Daniel Bia

Linear and Nonlinear Viscoelastic Modeling of Aorta and Carotid Pressure–Area Dynamics Under In Vivo and Ex Vivo Conditions

A better understanding of the biomechanical properties of the arterial wall provides important insight into arterial vascular biology under normal (healthy) and pathological conditions. This insight has potential to improve tracking of disease progression and to aid in vascular graft design and implementation. In this study, we use linear and nonlinear viscoelastic models to predict biomechanical properties of the thoracic descending aorta and the carotid artery under ex vivo and in vivo conditions in ovine and human arteries. Models analyzed include a four-parameter (linear) Kelvin viscoelastic model and two five-parameter nonlinear viscoelastic models (an arctangent and a sigmoid model) that relate changes in arterial blood pressure to the vessel cross-sectional area (via estimation of vessel strain). These models were developed using the framework of Quasilinear Viscoelasticity (QLV) theory and were validated using measurements from the thoracic descending aorta and the carotid artery obtained from human and ovine arteries. In vivo measurements were obtained from 10 ovine aortas and 10 human carotid arteries. Ex vivo measurements (from both locations) were made in 11 male Merino sheep. Biomechanical properties were obtained through constrained estimation of model parameters. To further investigate the parameter estimates, we computed standard errors and confidence intervals and we used analysis of variance to compare results within and between groups. Overall, our results indicate that optimal model selection depends on the artery type. Results showed that for the thoracic descending aorta (under both experimental conditions), the best predictions were obtained with the nonlinear sigmoid model, while under healthy physiological pressure loading the carotid arteries nonlinear stiffening with increasing pressure is negligible, and consequently, the linear (Kelvin) viscoelastic model better describes the pressure–area dynamics in this vessel. Results comparing biomechanical properties show that the Kelvin and sigmoid models were able to predict the zero-pressure vessel radius; that under ex vivo conditions vessels are more rigid, and comparatively, that the carotid artery is stiffer than the thoracic descending aorta; and that the viscoelastic gain and relaxation parameters do not differ significantly between vessels or experimental conditions. In conclusion, our study demonstrates that the proposed models can predict pressure–area dynamics and that model parameters can be extracted for further interpretation of biomechanical properties.

Regional Differences in Viscosity, Elasticity and Wall Buffering Function in Systemic Arteries: Pulse Wave Analysis of the Arterial Pressure-Diameter Relationship

INTRODUCTION AND OBJECTIVES Regional variations in the incidence of vascular diseases have been related to regional differences in arterial viscoelasticity. The aim of this study was to characterize the differences in the elastic and viscous modulus and in wall buffering function between central and peripheral systemic arteries, through a time-series analysis of the pressure-diameter relationship. MATERIAL AND METHOD Pressure and diameter were measured in seven arterial segments (carotid, brachiocephalic trunk, ascending aorta, proximal, middle and distal descending thoracic aorta, and femoral artery) from six sheep. Each segment was mounted on an in vitro mock circulatory system and perfused with Tyrode solution, with a pulse frequency of 1.8 Hz and systemic pressure levels. We used the Kelvin-Voigt model to calculate the pressure-diameter elastic (Epd, mmHg/mm) and viscous (Vpd, mmHg.s/mm) modulus, and to quantify the local wall buffering function (Vpd/Epd). We also calculated the incremental Youngs and pressure-strain elastic modulus and pulse wave velocity for each segment. RESULTS The elastic and viscous modulus increased from proximal to distal segments. The wall buffering function did not differ significantly between arteries. The lower rigidity of the central arteries compared to the distal ones may indicate that the systolic arterial compliance function is concentrated in the central arterial segments. On the other hand, the greater viscosity in the distal segments may indicate that viscous energy loss is concentrated in these segments. CONCLUSIONS Arterial elasticity and viscosity can be interpreted as properties that are dependent on the region of the vessel, whereas wall buffering function can be considered region-independent.

