Mechanical Properties Of Vessel Wall Research Articles

Statistical evaluation of measured biomechanical properties of human brain aneurysm samples.

<p>Human brain aneurysms may often prove fatal if not re&shy;cognized in time and treated accordingly. The understanding of development and rupture of aneurysms can significantly be improved by the application of numerical modelling, which in turn, requires the knowledge of mechanical properties of vessel wall. This study aims to identify assumed differences with respect to age, sex, spatial orientation, and rupture by utilizing detailed statistical analysis of uniaxial tensile measurements of human brain aneurysm samples, performed by the authors in a previous project.</p>. <p>At surgery of 42 patients, aneu&shy;rysm fundi were cut distally to the clip. In each case, depending on size, varying number of stripes (altogether 88) were prepared and uniaxial stress-strain measurements were performed. Quantities related to the capacity, energy absorption or stiffness were determined and statistically analysed.</p>. <p>The number of specimens in the aneurysm sample was sufficient to establish statistical differences with respect to sex and rupture (p&lt;0.05). No significant differences were detected in orientation, though higher values of stresses and deformations were ob&shy;tained in the circumferential direction com&shy;pared to the meridional direction.&nbsp;</p>. <p>Significant differences bet&shy;ween sexes with respect to ultimate deformations were demonstrated according to expectation, and the hypothesis on equality of energy capacity could be supported. Similarity of curves with respect to specimen orientation was also observed and ruptured aneurysm sacs tended to be smaller in size. It seems that differences and trends described in this paper are realistic and need to be applied in numerical modelling.</p>.

Specimen width affects vascular tissue integrity for in-vitro characterisation

The preparation of slender specimens for in-vitro tissue characterisation could potentially alter mechanical tissue properties. To investigate this factor, rectangular specimens were prepared from the wall of the porcine aorta for uniaxial tensile loading. Varying strip widths of 16 mm, 8 mm, and 4 mm were achieved by excising zero, one, and three cuts within the specimen along the loading direction, respectively. While specimens loaded along the vessel’s circumferential direction acquired consistent tissue properties, the width of test specimens influenced the results of axially loaded tissue; vascular wall stiffness was reduced by approximately 40% in specimens with strips 4 mm wide. In addition, the cross-loading stretch was strongly influenced by specimen strip width, and fiber sliding contributed to the softening of slender tensile specimens, an outcome from finite element analysis of test specimens. We may, therefore, conclude that cutting orthogonal to the main direction of collagen fibers introduces mechanical trauma that weakens slender tensile specimens, compromising the determination of representative mechanical vessel wall properties.

A comparison of passive and active wall mechanics between elastic and muscular arteries of juvenile and adult rats

The elastic abdominal aorta and muscular femoral artery are susceptible to aneurysm and atherosclerosis, respectively. The vessel wall mechanics should be an important element for the difference. The objective of the study is to demonstrate a comparison of vessel wall mechanics between elastic and muscular arteries of juvenile and adult rats to show the changes of mechanical properties relevant to aging. The passive and active mechanical tests, theoretical analysis, and histological evaluation were carried out to investigate mechanical properties of vessel walls in the abdominal aorta and carotid and femoral arteries of young and adult rats. There are stiffening femoral artery, unchanged carotid artery, and distensible abdominal aorta in adult rats as compared with the young. The opening angle has values of 54 ± 13°, 82 ± 13°, and 94 ± 13° in the abdominal aorta and carotid and femoral arteries of adult rats, respectively, as well as 80 ± 22°, 93 ± 19°, and 100 ± 23° in the young. The findings are explained by the significantly reduced width of collagen fibers in the abdominal aorta, relatively unchanged width in the carotid artery, and significantly increased width in the femoral artery of adult rats as compared with the young. In conjunction with available literatures, we concluded that inconsistency for nonlinear age-related changes of artery wall mechanics occurs between arteries of different types, which may be a risk factor for the occurrence of abdominal aorta aneurysm and femoral artery atherosclerosis.

