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At least 595 records · Page 33Linked to original sources
PubMed1999

[Cardiovascular neuroregulation and rhythms of the autonomic nervous system: frequency domain analysis].

Studies using spectral analysis of cardiovascular variability series, as a non invasive tool for assessing the autonomic nervous system activity, have attracted growing interest in the last 20 years. Short-term recordings of heart rate variability distinguish two main spectral components: a high-frequency (HF) component (ranging between 0.15-0.40 Hz), and a low-frequency (LF) component (ranging between 0.04-0.15 Hz), respectively considered markers of parasympathetic and sympathetic control. Spectral analysis of microcirculatory blood flow, by using laser Doppler flowmetry, recently disclosed the presence of similar rhythms. Although a general agreement about the amplitude and frequency of these spectral components has not been gained yet, evidences of an autonomic nervous system role have been collected. Also the pupil diameter spontaneously fluctuates in the HF range. The aim of this study was to design an experimental set up for the simultaneous and continuous recording of heart rate, blood pressure, respiratory activity, peripheral blood flow, and pupil diameter fluctuations in order to investigate whether common spontaneous rhythms could be detected into the variability series. We enrolled 10 normal volunteers (range 24-28 years). The variability series of heart rate, blood pressure, respiratory activity, peripheral blood flow and pupil diameter were obtained at rest and during sympathetic activation by head-up tilt test 70 degrees. Subjects were instructed to breath at 15 breath/min following an audio signal and adjusting their tidal volume to a comfortable level. Peripheral blood flow was non invasively monitored with laser Doppler technique, with the two probes located in the palmar site of the hand and in the sternum. We continuously assess the pupil diameter fluctuations by a custom, portable, infrared TV pupillometer. Spectral density was estimated by autoregressive modeling on 250-point segments. The respiratory rhythm was clearly detected in all the variability series. Oscillations in the LF band were detected in heart rate, blood pressure and in the laser Doppler signals of the hand, as well as in the pupil diameter, but not always in the laser Doppler flow of the sternum. In this last signal an harmonic component at 0.16-0.17 Hz was detected. A reduction in the total variability, with a relative increase in the LF component was observed during head-up tilt test in the heart rate, blood pressure, peripheral blood flow and pupil diameter signals. These findings are consistent with the increased sympathetic modulation induced by the head-up tilt test. These data confirm previous findings and observations, collected with different techniques and for different purposes. Whether these rhythms can be expression of central common oscillators or reflex mechanical factors is of primary importance for possible clinical applications of this approach.

Adult↗
PubMed1999

[Changes in the fractal component of spectral analysis of heart rate variability and systolic blood pressure variability during the head-up tilt test].

Blood pressure and heart rate change are related to the level of physical activity, and are correlated with each other. Heart rate and blood pressure signals were investigated by coarse graining spectral analysis and changes in the harmonic and non-harmonic (fractal) power were examined during the head-up tilt test. Fourteen healthy subjects, 9 men and 5 women (mean age 30.4 +/- 1.0 years) completed the test protocol of 15 min supine rest followed by the head-up tilt (80 degrees) test. Heart rate was measured continuously with standard bipolar leads and electrocardiography. A finger cuff was placed on the left index finger for beat-by-beat recording of systolic blood pressure based on the continuous noninvasive method, and the impulse train was stored on a personal computer for spectral analysis. The harmonic component, the integrated powers in the low-frequency and high-frequency regions, and the fractal component were then calculated. The fractal component was plotted on a log power versus log frequency plane with spectral index beta estimated as the slope of the linear regression of this 1/f beta plot. RR-interval was significantly shorter during the head-up tilt position than in the supine rest position with a marked reduction in the high-frequency power. The ratio of fractal component for total power was increased and the slope beta of the 1/f beta relationship was significantly greater in the head-up tilt (1.61 +/- 0.05) than in the supine rest (0.92 +/- 0.07) position. Systolic blood pressure showed a significant increase during head-up tilt, and marked increases in high-frequency power and fractal power. However, both the ratio of fractal component for total power and the slope beta remained unchanged. Further studies are needed to clarify whether the slope beta is essentially stable or variable in some conditions.

Adult↗
PubMed1989

Infrared emission from comets.

