Revistas

Revista:
FUZZY SETS AND SYSTEMS
ISSN:
0165-0114

Año:
2023
Vol.:
473
Págs.:
108717

In this work, we study when an aggregation operator preserves the structure of T-subgroup of groups whose subgroup lattice is a chain. There are two widely used ways of defining the aggregation of structures in fuzzy logic, previously named on sets and on products. We will focus our attention on the one called aggregation on products. When the lattice of subgroups is not a chain, it is known that the dominance relation between the aggregation operator and the t-norm is crucial. We show that this property is again important for some of the groups in this study. However, for the rest of them, we must define a new property weaker than domination, that will allow us to characterize those operators which preserve T-subgroups.

Revista:
PLOS ONE
ISSN:
1932-6203

Año:
2023
Vol.:
18
N°:
6
Págs.:
e0286884

PurposeTo investigate the possible risk factors for treatment failure in patients who had undergone Preserflo Microshunt (PMS) implantation, using anterior-segment optical coherence tomography (AS-OCT) to analyze the internal structures of the bleb. MethodsThe PMS blebs of 54 patients were evaluated with AS-OCT. A mathematical model was used to calculate the total filtering surface of the episcleral fluid cavity (EFC) and the hydraulic conductivity (HC) of the bleb wall. Complete and qualified success were defined as IOP between 6 and 17 mmHg with or without glaucoma medication. The relation between baseline characteristics and probability of bleb success was analyzed by bivariate and multivariate logistic regression. The main outcome measures were mean bleb wall thickness (BWT), reflectivity (BWR), HC, mean horizontal and vertical diameter and total filtering surface (TFS) of the EFC. ResultsBlebs from 74% patients were considered as complete success and 26% as failure. BWR and BWT increased linearly up to the first year in both groups. BWR was higher in the group failure (p = 0.02) and BWT in the group success (p<0.001). EFC was wider and shorter in the success group (p = 0.009, p = 0.03). Higher TFS showed a negative correlation with IOP (r = -0.4, p = 0.002). Higher baseline IOP was associated with success of PMS by multivariate analysis (p = 0.01). Mean HC, 0.034 +/- 0.008 (mu L/min)/mm(2)/mmHg, was negatively correlated with bleb surface (r = -0.5, p<0.0001) and wall ' s thickness (r = -0.3, p = 0.01). ConclusionsAS-OCT revealed that successful PMS blebs could show either thick hyporreflective walls or wide filtering surfaces with thin capsules. A higher baseline IOP increased the probability of surgical success.

Revista:
ENERGIES
ISSN:
1996-1073

Año:
2023
Vol.:
16
N°:
5
Págs.:
2365

The present article investigates viscous fluid flow's heat and mass transfers over a stretching/shrinking sheet using the single and multi-wall carbon nanotube models. The analysis considers the effects of thermal radiation, induced slip, mass transpiration, and inclined magnetic force. The effect of the carbon nanotube model on fluid flow has not been considered in previous studies. By exploiting the similarity variable, the governing nonlinear partial differential equations are converted into nonlinear ordinary differential equation. The derived equations are solved analytically, and we obtained an exact solution for the velocity and energy conservation equation. The physical parameters of interest such as induced slip parameter, suction/injection, magnetic field, thermal radiation, and shear stress are analyzed and presented graphically. In particular, we show that the fluid flow in a single wall carbon nanotube transfers more energy than the multivalued nanotubes.

Revista:
PLOS COMPUTATIONAL BIOLOGY
ISSN:
1553-7358

Año:
2023
Vol.:
19
N°:
10
Págs.:
e1011507

Mathematical modeling of unperturbed and perturbed tumor growth dynamics (TGD) in preclinical experiments provides an opportunity to establish translational frameworks. The most commonly used unperturbed tumor growth models (i.e. linear, exponential, Gompertz and Simeoni) describe a monotonic increase and although they capture the mean trend of the data reasonably well, systematic model misspecifications can be identified. This represents an opportunity to investigate possible underlying mechanisms controlling tumor growth dynamics through a mathematical framework. The overall goal of this work is to develop a data-driven semi-mechanistic model describing non-monotonic tumor growth in untreated mice. For this purpose, longitudinal tumor volume profiles from different tumor types and cell lines were pooled together and analyzed using the population approach. After characterizing the oscillatory patterns (oscillator half-periods between 8-11 days) and confirming that they were systematically observed across the different preclinical experiments available (p<10-9), a tumor growth model was built including the interplay between resources (i.e. oxygen or nutrients), angiogenesis and cancer cells. The new structure, in addition to improving the model diagnostic compared to the previously used tumor growth models (i.e. AIC reduction of 71.48 and absence of autocorrelation in the residuals (p>0.05)), allows the evaluation of the different oncologic treatments in a mechanistic way. Drug effects can potentially, be included in relevant processes taking place during tumor growth. In brief, the new model, in addition to describing non-monotonic tumor growth and the interaction between biological factors of the tumor microenvironment, can be used to explore different drug scenarios in monotherapy or combination during preclinical drug development.