In Vivo Non-Invasive Analysis of the Mechanical Properties of Vessel Walls using Vibrational Optical Coherence Tomography

In this study we use vibrational optical coherence tomography and ultrasound to image and measure the mechanical properties of arteries and veins in vivo at rest and after running. Our results suggest that several arteries and veins are tethered together via the surrounding ECM, both laterally and longitudinally. The collagen fibers and smooth muscle cells of arterial and venous walls give rise to a mechano-vibrational peak at 140-160 Hz with a modulus of between 4 and 5 MPa. Exercise increases the arterial modulus by about 2 MPa and can influence the properties of surrounding arteries and veins via the adjacent extracellular matrix. It is concluded that lateral vascular tethering results in effective stress transfer between arteries and neighboring veins. This limits stress concentrations and transmits mechanical information between the arterial and venous sides of the vascular system. Longitudinal tethering provides mechanical coupling up and down the arterial system that conveys loading information via the wall shear stress wave that propagates through the vascular tree. In this manner local changes in external and internal mechanical loading can influence anabolic and catabolic responses in other segments of the vasculature as a result of mechanotransduction.

Experimental measurement and modeling analysis of active and passive mechanical properties of arterial vessel wall

Coronary artery diseases (CAD) have always been serious threats to human health. The measurement, constitutive modeling, and analysis of mechanical properties of the blood vessel wall can provide a tool for disease diagnosis, stent implantation, and artificial artery design. The vessel wall has both active and passive mechanical properties. The passive mechanical properties are mainly determined by elastic and collagen fibers, and the active mechanical properties are determined by the contraction of vascular smooth muscle cells (VSMC). Substantial studies have shown that, the two-layer model of the vessel wall can feature the mechanical properties well, and the circumferential, axial and radial strain and stress are of great significance in arterial wall mechanics. This study reviewed recent investigations of mechanical properties of the vessel wall. Challenges and opportunities in this area are discussed relevant to the clinical treatment of coronary artery diseases.

Wire myography in modern scientific researches: methodical aspects

In the present work modern approaches to investigation of isolated vessel's vasoactive properties with myography method were described. Authors give detailed illustrated description to the vessels preparation technique and order of actions during myography based on own experience and published data. Authors described approaches to investigate mechanical properties of vessel wall and vasoactive properties with wire myography method.

Aortic artery distensibility shows inverse correlation with heart rate variability in elderly non-hypertensive, cardiovascular disease-free individuals: the Ikaria Study

The aim of this study was to evaluate the relation between aortic artery distensibility, expressed by pulse pressure (PP) and autonomic nervous system function expressed by heart rate variability (HRV), among elders. During 2009, 469 individuals (50 % males, mean age 75 ± 6 years old), permanent inhabitants of Ikaria Island, were enrolled. Among several sociodemographic, bioclinical, lifestyle, and dietary characteristics, and biochemical parameters related to cardiovascular risk, a digital, standard, 12-lead electrocardiogram (ECG) and 5-min ECG monitoring were recorded. Smart ECG measurement and interpretation programs were used for the automated measurement and interpretation of ECG intervals. Autonomic nervous system tone was estimated with the HRV-standard deviation of normal to normal intervals (SDNN)-in surface ECG. Pulse pressure was calculated as the difference between systolic and diastolic blood pressures. Pulse pressure showed a positive association with age, systolic blood pressure levels, and presence of diabetes mellitus (all P < 0.001) and an inverse correlation with logSDNN. Multiadjusted logistic regression analysis revealed that 10-unit increase in SDNN values decreases by 26 % the likelihood of having PP above 70 mmHg; when the analysis was stratified according to hypertension status, the relationship remained significant only among non-hypertensive individuals. These data support the hypothesis that cardiac sympathovagal disturbance correlates with impaired elasticity properties of aorta, indicating parallel impairment in cardiac autonomic modulation and mechanical vessel wall properties in elderly non-hypertensive individuals.

Systemic Dilation Diathesis in Patients with Abdominal Aortic Aneurysms: A Role for Matrix Metalloproteinase-9?

