Search

The Detrended Fluctuation Analysis (DFA) is a popular method for quantifying the self-similarity of the heart rate that may reveal complexity aspects in cardiovascular regulation. However, the self-similarity coefficients provided by DFA may be affected by an overestimation error associated with the shortest scales. Recently, the DFA has been extended to calculate the multifractal-multiscale self-similarity and some evidence suggests that overestimation errors may affect different multifractal orders. If this is the case, the error might alter substantially the multifractal-multiscale representation of the cardiovascular self-similarity. The aim of this work is 1) to describe how this error depends on the multifractal orders and scales and 2) to propose a way to mitigate this error applicable to real cardiovascular series.Clinical Relevance- The proposed correction method may extend the multifractal analysis at the shortest scales, thus allowing to better assess complexity alterations in the cardiac autonomic regulation and to increase the clinical value of DFA.

The Detrended Fluctuation Analysis (DFA) is widely employed to quantify the fractal dynamics of R-R intervals (RRI). This is usually done by estimating a short- and a long-term coefficient, but it is still unclear how much the information provided by such a bi-scale DFA is independent of that of traditional spectral indices. However, more sophisticated DFA approaches have been recently proposed, including the multifractal-multiscale DFA and the DFA for magnitude and sign of RRI changes. The aim of our work is to investigate whether novel DFA approaches allow extracting the information on the nonlinear RRI dynamics that traditional spectral methods cannot retrieve.We selected 4-hour segments of beat-by-beat RRI series from a 24-hour Holter recording, one during daytime (wake), one at night (sleep) in a healthy volunteer. From the wake segment, we generated 100 surrogate series shuffling the phases but preserving the power spectrum, and then from each of the resulting RRI series, we generated the series of the sign and the series of the magnitude of successive RRI changes. We generated similar series from the sleep recording. Thus, we finally obtained 6 original beat-to-beat series to be compared with 600 surrogate series, each of 4-hour duration.The comparison between original and surrogate series showed that for this experimental setting, the traditional monofractal DFA provides the same information retrievable by the power spectrum. However, specific components of the multifractal DFA reveal information not detectable by the power spectrum, particularly in the sleep condition. Furthermore, the DFA of the magnitude of RRI changes reflects important nonlinear components. Therefore, these more sophisticated DFA approaches might effectively improve the clinical value of RRI variability analysis.

Some physiological series, like the cardiovascular signals, show multifractal structures that depend on the temporal scale. Thus, their fractal nature is better assessed by a Detrended Fluctuation Analysis (DFA) approach that provides multifractal coefficients scale by scale. Our aim is to compare two estimators of the degree of scale-by-scale multifractality based on the width of the singularity spectrum or on the statistical dispersion of the DFA coefficients. We synthesized 1000 series of white noise (monofractal and monoscale), of autoregressive noise (monofractal and multiscale) and of Cauchy-distributed noise (multifractal and monoscale) comparing the two estimators at scales between 8 and 228 samples. We found that the two estimators provide similar scale-by-scale profiles of multifractality. However, the statistical-dispersion estimator better distinguishes multifractal from monofractal noises at all the scales, thus appearing more suitable than the singularity-spectrum width to describe the fractal structure of physiological time series.

A feature of time-series variability that may reveal underlying complex dynamics is the degree of "convolutedness". For multivariate series of m components, convolutedness can be defined as the propensity of the trail of the time-series samples to fill the m-dimensional space. This work proposes different convolutedness indices and compare them on synthesized and real physiological signals. The indices are based on length L and planar extension d of the trail in m dimensions. The classical ones are: the L/d ratio, and the Mandelbrot's fractal dimension (FD) of a curve: FDM =log(L)/log(d). In this work we also consider a correction of the Katz's estimator of FDM, i.e., FDKC =log(N)/(log(N)+log(d/L)), with N the number of samples; and FDMC, an estimator of FDM based on FDKC calculated over a shorter running window Nw

