeling ex vivo

Physiological axial strains range between 40 and 60% in arteries, resulting in stresses comparable to those due to normal blood pressure or flow. To investigate the contribution of axial strain to arterial remodeling and function, porcine carotid arteries were cultured for 9 days at physiological and reduced axial stretch ratios in the presence of normotensive and hypertensive transmural pressures by ex vivo perfusion techniques. Consistent with previous in vivo studies, vessels cultured with physiological levels of axial strain and exposed to hypertensive pressure had greater mass, wall area, and outer diameter relative to those cultured at the same axial stretch ratio and normotensive pressure. Reducing the amount of axial strain resulted in mass loss and decreased cell proliferation. Culture in a hypertensive pressure environment at reduced axial strain produced arteries with greater contractility in response to norepinephrine. Arteries cultured at reduced axial strain with the matrix metalloproteinase inhibitor GM6001 maintained their masses over culture, indicating a possible mechanism for this model of axial stretch-dependent remodeling. Although not historically considered one of the primary stimuli for remodeling, multiple linear regression analysis revealed that axial strain had an impact similar to or greater than transmural pressure on various remodeling indexes (i.e., outer diameter, wall area, and wet mass), suggesting that axial strain is a primary mediator of vascular remodeling.

A structural model of the venous wall considering elastin anisotropy

The three-dimensional biomechanical behavior of the vascular wall is best described by means of strain energy functions. Significant effort has been devoted lately in the development of structure-based models of the vascular wall, which account for the individual contribution of each major structural component (elastin, collagen, and vascular smooth muscle). However, none of the currently proposed structural models succeeded in simultaneously and accurately describing both the pressure-radius and pressure-longitudinal force curves. We have hypothesized that shortcomings of the current models are, in part, due to unaccounted anisotropic properties of elastin. We extended our previously developed biomechanical model to account for elastin anisotropy. The experimental data were obtained from inflation-extension tests on facial veins of five young white New Zealand rabbits. Tests have been carried out under a fully relaxed state of smooth muscle cells for longitudinal stretch ratios ranging from 100% to 130% of the in vivo length. The experimental data (pressure-radius, pressure-force, and zero-stress-state geometries) provided a complete biaxial mechanical characterization of rabbit facial vein and served as the basis for validating the applicability and accuracy of the new biomechanical model of the venous wall. When only the pressure-radius curves were fitted, both the anisotropic and the isotropic models gave excellent results. However, when both pressure-radius and pressure-force curves are simultaneously fitted, the model with isotropic elastin shows an average weighted residual sum of squares of 8.94 and 23.9 in the outer radius and axial force, respectively, as compared to averages of 6.07 and 4.00, when anisotropic elastin is considered. Both the Alkaike information criterion and Schwartz criterion show that the model with the anisotropic elastin is more successful in predicting the data for a wide range of longitudinal stretch ratios. We conclude that anisotropic description of elastin is required for a full 3D characterization of the biomechanics of the venous wall.

Structural strain energy function applied to the ageing of the human aorta

Stiffening of the aorta with progressing age leads to decrease of aortic compliance and thus to an increase of pulse pressure amplitude. Using a strain energy function (SEF) which takes into account the composition of the arterial wall, we have studied the evolution of key structural components of the human thoracic aorta using data obtained from the literature. The SEF takes into account the wavy nature of collagen, which upon gradual inflation of the blood vessel is assumed to straighten out and become engaged in bearing load. The engagement of the individual fibers is assumed to be distributed log-logistically. The use of a SEF enables the consideration of axial stretch (lambda(z)) and residual strain (opening angle) in the biomechanical analysis. Both lambda(z) and opening angle are known to change with age. Results obtained from applying the SEF to the measurements of aortic pressure-diameter curves indicate that the changes in aortic biomechanics with progressing age are not to be sought in the elastic constants of elastin and collagen or their volume fractions of the aortic wall but moreover in alterations of the collagen mesh arrangement and the waviness of the collagen fibers. In old subjects, the collagen fiber ensemble engages in load bearing much more abruptly than in young subjects. Reasons for this change in collagen fiber dynamics may include fiber waviness remodeling or cross-linkage by advanced glycation end-products (AGE). The abruptness of collagen fiber engagement is also the model parameter that is most responsible for the decreased compliance at progressed ages.

Could increased axial wall stress be responsible for the development of atheroma in the proximal segment of myocardial bridges?