Revista:
COMPUTING IN CARDIOLOGY
ISSN:
2325-8861

Año:
2022
Vol.:
49
Págs.:
1 - 4

The aim of this work is to use biomarkers extracted from high-resolution voltage maps of atrial fibrillation (AF) patients in order to make predictions about future "redo" procedures. We collected maps of the left atrium of 122 patients, prior of being treated for AF The bipolar voltage maps were extracted with the Rythmia system from Boston Scientific and subsequently analyzed in the MATLAB environment. The present study focuses on three biomarkers extracted from those maps. Two are associated with the bipolar voltage measurements on the map, i.e., the mean voltage and the voltage dispersion on the map. The third indicator is the area of the atrium evaluated from the map. The data are used for feeding a supervised classification algorithm. The output variable is a binary variable that is set to 1 if the patient will need a "redo" procedure in the twelve months following the cardiac intervention and 0 otherwise. We show that the biomarkers have some statistical power in predicting future outcomes. Especially the mean voltage on the map is the best predictor of the future outcome. We determine the cutoff value for the mean voltage based on the best prediction accuracy of Vm=0.542 mV in agreement with previous studies. We discuss some extensions of this study that could allow improvements in predictive power.

Revista:
CHAOS (EEUU)
ISSN:
1054-1500

Año:
2021
Vol.:
31
N°:
7
Págs.:
073144

In this paper, we study the propagation of the cardiac action potential in a one-dimensional fiber, where cells are electrically coupled through gap junctions (GJs). We consider gap junctional gate dynamics that depend on the intercellular potential. We find that different GJs in the tissue can end up in two different states: a low conducting state and a high conducting state. We first present evidence of the dynamical multistability that occurs by setting specific parameters of the GJ dynamics. Subsequently, we explain how the multistability is a direct consequence of the GJ stability problem by reducing the dynamical system's dimensions. The conductance dispersion usually occurs on a large time scale, i.e., thousands of heartbeats. The full cardiac model simulations are computationally demanding, and we derive a simplified model that allows for a reduction in the computational cost of four orders of magnitude. This simplified model reproduces nearly quantitatively the results provided by the original full model. We explain the discrepancies between the two models due to the simplified model's lack of spatial correlations. This simplified model provides a valuable tool to explore cardiac dynamics over very long time scales. That is highly relevant in studying diseases that develop on a large time scale compared to the basic heartbeat. As in the brain, plasticity and tissue remodeling are crucial parameters in determining the action potential wave propagation's stability. (C) 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Revista:
FRONTIERS IN PHYSIOLOGY
ISSN:
1664-042X

Año:
2021
Vol.:
12
Págs.:
730933

With an estimated 64.3 million cases worldwide, heart failure (HF) imposes an enormous burden on healthcare systems. Sudden death from arrhythmia is the major cause of mortality in HF patients. Computational modeling of the failing heart provides insights into mechanisms of arrhythmogenesis, risk stratification of patients, and clinical treatment. However, the lack of a clinically informed approach to model cardiac tissues in HF hinders progress in developing patient-specific strategies. Here, we provide a microscopy-based foundation for modeling conduction in HF tissues. We acquired 2D images of left ventricular tissues from HF patients (n = 16) and donors (n = 5). The composition and heterogeneity of fibrosis were quantified at a sub-micrometer resolution over an area of 1 mm(2). From the images, we constructed computational bidomain models of tissue electrophysiology. We computed local upstroke velocities of the membrane voltage and anisotropic conduction velocities (CV). The non-myocyte volume fraction was higher in HF than donors (39.68 +/- 14.23 vs. 22.09 +/- 2.72%, p < 0.01), and higher in ischemic (IC) than nonischemic (NIC) cardiomyopathy (47.2 +/- 16.18 vs. 32.16 +/- 6.55%, p < 0.05). The heterogeneity of fibrosis within each subject was highest for IC (27.1 +/- 6.03%) and lowest for donors (7.47 +/- 1.37%) with NIC (15.69 +/- 5.76%) in between. K-means clustering of this heterogeneity discriminated IC and NIC with an accuracy of 81.25%. The heterogeneity in CV increased from donor to NIC to IC tissues. CV decreased with increasing fibrosis for longitudinal (R-2 = 0.28, p < 0.05) and transverse conduction (R-2 = 0.46, p < 0.01). The tilt angle of the CV vectors increased 2.1 degrees for longitudinal and 0.91 degrees for transverse conduction per 1% increase in fibrosis. Our study suggests that conduction fundamentally differs in the two etiologies due to the characteristics of fibrosis. Our study highlights the importance of the etiology-specific modeling of HF tissues and integration of medical history into electrophysiology models for personalized risk stratification and treatment planning.</p>

