128 research outputs found
Nonlinear trend removal should be carefully performed in heart rate variability analysis
Background : In Heart rate variability analysis, the rate-rate time
series suffer often from aperiodic non-stationarity, presence of ectopic beats
etc. It would be hard to extract helpful information from the original signals.
10 Problem : Trend removal methods are commonly practiced to reduce
the influence of the low frequency and aperiodic non-stationary in RR data.
This can unfortunately affect the signal and make the analysis on detrended
data less appropriate. Objective : Investigate the detrending effect
(linear \& nonlinear) in temporal / nonliear analysis of heart rate variability
of long-term RR data (in normal sinus rhythm, atrial fibrillation, 15
congestive heart failure and ventricular premature arrhythmia conditions).
Methods : Temporal method : standard measure SDNN; Nonlinear methods
: multi-scale Fractal Dimension (FD), Detrended Fluctuation Analysis (DFA) \&
Sample Entropy (Sam-pEn) analysis. Results : The linear detrending
affects little the global characteristics of the RR data, either 20 in temporal
analysis or in nonlinear complexity analysis. After linear detrending, the
SDNNs are just slightly shifted and all distributions are well preserved. The
cross-scale complexity remained almost the same as the ones for original RR
data or correlated. Nonlinear detrending changed not only the SDNNs
distribution, but also the order among different types of RR data. After this
processing, the SDNN became indistinguishable be-25 tween SDNN for normal sinus
rhythm and ventricular premature beats. Different RR data has different
complexity signature. Nonlinear detrending made the all RR data to be similar ,
in terms of complexity. It is thus impossible to distinguish them. The FD
showed that nonlinearly detrended RR data has a dimension close to 2, the
exponent from DFA is close to zero and SampEn is larger than 1.5 -- these
complexity values are very close to those for 30 random signal.
Conclusions : Pre-processing by linear detrending can be performed on RR data,
which has little influence on the corresponding analysis. Nonlinear detrending
could be harmful and it is not advisable to use this type of pre-processing.
Exceptions do exist, but only combined with other appropriate techniques to
avoid complete change of the signal's intrinsic dynamics. 35 Keywords
heart rate variability linear / nonlinear detrending
complexity analysis mul-tiscale analysis detrended
fluctuation analysis fractal dimension sample entropy
Description, modeling and forecasting of data with optimal wavelets
Cascade processes have been used to model many different self-similar systems, as they are able to accurately describe most of their global statistical properties. The so-called optimal wavelet basis allows to achieve a geometrical representation of the cascade process-named microcanonical cascade- that describes the behavior of local quantities and thus it helps to reveal the underlying dynamics of the system. In this context, we study the benefits of using the optimal wavelet in contrast to other wavelets when used to define cascade variables, and we provide an optimality degree estimator that is appropriate to determine the closest-to-optimal wavelet in real data. Particularizing the analysis to stock market series, we show that they can be represented by microcanonical cascades in both the logarithm of the price and the volatility. Also, as a promising application in forecasting, we derive the distribution of the value of next point of the series conditioned to the knowledge of past points and the cascade structure, i.e., the stochastic kernel of the cascade process.
