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Dive into the research topics where Ricardo L. Armentano is active.

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Featured researches published by Ricardo L. Armentano.


Circulation Research | 1995

Arterial Wall Mechanics in Conscious Dogs Assessment of Viscous, Inertial, and Elastic Moduli to Characterize Aortic Wall Behavior

Ricardo L. Armentano; Juan Gabriel Barra; Jaime Levenson; Alain Simon; Ricardo H. Pichel

To evaluate arterial physiopathology, complete arterial wall mechanical characterization is necessary. This study presents a model for determining the elastic response of elastin (sigma E, where sigma is stress), collagen (sigma C), and smooth muscle (sigma SM) fibers and viscous (sigma eta) and inertial (sigma M) aortic wall behaviors. Our work assumes that the total stress developed by the wall to resist stretching is governed by the elastic modulus of elastin fibers (EE), the elastic modulus of collagen (EC) affected by the fraction of collagen fibers (fC) recruited to support wall stress, and the elastic modulus of the maximally contracted vascular smooth muscle (ESM) affected by an activation function (fA). We constructed the constitutive equation of the aortic wall on the basis of three different hookean materials and two nonlinear functions, fA and fC: sigma = sigma E + sigma C + sigma SM + sigma eta + sigma M = EE. (epsilon - epsilon 0E) + EC.fC.epsilon + ESM.fA.epsilon + eta. [equation: see text] + M.[equation: see text] where epsilon is strain and epsilon 0E is strain at zero stress. Stress-strain relations in the control state and during activation of smooth muscle (phenylephrine, 5 micrograms.kg-1.min-1 IV) were obtained by transient occlusions of the descending aorta and the inferior vena cava in 15 conscious dogs by using descending thoracic aortic pressure (microtransducer) and diameter (sonomicrometry) measurements. The fC was not linear with strain, and at the onset of significant collagen participation in the elastic response (break point of the stress-strain relation), 6.02 +/- 2.6% collagen fibers were recruited at 23% of stretching of the unstressed diameter. The fA exhibited a skewed unimodal curve with a maximum level of activation at 28.3 +/- 7.9% of stretching. The aortic wall dynamic behavior was modified by activation increasing viscous (eta) and inertial (M) moduli from the control to active state (viscous, 3.8 +/- 1.3 x 10(4) to 7.8 +/- 1.1 x 10(4) dyne.s.cm-2, P < .0005; inertial, 61 +/- 42 to 91 +/- 23 dyne.s2.cm-2, P < .05). Finally, the purely elastic stress-strain relation was assessed by subtracting the viscous and inertial behaviors.


Circulation Research | 1993

Assessment of smooth muscle contribution to descending thoracic aortic elastic mechanics in conscious dogs.

Juan Gabriel Barra; Ricardo L. Armentano; Jaime Levenson; E. I. C. Fischer; Ricardo H. Pichel; Alain Simon

Early investigators found contradictory evidence that vascular smooth muscle activation reduces the elastic modulus of the arterial wall under isotonic conditions but increases it under isometric conditions, concomitant with increased pulse-wave velocity. We examined the individual contributions of aortic constituents to the elastic modulus of the aortic wall to determine if isobaric analysis produces an accurate assessment of vascular smooth muscle activation. We used a modified Maxwell model assuming an incremental elastic modulus (Einc) composed of the elastic modulus of elastin fibers (EE), the elastic modulus of collagen fibers (EC) affected by the fraction of collagen fibers (fC) recruited to support wall stress, and the elastic modulus of the vascular smooth muscle (ESM) according to the following formula: Einc = EE+EC x fC+ESM.Einc was assessed in eight conscious dogs using descending thoracic aortic pressure (microtransducer) and diameter (sonomicrometry) measurements. Stress-strain relations in the control state and during activation of smooth muscle by continuous administration of phenylephrine (5 micrograms.kg-1 x min-1) were obtained by transient occlusions of the descending aorta and inferior vena cava. Results were as follows: EE was 4.99 +/- 1.58 x 10(6) dynes/cm2 (mean +/- SD), and EC was 965.8 +/- 399.8 x 10(6) dynes/cm2, assessed during the control state. Phenylephrine administration increased the theoretical pulse-wave velocity (Moens-Korteweg equation) from 5.25 +/- 1.03 m/s during the control state to 7.57 +/- 2.53 m/s (P < .005). Active muscle exhibited a unimodal stress-strain curve with a maximum stress of 0.949 +/- 0.57 x 10(6) dynes/cm2 at a corresponding strain value of 1.299 +/- 0.083. The maximum value observed corresponded, on the pressure-diameter curve of the active artery, to a pressure of 234.28 +/- 46.6 mm Hg and a diameter of 17.94 +/- 1.6 mm. The maximum ESM derived from the stress-strain relation of the active muscle was 8.345 +/- 7.56 x 10(6) dynes/cm2 at a strain value of 1.283 +/- 0.079. This point was located at 208.01 +/- 40.8 mm Hg and 17.73 +/- 1.41 mm on the active pressure-diameter curve. During activation of vascular smooth muscle, Einc decreased (P < .05) when plotted against internal pressure but increased (P < .05) when plotted against strain, over the operative range.(ABSTRACT TRUNCATED AT 400 WORDS)


Ultrasound in Medicine and Biology | 1999

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

Sebastian Graf; Jerome Gariepy; Marc Massonneau; Ricardo L. Armentano; Souheil Mansour; J. G. Barra; Alain Simon; Jaime Levenson

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.