Revista:
TRANSLATIONAL VISION SCIENCE & TECHNOLOGY
ISSN:
2164-2591

Año:
2021
Vol.:
10
N°:
13
Págs.:
26

Purpose: To measure the in vitro flow properties of the PRESERFLO implant for comparison with the theoretical resistance to flow. Methods: The PRESERFLO was designed to control the flow of aqueous humor according to the Hagen-Poiseuille (HP) equation. Scanning electron microscopy (SEM) was performed to analyze the ultrastructure, and flow measurements were carried out using a gravity-flow setup. Results: SEM images of the PRESERFLO showed luminal diameters of 67.73 x 65.95 mu m and 63.66 x 70.54 mu m. The total diameter was 337.2 mu m, and the wall was 154 mu m wide. The theoretical calculation of the resistance to flow (R) for an aqueous humor (AH) viscosity of 0.7185 centipoises (cP) was 1.3 mm Hg/(mu L/min). Hence, assuming a constant AH flow of 2 mu L/min, the pressure differential across the device (AP) was estimated to be 2.6 mm Hg. The gravity-flow experiment allowed us to measure the experimental resistance to flow, which was R-E = 1.301 mm Hg/(mu L/min), in agreement with the theoretical resistance to flow R given by the HP equation. Conclusions: The experimental and theoretical flow testing showed that the pressure drop across this device would not be large enough to avoid hypotony unless the resistance to outflow of the sub-Tenon space was sufficient to control the intraocular pressure in the early postoperative period. Translational Relevance: The fluid properties of glaucoma subconjunctival drainage devices determine their specific bleb-forming capacity and ability to avoid hypotony and therefore their safety and efficacy profile. Translational Relevance: The fluid properties of glaucoma subconjunctival drainage devices determine their specific bleb-forming capacity and ability to avoid hypotony and therefore their safety and efficacy profile.

Revista:
REVISTA ESPAÑOLA DE CARDIOLOGIA
ISSN:
0300-8932

Año:
2021
Vol.:
74
N°:
1
Págs.:
65 - 71

Cardiovascular diseases currently have a major social and economic impact, constituting one of the leading causes of mortality and morbidity. Personalized computational models of the heart are demonstrating their usefulness both to help understand the mechanisms underlying cardiac disease, and to optimize their treatment and predict the patient's response. Within this framework, the Spanish Research Network for Cardiac Computational Modelling (VHeart-SN) has been launched. The general objective of the VHeart-SN network is the development of an integrated, modular and multiscale multiphysical computational model of the heart. This general objective is addressed through the following specific objectives: a) to integrate the different numerical methods and models taking into account the specificity of patients; b) to assist in advancing knowledge of the mechanisms associated with cardiac and vascular diseases; and c) to support the application of different personalized therapies. This article presents the current state of cardiac computational modelling and different scientific works conducted by the members of the network to gain greater understanding of the characteristics and usefulness of these models. (C) 2020 Sociedad Espanola de Cardiologia. Published by Elsevier Espana, S.L.U. All rights reserved.

Revista:
PHYSICA SCRIPTA
ISSN:
0031-8949

Año:
2021
Vol.:
96
N°:
12
Págs.:
124045

We have studied a route of chaos in the dissipative Landau-Lifshitz-Gilbert equation representing the magnetization dynamics of an anisotropic nanoparticle subjected to a time-variant magnetic field. This equation presents interesting chaotic dynamics. In the parameter space, for some forcing frequency and magnetic strength of the applied field, one observes a transition from a regular periodic behavior to chaotic dynamics. The chaotic dynamics, close to the bifurcation, are characterized by type-III intermittency. Long epochs of quasi-regular dynamics followed by turbulent bursts. The characterization of the intermittencies has been done through four different techniques. The first method is associated with the computation of the Lyapunov exponents that characterize the chaotic regime. The second and third methods are associated with the statistics of the duration of the laminar epochs prior to a turbulent burst. The fourth method is associated with the subharmonic instability present in those laminar epochs and quantified through a Poincare section method. At the end of the manuscript, we compare the result obtained by the different techniques and discuss the methods' limitations.

Autores:
Diaz, P.; Pérez, L. M.; Reyes, L. I.; et al.