IntroductionAccumulating evidence suggests that patients with abdominal aortic aneurysm (AAA) suffer from a systemic dilating condition affecting all arteries. Matrix metalloproteinases (MMPs) and their natural inhibitors, the tissue inhibitors of metalloproteinases (TIMPs), appear to be involved in aneurysm formation, as evidenced by increased aortic tissue MMP activity and plasma MMP levels in patients with AAA. Hypothesizing that an imbalance in plasma MMP/TIMP level might be associated with a systemic dilation diathesis, we studied mechanical vessel wall properties of non-affected arteries of patients with either AAA or aorto-iliac obstructive lesions in association with plasma MMP-9 and TIMP-1 levels. MethodsTwenty-two patients with AAA and 12 with aorto-iliac occlusive disease (AOD) were included. Diastolic diameter (d) and distension (Δd) were measured at the level of the common carotid artery (CCA) and suprarenal aorta (SA) using ultrasonography. Distensibility (DC) and compliance (CC) were calculated from d, Δd and brachial pulse pressure. Plasma MMP-9 and TIMP-1 were determined with specific immunoassays. ResultsThe average (±SD) age was 72.3±5.6 and 65.0±8.2 years for the AAA and AOD patients, respectively, (P=0.005). CCA diameter was 9.1±1.3mm in AAA patients and AOD 7.8±1.4mm in AOD patients, P=0.009. This difference persisted after correction for age. Plasma MMP-9 and TIMP-1 did not differ significantly between AAA and AOD patients. In the total 34 patients, the MMP-9/TIMP-1 ratio was correlated inversely with distensibility (r=−0.74, P=0.002) and to compliance (r=−0.58, P=0.024) of the suprarenal aorta. ConclusionsThe CCA diameter was larger in AAA patients compared to AOD patients. MMP-9/TIMP-1 ratio was associated with decreased distensibility and compliance of the suprarenal aorta. These data support the idea that AAA patients exhibit a systemic dilation diathesis, which might be attributable to MMP/TIMP imbalances.

Relationship between muscle sympathetic nerve activity and large artery mechanical vessel wall properties in renal transplant patients.

Renal transplant recipients (RTX) show a major impairment of large artery elastic wall properties. Sympathetic overactivity present in patients with renal disease has been shown to alter large artery elasticity; however, in RTX, this issue has not been addressed. The present study therefore investigated a possible relationship between sympathetic activity and large artery distensibility in RTX. In 32 patients treated with calcineurin inhibitors (RTX-CI, cyclosporine n = 16, tacrolimus n = 16) mean arterial pressure (MAP, automatic sphygmomanometer), muscle sympathetic nerve activity (MSNA, microneurography) and distensibility coefficients of the brachial and carotid arteries (pulsed Doppler) were measured. Sixteen healthy volunteers (CTR), six patients with calcinneurin inhibitor-free immunosuppression (RTX-AZA) and 12 transplant patients after native kidney nephrectomy (RTX-NC) served as control groups. RTX-CI significantly increased MSNA compared to CTR (36 +/- 3 versus 16 +/- 2 bursts/min, P < 0.05, mean +/- SEM). Both brachial and carotid artery distensibility were decreased in RTX-CI compared to CTR (7 +/- 1 versus 13 +/- 1 +/- 10(-3) /kPa and 17 +/- 1 versus 25 +/- 2 x 10(-3) /kPa, respectively, both P < 0.05). In RTX-CI, a significant inverse correlation between brachial, but not carotid artery distensibility and MSNA (r = -0.46, P < 0.01, r = -0.12, not significant, respectively) was found. Correlation between brachial artery distensibility and MSNA remained statistically significant on separate analysis of cyclosporine- or tacrolimus-treated RTX and after correction for arterial diameter, blood pressure, graft function, age and sex by stepwise multiple regression analysis. Results in RTX-AZA were similar to those in RTX-CI. In contrast, in RTX-NC with MSNA not significantly different from CON (16.6 +/- 2.0 bursts/min), brachial artery distensibility was significantly higher compared to RTX-CI and RTX-AZA (14.2 +/- 2.0 x 10(-3) /kPa, P < 0.05, respectively). Increased sympathetic nerve activity in renal transplant patients is related to decreased distensibility of the muscular type brachial artery, but not the elastic type carotid artery.

Acute effects of haemodialysis on endothelial function and large artery elasticity.

