1,721,090 research outputs found
Error estimates for an approximation of a problem of percolation in gently sloping beaches.
No full textWe prove error estimates for approximations of a parabolic variational inequality arising from a model of percolation in gently sloping beaches. We use piecewise linear finite element discretization in space and backward differences in time
Exploring anodal and cathodal make and break cardiac excitation mechanisms in a 3D anisotropic bidomain model
No full textPublished studies have investigated the relevance of cardiac virtual electrode responses to unipolar cathodal and anodal stimulations for explaining the make and break excitation mechanisms. Most of these studies have considered 2D bidomain models or cylindrical domains that by symmetry reduce to the 2D case, so the triggering mechanisms and onset of excitation have not yet been fully elucidated in 3D anisotropic models. The goal of this work is to revisit these excitation mechanisms with 3D bidomain simulations considering two tissue types with unequal anisotropy ratio, including transmural fiber rotation and augmenting the Luo–Rudy I membrane model with the so-called funny and the electroporation currents. In addition to usual snapshots of transmembrane potential patterns, we compute from the action potential waveforms the activation time and associated isochrone sequences, yielding a detailed 3D description of the instant and location of excitation origin, shape and propagation of activation wavefronts. A specific aim of this work is to detect the location of the excitation onset and whether its trigger mechanism is (a) electrotonic, i.e. originating from discharge diffusion of currents flowing between virtual cathodes and anods and/or (b) membrane-based, i.e. arising only from intrinsic depolarizing membrane currents. Our results show that the electrotonic mechanism is observed independently of the degree of unequal anisotropy in diastolic anode make and systolic cathode break. The membrane-based mechanism is observed in diastolic cathode make, diastolic anode break, only for a relative weak anisotropy, and systolic anode break. The excitation trigger mechanism, the location of the excitation origin and the pattern of the isochrone sequence are independent of the degree of anisotropy for diastolic cathode make, systolic cathode and anode break, while they might depend on the degree of anisotropy for diastolic anode make and break. Moreover, the tissue anisotropy has a strong influence on the threshold amplitude of the stimulation pulse triggering these mechanisms
Automatic control via thermostats of a hyperbolic Stefan problem with memory
No full textA hyperbolic Stefan problem based on the linearized Gurtin—Pipkin heat conduction law is considered. The temperature and free boundary are controlled by a thermostat acting on the boundary. This feedback control is based on temperature measurements performed by real thermal sensors located within the domain containing the two-phase system and/or at its boundary. Three different types of thermostats are analyzed: simple switch, relay switch, and a Preisach hysteresis operator. The resulting models lead to integrodifferential hyperbolic Stefan problems with nonlinear and nonlocal boundary conditions. Existence results are proved in all the cases. Uniqueness is also shown, except in the situation corresponding to the ideal switch
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
A reliability analysis of cardiac repolarization time markers
No full textOnly a limited number of studies have addressed the reliability of extracellular markers of cardiac repolarization time, such as the classical marker RTeg defined as the time of maximum upslope of the electrogram T wave. This work presents an extensive three-dimensional simulation study of cardiac repolarization time, extending the previous one-dimensional simulation study of a myocardial strand by Steinhaus [B.M. Steinhaus, Estimating cardiac transmembrane activation and recovery times from unipolar and bipolar extracellular electrograms: a simulation study, Circ. Res. 64 (3) (1989) 449]. The simulations are based on the bidomain - Luo-Rudy phase I system with rotational fiber anisotropy and homogeneous or heterogeneous transmural intrinsic membrane properties. The classical extracellular marker RTeg is compared with the gold standard of fastest repolarization time RTtap, defined as the time of minimum derivative during the downstroke of the transmembrane action potential (TAP). Additionally, a new extracellular marker RT 90eg is compared with the gold standard of late repolarization time RT 90tap, defined as the time when the TAP reaches 90% of its resting value. The results show a good global match between the extracellular and transmembrane repolarization markers, with small relative mean discrepancy (≤ 1.6 %) and high correlation coefficients (≥ 0.92), ensuring a reasonably good global match between the associated repolarization sequences. However, large local discrepancies of the extracellular versus transmembrane markers may ensue in regions where the curvature of the repolarization front changes abruptly (e.g. near front collisions) or is negligible (e.g. where repolarization proceeds almost uniformly across fiber). As a consequence, the spatial distribution of activation-recovery intervals (ARI) may provide an inaccurate estimate of (and weakly correlated with) the spatial distribution of action potential durations (APD)
Computing cardiac recovery maps from electrograms and monophasic action potentials under heterogeneous and ischemic conditions
No full textThe currently available techniques to investigate the 3D sequence of activation and recovery in the cardiac atria and ventricles, with high spatial resolution, are based on extracellular electrical recordings. The goal of the present work is to provide an extensive quantitative analysis of the accuracy level of commonly used recovery time (RT) markers, under heterogeneous and pathological conditions of the myocardial tissue, such as myocardial ischemia. A widely used technique is based on unipolar electrograms (EGs); an alternative technique is based on hybrid monophasic action potentials (HMAPs), obtained as the potential difference between a permanently depolarized site and an exploring site. The RT markers derived from EGs and HMAPs are compared with two transmembrane action potential (TAP) markers considered here as gold standards for the fastest and final recovery phase, respectively. The analysis is based on 3D numerical simulations of the action potential propagation in anisotropic and insulated cardiac blocks, modeled by the Bidomain system coupled with the Luo–Rudy I membrane model. These demanding simulations have been made possible by recent advances in computing power and multilevel Bidomain solvers. The results show that the extracellular RT markers considered are reliable estimates of the gold standard TAP markers, with low relative mean discrepancies and high correlation coefficients. We also investigate the capability of the markers to discriminate different transmural dispersions of recovery times and action potential durations. In some specific pathological cases when the EG markers fail, the HMAP markers may offer reliable alternatives
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