Revista:
CHAOS SOLITONS AND FRACTALS
ISSN:
0960-0779

Año:
2021
Vol.:
153
N°:
P1
Págs.:
111416

This work presents a study of the Faraday instability in a parametrically forced Fermi-Fermi mixture. The condensate is confined in the transversal spatial dimension with a strong parametric confinement potential and in the longitudinal spatial dimension with a weaker potential. The theoretical description is done using the mean-field theory with two amplitude equations that represent each spin state. In order to stabilize the Faraday patterns, a phenomenological damping term is introduced. The influence of the Zeeman interaction is analyzed in detail. In particular, phase diagrams of the existence and stability of the Faraday waves are calculated as a function of the Zeeman interaction, the coupling parameter, and the forcing amplitude. The degree of segregation of the two fields and their synchronization level is also calculated as a function of the Zeeman parameter. In addition, we examine how the pattern wavelength varies as a function of the Zeeman parameter and the forcing frequency

Revista:
PLOS ONE
ISSN:
1932-6203

Año:
2020
Vol.:
15
N°:
3
Págs.:
e0229856

Purpose To analyse nocturnal intraocular pressure (IOP) fluctuations in patients with obstructive sleep apnea syndrome (OSAS) using a contact lens sensor (CLS) and to identify associations between the OSAS parameters determined by polysomnographic study (PSG) and IOP changes. Method Prospective, observational study. Twenty participants suspected of having OSAS were recruited. During PSG study, IOP was monitored using a CLS placed in the eye of the patient. The patients were classified according to the apnea-hypopnea index (AHI) in two categories, severe (> 30) or mild/moderate (< 30) OSAS. We evaluated several parameters determined by the IOP curves, including nocturnal elevations (acrophase) and plateau times in acrophase (PTs) defined by mathematical and visual methods. Results The IOP curves exhibited a nocturnal acrophase followed by PTs of varying extents at which the IOP remained higher than daytime measurement with small variations. We found significant differences in the length of the PTs in patients with severe OSAS compared to those with mild/moderate disease (P = 0.032/P = 0.028). We found a positive correlation between PTs and OSAS severity measured by the total number of apneic events (r = 0.681/ 0.751 P = 0.004/0.001) and AHI (r = 0.674/0.710, P = 0.004/0.002). Respiratory-related arousal and oxygen saturation also were associated significantly with the IOP PT length. Conclusions Periods of nocturnal IOP elevation lasted longer in severe OSAS patients than those with mild/moderate OSAS and correlate with the severity of the disease. The length of the nocturnal PT is also associated to respiratory parameters altered in patients with OSAS.

Revista:
CHAOS (EEUU)
ISSN:
1054-1500

Año:
2020
Vol.:
30
N°:
9
Págs.:
093112

In this work, we study numerically the periodicity of regular regions embedded in chaotic states for the case of an anisotropic magnetic particle. The particle is in the monodomain regime and subject to an applied magnetic field that depends on time. The dissipative Landau-Lifshitz-Gilbert equation models the particle. To perform the characterization, we compute several two-dimensional phase diagrams in the parameter space for the Lyapunov exponents and the isospikes. We observe multiple transitions among periodic states, revealing complex topological structures in the parameter space typical of dynamic systems. To show the finer details of the regular structures, iterative zooms are performed. In particular, we find islands of synchronization for the magnetization and the driven field and several shrimp structures with different periods.

Revista:
JOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY
ISSN:
1045-3873

Año:
2019
Vol.:
30
N°:
8
Págs.:
1231 - 1240

Introduction Ultrahigh-density-voltage mapping (uHD(V)M) is a new tool that can add new insights into the pathophysiology of atrial fibrillation (AF). The aim of this study was to evaluate the performance of uHD(V)M in predicting postablation AF recurrence (AFR). Methods and Results We included 98 consecutive patients undergoing pulmonary vein isolation for AF (40.8% persistent) using an uHD(V)M system and followed for 1 year. The left atrium (LA) mean voltage (V-m) and the V-slope (slope of the voltage histogram calculated by linear interpolation, with the relative frequency on the vertical axis and the bipolar potential on the horizontal axis) were calculated from 12 567 +/- 5486 points per map. Patients with AFR (N = 29) had lower V-m and higher V-slope as compared with patients without AFR (N = 69). Receiver operating characteristic curves identified V-m as the strongest predictor of AFR, with a higher incidence of AFR in patients with V-m 0.758 mV (57.6%) or lower than patients with V-m higher than 0.758 mV (15.4%; P < .0001). Among patients with V-m higher than 0.758 mV, patients with V-slope 0.637 or higher exhibited higher (P = .043) AFR incidence (31.3%) than patients with V-slope lower than 0.637 (10.2%). This classification showed incremental predictive value over relevant covariables. V-m values were lower and V-slope values were higher in patients that progressed from paroxysmal to persistent AF. Patients with V-slope 0.637 or higher had a 14.2% incidence of postablation atypical atrial flutter, whereas patients with V-slope lower than 0.637 did not present this outcome. Conclusions The risk of AFR, atrial flutter, and progression from paroxysmal to persistent AF can be detected by quantitative analysis of LA uHD(V)M identifying diverse patterns of atrial substrate alterations.

