Damian Craiem

Fractional-order viscoelasticity applied to describe uniaxial stress relaxation of human arteries

Viscoelastic models can be used to better understand arterial wall mechanics in physiological and pathological conditions. The arterial wall reveals very slow time-dependent decays in uniaxial stress-relaxation experiments, coherent with weak power-law functions. Quasi-linear viscoelastic (QLV) theory was successfully applied to modeling such responses, but an accurate estimation of the reduced relaxation function parameters can be very difficult. In this work, an alternative relaxation function based on fractional calculus theory is proposed to describe stress relaxation experiments in strips cut from healthy human aortas. Stress relaxation (1 h) was registered at three incremental stress levels. The novel relaxation function with three parameters was integrated into the QLV theory to fit experimental data. It was based in a modified Voigt model, including a fractional element of order alpha, called spring-pot. The stress-relaxation prediction was accurate and fast. Sensitivity plots for each parameter presented a minimum near their optimal values. Least-squares errors remained below 2%. Values of order alpha = 0.1-0.3 confirmed a predominant elastic behavior. The other two parameters of the model can be associated to elastic and viscous constants that explain the time course of the observed relaxation function. The fractional-order model integrated into the QLV theory proved to capture the essential features of the arterial wall mechanical response.

Fractional order models of viscoelasticity as an alternative in the analysis of red blood cell (RBC) membrane mechanics

New lumped-element models of red blood cell mechanics can be constructed using fractional order generalizations of springs and dashpots. Such spring-pots exhibit a fractional order viscoelastic behavior that captures a wide spectrum of experimental results through power-law expressions in both the time and frequency domains. The system dynamics is fully described by linear fractional order differential equations derived from first order stress-strain relationships using the tools of fractional calculus. Changes in the composition or structure of the membrane are conveniently expressed in the fractional order of the model system. This approach provides a concise way to describe and quantify the biomechanical behavior of membranes, cells and tissues.

Aging Impact on Thoracic Aorta 3D Morphometry in Intermediate-Risk Subjects: Looking Beyond Coronary Arteries with Non-Contrast Cardiac CT

An increasing number of intermediate risk asymptomatic subjects benefit from measures of atherosclerosis burden like coronary artery calcification studies with non-contrast heart computed tomography (CT). However, additional information can be derived from these studies, looking beyond the coronary arteries and without exposing the patients to further radiation. We report a semi-automatic method that objectively assesses ascending, arch and descending aorta dimension and shape from non-contrast CT datasets to investigate the effect of aging on thoracic aorta geometry. First, the segmentation process identifies the vessel centerline coordinates following a toroidal path for the curvilinear portion and axial planes for descending aorta. Then, reconstructing oblique planes orthogonal to the centerline direction, it iteratively fits circles inside the vessel cross-section. Finally, regional thoracic aorta dimensions (diameter, volume and length) and shape (vessel curvature and tortuosity) are calculated. A population of 200 normotensive men was recruited. Length, mean diameter and volume differed by 1.2xa0cm, 0.13xa0cm and 21xa0cm3 per decade of life, respectively. Aortic shape uncoiled with aging, reducing its tortuosity and increasing its radius of curvature. The arch was the most affected segment. In conclusion, non-contrast cardiac CT imaging can be successfully employed to assess thoracic aorta 3D morphometry.

Reactive hyperemia-related changes in carotid-radial pulse wave velocity as a potential tool to characterize the endothelial dynamics