Journal of the American College of Cardiology | 2001

Gender Differences in Wall Shear-Mediated Brachial Artery Vasoconstriction and Vasodilation

Jaime Levenson; Franco Pessana; Jerome Gariepy; Ricardo L. Armentano; Alain Simon

OBJECTIVES We sought to investigate wall shear rate (WSR) and brachial artery diameter (BAD) changes simultaneously and to determine whether any gender differences exist in arterial reactivity. BACKGROUND Wall shear rate/stress and arterial reactivity are rarely assessed at the same time. Furthermore, flow-mediated vasoconstriction has received less attention than flow-mediated vasodilation in humans. METHODS A new noninvasive evaluation of WSR in the brachial artery, using multigated, pulsed Doppler velocimeter and a double-transducer probe moved and fixed by a robotic system, was developed. RESULTS The validity of the system was tested in vitro with calibrated tubes and showed a high correlation (r = 0.98, p < 0.001). In 10 men and 10 women of similar age, induction of low and high shear rates by forearm occlusion produced significant vasoconstriction and vasodilation, respectively. The time lag for maximal BAD changes was 3 min for vasoconstriction and 1 min for vasodilation. A greater half-time for vasodilation (96 +/- 6 for men and 86 +/- 12 s for women) than for shear rate (31 +/- 5 s for men and 34 +/- 4 s for women) was observed after discontinuation of occlusion. Relative BAD was correlated with WSR changes, showing a significantly higher slope in women than in men (p < 0.01). Moreover, a larger normalized arterial diameter per shear rate was observed for vasoconstriction (p < 0.01) and vasodilation (p < 0.01) in women than in men. CONCLUSIONS Shear-mediated arterial vasodilation and vasoconstriction were more pronounced in women than in men, suggesting different gender-related sensitivity in the regulation of large-artery vascular tone.


Hypertension | 1998

Carotid Wall Viscosity Increase Is Related to Intima-Media Thickening in Hypertensive Patients

Ricardo L. Armentano; Sebastian Graf; Juan Gabriel Barra; Gerardo Velikovsky; Hugo Baglivo; Ramiro Sanchez; Alain Simon; Ricardo H. Pichel; Jaime Levenson

Increases in arterial wall viscosity and intima-media thickness (IMT) were found in hypertensive patients. Because smooth muscle cells are responsible for the viscous behavior of the arterial wall and they are involved in the process of thickening of the intima-media complex, this study evaluates the relationship between carotid thickness and wall viscosity. The simultaneous and noninvasive assessment of the intima-media complex and arterial diameter waveform was performed using high-resolution ultrasonography. This technique was contrasted against sonomicrometry in sheep, showing that the waveforms obtained by both methods were similar. The common carotid arteries of 11 normotensive subjects (NTA) and 11 patients with mild to moderate essential hypertension (HTA) were measured noninvasively by using tonometry and an automatic densitometric analysis of B-mode images to obtain IMT and instantaneous pressure and diameter loops. A viscoelastic model was used to derive the wall viscosity index (eta) using the hysteresis loop elimination criteria. In NTA, eta was 2.73+/-1.66 (mm Hg x s/mm) and IMT was 0.58+/-0.08 (mm), whereas in HTA, eta was 5.91+/-2.34 (P<.025) and IMT was 0.70+/-0.12 (P<.025), respectively. When all data of eta versus IMT of NTA and HTA were pooled in a linear regression analysis, a correlation coefficient of r=.71 (P<.05) was obtained. Partial correlation between eta and IMT holding constant pressure was r=.59 (P<.05). In conclusion, wall viscosity increase was associated with a higher IMT even maintaining blood pressure fixed, suggesting that the intima-media thickening might be related to smooth muscle alterations manifested as an increase in viscous behavior.


Physics in Medicine and Biology | 2008

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

Damian Craiem; Francisco J. Rojo; J. M. Atienza; Ricardo L. Armentano; Gustavo V. Guinea

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.


IEEE Engineering in Medicine and Biology Magazine | 2007

Smart smooth muscle spring-dampers

Ricardo L. Armentano; J.G. Barra; F.M. Pessana; D.O. Craiem; S. Graf; D.B. Santana; R.A. Sanchez