Revista:
JOURNAL OF THE AMERICAN COLLEGE OF CARDIOLOGY
ISSN:
0735-1097

Año:
2019
Vol.:
73
N°:
12
Págs.:
1398 - 1410

Background A combination of circulating biomarkers associated with excessive myocardial collagen type-I cross-linking or CCL+ (i.e., decreased carboxy-terminal telopeptide of collagen type-I to matrix metalloproteinase-1 ratio) and with excessive myocardial collagen type-I deposition or CD+ (i.e., increased carboxy-terminal propeptide of procollagen type-I) has been described in heart failure (HF) patients and associates with poor outcomes.
Objectives The purpose of this study was to investigate whether the CCL+CD+ combination of biomarkers associates with atrial fibrillation (AF).
Methods Biomarkers were analyzed in serum samples from 242 HF patients (study 1) and 150 patients referred for AF ablation (study 2). Patients were classified into 3 groups (CCL¿CD¿, CCL+CD¿ or CCL¿CD+, and CCL+CD+) in accordance to biomarker threshold values. Left atrial electroanatomic high-density mapping was performed in 71 patients from study 2.
Results In study 1, 53.7% patients had AF at baseline and 19.6% developed AF (median follow-up 5.5 years). Adjusted odds and hazard ratios associated with baseline and new-onset AF, respectively, were both ¿3.3 (p ¿ 0.050) in CCL+CD+ patients compared with CCL¿CD¿ patients, with nonsignificant changes in the other group. In study 2, 29.3% patients had AF recurrence during 1-year post-ablation.

Revista:
FRONTIERS IN PSYCHOLOGY
ISSN:
1664-1078

Año:
2019
Vol.:
10
Págs.:
337

Defibrillation is a well-established therapy for atrial and ventricular arrhythmia. Here, we shed light on defibrillation in the fibrotic heart. Using the extended bidomain model of electrical conduction in cardiac tissue, we assessed the influence of fibrosis on the strength of virtual electrodes caused by extracellular electrical current. We created one-dimensional models of rabbit ventricular tissue with a central patch of fibrosis. The fibrosis was incorporated by altering volume fractions for extracellular, myocyte and fibroblast domains. In our prior work, we calculated these volume fractions from microscopic images at the infarct border zone of rabbit hearts. An average and a large degree of fibrosis were modeled. We simulated defibrillation by application of an extracellular current for a short duration (5 ms). We explored the effects of myocyte-fibroblast coupling, intra-fibroblast conductivity and patch length on the strength of the virtual electrodes present at the borders of the normal and fibrotic tissue. We discriminated between effects on myocyte and fibroblast membranes at both borders of the patch. Similarly, we studied defibrillation in two-dimensional models of fibrotic tissue. Square and disk-like patches of fibrotic tissue were embedded in control tissue. We quantified the influence of the geometry and fibrosis composition on virtual electrode strength.

Revista:
INTERNATIONAL JOURNAL OF BIFURCATION AND CHAOS
ISSN:
0218-1274

Año:
2019
Vol.:
29
N°:
8
Págs.:
1930021

Connexins are specialized ionic channels that control the action potential propagation between cardiac myocytes. In this paper, we study the connexin dynamics in a one-dimensional model of cardiac tissue. We show that the connexin dynamics may lead to a spatial organization of the gap junction conductance. In the numerical simulations presented in this paper we have found two different regimes for the spatial organization of the conductances: (a) a spatially uniform conductance; (b) a spatially complex pattern of local values of high and low conductances. in addition, we have observed that, locally, the two final states are limit cycles with a period equal to the period associated with the external excitation of the tissue strand. The conductance dispersion usually takes place on a very large time scale, i.e. thousands of heart beats, and on a very short spatial scale. Due to its simplicity, the one-dimensional setting allows a detailed study of the emerging structure and in particular very long simulations. We have studied the transition between the two aforementioned states as a function of the gap junction conductance characteristics. Furthermore, we have studied the effect of initially added noises on the outcome of the system. Finally, using spatial autocorrelation functions we have characterized the spatial dispersion in conductance values.

Autores:
Baghramyan, H. M. (Autor de correspondencia); Barseghyan, M. G.; Kirakosyan, A. A.; et al.