Current methods used to evaluate the endothelial function have limitations. The analysis of the pulse wave velocity (PWV) response to transient ischaemia could be an alternative to evaluate the endothelial dynamics. Aims: To analyze (a) the carotid-radial PWV temporal profile during flow mediated dilatation test, and (b) the PWV changes considering its main vascular geometrical (diameter) and intrinsic (elastic modulus) determinants. Methods: Sixteen healthy young adults were included. The carotid-radial PWV (strain gauge mechano-transducers), wall thickness and brachial diameter (B-Mode ultrasound) were measured before (basal state), during a forearm cuff inflation (5 minutes) and after its deflation (10 minutes). The PWV, brachial diameter and elastic modulus changes and temporal profile were analyzed (basal state, 15, 30, 45, and 60 seconds after cuff deflation). Results: Transient ischaemia was associated with arterial stiffness changes, evidenced by carotid-radial PWV variations. The PWV and diastolic diameter changes, and temporal profiles differed. The arterial stiffness changes could not be explained only by geometrical (diameter) changes. Conclusion: The carotid-radial PWV analysis, evaluated using robust and simple available techniques, could be used in the clinical practice to study the vascular response to transient ischaemia and the endothelial function.

Coronary Arteries Simplified with 3D Cylinders to Assess True Bifurcation Angles in Atherosclerotic Patients

The geometry of coronary arteries affects regional atherogenic processes. Accurate images can be assessed using multislice computer tomography (MSCT) to estimate bifurcations angles. We propose a three-dimensional (3D) method to measure true bifurcation angles of coronary arteries and to determine possible correlations between plaque presence and angulations. The left main (LM) coronary artery, left anterior descendent (LAD) and left circumflex artery (LCX) were imaged in 40 atherosclerotic and 35 healthy patients, using 64-rows MSCT. This Y-junction was simplified fitting a 3D cylinder to each vessel to estimate true bifurcation angles and diameters. The method was tested in phantoms and interobserver variability was assessed. Geometrical results were compared between groups using an unpaired t-test. The cylinders fitted reasonably well with mean distances to measured points below 0.4xa0mm. LAD–LCX bifurcation angles were wider in the atherosclerotic group (pxa0<xa00.01). LAD (pxa0<xa00.01) and LCX (pxa0<xa00.05) diameters were also larger. In phantoms mean absolute difference between true and estimated angles (Nxa0=xa027) was 0.44xa0±xa00.54°. Interobserver mean difference (Nxa0=xa0135) was 1.8xa0±xa05.8°. Simplifying coronary bifurcation with cylinders results in a reliable technique to assess coronary artery geometry in 3D, avoiding planar projections and decreasing interobserver variability. Geometrical risk factors should be incorporated to properly predict atherosclerosis processes.

The physiological impact of the nonlinearity of arterial elasticity in the ambulatory arterial stiffness index

The ambulatory arterial stiffness index (AASI) is claimed to be a new estimator for arterial rigidity. It was recently defined as one minus the slope of the linear regression of systolic to diastolic ambulatory pressure during 24 h. Although several reports testify its clinical relevance, the explanation of how this new index is conceptually associated with arterial stiffness remains controversial. In this work we hypothesize that nonlinear arterial elasticity is behind AASI physiological principles. To that end, random number generators were used to emulate arterial cross-sectional area (CSA) during 24 h. Pressure values were calculated using linear and nonlinear elasticity models for rigid and compliant arteries. The AASI was calculated from simulated pressures and also analytically predicted for each model. Additionally, invasive aortic pressure and CSA were continuously measured in a conscious sheep during 24 h to test the nonlinear model. We found that analytical solutions agreed with simulation outcomes; for the nonlinear model, the AASI was higher in rigid arteries with respect to compliant arteries (0.51 versus 0.38) and the linear model systematically predicted AASI = 0. For in vivo pressure measurements, AASI was 0.31. Using the measured pulsatile CSA and an estimation of the elastic constant for the nonlinear model, the AASI was accurately predicted with errors below 5%. We conclude that the AASI is higher in stiffer arteries due to the nonlinear behavior of the arterial wall. With a nonlinear arterial function, the slope of the linear regression of diastolic to systolic pressures during 24 h depends on the product of an elastic constant by the pulsatile CSA. As the elastic constant dominates the product, the reported associations between the AASI and arterial stiffness indices now have a consistent explanation.