Assessment of training-dependent changes in the left ventricle torsion dynamics of professional soccer players using speckle-tracking echocardiography

Recently, it has been proposed the use of speckle- tracking echography (STE) to study the left ventricle (LV) torsion dynamics, which would make LV torsion assessment more available in clinical and research cardiology. LV torsion has been described during exercise and in some sportsmen, but so far, its dynamics has not been studied in soccer players. The aims were to characterize and to compare LV apical and basal rotation, and to analyze LV torsion in professional soccer players using STE, and to determine the main differences in torsion between soccer players and age-matched non-trained individuals. The STE allowed characterizing LV rotation and torsion in both groups. LV torsion level and velocities were lesser in soccer players than in non-trained individuals. Changes in torsion in soccer players could represent physiological adaptations to training.

Diferencias regionales en viscosidad, elasticidad y amortiguamiento parietal de arterias sistémicas: análisis isopulsátil de la relación presión-diámetro arterial

Introduccion y objetivos Variaciones regionales en la incidencia de diversas afecciones vasculares se han relacionado con diferencias regionales en la viscoelasticidad arterial. El objetivo de este trabajo fue caracterizar las diferencias regionales en el modulo elastico, viscoso y en el amortiguamiento parietal de arterias sistemicas, centrales y perifericas, mediante el analisis de la relacion instantanea presion-diametro arterial en el dominio temporal. Material y metodo Se midieron la presion y el diametro en 7 segmentos arteriales extraidos de 6 ovejas: carotida, tronco braquiocefalico, aorta ascendente, aorta toracica descendente proximal, media y distal, y arteria femoral. Cada segmento fue montado en un sistema circulatorio in vitro y perfundido con solucion Tyrode, con frecuencia de estimulacion de 1,8 Hz y valores de presion sistemica. Utilizando un modelo Kelvin-Voigt, se obtuvieron el modulo presion-diametro elastico (Epd, mmHg/mm) y viscoso (Vpd, mmHg×s/mm), y se cuantifico la funcion de amortiguamiento parietal (FAP) como Vpd/Epd. Adicionalmente, se calculo el modulo de Young incremental y elastico presion-deformacion y la velocidad de onda del pulso de cada segmento. Resultados Los modulos elasticos y viscoso aumentaron hacia los segmentos perifericos, mientras que la FAP no mostro diferencias entre segmentos. La menor rigidez en las arterias centrales y la mayor viscosidad en las arterias perifericas podrian indicar que la funcion de reservorio arterial sistolico se concentra en las primeras y la disipacion viscosa de energia en las segundas. Conclusiones La respuesta elastica y viscosa arterial podrian considerarse dependientes de la region arterial, mientras que la constante de FAP seria un indicador independiente de la region arterial.

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

Aim:  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.

Increased reversal and oscillatory shear stress cause smooth muscle contraction-dependent changes in sheep aortic dynamics: role in aortic balloon pump circulatory support

Aims:  The intra‐aortic balloon pumping (IABP) changes pressure and increases the aorta shear stress reversal (SSR) and oscillatory (SSO) components. Hence, IABP‐dependent changes in aortic biomechanics would be expected, because of vascular smooth muscle (VSM) tone (i.e. flow‐induced endothelium‐dependent response, related to SSR and SSO variations) and/or pressure changes. To characterize: (i) the IABP effects on the aortic and global (systemic circulation) biomechanics, analysing their dependence on pressure and VSM basic tone changes and (ii) the relation between the SSR and SSO and the aortic biomechanical changes associated with the VSM tone variations.