A brief discussion of the infrared observations from 4 to 20 micrometers of seven comets is presented. The observed infrared emission from comets depends primarily on their heliocentric distance. A model based on grain populations composed of a mixture of silicate and amorphous carbon particles in the mass ratio of about 40 to 1, with a power-law size distribution similar to that inferred for comet Halley, is applied to the observations. The model provides a good match to the observed heliocentric variation of both the 10 micrometers feature and the overall thermal emission from comets West and Halley. Matches to the observations of comet IRAS-Araki-Alcock and the antitail of comet Kohoutek require slightly larger grains. While the model does not match the exact profile and position of the 3.4 micrometers feature discovered in comet Halley, it does produce a qualitative fit to the observed variation of the feature's strength as a function of heliocentric distance. The calculations predict that the continuum under the 3.4 micrometers feature is due primarily to thermal emission from the comet dust when the comet is close to the Sun and to scattered solar radiation at large heliocentric distances, as is observed. A brief discussion of the determination of cometary grain temperatures from the observed infrared emission is presented. It is found that the observed shape of the emission curve from about 4 to 8 micrometers provides the best spectral region for estimating the cometary grain temperature distribution.

Astronomical Phenomena↗
PubMed1988

Comparison of four digital maximum frequency estimators for Doppler ultrasound.

The performance of four methods for digitally estimating the maximum frequency waveform from the Doppler ultrasound spectrum, are described. The methods investigated are: a percentile method, D'Alessio's threshold crossing method [D'Alessio T. (1985) "Objective" algorithm for maximum frequency estimation in Doppler spectral analysers. Med. Biol. Engng and Comput. 23, 63-68.], a modified threshold crossing method, and a new hybrid algorithm. Evaluations of the variance and bias were performed using stationary simulated continuous wave (CW) Doppler signals of different bandwidths and signal/noise ratios (SNR) of 9 and 17 dB. Furthermore, a simulated nonstationary Doppler signal, similar to that from a normal internal carotid artery, was also used to compare the various methods. Overall, it was found that the modified threshold method and the new hybrid method have the best performance over a wide range of signal and noise hybrid method have the best performance over a wide range of signal and noise conditions; however, D'Alessio's method also performs well for low SNR's.

Computer Simulation↗
PubMed1984

Power spectral density of heart rate variability as an index of sympatho-vagal interaction in normal and hypertensive subjects.

Instantaneous heart rate reflects sympatho-vagal influences on pace-maker activity. Hence computer analysis of heart rate variability might provide a quantitative index of that interaction. The power spectral density (PSD) estimate of heart rate variability was obtained in normal controls and in uncomplicated hypertensives, both at rest and during a non-hypotensive sympathetic stimulus (tilting). In normal controls PSD shows three major peaks of frequencies P1 = 0.07, P2 = 0.12, P3 = 0.25 cycles/beat. P1, which is associated with sympathetic activity, represents only a minor portion of total variability at rest, while becoming predominant with tilting. P2 and P3 are associated with vagal activity, and represent the major part of variability at rest, while they are reduced by tilting. In hypertensive patients PSD is altered, as P1 is already predominant at rest and increases only slightly with tilting. Thus PSD of heart rate variability is capable of detecting an early alteration in sympatho-vagal balance of cardiac control present in uncomplicated hypertension.

Adult↗
PubMed1995

[Spectro-temporal mapping (STM)].

In recent years, various kinds of time-varying spectral analysis which estimate the frequency content of a signal as a function of time have been proposed. Spectro-temporal mapping (STM) is one of such methods with elaborate modification of a simple fixed period analysis. Merits and demerits of this method in clinical application are described in this chapter. Certain technical aspects pertinent to the interpretation of STM were also discussed. In conclusion, STM in present form is inferior to time domain analysis for the prediction of serious ventricular arrhythmias. But it has some usefulness to supplement time domain analysis in noise detection and application to patients with broad QRS complex.

Body Surface Potential Mapping↗
PubMedDate not supplied

[Interaction of antioxidants with differing chemical structure with a phospholipid bilayer].

Interaction of antioxidant (derivatives of phenol and 1,4-dihydropyridine) with bilayer from lecithin was studied by means of electronic spectroscopy and microcalorimetry. The connection of warm effects and spectral properties was estimated by chemical structure of antioxidants. The effect of physiologically active compounds on the structural and dynamic parameters of the bilayer was determined by means of fluorescent probing.

Antioxidants↗
PubMed1993

Fundamental sources of error and spectral broadening in Doppler ultrasound signals.

Analysis of the signals, spectra and error bounds for Doppler ultrasound signals is challenging and involves numerous concepts in signal analysis, probability, acoustics, and fluid mechanics. Nonetheless, the results of this analysis must be accessible to both engineers and clinicians who work with ultrasound technology. The engineer who designs, builds, or maintains equipment must know whether specific artifacts are fundamental or can be eliminated. The clinician must be able to interpret whether specific signal features accurately represent the flow field or result from limitations of Doppler ultrasound. This article reviews recent advances in both conceptual and numerical models of the Doppler ultrasound process, and relates these advances to practical aspects such as spectral broadening, velocity estimation error, and data analysis error. It then reviews recent innovations in system implementation and signal analysis which are indicative of the future potential of Doppler ultrasound instrumentation.