Angle estimation of human femora in a three-dimensional virtual environment

The estimation of human femur morphology and angulation provide useful information for assisted surgery, follow-up evaluation and prosthesis design, cerebral palsy management, congenital dislocation of the hip and fractures of the femur. Conventional methods that estimate femoral neck anteversion employ planar projections because accurate 3D estimations require complex reconstruction routines. In a recent work, we proposed a cylinder fitting method to estimate bifurcation angles in coronary arteries and we thought to test it in the estimation of femoral neck anteversion, valgus and shaft-neck angles. Femora from 10 patients were scanned using multisliced computed tomography. Virtual cylinders were fitted to 3 regions of the bone painted by the user to automatically estimate the femoral angles. Comparisons were made with a conventional manual method. Inter- and intra-reading measurements were evaluated for each method. We found femoral angles from both methods strongly correlated. Average anteversion, neck-shaft and valgus angles were 17.5°, 139.5°, 99.1°, respectively. The repeatability and reproducibility of the automated method showed a 5-fold reduction in inter- and intra-reading variability. Accordingly, the coefficients of variation for the manual method were below 25% whereas for the automated method were below 6%. The valgus angle assessment was globally the most accurate with differences below 1°. Maximum distances from true surface bone points and fitting cylinders attained 6 mm. The employment of virtual cylinders fitted to different regions of human femora consistently helped to assess true 3D angulations.

Cardiac resynchronization results in aortic blood flow-associated changes in the arterial load components: Basal biomechanical conditions determine the load changes

The cardiac resynchronization therapy (CRT) effects on the arterial load components, the mechanisms (i.e. haemodynamic changes-dependence) involved in the load reduction and the factors (i.e. basal load conditions) associated with the load changes after CRT, are to be evaluated. Aims: a) to analyze the potential changes in the arterial load components (peripheral resistances, arterial compliance and impedance) associated with the CRT, b) to determine if the load components changes are associated with variations in haemodynamic variables (pressure, heart rate or blood flow), c) to analyze the relationship between the load components basal state and their changes after CRT. To fulfill these aims cardiac and arterial structural and mechanical parameters were non-invasively evaluated in 8 heart failure patients, pre- and post-CRT (23±8 days). The main results were that short-term after CRT: 1) there were changes in the static and dynamic determinants of the arterial load; 2) the changes in the load components were not associated with heart rate or pressure variations, but with blood flow changes, and 3) the load components basal levels and their changes after CRT were associated.

Association between mechanics and structure in arteries and veins: Theoretical approach to vascular graft confection

Biomechanical and functional properties of tissue engineered vascular grafts must be similar to those observed in native vessels. This supposes a complete mechanical and structural characterization of the blood vessels. To this end, static and dynamic mechanical tests performed in the sheep thoracic and abdominal aorta and the cava vein were contrasted with histological quantification of their main constituents: elastin, collagen and muscle cells. Our results demonstrate that in order to obtain adequate engineered vascular grafts, the absolute amount of collagen fibers, the collagen/elastin ratio, the amount of muscle cells and the muscle cells/elastic fibers ratio are necessary to be determined in order to ensure adequate elastic modulus capable of resisting high stretches, an adequate elastic modulus at low and normal stretch values, the correct viscous energy dissipation, and a good dissipation factor and buffering function, respectively.

Dynamics of cryopreserved human carotid arteries

The viscoelastic properties of the arterial wall are responsible for their functional role in the arterial system. Cryopreservation is widely used to preserve blood vessels for vascular reconstruction but is controversially suspected to affect the dynamic behaviour of these allografts. The aim of this study was to determine whether differences in the dynamic behaviour exist or not between fresh and cryopreserved human common carotid arteries (CCA). Using a previously developed mock circulation system, dynamic pressure-diameter tests were performed on segments of human fresh (n=10) and cryopreserved arteries (n=7). A diameter-pressure transfer function was designed to evaluate the wall dynamics. An adaptive model was fit to obtain its frequency response. Three models were tested. Results show that non-significant differences exist between wall dynamics of fresh and cryopreserved segments of human CCA.