Revista:
SCIENTIFIC REPORTS
ISSN:
2045-2322

Año:
2018
Vol.:
8
N°:
6145

The rendering of different shapes of just a single sample of a concentric double quantum ring is demonstrated realizable with a terahertz laser field, that in turn, allows the manipulation of electronic and optical properties of a sample. It is shown that by changing the intensity or frequency of laser field, one can come to a new set of degenerated levels in double quantum rings and switch the charge distribution between the rings. In addition, depending on the direction of an additional static electric field, the linear and quadratic quantum confined Stark effects are observed. The absorption spectrum shifts and the additive absorption coefficient variations affected by laser and electric fields are discussed. Finally, anisotropic electronic and optical properties of isotropic concentric double quantum rings are modeled with the help of terahertz laser field.

Revista:
JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS
ISSN:
0304-8853

Año:
2017
Vol.:
444
Págs.:
432 - 438

In magnetic fluids the viscosity can depend on an external magnetic field. We theoretically investigate the influence of this magneto-viscous effect on the thermal convection thresholds for viscoelastic fer-rofluids, which are described by a linear Oldroyd model. Such a system is influenced by a static magnetic field not only via the Kelvin force, but also through the magneto-viscous effect. In particular, we find that these two contributions compete oppositely when the threshold for the oscillatory instability is considered. While the Kelvin force tends to decrease the critical Rayleigh number, the magneto-viscous effect increases it. The critical properties at the onset of the oscillatory instability are discussed as a function of the viscoelastic parameters, the external field strength, and the magneto-viscous coefficient. The transition between the stationary and the oscillatory instability is only slightly affected by the magneto-viscous effect. Examples for codimension-2 lines are given. (C) 2017 Elsevier B.V. All rights reserved.

Revista:
CHAOS (EEUU)
ISSN:
1054-1500

Año:
2017
Vol.:
27
N°:
9
Págs.:
093924

This article characterizes the cardiac autonomous electrical activity induced by the mechanical deformations in the cardiac tissue through the mechano-electric feedback. A simplified and qualitative model is used to describe the system and we also account for temperature effects. The analysis emphasizes a very rich dynamics for the system, with periodic solutions, alternans, chaotic behaviors, etc. The possibility of self-sustained oscillations is analyzed in detail, particularly in terms of the values of important parameters such as the dimension of the system and the importance of the stretch-activated currents. It is also shown that high temperatures notably increase the parameter ranges for which self-sustained oscillations are observed and that several attractors can appear, depending on the location of the initial excitation of the system. Finally, the instability mechanisms by which the periodic solutions are destabilized have been studied by a Floquet analysis, which has revealed period-doubling phenomena and transient intermittencies. Published by AIP Publishing.

Revista:
CHAOS SOLITONS AND FRACTALS
ISSN:
0960-0779

Año:
2016
Vol.:
83
Págs.:
140 - 146

We study a spatial model of social interactions. Though the properties of the spatial equilibrium have been largely discussed in the existing literature, the stability of equilibrium remains an unaddressed issue. Our aim is to fill up this gap by introducing dynamics in the model and by determining the stability of equilibrium. First we derive a variational equation useful for the stability analysis. This allows to study the corresponding eigenvalue problem. While odd modes are shown to be always stable, there is a single even mode of which stability depends on the model parameters. Finally various numerical simulations illustrate our theoretical results.

Revista:
PHYSICAL REVIEW E
ISSN:
2470-0045

Año:
2015
Vol.:
92
N°:
6
Págs.:
2919

Defibrillation is the standard clinical treatment used to stop ventricular fibrillation. An electrical device delivers a controlled amount of electrical energy via a pair of electrodes in order to reestablish a normal heart rate. We propose a technique that is a combination of biphasic shocks applied with a four-electrode system rather than the standard two-electrode system. We use a numerical model of a one-dimensional ring of cardiac tissue in order to test and evaluate the benefit of this technique. We compare three different shock protocols, namely a monophasic and two types of biphasic shocks. The results obtained by using a four-electrode system are compared quantitatively with those obtained with the standard two-electrode system. We find that a huge reduction in defibrillation threshold is achieved with the four-electrode system. For the most efficient protocol (asymmetric biphasic), we obtain a reduction in excess of 80% in the energy required for a defibrillation success rate of 90%. The mechanisms of successful defibrillation are also analyzed. This reveals that the advantage of asymmetric biphasic shocks with four electrodes lies in the duration of the cathodal and anodal phase of the shock.

Revista:
NETWORKS AND HETEROGENEOUS MEDIA
ISSN:
1556-1801

Año:
2015
Vol.:
10
N°:
1 Supl. 1
Págs.:
209 - 221

We report a systematic investigation of the magnetic anisotropy effects observed in the deterministic spin dynamics of a magnetic particle in the presence of a time-dependent magnetic field. The system is modeled by the Landau-Lifshitz-Gilbert equation and the magnetic field consists of two terms, a constant term and a term involving a harmonic time modulation. We consider a general quadratic anisotropic energy with three different preferential axes. The dynamical behavior of the system is represented in Lyapunov phase diagrams, and by calculating bifurcation diagrams, Poincare sections and Fourier spectra. We find an intricate distribution of shrimp-shaped regular island embedded in wide chaotic phases. Anisotropy effects are found to play a key role in defining the symmetries of regular and chaotic stability phases.