Disturbances of functional properties of large arteries contribute to increased cardiovascular morbidity and mortality in patients with end-stage renal disease. However, it is not clear whether haemodialysis per se acutely affects mechanical vessel wall properties or endothelial function. Twenty-five chronic haemodialysis patients (mean+/-standard error of the mean (SEM): age 52+/-5 years; time on dialysis 63+/-7 months; blood pressure 132+/-4/72+/-2 mmHg) were studied before and immediately after a haemodialysis (HD) session using a polysulphone dialyser (ultrafiltration 1460+/-54 ml), as well as on the following day. Blood pressure was measured with an automatic sphygmomanometer and applanation tonometry. End-diastolic diameter and distension of the brachial and carotid arteries were measured by Doppler frequency analysis of vessel wall movements in M-mode using a multigate pulsed Doppler system and aortic pulse wave velocity (PWV) by an automatic device (Complior). Endothelial function was determined as brachial artery flow-mediated dilation (FMD) and compared with endothelium-independent nitroglycerine-induced dilation (NMD). FMD was 7.9+/-1.8% in patients before HD and did not change significantly after HD or in the dialysis-free intervall (6.7+/-2.1 and 7.1+/-2.0%, respectively; NS). The same was true for NMD and PWV (12.6+/-0.8 m/s before HD, 12.8+/-0.8 m/s after HD, and 11.9+/-0.7 m/s on the HD-free day). Carotid distensibility coefficients decreased significantly during HD (from 18.1+/-1.9 x 10(-3)/kPa to 16.7+/-2.2 x 10(-3)/kPa, P<0.05) and increased again on the HD-free day (19.8+/-2.4 x 10(-3)/kPa). However, when corrected for blood pressure by tonometry, isobaric carotid distensibility did not change significantly. Brachial artery distensibility also did not show significant acute changes. Haemodialysis per se did not have a significant effect on endothelial function or large artery mechanical vessel wall properties in patients on maintenance dialysis therapy.

Gefäßwandveränderungen bei Niereninsuffizienz

Cardiovascular events are the main cause of death in patients with end-stage renal disease. Functional and structural alterations of the arterial system substantially contribute to the high cardiovascular mortality in these patients. Structural alterations of the arterial wall comprise intima-media thickening and atherosclerotic plaque formation. Moreover, mechanical vessel wall properties of large arteries are significantly disturbed. This is already observed in young patients. Reduced arterial distensibility impairs large artery cushioning function. This results in increased ventricular afterload promoting left ventricular hypertrophy and in reduced coronary perfusion. After renal transplantation, structural alterations of the arterial wall and disturbed mechanical vessel wall properties persist. Moreover, renal failure is characterized by a severe impairment of endothelial function. Disturbed endothelial function results from reduced production of endothelium-dependent endogenous vasodilators and/or blunted vascular effects of these substances. Uremia is associated with the accumulation of an endogenous inhibitor of the endothelial nitric oxide synthase. Impaired endothelial function in renal failure promotes the progression of structural lesions of the arterial wall. Also in renal transplant recipients, substantially impaired endothelial function is observed despite correction of uremia. Hyperparathyroidism commonly observed in renal failure contributes to the disturbed functional vessel wall properties of large arteries. In patients with end-stage renal disease, decreased large artery distensibility is an independent risk factor for increased cardiovascular mortality. This may apply also to intima-media thickening and to impaired endothelial function.

Flow-mediated vasodilation and distensibility in relation to intima-media thickness of large arteries in mild essential hypertension