Pulsatile pressure in arteries produces mechanical oscillations. Highfrequency vibrations tend to produce mechanical structure injures. Vascular smooth muscle (VSM) could react modulating viscoelasticity to protect the arterial wall, filtering the highest harmonics component present in the large and rapid slope of blood pressure. The aim of this study was to evaluate the filtering performance exerted by VSM in the human common carotid artery (CCA) in normotensive (NT, smart springdampers turned on), hypertensive (HT, smart spring-dampers in action) and angiotensin converting enzyme (ACE) inhibitors-treated hypertensive patients (HT-treated, smart spring-dampers tuned), and in harvested human CCA segments (smart spring-dampers turned off). Human carotid arteries in vitro experiments (n = 14) and in vivo studies (n = 24) were performed, using adaptive modeling techniques to calculate mechanical impedance and creep (τC) and stress relaxation (τSR) time constants. This adaptive procedure was tested in vitro in harvested CCA mounted in a circulation mock. A confirmatory measure of damping was achieved by using the half-power bandwidth method ( fC) derived from the pressure-diameter frequency dependence using Bode diagrams, i.e., a compliance transfer function (diameter/pressure). Energy dissipation was calculated from the imaginary part of this function. Low-pass frequency responses were verified with a flat plateau up to a relatively stable frequency corner fC in the Bode diagram of the complete third-order model. Simplified first-order model cutoff frequencies were 2.7, 2.8, and 3.0 Hz for NT, HT, and HT-treated, respectively, showing an interesting constancy between groups. Smooth muscle tonus proved to preserve fC as well as τC. Energy dissipation in hypertensive patients (n = 12) three-folded NT values and tended to be restored in HT-treated by means of a decrease in τSR. VSM acts as smart spring-dampers, dissipating high-frequency components that might have damaging effects. VSM alterations found in HT patients could have a mandatory directive of preserving dynamic range near NT values, suggesting that VSM modulates its degree of activation and/or the vessel wall remodeling in order to ensure a suitable protection role.


Hypertension | 2006

Smart Damping Modulation of Carotid Wall Energetics in Human Hypertension Effects of Angiotensin-Converting Enzyme Inhibition

Ricardo L. Armentano; Juan Gabriel Barra; Daniel Bia Santana; Franco Pessana; Sebastian Graf; Damian Craiem; Laura Brandani; Hugo Baglivo; Ramiro Sanchez

Damping is the conversion of mechanical energy of a structure into thermal energy, and it is related to the material viscous behavior. To evaluate the role of damping in the common carotid artery (CCA) wall in human hypertension and the possible improvement of angiotensin-converting enzyme (ACE) inhibition, we used noninvasive CCA pressure (tonometry) and diameter (B-mode echography) waveforms in normotensive subjects (NT group; n=12) and in hypertensive patients (HT group; n=22) single-blind randomized into HT–placebo (n=10) or HT-treated (ramipril, 5 to 10 mg/d during 3 months; n=12). Vascular smooth muscle (VSM) null tonus condition was achieved from in vitro pressure and diameter waveforms (Konigsberg microtransducer and sonomicrometry) measured in explanted human CCA (n=14). Arterial wall dynamics was described by viscous (&eegr;), inertial (M), and compliance (C) parameters, mean circumferential wall stress, viscous energy dissipation (WD), peak strain energy (WSt), damping ratio (&xgr;=WD/WSt), and modeling isobaric indexes CIso and WSt(Iso). The lack of VSM tonus isobarically increased wall stress and reduced &eegr;, CIso, and damping (P<0.01). Wall stress, &eegr;, and WD were greater in HT than in NT (P<0.015) and arrived near normal in HT-treated (P<0.032 respect to HT), with no changes in HT–placebo. Whereas CIso increased in HT-treated (P<0.01) approaching the NT level, &xgr; did not vary among groups. During hypertension, because of the WSt increase, the arterial wall reacts increasing WD to maintain &xgr;. ACE inhibition modulates VSM activation and vessel wall remodeling, significantly improving wall energetics and wall stress. This protective vascular action reduces extra load to the heart and maintains enhanced arterial wall damping.


IEEE Transactions on Biomedical Engineering | 2009

Analysis of Viscoelastic Wall Properties in Ovine Arteries

Daniela Valdez-Jasso; Mansoor A. Haider; Harvey Thomas Banks; Daniel Bia Santana; Yanina Zócalo Germán; Ricardo L. Armentano; Mette S. Olufsen

In this paper, we analyze how elastic and viscoelastic properties differ across seven locations along the large arteries in 11 sheep. We employ a two-parameter elastic model and a four-parameter Kelvin viscoelastic model to analyze experimental measurements of vessel diameter and blood pressure obtained in vitro at conditions mimicking in vivo dynamics. Elastic and viscoelastic wall properties were assessed via solutions to the associated inverse problem. We use sensitivity analysis to rank the model parameters from the most to the least sensitive, as well as to compute standard errors and confidence intervals. Results reveal that elastic properties in both models (including Youngs modulus and the viscoelastic relaxation parameters) vary across locations (smaller arteries are stiffer than larger arteries). We also show that for all locations, the inclusion of viscoelastic behavior is important to capture pressure-area dynamics.


Annals of Biomedical Engineering | 2011

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

Daniela Valdez-Jasso; Daniel Bia; Yanina Zócalo; Ricardo L. Armentano; Mansoor A. Haider; Mette S. Olufsen

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.

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Daniel Bia

University of the Republic

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Yanina Zócalo

University of the Republic

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Damian Craiem

Facultad de Ciencias Exactas y Naturales

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Juan Torrado

University of the Republic

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Ignacio Farro

University of the Republic

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