BACKGROUND

A recent model describing the mechanical interaction between a stenosis and the vessel wall has shown that axial wall stress can considerably increase in the region immediately proximal to the stenosis during the (forward) flow phases, so that abnormal biological processes and wall damages are likely to be induced in that region. Our objective was to examine what this model predicts when applied to myocardial bridges.

METHOD

The model was adapted to the hemodynamic particularities of myocardial bridges and used to estimate by means of a numerical example the cyclic increase in axial wall stress in the vessel segment proximal to the bridge. The consistence of the results with reported observations on the presence of atheroma in the proximal, tunneled, and distal vessel segments of bridged coronary arteries was also examined.

RESULTS

1) Axial wall stress can markedly increase in the entrance region of the bridge during the cardiac cycle. 2) This is consistent with reported observations showing that this region is particularly prone to atherosclerosis.

CONCLUSION

The proposed mechanical explanation of atherosclerosis in bridged coronary arteries indicates that angioplasty and other similar interventions will not stop the development of atherosclerosis at the bridge entrance and in the proximal epicardial segment if the decrease of the lumen of the tunneled segment during systole is not considerably reduced.

A two-layer model of the static behaviour of blood vessel walls

A model is presented that simulates the steady state two-layer nature of blood vessel walls. The model comprises an intimal-medial layer and an adventitial layer, both of which are modeled using previously validated separate models. By including both models, different types of blood vessel can be modeled within a common framework. The necessary relationships that link the two models are presented and simulations of the complete model shown. These illustrate how different types of vessel exhibit different non-linear behaviour. The models include both mechanical and biochemical aspects of the vessel wall behaviour, which will enable the model to be used to simulate more advanced behaviour such as local autoregulatory processes.

Effect of elastin degradation on carotid wall mechanics as assessed by a constituent-based biomechanical model

Arteries display a nonlinear anisotropic behavior dictated by the elastic properties and structural arrangement of its main constituents, elastin, collagen, and vascular smooth muscle. Elastin provides for structural integrity and for the compliance of the vessel at low pressure, whereas collagen gives the tensile resistance required at high pressures. Based on the model of Zulliger et al. (Zulliger MA, Rachev A, Stergiopulos N. Am J Physiol Heart Circ Physiol 287: H1335-H1343, 2004), which considers the contributions of elastin, collagen, and vascular smooth muscle cells (VSM) in an explicit form, we assessed the effects of enzymatic degradation of elastin on biomechanical properties of rabbit carotids. Pressure-diameter curves were obtained for controls and after elastin degradation, from which elastic and structural properties were derived. Data were fitted into the model of Zulliger et al. to assess elastic constants of elastin and collagen as well as the characteristics of the collagen engagement profile. The arterial segments were also prepared for histology to visualize and quantify elastin and collagen. Elastase treatment leads to a diameter enlargement, suggesting the existence of significant compressive prestresses within the wall. The elastic modulus was more ductile in treated arteries at low circumferential stretches and significantly greater at elevated circumferential stretches. Abrupt collagen fiber recruitment in elastase-treated arteries leads to a much stiffer vessel at high extensions. This change in collagen engagement properties results from structural alterations provoked by the degradation of elastin, suggesting a clear interaction between elastin and collagen, often neglected in previous constituent-based models of the arterial wall.

Oxygen Mass Transport in a Compliant Carotid Bifurcation Model

The purpose of the present study was to investigate oxygen mass transfer in the human carotid bifurcation, focusing on the effects of the wall compliance and flow field on the temporal variation and spatial distribution of the oxygen wall flux. Details of unsteady convective-diffusive oxygen transport were examined numerically using a compliant model of the human carotid bifurcation and realistic blood flow waveforms. Results reveal that axial flow separation at the outer common-internal carotid wall can significantly alter the flow field, oxygen tension field, and oxygen wall flux distribution. At the outer wall of the sinus, the Sherwood number, Sh (non-dimensional oxygen wall flux), takes on significantly lower values than at other sites due to the attenuation of transport rates by convective flow away from wall. More specifically, the lowest value of Sh was Sh approximately 6 (in the sinus), which is much lower than the value of the non-dimensional oxygen consumption rate (Damkohler number, Da) in the reactive wall tissue (Da=29-39). At the inner wall of the sinus, Sh approximately 170 is far above the expected value of Da. This implies that flow separation on the outer wall of the sinus provides a very strong fluid mechanical barrier to oxygen transport; whereas at the inner wall of the sinus, the mechanism of transport is controlled by the wall consumption rate.