— Cardiorespiratory polygraphies do not allow the traditional sleep scoring. Therefore this study evaluated whether fractal dimension (FD) analysis of ECG and respiration (RSP) provides information on sleep stages. We considered two sleep-scored overnight full-polysomnographies. R-R intervals (RRI) from the ECG and RSP were resampled (4 Hz) and normalized to unit variance. FD of a segment of N samples of m signals is log( N-1 )/[log( N-1 )+log( d/L )], with L length of the trajectory in the m -dimensional space, d its extension. Monovariate ( m =1, FD RRI and FD RSP ) and bivariate ( m =2, FD RRI,RSP ) fractal dimensions were estimated over running windows of 15 s, and averaged over wake, lighter and deeper NREM stages, and REM. Unlike FD RRI and FD RSP , the bivariate FD RRI,RSP showed the same behavior in both subjects, being lowest in wake, increasing with the depth of NREM sleep and decreasing slightly in REM. This suggests that bivariate FD can provide information for sleep scoring of cardiorespiratory polygraphies.

The low-frequency fluctuations of tunneling current in GaAs/AlAs/GaAs single-barrier heterostructure devices were measured for three samples with different Si-planar-doping in the center of 10.2 nm-thick barrier. The diameter of the mesas were close to 100 micrometers. Upon changes of the biasing voltage across the barrier the character of noise spectra also changed - depending on various electron transport mechanisms in the barrier. We emphasise that noise measurement is much more sensitive than simple current-voltage characteristics.

Seismocardiography is the body-surface recording of vibrations produced by the beating heart. A high frequency (HF) accelerometric component of the seismocardiogram (SCG) is related to the heart sounds generated by the closure of atrio-ventricular and semilunar valves. This paper evaluates the feasibility of recording the SCG component associated to cardiac sounds by means of a wearable device originally designed for monitoring ECG, respiratory movements, body accelerations and posture in freely moving subjects. The method is based on the averaging of the HF component of the acceleration vector measured by the wearable system, and on the subsequent extraction of features from its envelope. The method is applied on data recorded in healthy volunteers in different postures and during sleep. Results indicate that it is possible to reliably identify the time of occurrence of the first and second heart sound within the cardiac cycle. They also show significant differences in the HF component of SCG between supine and standing postures. Analyzing the HF SCG in a volunteer sleeping at high altitude (4554 m asl) substantial differences were also found among three body positions (lying supine or on the left or right side). These differences are likely to reflect changes in cardiac mechanics induced by different postures of the body.

Seismocardiogram (SCG) is the recording of the minute body accelerations induced by the heart activity, and reflects mechanical aspects of heart contraction and blood ejection. So far, most of the available systems for the SCG assessment are designed to be used in a laboratory or in controlled behavioral and environmental conditions. In this paper we propose a modified version of a textile-based wearable device for the unobtrusive recording of ECG, respiration and accelerometric data (the MagIC system), to assess the 3d sternal SCG in daily life. SCG is characterized by an extremely low magnitude of the accelerations (in the order of g × 10(-3)), and is masked by major body accelerations induced by locomotion. Thus in daily life recordings, SCG can be measured whenever the subject is still. We observed that about 30 seconds of motionless behavior are sufficient for a stable estimate of the average SCG waveform, independently from the subject's posture. Since it is likely that during spontaneous behavior the subject may stay still for at least 30 seconds several times in a day, it is expected that the SCG could be repeatedly estimated and tracked over time through a prolonged data recording. These observations represent the first testing of the system in the assessment of SCG out of a laboratory environment, and open the possibility to perform SCG studies in a wide range of everyday conditions without interfering with the subject's activity tasks.

We show that we can detect the direction of the magnetization in a vortex core by just using electrical transport measurements. The shape of the homodyne peak when a vortex is resonantly excited by an AC current is found to be a direct measure of the out-of-plane component in the vortex core and can thus serve as a detector of a vortex core reversal.

