Benjamin Buard

Multifractal analysis of heart rate variability and laser Doppler flowmetry fluctuations:comparison of results from different numerical methods

To contribute to the understanding of the complex dynamics in the cardiovascular system (CVS), the central CVS has previously been analyzed through multifractal analyses of heart rate variability (HRV) signals that were shown to bring useful contributions. Similar approaches for the peripheral CVS through the analysis of laser Doppler flowmetry (LDF) signals are comparatively very recent. In this direction, we propose here a study of the peripheral CVS through a multifractal analysis of LDF fluctuations, together with a comparison of the results with those obtained on HRV fluctuations simultaneously recorded. To perform these investigations concerning the biophysics of the CVS, first we have to address the problem of selecting a suitable methodology for multifractal analysis, allowing us to extract meaningful interpretations on biophysical signals. For this purpose, we test four existing methodologies of multifractal analysis. We also present a comparison of their applicability and interpretability when implemented on both simulated multifractal signals of reference and on experimental signals from the CVS. One essential outcome of the study is that the multifractal properties observed from both the LDF fluctuations (peripheral CVS) and the HRV fluctuations (central CVS) appear very close and similar over the studied range of scales relevant to physiology.

Multifractal analysis of central (electrocardiography) and peripheral (laser Doppler flowmetry) cardiovascular time series from healthy human subjects.

Analysis of the cardiovascular system (CVS) activity is important for several purposes, including better understanding of heart physiology, diagnosis and forecast of cardiac events. The central CVS, through the study of heart rate variability (HRV), has been shown to exhibit multifractal properties, possibly evolving with physiologic or pathologic states of the organism. An additional viewpoint on the CVS is provided at the peripheral level by laser Doppler flowmetry (LDF), which enables local blood perfusion monitoring. We report here for the first time a multifractal analysis of LDF signals through the computation of their multifractal spectra. The method for estimation of the multifractal spectra, based on the box method, is first described and tested on a priori known synthetic multifractal signals, before application to LDF data. Moreover, simultaneous recordings of both central HRV and peripheral LDF signals, and corresponding multifractal analyses, are performed to confront their properties. With the scales chosen on the partition functions to compute Renyi exponents, LDF signals appear to have broader multifractal spectra compared to HRV. Various conditions for LDF acquisitions are tested showing larger multifractal spectra for signals recorded on fingers than on forearms. The results uncover complex interactions at central and peripheral CVS levels.

Laser speckle contrast imaging of the skin: interest in processing the perfusion data

Laser speckle contrast imaging (LSCI) is a recent clinical powerful tool to obtain full-field images of microvascular blood perfusion. The technique relies on laser speckle obtained by the interactions between coherent monochromatic radiations and the tissues under study. From these speckle images, contrast values are determined and instantaneous map of the perfusion are computed. LSCI has gained increased attention in the last years and is now additional to laser Doppler flowmetry (LDF). In spite of the growing interest for LSCI in skin clinical research, very few LSCI perfusion data processing have been published from now to extract physiologically-linked indices. By opposition, numerous signal processing works have been dedicated to the processing of LDF signals. The latter works proposed methodological processing procedures to extract information reflecting underlying microvascular mechanisms such as myogenic, neurogenic and endothelial activities. Our goal herein is to report on the potentialities of studies dedicated to the processing of LSCI perfusion data. Linear and nonlinear analyses could be of interest to improve the understanding of LSCI images.

Generalized fractal dimensions of laser Doppler flowmetry signals recorded from glabrous and nonglabrous skin

PURPOSE The technique of laser Doppler flowmetry (LDF) is commonly used to have a peripheral view of the cardiovascular system. To better understand the microvascular perfusion signals, the authors herein propose to analyze and compare the complexity of LDF data recorded simultaneously in glabrous and nonglabrous skin. Glabrous zones are physiologically different from the others partly due to the presence of a high density of arteriovenous anastomoses. METHODS For this purpose, a multifractal analysis based on the partition function and generalized fractal dimensions computation is proposed. The LDF data processed are recorded simultaneously on the right and left forearms and on the right and left hand palms of healthy subjects. The signal processing method is first tested on a multifractal binomial measure. The generalized fractal dimensions of the normalized LDF signals are then estimated. Furthermore, for the first time, the authors estimate the generalized fractal dimensions from a range of scales corresponding to factors influencing the microcirculation flow (cardiac, respiratory, myogenic, neurogenic, and endothelial). RESULTS Different multifractal behaviors are found between normalized LDF signals recorded in the forearms and in the hand palms of healthy subjects. Thus, the variations in the estimated generalized fractal dimensions of LDF signals recorded in the hand palms are higher than those of LDF signals recorded in the forearms. This shows that LDF signals recorded in glabrous zones may be more complex than those recorded in nonglabrous zones. Furthermore, the results show that the complexity in the hand palms could be more important at scales corresponding to the myogenic control mechanism than at the other studied scales. CONCLUSIONS These findings suggest that the multifractality of the normalized LDF signals is different on glabrous and nonglabrous skin. This difference may rely on the density of arteriovenous anastomoses and differences in nerve supply or biochemical properties. This study provides useful information for an in-depth understanding of LDF data and a more detailed knowledge of the peripheral cardiovascular system.