Hemodynamic conditions alter axial and circumferential remodeling of arteries engineered ex vivo

We previously demonstrated that growth and remodeling was stimulated in arteries elongated ex vivo using step increases in axial strain. Viability and vasoactivity were similar to fresh arteries, however there was a substantial decrease in the ultimate circumferential stress. To test the hypothesis that the subphysiological perfusion conditions (i.e., low pressure and flow) previously used caused the reduction, arteries were subjected to the identical elongation protocol (50% increase over 9 days) while being perfused with physiological levels of flow, viscosity and pulsatile pressure. A significant increase in unloaded length was achieved by elongation under both perfusion conditions, although the increase was less under physiological (7 +/- 1%) than under subphysiological conditions (19 +/- 2%, p < 0.005). When length at physiological stress was estimated using mechanical testing data the values were similar. The ultimate circumferential stress of arteries elongated under physiological conditions was increased (33%), whereas the ultimate axial stress was decreased (50%) as compared with arteries elongated under subphysiological conditions. Elongated arteries under both perfusion conditions showed significant increases in proliferation and collagen mass, and similar viability and appearance to fresh arteries. These data suggest that there is substantial cross-talk between perfusion conditions and axial strain that modulates arterial remodeling and length.

Biomechanical proprieties of decellularized porcine common carotid arteries

To analyze the effects of decellularization on the biomechanical properties of porcine common carotid arteries, decellularization was performed by a detergent-enzymatic procedure that preserves extracellular matrix scaffold. Internal diameter, external diameter, and wall thickness were measured by optical microscopy on neighboring histological sections before and after decellularization. Rupture tests were conducted. Inner diameter and wall thickness were measured by echo tracking during pressure inflation from 10 to 145 mmHg. Distensibility and incremental elastic modulus were computed. At 10 mmHg, mean diameter of decellularized arteries was 5.38 mm, substantially higher than controls (4.1 mm), whereas decellularized and control arteries reached the same internal diameter (6.7 mm) at 145 mmHg. Wall thickness decreased 16% for decellularized and 32% for normal arteries after pressure was increased from 10 to 145 mmHg. Decellularized arteries withstood pressure >2,200 mmHg before rupture. At 145 mmHg, decellularization reduced compliance by 66% and increased incremental elastic modulus by 54%. Removal of cellular elements from media led to changes in arterial dimensions. Collagen fibers engaged more rapidly during inflation, yielding a stiffer vessel. Distensibility was therefore significantly lower (by a factor of 3) in decellularized than in normal vessels: reduced in the physiological range of pressures. In conclusion, decellularization yields vessels that can withstand high inflation pressures with, however, markedly different geometrical and biomechanical properties. This may mean that the potential use of a decellularized artery as a scaffold for the creation of xenografts may be compromised because of geometrical and compliance mismatch.

A Computational Study of Flow in a Compliant Carotid Bifurcation–Stress Phase Angle Correlation with Shear Stress

The present study presents a three-dimensional, unsteady supercomputer simulation of the coupled fluid-solid interaction problem associated with flow through a compliant model of the bifurcation of the common carotid artery into the internal and external carotid arteries. The fluid wall shear stress (WSS) and solid circumferential stress/strain (CS) are computed and analyzed for the first time using the complex ratio of CS to WSS (CS/WSS). This analysis reveals a large negative phase angle between CS and WSS (stress phase angle--SPA) on the outer wall of the carotid sinus where atherosclerotic plaques are localized. This finding is consistent with other measurements and computations of the SPA in coronary arteries and the aortic bifurcation that show large negative SPA correlating with sites of plaque location and in vitro studies of endothelial cells showing that large negative SPA induces pro-atherogenic gene expression and metabolite release profiles.

Arterial Stiffening Influence of Sympathetic Nerve Activity

Studies in animals and humans suggest that sympathetic activity exerts a stiffening influence on large and middle-sized artery walls. We sought to obtain further evidence on this issue by measuring radial artery distensibility in an allotransplanted and thus denervated hand using the contralateral artery as control. In 2 men, blood pressure was measured by a semiautomatic device (Dinamap). Diastolic diameter, systo-diastolic diameter excursion (ultrasound Wall Track system), and distensibility (Reneman formula) of both radial arteries were measured at a level corresponding to 4 cm below the suture of the transplanted hand 40 days after surgery and every 4 weeks for the next 6 months. After surgery, systo-diastolic diameter excursion and distensibility were much greater in the transplanted radial artery than in the contralateral vessel, reaching values similar to the contralateral ones after 4 months, when signs of reinnervation of the transplanted hands had appeared. Radial deinnervation was accompanied by an increased arterial distensibility, which provides further evidence of the sympathetic stiffening effect on arterial wall in humans.

