J. G. Barra

Experimental and clinical validation of arterial diameter waveform and intimal media thickness obtained from B-mode ultrasound image processing

A new automated computerized system (IôTEC) that assesses concomitantly the instantaneous temporal arterial diameter and intimal media thickness (IMT) obtained from B-mode ultrasound (US) images was validated by sonomicrometry in sheep, by an echo-tracking system in humans, and by a Lucite phantom in vitro. Differences between methods for diameter measurements did not vary in any systematic way, with no significant differences in the lower frequency range. Ultrasonic measurements of the true phantom gap sizes showed high correlation (r2 = 0.98,p < 0.001) with no systematic errors. Carotid and femoral arteries in humans were strongly related between IôTEC and echo-tracking device (r2 = 0.94 carotid; R2 = 0.88 femoral, p < 0.001), with a Gaussian distribution of the errors. This new method showed high intra- and interobserver repeatability of arterial diameter and IMT, allowing consistent characterization of arterial dynamics in humans.

Identification of Arterial Wall Dynamics in Conscious Dogs

Viscoelastic properties determine the dynamic behaviour of the arterial wall under pulsatile pressure and flow, suggesting time‐ or frequency‐dependent responses to changes in wall stress and strain. The objectives of the present study were: (i) to develop a simplified model to derive simultaneously the elastic, viscous and inertial wall moduli; (ii) to assess Youngs modulus as a function of frequency, in conscious, chronically instrumented dogs. Parametric discrete time models were used to characterise the dynamics of the arterial system based on thoracic aortic pressure (microtransducer) and diameter (sonomicrometry) measurements in control steady state and during activation of smooth muscle with the α‐adrenoceptor agonist phenylephrine (5 μg kg−1 min−1, I.V.), in eight conscious dogs. The linear autoregressive model and a physically motivated non‐linear model were fitted to the input‐output (stress‐strain) relationship. The aortic buffering function (complex Youngs modulus) was obtained in vivo from the identified linear model. Elastic, viscous and inertial moduli were significantly increased from control state ((44.5 ± 7.7) × 104 Pa; (12.3 ± 4.7) × 104 Pa s; (0.048 ± 0.028) × 104 Pa s2) to active state ((85.3 ± 29.5) × 104 Pa, P < 0.001; (22.4 ± 8.3) × 104 Pa s, P < 0.05; (0.148 ± 0.060) × 104 Pa s2, P < 0.05). These moduli, obtained using the linear model, did not present significant differences compared with those derived using the non‐linear model. In control conditions, the magnitude of the normalised complex Youngs modulus was found to be similar to that reported in previous animal studies ranging from 1 to 10 Hz. During vascular smooth muscle activation, this modulus was found to be increased with regard to control conditions (P < 0.01) in the frequency range used in this study. The frequency‐dependent Youngs modulus of the aortic wall was obtained for the first time in conscious, unsedated dogs. The parametric modelling approach allows us to verify that vascular smooth muscle activation increases the elastic, viscous and inertial moduli with the advantage of being able to track their time evolution. Furthermore, under activation, the aortic wall remains stiff in the physiological frequency range, suggesting the impairment of the arterial buffering function.

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.

A patient-specific method for the evaluation of wall shear stress in human coronary arteries

Atherosclerotic plaques form at specific sites of the arterial tree, an observation that has led to the “geometric risk factor” hypothesis for atherogenesis. It is accepted that the location of atherosclerotic plaques is correlated with sites subjected to low abnormal values of wall shear stress (WSS), which is in turn determined by the specific geometry of the arterial segment. In particular, the left coronary artery (LCA) is one of the most important sites of plaque formation and its progression may lead to stroke. However, little is known about hemodynamics and WSS distributions in the LCA. The purpose of this work is to set up a method to evaluate flow patterns and WSS distributions in the human LCA based on real patient-specific geometries reconstructed from medical images.

Arterial pressure fractality is highly dependent on wave reflection

Wave reflection is an important factor that influences pressure wave morphology and becomes more significant with aging, when cardiovascular risk increases. A pressure wave, measured at any location in the arterial tree, can be decomposed into its forward and backward components and depends on the corresponding amplitude and shifting time delays. Fractal dimension (FD) quantifies the time series complexity defined by its geometrical representation. Objective: The aim of this study was to evaluate the arterial pressure and diameter time series in order to assess the relationship between wave reflection and arterial pressure fractal dimension (FD). Methods: Simultaneous aortic pressure and diameter were measured in 14 conscious dogs. A pair of ultrasonic crystals, a pressure microtransducer and a pneumatic cuff occluder were positioned in the upper third of the descending aorta. Results: Total reflection induced by the occlusion maneuver decreased FD concomitant to the aortic stiffening. Conclusion: Arterial pressure fractality is highly dependent on wave reflection.

Contribution of arterial tree structure to the arterial pressure fractal behavior

Arterial vascular beds can be characterized considering the arterial segments in terms of their physical properties. These and other trees have an open structure based on repeated bifurcations, following fractal rules. Fractal dimension (FD) quantifies the time series complexity defined by its geometrical representation. Objective: To evaluate the arterial pressure and diameter time series in order to assess the influence of arterial tree structure in arterial pressure fractal dimension (FD). Methods: Simultaneous aortic pressure and diameter were measured in 14 conscious dogs. A pair of ultrasonic crystals, a pressure microtransducer and a pneumatic cuff occluder were positioned in the upper third of the descending aorta. Results: Total reflection induced by the occlusion maneuver decreased FD concomitant to the aortic stiffening and early wave reflection. Conclusion: Arterial pressure fractality is highly dependent on the arterial tree structure.

Assessment of pulsatile wall shear stress in compliant arteries: Numerical model, validation and experimental data

There is evidence that wall shear stress (WSS) is associated with vascular disease. In particular, it is widely accepted that vascular segments with low or oscillatory values of WSS are more probable to develop vascular disease. It is then necessary to establish a realistic model of the blood flow in blood vessels in order to determine precisely WSS. We proposed a numerical 1D model which takes into account the pulsatile nature of blood flow, the elasticity of the vessel, and its geometry. The model allows the calculation of shear stress. It was validated for stationary situations. Then, we computed the time-dependent WSS distribution from experimental data in the sheep thoracic aorta. Results showed that mean WSS calculated through steady flow and rigid walls models is overestimated. Peak WSS values for pulsatile flow must be considered since they resulted to be at least one order higher than mean values. Oscillations in shear stress in a period showed to be approximately of 40%. These findings show that the proposed model is suitable for estimating time-dependent WSS distributions, and confirm the need of using this kind of model when trying to evaluate realistic WSS in blood vessels.

Arterial complex elastic modulus was preserved after an intercontinental cryoconserved exchange

There is a pressing need to obtain adequate vascular substitutes for arterial by-pass or reconstruction. Since the performance of venous and commercially prosthetic grafts is not ideal and the availability of autologous arteries is limited, the use of cryopreserved arteries has emerged as a very attractive alternative. In this sense, the development of an inter-continental network for cryopreserved tissue exchange would improve international cooperation increasing the possibilities of obtaining the requested materials. In this work, the effects of an inter-continental shipment, which includes cryopreservation, on the biomechanical properties of sheep aortas were evaluated by means of the arterial complex elastic modulus. It is shown that these properties were preserved after the shipment. The actual possibilities of establishing a network for arterial exchange for the international cooperation are discussed.