Whether endothelial dysfunction in essential hypertension is a cause or a consequence of structural vessel wall alterations is not known. The purpose of the present study was to compare flow-mediated vasodilation and mechanical vessel wall properties of large arteries between never treated mild essential hypertensive patients with normal intima-media thickness (IMT) and those exhibiting intima-media thickening. We measured brachial and carotid artery diameter and distension by Doppler frequency analysis of vessel wall movements in M-mode in ten essential hypertensive patients with normal carotid artery IMT (HYP1), in ten patients with increased IMT (HYP2), and in 13 normotensive control subjects (CON). Thereafter, we measured changes in brachial artery (BA) diameters during distal reactive hyperemia after 4 min of forearm occlusion. Nitroglycerin-mediated vasodilation was measured to assess endothelium-independent vasodilation, and BA blood flow was estimated using a pulsed Doppler system. Intima-media thickness of the carotid arteries was examined by high resolution B-mode ultrasound. IMT was 0.66 ± 0.02 mm in the HYP1 group, 0.84 ± 0.03 mm in the HYP2 group ( P < .01 v HYP1, P < .01 v CON), and 0.71 ± 0.04 mm in the CON group. Forearm occlusion was reduced in both the HYP1 group (3.4% ± 3.6%, P < .01 v CON) and the HYP2 group (6.4% ± 1.5%, P < .05 v CON) when compared with the CON group (16.5% ± 2.8%). Nitroglycerin-mediated vasodilation and BA blood flow were not different between study groups. BA distension (as well as carotid artery distension) was significantly lower in the HYP1 group (52 ± 6 μm, P < .05 v CON), but not in the HYP2 group (72 ± 10 μm) when compared with the CON group (88 ± 13 μm). The data suggest that endothelial dysfunction and reduced distensibility of large arteries in patients with essential hypertension occur in the absence of structural vessel wall alterations.

Studies on cardiac sympathovagal balance and large artery distensibility in patients with untreated essential hypertension.

Power spectral analysis of heart rate variability and arterial distensibility are non-invasive measures of cardiac autonomic modulation and mechanical vessel wall properties, respectively. The aim of the present study was to assess cardiac sympathovagal balance, carotid and brachial artery distensibility and a possible relation between these parameters in mildly hypertensive patients as compared to normotensive controls. Total power (TP, 0.01 to 0.5 Hz) and spectral components (low frequency 0.04-0.15 Hz, mainly sympathetic cardiac modulation; high frequency 0.15-0.4 Hz, mainly vagal cardiac modulation) and cardiac sympathovagal balance (LF/LH ratio) of short term heart rate variability (ECG-recording) were calculated in 15 untreated essential hypertensive patients (HYP) and 15 age- and sex-matched healthy controls (CON). Brachial and carotid artery distensibility coefficient (DC) was measured with a multigate doppler system (echo-tracking). TP (ms2 x 10(-3)) (11.2 +/- 0.8 vs 13.6 +/- 0.9, P < 0.03), LF/HF ratio (1.07 +/- 0.08 vs 0.75 +/- 0.07, P < 0.01) and HF (ms2 x 10(-3)/%) (0.7 +/- 0.1/49 +/- 2 vs 1.3 +/- 0.2/58 +/- 2, P < 0.01/P < 0.01) were significantly reduced in HYP compared to CON subjects. LF (ms2 x 10(-3)/%) was 0.7 +/- 0.1/50 +/- 2 vs 0.9 +/- 0.1/41 +/- 2, P = 0.16/P < 0.01. Carotid artery DC (15 +/- 2 vs 26 +/- 2, P < 0.001) and brachial artery DC (4.7 +/- 0.6 vs 9 +/- 1.0, P < 0.001) were significantly reduced in HYP. There was a significant correlation between carotid DC and LF/HF (rho = -0.41, P < 0.03). The data shows reduced heart rate variability and altered cardiac sympathovagal balance as well as impaired arterial distensibility in untreated mildly hypertensive patients. The relative increase in sympathetic modulation and decreased carotid distensibility appear to be related.

Role of physical properties of resistance vessel wall in regulating peripheral blood flow—II. Structural and mechanical behavior

In order to examine the structural and mechanical properties of the vessel wall resistance when subjected to autoregulatory flow control, a mechanical model for the vascular wall was derived from a mathematical model. The mechanical model was an analogue model which connected in series the Maxwell model (elastic modulus: K 3) with the parallel elements of Hill's model (elastic modules: K 2) and Hooke's elastic model (elastic modulus: K 1); it was also mathematically equivalent to the Spring model (see part I). The structural and mechanical properties of the resistance vessel wall were characterized by the three elastic moduli ( K 1, αK 2 and K 3) [mmHg]. The parameter α was a modification factor of the elastic modulus K 2 given by the myogenic mechanism. After a numerical analysis of the experimental data given by the mechanical model, we confirmed that the arterial pressure range for autoregulatory flow controls shifted to the upper region with an increase of the elastic modulus K 1 and the flow regulation was reduced.