A constitutive formulation of arterial mechanics including vascular smooth muscle tone

A pseudo-strain energy function (pseudo-SEF) describing the biomechanical properties of large conduit arteries under the influence of vascular smooth muscle (VSM) tone is proposed. In contrast to previous models that include the effects of smooth muscle contraction through generation of an active stress, in this study we consider the vascular muscle as a structural element whose contribution to load bearing is modulated by the contraction. This novel pseudo-SEF models not only arterial mechanics at maximal VSM contraction but also the myogenic contraction of the VSM in response to local increases in stretch. The proposed pseudo-SEF was verified with experimentally obtained pressure-radius curves and zero-stress state configurations from rat carotid arteries displaying distinct differences in VSM tone: arteries from normotensive rats displaying minimal VSM tone and arteries from hypertensive rats exhibiting significant VSM tone. The pressure-radius curves were measured in three different VSM states: fully relaxed, maximally contracted, and normal VSM tone. The model fitted the experimental data very well ( r2> 0.99) in both the normo- and hypertensive groups for all three states of VSM activation. The pseudo-SEF was used to illustrate the localized reduction of circumferential stress in the arterial wall due to normal VSM tone, suggesting that the proposed pseudo-SEF can be of general utility for describing stress distribution not only under passive VSM conditions, as most SEFs proposed so far, but also under physiological and pathological conditions with varying levels of VSM tone.

Functional, mechanical and geometrical adaptation of the arterial wall of a non-axisymmetric artery in vitro

OBJECTIVE

Vascular remodeling is an adaptive response to variations in the hemodynamic environment acting on the arterial wall. Remodeling translates into changes of structure, geometry and mechanical properties of the artery. Our aim was to study the remodeling response of pig right common carotid arteries in vitro.

METHODS

In vivo right carotid arteries are exposed to a non-uniform hemodynamic environment and exhibit a strong wall asymmetry in the circumferential direction that allows the study of two regions separately, as the artery remodels under in vitro perfusion. Porcine right common carotid arteries were cultured during 1 day (n = 6), 3 days (n = 6) or 8 days (n = 6) in an in vitro organ culture system, at a constant perfusion pressure of 100 mmHg. Geometrical, histological, biomechanical and biological analysis of the perfused segments was performed at the end of each study.

RESULTS

Smooth muscle cell nuclei density and wall thickness remain constant along the culture periods. Elastin and collagen are significantly redistributed to equilibrate their relative content along the vessel circumference. The distensibility profile is significantly different at day 8. Matrix metalloproteinase-2 expression and activity increase significantly at days 3 and 8.

CONCLUSION

The non-axisymmetric arterial wall adapts to a uniform hemodynamic environment by redistributing the structural components of the extracellular matrix. The changes of collagen and elastin density may result from a vascular remodeling process involving matrix metalloproteinase-2 up-regulation and enzymatic activity. The remodeling response results in a new vascular wall configuration that is more distensible at physiological pressures (30-120 mmHg) and stiffer at higher pressures.

Development of an experimental system for the study of urethral biomechanical function

Despite its principal mechanical function in the storage and release of urine, the biomechanical properties of the urethra have remained largely unexplored. The purpose of this study was to develop and validate an experimental model that can be used for evaluating whole urethral tissue in such a manner. Bladder-urethral specimens were excised from halothane-anesthetized female rats and mounted at in vivo length within the experimental apparatus consisting of a tissue perfusion chamber, an adjustable fluid column, and a laser micrometer. Outer diameter measurements were made at proximal, mid, and distal axial locations in response to increases in intraluminal pressure and after addition of various muscle-responsive agents. Basal smooth muscle tone and regional variations in compliance were detected through pressure-diameter responses. Chemically evoked contractile responses were measured and correspond to regional compositions of intrinsic smooth and striated muscle components. The results presented illustrate the utility of this system, which should permit a more thorough characterization of structure-function relationships and urethral biomechanical function in relation to normal and dysfunctional tissue states.