Artifacts↗
PubMedDate not supplied

[Study of radiation-induced change in the membrane structure using a fluorescent probe].

The effect of radiation on erythrocyte membrane structure has been investigated using fluorescent probe 4-(n-dimethyl aminostyryl)-1-methylpyridinium n-toluolsulfonate. The form of fluorescence spectra was analyzed and inhomogeneous broadening parameters of spectral components were estimated. It was shown that in erythrocyte membrane there were three types of DSM binding sites. The physico-chemical properties of DSM surrounding and probes distribution were found to change after irradiation.

Animals↗
PubMedDate not supplied

[Interaction of 4-(n-dimethylaminostyryl)-1-methylpyridinium-n-toluolsulfonate with liposomes: analysis of fluorescence spectra].

Using fluorescent probe 4-(n-dimethylaminostyryl)-1-methylpyridinium n-toluenesulfonate the radiation effect on the structure of liposomes composed of phosphatidylcholine and diphosphatidylgycerol has been investigated. The from of fluorescence spectra was analyzed and inhomogeneous broadening parameters of spectral components were estimated. The polarity of DSM environment and probe distribution between different sites were found to change after irradiation.

Liposomes↗
PubMed2006

A spectral color correction framework for medical applications.

This paper presents a new spectral approach to color correction for medical image analysis applications. Linear estimation with regularization by a constrained principal eigenvector method is used for calibration of the camera system and estimation of the illumination spectrum while spectral surface reflectivities are determined by Wiener inverse estimation. Nonlinear devices are handled by piecewise linear interpolation and any linear color preprocessing inside the camera is explicitly modeled. All measurement and estimation processes are combined into a spectral calibration framework for practical application in computer-assisted image analysis. The novelty of our approach lies in the generalization of the image formation model allowing for linear preprocessing inside the camera system. Such transforms would lead to erroneous results with positivity constraint based algorithms or a monochromator based measurement. We provide experimental results from a comprehensive set of reference measurements acquired with a video endoscopy system for gastroscopic application.

Algorithms↗
PubMed1998

[The spectral broadening correction in peak blood flow velocity estimation].

Numerous effects contribute to the ultimate shape of output power spectrum in Doppler ultrasound blood flow measurement. The output Doppler spectrum is broadened by physiological sources related to blood flow velocity as well as some unexpected broaden components due to the nonideal acoustical factors of the ultrasound unit. Therefore, by using the classical Doppler equation, it will lead to overestimation of the maximum velocity in conventional Doppler blood flow measurement. In this paper, we present a modified Doppler equation which takes the intrinsic spectral broadening into account. The foundation of this method is introduced and the results of the in vivo experiment using this corrected equation are reported.

Algorithms↗
PubMed1995

Objective analysis versus subjective assessment of vowels pronounced by deaf and normal-hearing children.

Objective whole-spectrum and formant analyses have been performed on all 15 Dutch vowels pronounced in /C1VC2/ words by 24 deaf and 24 normal-hearing children, in order to develop a model of pronunciation quality for evaluating (deaf) speech; the results as obtained for adult males by Bakkum et al. [J. Acoust. Soc. Am. 94, 1989-2004 (1993)] have been verified and extended. Spectral representations of the vowels were created by determining the output levels of a bank of 16 filters (90-7200 Hz), with 1/3-oct bandwidths and logarithmic spacing of their center frequencies. Spectral differences agree well with subjective differences in pronunciation quality obtained from magnitude estimation and identification experiments. Spectral differences not related to pronunciation quality judgments arise as a consequence of physiological interspeaker differences and variation in fundamental frequency, but these differences can be compensated for by speaker-normalization and F0-compensation procedures. Using principal components analysis (PCA), the vowel spectra can be described by a limited number of dimensions, without losing much information; a description in a two-dimensional PCA subspace still agrees well with the subjective judgments and it also agrees with a description by the first two formants. The whole-spectrum approach provides a determinate, readily interpretable model of pronunciation quality for evaluating vowels. As a practical advantage, its computational requirements are modest and, in conjunction with PCA, the vowel dynamics can be visualized, which makes the approach suitable for vowel training and diagnostics.

Child↗
PubMed1991

Modern spectral analysis techniques for blood flow velocity and spectral measurements with pulsed Doppler ultrasound.

