1,721,041 research outputs found
Analysis of scaling behaviour of ECG signal during atrial fibrillation
Full text linkIn this study, surface ECG signals recoded during Atrial Fibrillation (AF) episodes have been investigated to assess the presence of scaling behaviors. The ECG signals consisted of one minutes recordings coming from the 2004 Computers in Cardiology Challenge database. After having removed the ventricular related activity, the residual ECG signal (rECG) was analyzed using Detrended Fluctuation Analysis (DFA). We documented the existence of two scaling regions in the signals. Over long time scales (n >= 32, approximate to 234ms) the rECG fluctuations resembled a white or slightly persistent noise (scaling A=0.64 +/- 0.17), whereas over short scales (n <= 16, approximate to 125ms) slowly decaying antipersistent correlations were evident (scaling A=1.20 +/- 0.20).
In addition, we searched for possible differences in the scaling exponents between terminating (T) and nonterminating (N) AF episodes. We observed that both short-term (A1: 1.29 +/- 0.15 for T vs. 1.12 +/- 0.18 for N, p < 0.005) and long-term (A2: 0.54 +/- 0.16 vs. 0.70 +/- 0.14, p < 0.005) scaling exponents were significantly different (t-test)
Editing RR Series and Computation of Long-Term Scaling Parameters
Full text linkThe editing of heart rate variability (HRV) sequences is largely employed in presence of biological (ectopies, arrhythmias) and technical artifacts. Little is know about the effects of these corrections on the estimation of the long-term scaling exponents, especially for long artifacts. We therefore investigated the robustness of three popular scaling exponent estimators (DFA, a-slope and Dispersional Analysis) with an increasing number of missing RR samples. We tested three editing methods: (i) substitution with local mean value, (ii) linear interpolation and (iii) deletion. Starting from long uncorrupted (≫ 10000 points) NN series, we artificially inserted artifacts. We then evaluated the effect of the editing methods on the estimation of the scaling exponents. As a reference, the same computation was performed simply removing an equivalent number of points at the extreme of the series. The simulations suggest a negligible effect of the corrections, at least as long as the number of points edited is relatively small
Analysis of surface atrial signals using spectral methods for time series with missing data
Full text linkIn this work, the analysis of atrial signals recorded during atrial fibrillation was pursued using two spectral estimators designed for series with missing data: the Lomb periodogram (LP) and the Iterative Singular Spectrum Analysis (ISSA). The main aim is to verify if subtraction ofthe ventricular activity might be avoided by performing spectral analysis on those ECG intervals where such activity is absent, (i.e. the T-Q intervals), at least to estimate the dominant atrial Fibrillatory Frequency (FF). Recordings coming from the 2004 Computers in Cardiology Termination Challenge Database were analyzed. Fibrillatory frequencies were then compared with those obtained from the analysis ofthe correspondent atrial signals extracted using a modified Average Beat Substraction (ABS) technique. We observed that the mean absolute difference was 0.42 ± 0.66 Hz for LP, (mean±SD), and 0.39 ± 0.64 Hz for ISSA. We concluded that estimation of FF is feasible without applying QRS-T subtraction
Analysis of T-Wave Alternans Using the Ramanujan Transform,
Full text linkIn this study the use of Ramanujan transform (RT) for the assessment of T-Wave alternans (TWA) is investigated. The RT projects a signal on particular basis functions constituted by Ramanujan sums (RS). The resulting components highlight the presence of 1/q resonances, being q an integer number. Thus, the transform is suitable for detecting the typical 1/2 pattern of TWA. After alignment, the successive T-waves are filtered using the Amplitude Adjusted T-Average method. For each point of the obtained waveforms, the series of corresponding T amplitudes is transformed using RT. Next, the quantification of TWA is obtained through the RT coefficients. Performances of the novel approach are evaluated on the Computers in Cardiology Challenge dataset 2008. The final score was encouraging (0.641), thus evidencing that RT might be a valuable tool for the detection of TWA
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
Analysis of T-wave alternans using the dominant T wave
No full textThe dominant T wave (DTW) reflects the derivative of the repolarization phase of the transmembrane potential of myocytes. T-wave alternans (TWA) is defined as an alteration of this repolarization that repeats every other beat. We investigate if the DTW can offer new insight on TWA. We first proved that the DTW estimate obtained through singular value decomposition is optimal, because it minimizes the norm of the residuals. Then we suggested an optimal estimate of the vector of lead factors, in the case in which the DTW is given. Finally, we derived a mathematical relationship between observable TWA on electrocardiogram and DTW morphology. The relationship depends on the slope of the repolarization phase of the myocytes' transmembrane potentials and on the dispersion of the repolarization times. Based on this finding, a new index meant to quantify TWA was defined and termed amplitude of dominant T-wave alternans (ADTWA). A preliminary validation of the index was performed using the synthetic records contained in the Computers in Cardiology 2008 data set. They were obtained from 5 electrocardiogram models to which TWA was added at different extents. We found a linear relationship between the TWA amplitude and the ADTWA metric (R 2 = 0.9898 ± 0.100 across all models). Moreover, the root mean square error between actual and estimated TWA amplitudes was 10.9 μV (ADTWA) vs 12.9 μV obtained with the classical spectral method
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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