Residual-based stabilization of the finite element approximation to the acoustic perturbation equations for low Mach number aeroacoustics
This is the peer reviewed version of the following article: [Guasch, O., SĂĄnchez-MartĂn, P., Pont, A., Baiges, J., and Codina, R. (2016) Residual-based stabilization of the finite element approximation to the acoustic perturbation equations for low Mach number aeroacoustics. Int. J. Numer. Meth. Fluids, 82: 839â857. doi: 10.1002/fld.4243], which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1002/fld.4243/abstract. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.The acoustic perturbation equations (APE) are suitable to predict aerodynamic noise in the presence of a non-uniform mean flow. As for any hybrid computational aeroacoustics approach, a first computational fluid dynamics simulation is carried out from which the mean flow characteristics and acoustic sources are obtained. In a second step, the APE are solved to get the acoustic pressure and particle velocity fields. However, resorting to the finite element method (FEM) for that purpose is not straightforward. Whereas mixed finite elements satisfying an appropriate infâsup compatibility condition can be built in the case of no mean flow, that is, for the standard wave equation in mixed form, these are difficult to implement and their good performance is yet to be checked for more complex wave operators. As a consequence, strong simplifying assumptions are usually considered when solving the APE with FEM. It is possible to avoid them by resorting to stabilized formulations. In this work, a residual-based stabilized FEM is presented for the APE at low Mach numbers, which allows one to deal with the APE convective and reaction terms in its full extent. The key of the approach resides in the design of the matrix of stabilization parameters. The performance of the formulation and the contributions of the different terms in the equations are tested for an acoustic pulse propagating in sheared-solenoidal mean flow, and for the aeolian tone generated by flow past a two-dimensional cylinder.Peer ReviewedPostprint (author's final draft
Characterizing Complexity of Atrial Arrhythmias through Effective Dynamics from Electric Potential Measures
International audienceThe cardiac electrical activity follows a complex dynamics whose accurate description is crucial to characterize arrythmias and classify their complexity. Rhythm reflects the connection topology of pacemaker cells at their source. Hence, characterizing the attractors as nonlinear, effective dynamics can capture the key parameters without imposing any particular model on the empirical signals. A dynamic phase-space reconstruction from appropriate embedding can be made robust and numerically stable with the presented method. \textbf{Methods:} Time series evolution is mapped to an object embedded in a phase space in abstract coordinates. independent observations construct an phase space, as per the ebmedding theorem. The dimension is the least one that embeds the dynamics (which is twice plus one the Minkowski dimension of its attractor set) and the time lag is the shortest for which the coordinates do not mutually interfere. With appropriate filtering, the method is robust and adapted to empirical signals. The result is a compact dynamical description that characterizes complexity degree and information distribution. \textbf{Results and Conclusion:} Nonlinear analysis provides appropriate tools to characterize cardiac dynamics. Singularity analysis and phase-space reconstruction are physically meaningful complexity measures with minimal assumptions on the underlying models. We validate our approach on ECG, endocavitary catheter measures and electrocardiographic maps. Key parameters vary infrequently and exhibit sharp transitions, which show where information concentrates and correspond to actual dynamical regime changes. In space domain, extreme values highlight arrhythmogenic areas whose ablation stopped the fibrillation. We observe a correspondence of time lag fluctuations of phase-space reconstructions with atrial fibrillation episodes in the same way as with the dynamical changes coming from singularity exponents. This opens the way for improved model-independent complexity descriptors to be used in non-invasive, automatic diagnosis support and ablation guide for electrical insulation therapy, in cases of arrhythmias such as atrial flutter and fibrillation
Microcanonical processing methodology for ECG and intracardial potential: application to atrial fibrillation
Cardiac diseases are the principal cause of human morbidity and mortality in
the western world. The electric potential of the heart is a highly complex
signal emerging as a result of nontrivial flow conduction, hierarchical
structuring and multiple regulation mechanisms. Its proper accurate analysis
becomes of crucial importance in order to detect and treat arrhythmias or other
abnormal dynamics that could lead to life-threatening conditions. To achieve
this, advanced nonlinear processing methods are needed: one example here is the
case of recent advances in the Microcanonical Multiscale Formalism. The aim of
the present paper is to recapitulate those advances and extend the analyses
performed, specially looking at the case of atrial fibrillation. We show that
both ECG and intracardial potential signals can be described in a model-free
way as a fast dynamics combined with a slow dynamics. Sharp differences in the
key parameters of the fast dynamics appear in different regimes of transition
between atrial fibrillation and healthy cases. Therefore, this type of analysis
could be used for automated early warning, also in the treatment of atrial
fibrillation particularly to guide radiofrequency ablation procedures.Comment: Transactions on Mass-Data Analysis of Images and Signals 4, 1 (2012).