Four spectral analysis techniques were applied to pulsed Doppler ultrasonic quadrature signals to compare the relative merits of each technique for estimation of flow velocity and Doppler spectra. The four techniques were 1) the fast Fourier transform method, 2) the maximum likelihood method, 3) the Burg autoregressive algorithm, and 4) the modified covariance approach to autoregressive modeling. Both simulated signals and signals obtained from an in vitro flow system were studied. Optimal parameter values (e.g., model orders) were determined for each method, and the effects of signal-to-noise ratio and signal bandwidth were investigated. The modern spectral analysis techniques were shown to be superior to Fourier techniques in most circumstances, provided the model order was chosen appropriately. Robustness considerations tended to recommend the maximum likelihood method for both velocity and spectral estimation. Despite the restrictions of steady laminar flow, the results provide important basic information concerning the applicability of modern spectral analysis techniques to Doppler ultrasonic evaluation of arterial disease.

Algorithms↗
PubMed2001

Normal pericardial fluid in the fetus: color and spectral Doppler analysis.

OBJECTIVES: To estimate the incidence of sonographic identification of pericardial fluid in normal fetuses and to evaluate the flow pattern of pericardial fluid by using color and spectral Doppler techniques. METHODS: We evaluated 27 normal fetuses for the presence of pericardial fluid by using gray-scale two-dimensional and M-mode ultrasound, and color and spectral Doppler techniques. RESULTS: Pericardial fluid was detected in 52% of cases by two-dimensional and M-mode ultrasound and in 81% of cases by color Doppler. The pericardial fluid moved towards the ventricles during systole and towards the atria during diastole. In 9 of 22 fetuses with pericardial fluid identified by color Doppler, spectral waveforms were obtained. The waveforms confirmed the bidirectional flow pattern identified at color Doppler. In six cases there was monophasic systolic and biphasic diastolic flow. In the remaining three cases, the flow was monophasic during both systole and diastole. CONCLUSIONS: Pericardial fluid can be identified with color Doppler in the majority of normal fetuses. It characteristically shows bidirectional flow as it moves with ventricular systole and diastole. Spectral waveforms can be obtained from the pericardial fluid. The presence of pericardial fluid per se should not be considered as abnormal. Color-coded pericardial fluid should not be mistaken for coronary artery blood flow.

Body Fluids↗
PubMed1993

Preventing errors when estimating single channel properties from the analysis of current fluctuations.

The conductance, number, and mean open time of ion channels can be estimated from fluctuations in membrane current. To examine potential errors associated with fluctuation analysis, we simulated ensemble currents and estimated single channel properties. The number (N) and amplitude (i) of the underlying single channels were estimated using nonstationary fluctuation analysis, while mean open time was estimated using covariance and spectral analysis. Both excessive filtering and the analysis of segments of current that were too brief led to underestimates of i and overestimates of N. Setting the low-pass cut-off frequency of the filter to greater than five times the inverse of the effective mean channel open time (burst duration) and analyzing segments of current that were at least 80 times the effective mean channel open time reduced the errors to < 2%. With excessive filtering, Butterworth filtering gave up to 10% less error in estimating i and N than Bessel filtering. Estimates of mean open time obtained from the time constant of decay of the covariance, tau obs, at low open probabilities (Po) were much less sensitive to filtering than estimates of i and N. Extrapolating plots of tau obs versus mean current to the ordinate provided a method to estimate mean open time from data obtained at higher Po, where tau obs no longer represents mean open time. Bessel filtering gave the least error when estimating tau obs from the decay of the covariance function, and Butterworth filtering gave the least error when estimating tau obs from spectral density functions.

Analysis of Variance↗
PubMed1996

Estimation of certain parameters of a stationary hybrid process involving a time series and a point process.

A method is presented for estimating the cross-spectral density of a hybrid process involving a time series and a point process. The method is based on the generalized cross-periodogram statistic, which is smoothed by splitting the whole record of the data into a number of disjoint subrecords. Estimates of the coherence function and the cross-covariance function can also be obtained by using the estimate of the cross-spectral density. The distribution of the cross-covariance function between a time series and a point process is shown to be asymptotically normal. The theoretical results are used in the study of a complex physiological system. It is shown that the presence of a gamma motor neuron (gamma stimulation) modifies the effect of the length changes on the complex system at low frequencies (the length changes, and the response of the system become uncorrelated in the range 3-30 Hz) while the effect remains unchanged at higher frequencies. As a comparison it is shown that the presence of the length changes weakens the effect of the gamma stimulation on the complex system.

Analysis of Variance↗