Autores:
Barseghyan, M.; Baghramyan, H.; Laroze, D.; et al.

Revista:
PHYSICA E-LOW-DIMENSIONAL SYSTEMS AND NANOSTRUCTURES
ISSN:
1386-9477

Año:
2015
Vol.:
74
Págs.:
421 - 425

The effects of a lateral electric field on intraband absorption in GaAs/GaAlAs two-dimensional coupled quantum dot-ring structure with an on-center hydrogenic donor impurity are investigated. The confining potential of the system consists of two parabolas with various confinement energies. The calculations are made using the exact diagonalization technique. A selection rule for intraband transitions was found for x-polarized incident. light. The absorption spectrum mainly exhibits a redshift with the increment of electric field strength. On the other hand, the absorption spectrum can exhibit either a blue- or redshift depending on the values of confinement energies of dot and ring. Additionally, electric field changes the energetic shift direction influenced by the variation of barrier thickness of the structure.

Revista:
INFORMATION SCIENCES
ISSN:
0020-0255

Año:
2015
Vol.:
310
Págs.:
36 - 51

Fuzzy operators are an essential tool in many fields and the operation of composition is often needed. In general, composition is not a commutative operation. However, it is very useful to have operators for which the order of composition does not affect the result. In this paper, we analyze when permutability appears. That is, when the order of application of the operators does not change the outcome. We characterize permutability in the case of the composition of fuzzy consequence operators and the dual case of fuzzy interior operators. We prove that for these cases, permutability is completely connected to the preservation of the operator type.
We also study the particular case of fuzzy operators induced by fuzzy relations through Zadeh¿s compositional rule and the inf-¿¿ composition. For this cases, we connect permutability of the fuzzy relations (using the sup-¿ composition) with permutability of the induced operators. Special attention is paid to the cases of operators induced by fuzzy preorders and similarities. Finally, we use these results to relate the operator induced by the transitive closure of the composition of two reflexive fuzzy relations with the closure of the operator this composition induces.

Revista:
FUZZY SETS AND SYSTEMS
ISSN:
0165-0114

Año:
2013
Vol.:
218
Págs.:
73 - 89

In a previous paper, Elorza and Burillo explored the coherence property in fuzzy consequence operators. In this paper we show that fuzzy closing operators of mathematical morphology are always coherent operators. We also show that the coherence property is the key to link the four following families: fuzzy closing morphological operators, fuzzy consequence operators, fuzzy preorders and fuzzy closure and co-closure systems. This will allow to translate important well-known properties from the field of approximate reasoning to the field of image processing.

Revista:
LECTURE NOTES IN COMPUTER SCIENCE
ISSN:
1611-3349

Año:
2013
Vol.:
8109
Págs.:
62 - 69

In this paper we study the permutability of the composition of fuzzy consequence operators (fuzzy closings) and fuzzy interior operators (fuzzy openings). We establish several characterizations and we show the relation of permutability with the fuzzy closure and fuzzy interior of a fuzzy operator. We also study the connection between permutability and the preservation of the operator type through the composition. More precisely, when the composition of two openings is an opening and the composition of two closings is a closing.

Revista:
LECTURE NOTES IN COMPUTER SCIENCE
ISSN:
1611-3349

Año:
2013
Vol.:
8234
Págs.:
237 - 247

In this paper we study the permutability of the composition of fuzzy consequence operators when they are induced by fuzzy relations using the usual Zadeh's compositional rule. In particular, we study the case of fuzzy indistinguishability operators and fuzzy preorders. We study the connection between the permutability of the fuzzy relations and the permutability of their induced fuzzy operators

Revista:
CHAOS (EEUU)
ISSN:
1054-1500

Año:
2013
Vol.:
23
Págs.:
043119

In this article, we compare quantitatively the efficiency of three different protocols commonly used in commercial defibrillators. These are based on monophasic and both symmetric and asymmetric biphasic shocks. A numerical one¿dimensional model of cardiac tissue using the bidomain formulation is used in order to test the different protocols. In particular, we performed a total of 4.8 × 106 simulations by varying shock waveform, shock energy, initial conditions, and heterogeneity in internal electrical conductivity. Whenever the shock successfully removed the reentrant dynamics in the tissue, we classified the mechanism. The analysis of the numerical data shows that biphasic shocks are significantly more efficient (by about 25%) than the corresponding monophasic ones. We determine that the increase in efficiency of the biphasic shocks can be explained by the higher proportion of newly excited tissue through the mechanism of direct activation.