Non-Invasive Biomechanical Evaluation of Implanted Human Cryopreserved Arterial Homografts : Comparison with Pre-Implanted Cryografts and Arteries from Human Donors and Recipients ()

Native vessels–grafts biomechanical mismatch (BM) is related to graft failure. The BM could be reduced using human cryopreserved/defrosted arteries (cryografts), but post-thaw cryografts’ recovery could be associated with an impaired biomechanical behavior. In vitro, we demonstrated that our cryopreservation methods do not affect arteries’ biomechanics, but only post-implant studies would allow determining the cryografts’ biomechanical performance. Aim To characterize the biomechanical properties of implanted cryografts, and to compare them with cryografts pre-implant, recipients’ native arteries, and arteries from subjects with characteristics similar to those of the recipients and multiorgan donors (MOD) whose arteries were cryopreserved. Methods Native femoral arteries anastomosed to cryografts, implanted cryografts, and arteries from subjects, recipient-like and MOD-like, were studied. In vitro (pre-implant cryografts) and in vivo non-invasive studies were performed. Arterial pressure, diameter, and wall thickness were obtained to quantify local and regional biomechanical parameters, and to evaluate the arterial remodeling. Conclusion Implanted cryografts were remodeled, with an increased wall thickness, wall-to-lumen ratio, and wall cross-sectional area. The proximal–distal gradual transition in stiffness remained unchanged. Implanted cryografts were stiffer than MOD-like arteries, but more compliant than recipients’ arteries. The cryografts–native arteries biomechanical differences were lesser than those described for venous grafts or expanded polytetrafluoroethylene.

Pulse wave velocity as marker of preclinical arterial disease: reference levels in a uruguayan population considering wave detection algorithms, path lengths, aging, and blood pressure.

Carotid-femoral pulse wave velocity (PWV) has emerged as the gold standard for non-invasive evaluation of aortic stiffness; absence of standardized methodologies of study and lack of normal and reference values have limited a wider clinical implementation. This work was carried out in a Uruguayan (South American) population in order to characterize normal, reference, and threshold levels of PWV considering normal age-related changes in PWV and the prevailing blood pressure level during the study. A conservative approach was used, and we excluded symptomatic subjects; subjects with history of cardiovascular (CV) disease, diabetes mellitus or renal failure; subjects with traditional CV risk factors (other than age and gender); asymptomatic subjects with atherosclerotic plaques in carotid arteries; patients taking anti-hypertensives or lipid-lowering medications. The included subjects (n = 429) were categorized according to the age decade and the blood pressure levels (at study time). All subjects represented the “reference population”; the group of subjects with optimal/normal blood pressures levels at study time represented the “normal population.” Results. Normal and reference PWV levels were obtained. Differences in PWV levels and aging-associated changes were obtained. The obtained data could be used to define vascular aging and abnormal or disease-related arterial changes.

Integrated Evaluation of Age-Related Changes in Structural and Functional Vascular Parameters Used to Assess Arterial Aging, Subclinical Atherosclerosis, and Cardiovascular Risk in Uruguayan Adults: CUiiDARTE Project

This work was carried out in a Uruguayan (South American) population to characterize aging-associated physiological arterial changes. Parameters markers of subclinical atherosclerosis and that associate age-related changes were evaluated in healthy people. A conservative approach was used and people with nonphysiological and pathological conditions were excluded. Then, we excluded subjects with (a) cardiovascular (CV) symptoms, (b) CV disease, (c) diabetes mellitus or renal failure, and (d) traditional CV risk factors (other than age and gender). Subjects (n = 388) were submitted to non-invasive vascular studies (gold-standard techniques), to evaluate (1) common (CCA), internal, and external carotid plaque prevalence, (2) CCA intima-media thickness and diameter, (3) CCA stiffness (percentual pulsatility, compliance, distensibility, and stiffness index), (4) aortic stiffness (carotid-femoral pulse wave velocity), and (5) peripheral and central pressure wave-derived parameters. Age groups: ≤20, 21–30, 31–40, 41–50, 51–60, 61–70, and 71–80 years old. Age-related structural and functional vascular parameters profiles were obtained and analyzed considering data from other populations. The work has the strength of being the first, in Latin America, that uses an integrative approach to characterize vascular aging-related changes. Data could be used to define vascular aging and abnormal or disease-related changes.

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.