The underlying mechanisms for domain wall propagation induced by spin-polarized currents were studied. The critical parameters for current-induced wall motion such as the threshold current density at which the motion sets in, the wall velocity and their temperature dependence, were obtained experimentally. Permalloy rings with nonmagnetic contacts were fabricated for transport measurements.

In this paper, current-induced domain wall displacement experiments that are combined with high resolution imaging of the domain walls are reported. A spin-polarized scanning electron microscope is used to determine topography and magnetization distribution of Fe/sub 20/Ni/sub 80/ sub-micron wires with a zigzag geometry. Injection of a single 10 /spl mu/s current pulse through the wire induces the displacement of both head-to-head and tail-to-tail walls in the same direction, which is the direction of the electron flow. The current density of 3 /spl times/10/sup 12/ A/m/sup 2/ is about 10% higher than the threshold current density at which domain wall motion sets in for these structures. This observation confirms unambiguously that the spin-torque effect is the driving force for the domain wall motion. The variation of the domain-wall velocity is correlated with the domain-wall configuration induced by the current. The evolution of domain wall velocities has been determined after consecutive current injections with the same current density. Additional experiments on wires with different dimensions show that the wall velocity depends on the wire width and hence on the domain wall width for constant current density. Simple 1-D models of the domain walls are not sufficient for an adequate description of current-induced domain-wall motion and spin-structure distortion. Thus, the domain walls have to be treated as 2-D objects.

Permalloy ring structures with nonmagnetic contacts have been fabricated for transport measurements. The position of a domain wall is determined by low-temperature measurements of the resistance between different pairs of contacts. By monitoring the resistance, the threshold current density at which the current moves the domain wall is determined. Injecting a single 20 /spl mu/s pulse with a current density of 3/spl times/10/sup 12/ A/m into a 200 nm wide and 27 nm thick line moves the domain wall by 20 /spl mu/m, with an effective velocity of 1 m/s. It is found that the domain wall velocity is largely independent of the pulse duration but varies between 0.2 m/s and 1.3 m/s with the wire dimensions. Also, the velocity is shown to increase with current density. In addition, the threshold current density and velocity are determined to be dependent on the geometry of the structure and on the resulting domain wall type. Theoretical predictions reveal that the velocity is dependent on material parameters but not on the geometry of the structure. This implies that the simple one-dimensional model of a domain wall used in current calculations in this work is not sufficient to explain the observations.

The new generation of Implantable Cardioverter Defibrillators (ICDs) allows storage of intracardiac electrograms (EGMs) recorded immediately before the onset of malignant ventricular tachyarrhythmias (VTs) and during their course. Although all devices base most of diagnostic parameters on the bipolar EGMs, some of them allow two types of EGM recordings: bipolar (potential is obtained between two electrodes at the tip of the electrocatheter) and far-field (potential is recorded between the tip of the electrocatheter and the active can device). The main objective of this work is to study which differences can be observed from the analysis of bipolar or far-field EGMs when deterministic chaos methods are being applied to bipolar or far-field EGM recordings. For this analysis we considered 20 EGMs obtained from 6 patients with ICDs. Some of the EGMs contain VT episodes, other are collected in basal heart conditions. Within the limited set of EGM considered in this work, results indicate that far field recordings, monitoring the whole heart activity, allow better characterization of heart substrate, and they are more indicated when deterministic chaos analysis would be applied.

The installation of the 25-MeV superconducting electron linac LISA is in progress as Frascati INFN National Laboratory. The installation is scheduled to be completed in Summer 1991 and commissioning will follow immediately. The status of the project is reviewed, and the results of the first tests on various components are presented. >

An overview of the Frascati Phi -factory and the salient project features are presented. The engineering design of vacuum and magnetic components, diagnostics, power supplies of the accumulator ring and of the main rings is in progress. The main concern, at this time, is how to handle the multibunch instability that is the most harmful problem in the design of high current storage rings and the most serious limitation for the luminosity. The parasitic high-order modes of the RF cavities are responsible for such an instability. >