Accepte
Unified solver for fluid dynamics and aeroacoustics in isentropic gas flows
The high computational cost of solving numerically the fully compressible NavierâStokes equations, together with the poor performance of most numerical formulations for compressible flow in the low Mach number regime, has led to the necessity for more affordable numerical models for Computational Aeroacoustics. For low Mach number subsonic flows with neither shocks nor thermal coupling, both flow dynamics and wave propagation can be considered isentropic. Therefore, a joint isentropic formulation for flow and aeroacoustics can be devised which avoids the need for segregating flow and acoustic scales. Under these assumptions density and pressure fluctuations are directly proportional, and a two field velocity-pressure compressible formulation can be derived as an extension of an incompressible solver. Moreover, the linear system of equations which arises from the proposed isentropic formulation is better conditioned than the homologous incompressible one due to the presence of a pressure time derivative. Similarly to other compressible formulations the prescription of boundary conditions will have to deal with the backscattering of acoustic waves. In this sense, a separated imposition of boundary conditions for flow and acoustic scales which allows the evacuation of waves through Dirichlet boundaries without using any tailored damping model will be presented.Peer ReviewedPostprint (author's final draft
Arrhythmic dynamics from singularity analysis of electrocardiographic maps
International audienceFrom a point view of nonlinear dynamics, the electrical activity of the heart is a complex dynamical system, whose dynamics reflects the actual state of health of the heart. Nonlinear signal-processing methods are needed in order to accurately characterize these signals and improve understanding of cardiac arrhythmias. Recent developments on reconstructible signals and multiscale information content show that an analysis in terms of singularity exponents provides compact and meaningful descriptors of the structure and dynamics of the system. Such approach gives a compact representation atrial arrhythmic dynamics, which can sharply highlight regime transitions and arrhythmogenic areas
In Vitro Arrhythmia Generation by Mild Hypothermia - a Pitchfork Bifurcation Type Process
International audienceThe neurological damage after cardiac arrest (CA) constitutes a big challenge of hospital discharge. The therapeutic hypothermia (34°C-32°C) has shown its benefit to reduce cerebral oxygen demand and improve neurological outcomes after the cardiac arrest. However, it can have many adverse effects, among them the cardiac arrhythmia generation represents an important part (up to 34%, according different clinical studies). Monolayer cardiac culture is prepared with cardiomyocytes from new-born rat directly on the multi-electrodes array, which allows acquiring the extracellular potential of the culture. The temperature range is 37°C - 30°C - 37°C, representing the cooling and rewarming process in the therapeutic hypothermia. Both experiments showed that at 35°C, the acquired signals are characterized by period-doubling phenomenon, compared to signals at any other temperatures. Spiral waves, commonly considered as a sign of cardiac arrhythmia, are observed in the reconstructed activation map. With an approach from nonlinear dynamics, phase space reconstruction, it is shown that at 35°C, the trajectories of these signals formed a bifurcation, even trifurcation. Another transit point is found between 30°C - 33°C, which agreed with other clinical studies that induced hypothermia after cardiac arrest should not be below 32°C. The therapeutic hypothermia after cardiac arrest can be represented by a Pitchfork bifurcation, which could explain the different ratio of arrhythmia among the adverse effects after this therapy. This nonlinear dynamics suggests that a variable speed of cooling / rewarming, especially when passing 35°C, would help to decrease the ratio of post-hypothermia arrhythmia and then improve the hospital output
On optimal wavelet bases for the realization of microcanonical cascade processes
International Journal of Wavelets, Multiresolution and Information ProcessingInternational audienceMultiplicative cascades are often used to represent the structure of turbulence. Under the action of a multiplicative cascade, the relevant variables of the system can be understood as the result of a successive transfer of information in cascade from large to small scales. However, to make this cascade transfer explicit (i.e, being able to decompose each variable as the product of larger scale contributions) is only achieved when signals are represented in an optimal wavelet basis. Finding such a basis is a data-demanding, highly-complex task. In this paper we propose a formalism that allows to find the optimal wavelet of a signal in an efficient, little data-demanding way. We confirm the appropriateness of this approach by analyzing the results on synthetic signals constructed with prescribed optimal bases. We show the validity of our approach constrained to given families of wavelets, though it can be generalized for a continuous unconstrained search scheme
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