Revista:
THEORETICAL BIOLOGY AND MEDICAL MODELLING
ISSN:
1742-4682

This paper analyzes a new semiphysiological ionic model, used recently to study reexitations and reentry in cardiac tissue [I.R. Cantalapiedra et al, PRE 82 011907 (2010)]. The aim of the model is to reproduce action potencial morphologies and restitution curves obtained, either from experimental data, or from more complex electrophysiological models. The model divides all ion currents into four groups according to their function, thus resulting into fast-slow and inward-outward currents. We show that this simplified model is flexible enough as to accurately capture the electrical properties of cardiac myocytes, having the advantage of being less computational demanding than detailed electrophysiological models. Under some conditions, it has been shown to be amenable to mathematical analysis. The model reproduces the action potential (AP) change with stimulation rate observed both experimentally and in realistic models of healthy human and guinea pig myocytes (TNNP and LRd models, respectively). When simulated in a cable it also gives the right dependence of the conduction velocity (CV) with stimulation rate. Besides reproducing correctly these restitution properties, it also gives a good fit for the morphology of the AP, including the notch typical of phase 1. Finally, we perform simulations in a realistic geometric model of the rabbit's ventricles, finding a good qualitative agreement in AP propagation and the ECG. Thus, this simplified model represents an alternative to more complex models when studying instabilities in wave propagation.

Revista:
Magnetohydrodynamics
ISSN:
0024-998X

Año:
2011
Vol.:
47
N°:
2
Págs.:
159 - 165

We report both theoretical and numerical results on convection for a magnetic fluid under rotation in a viscoelastic carrier liquid. The viscoelastic properties are given by the Oldroyd model. We focus on the stationary convection for idealized boundary conditions. Close to the first bifurcation, the coefficients of the corresponding amplitude equation are determined. The effect of the Kelvin force and of the rotation on instability thresholds for a diluted suspension are also emphasized.

Revista:
PHYSICAL REVIEW E
ISSN:
1539-3755

Año:
2011
Vol.:
84
N°:
3
Págs.:
037202

We study the deterministic spin dynamics of an anisotropic magnetic particle in the presence of a magnetic field with a constant longitudinal and a time-dependent transverse component using the Landau-Lifshitz-Gilbert equation. We characterize the dynamical behavior of the system through calculation of the Lyapunov exponents, Poincare sections, bifurcation diagrams, and Fourier power spectra. In particular we explore the positivity of the largest Lyapunov exponent as a function of the magnitude and frequency of the applied magnetic field and its direction with respect to the main anisotropy axis of the magnetic particle. We find that the system presents multiple transitions between regular and chaotic behaviors. We show that the dynamical phases display a very complicated structure of intricately intermingled chaotic and regular phases.

Revista:
IEEE TRANSACTIONS ON MAGNETICS
ISSN:
0018-9464

Año:
2011
Vol.:
47
N°:
10
Págs.:
3032-3035

Revista:
BMC SYSTEMS BIOLOGY
ISSN:
1752-0509

Año:
2011
Vol.:
5
N°:
1
Págs.:
114

Revista:
JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS
ISSN:
0304-8853

Año:
2011
Vol.:
323
N°:
6
Págs.:
691 - 698

Autores:
Cantalapiedra, I. R.; Peñaranda, A.; Echebarria, B.; et al.

Revista:
PHYSICAL REVIEW E
ISSN:
1539-3755

Año:
2010
Vol.:
82
N°:
1
Págs.:
11907

The involvement of the pedunculopontine nucleus (PPN) and the adjacent cuneiform nucleus (CuN), known as the mesencephalic locomotor area, in the pathophysiology of parkinsonian symptoms is receiving increasing attention. Taking into account the role of dopamine (DA) in motor control and its degeneration in Parkinson's disease, this neurotransmitter could induce dysfunction in the PPN and CuN through a direct dopaminergic innervation of these brainstem structures. This study provides the first demonstration that the PPN and CuN are innervated by dopamine transporter-bearing fibres in normal monkeys, which points to a novel dopaminergic system that targets the lower brainstem. Intoxication with MPTP induced a significant loss of dopamine transporter-positive fibres in the PPN and CuN of young (3-5 years old) acutely or chronically intoxicated monkeys compared with control animals. The more severe DA depletion found after chronic intoxication may explain, at least in part, deficits that appear late in the evolution of Parkinson's disease. A drastic loss of DA fibres was also observed in aged acutely intoxicated monkeys (about 30 years old) suggesting that age- and disease-related loss of dopaminergic fibres might be responsible for symptoms, such as gait disorders, that are more severe in elderly parkinsonian patients.