Laser Doppler flowmetry signals: pointwise Hölder exponents of experimental signals from young healthy subjects and numerically simulated data

We analyze the complexity of laser Doppler flowmetry (LDF) signals which give a peripheral view of the cardiovascular system. For this purpose, experimental and numerically simulated LDF signals are processed. The experi- mental signals are recorded in young healthy subjects. The numerically simulated LDF data are computed from a model containing six nonlinear coupled oscillators reflecting six al- most periodic rhythmic activities present in experimental LDF signals. In the model, the oscillators are coupled with both linear and parametric couplings in order to represent cardio- vascular system behaviors. To our knowledge this modeling has never been proposed yet. The complexity of all the experi- mental and simulated signals is studied by the computation of pointwise Holder exponents. The latter identify the possible multifractal characteristics of data. The pointwise Holder exponents are determined with a parametric generalized quadratic variation based estimation method first calibrated from white noise measures. The results of our signal process- ing analysis show that experimental LDF signals are weakly multifractal for young healthy subjects at rest. Furthermore, our findings together with another recent work of our group show that pointwise Holder exponents of the simulated data do not describe the ones of the young healthy subjects but are closer to the ones of elderly healthy people. This paper pro- vides useful information to go deeper into the modeling of LDF data, that could bring enlightenment for a better understand- ing of the peripheral cardiovascular system.

Multifractal Analysis of Laser Doppler Flowmetry Signals: Partition Function and Generalized Dimensions of Data Recorded before and after Local Heating

Laser Doppler flowmetry (LDF) signals - that reflect the peripheral cardiovascular system - are now widespread in blood microcirculation research. Over the last few years, the central cardiovascular system has been the subject of many fractal and multifractal works. However, only very few multifractal studies of LDF signals have been published. Such multifractal analyses have shown that LDF data can be weakly multifractal but the origin of such characteristics are still unknown. We therefore herein propose a multifractal analysis of LDF signals recorded on the forearm of twelve healthy subjects, before and after skin local heating. The results show that the partition functions for all the signals have power-law characteristics. Moreover, generalized dimensions present very few variations with q for the signals recorded before heating; these variations are larger 20 minutes after local heating. Physiological activities may therefore play a role in the weak multifractal properties of LDF data.

Laser Doppler flowmetry: multifractal spectra of signals recorded in hand of young healthy subjects before and after local heating

Laser Doppler flowmetry (LDF) signals give a peripheral view of the cardiovascular system. We herein propose to analyze the complexity of LDF signals recorded in the palm side of the hand and to compare the results with those obtained in the forearm. Moreover, we also study the possible impact of local heating (40°C) on the width of the multifractal spectra for the hand. For this purpose, LDF signals are recorded simultaneously in the hand and forearm. A local heating of 40°C is performed in the hand palm, leading to an increase of the local skin blood flow via, among others, the production of nitric oxide. LDF data recorded before and during the local heating are processed in order to obtain their multifractal spectra. The latter are computed, without normalization of the signals amplitude, by first estimating the discrete partition function of the data, then by determining their Renyi exponents with a linear regression, and finally by computing their Legendre transform. The results show that, at rest, the average multifractal spectrum of signals recorded in the hand palm is larger and more asymmetric than the one of data from the forearm. Furthermore, without normalization of the signals amplitude, local heating in the hand palm leads to a slightly narrower and more symmetric average multifractal spectrum for this site. This study brings information on the multifractal spectra of LDF signals and is a first step in order to have more knowledge on the potential implication of the endothelium in the complexity of LDF signals in the hand palm.