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PubMed2006

Optimal frequency ranges for extracting information on cardiovascular autonomic control from the blood pressure and pulse interval spectrograms in mice.

The analysis of blood pressure (BP) and heart rate (HR) variability by spectral methods has proven a useful tool in many animal species for the assessment of the vagal and sympathetic contributions to oscillations of BP and HR. Continuous BP measurements obtained in mice by telemetry were used to characterize the spectral bandwidths of autonomic relevance by using an approach with no a priori. The paradigm was based on the autonomic blockades obtained with conventional drugs (atropine, prazosin, atenolol). The spectral changes were estimated in all of the combinations of spectral bandwidths. The effect of hydralazine was also tested using the same systematic analysis, to detect the zones of sympathetic activation resulting reflexly from the vasodilatory action of the drug. Two zones of interest in the study of the autonomic control of BP and HR were observed. The first zone covered the 0.15-0.60 Hz range of the systolic BP spectrum and corresponds to the low-frequency zone (or Mayer waves). This zone reflects sympathetic control since the power spectral density of this zone was significantly reduced with alpha1-adrenoceptor blockade (prazosin), while it was significantly amplified as a result of a reflex sympathetic activation (hydralazine). The second zone covered the 2.5-5.0 Hz range of the pulse interval spectrum and corresponded to the high-frequency zone (respiratory sinus arrhythmia) under vagal control (blocked by atropine). These zones are recommended for testing the autonomic control of circulation in mice.

Animals↗
PubMed2003

Band-target entropy minimization. A robust algorithm for pure component spectral recovery. Application to complex randomized mixtures of six components.

A newly developed self-modeling curve resolution method, band-target entropy minimization (BTEM), is described. This method starts with the data decomposition of a set of spectroscopic mixture data using singular value decomposition. It is followed by the transformation of the orthonormal basis vectors/loading vectors into individual pure component spectra one at a time. The transformation is based in part on some seminal ideas borrowed from information entropy theory with the desire to maximize the simplicity of the recovered pure component spectrum. Thus, the proper estimate is obtained via minimization of the proposed information entropy function or via minimization of derivative and area of the spectral estimate. Nonnegativity constraints are also imposed on the recovered pure component spectral estimate and its corresponding concentrations. As its name suggests, in this method, one targets a spectral feature readily observed in loading vectors to retain, and then combinations of the loading vectors are searched to achieve the global minimum value of an appropriate objective function. The major advantage of this method is its one spectrum at a time approach and its capability of recovering minor components having low spectroscopic signals. To illustrate the application of BTEM, spectral resolution was performed on FT-IR measurements of very highly overlapping mixture spectra containing six organic species with a two-component background interference (air). BTEM estimates were also compared with the estimates obtained using other self-modeling curve resolution techniques, i.e., SIMPLISMA, IPCA, OPA-ALS, and SIMPLISMA-ALS.

Journal Article↗
PubMedDate not supplied

A 4-MV CT scanner for radiation therapy; spectral properties of the therapy beam.

A Clinac-4 therapy accelerator has been modified to function also as a computed tomography (CT) scanner. We report on measurements made to calibrate the beam. 104 Bismuth-germanate silicon-photodiode detectors were placed in a circular arc of radius 180 cm, centered on the source, in the arrangement of a third-generation CT scanner. With the beam collimated to a fan section and just filling the detectors, the signal S(chi), obtained with thickness chi of plastic phantom, closely followed the relationship S(chi)/S(0) = exp [- (alpha chi + beta chi 2)], where coefficients alpha and beta varied systematically over the extent of the beam. This result agrees with results of similar work by others. It may be explained in terms of the spectral changes that are known to be an inherent property of such beams. Expressions relating alpha and beta to the source spectral density are derived; also estimates of the source spectral density are calculated using the method of Laplace inversion. It is shown that scattered radiation arising from the fan beam within the scattering medium does not contribute significantly to the values of alpha and beta.

Mathematics↗
PubMed1995

Spectral lineshape determination by self-deconvolution.

A data-processing method is described for the determination of spectral lineshapes using deconvolution of the data with an initial estimate of the same spectrum, referred to as self-deconvolution. The method is demonstrated using computer-simulation studies and experimental data, and is shown to accurately determine amplitude and phase lineshape distortions which may be caused by field inhomogeneity and gradient eddy-current effects. The results indicate that the method is robust in the presence of noise and errors in the initial spectral estimate. Once the spectral lineshape is determined it can be incorporated into a parametric spectral-analysis procedure, thereby reducing the number of parameters to be determined and improving the accuracy of the fit. A proposed application of the method is for spatially resolved in vivo NMR studies where local susceptibility effects and gradient eddy-current effects cause significant deviation of the spectral lineshape from a Lorentzian lineshape.

Algorithms↗
PubMed2000

Establishing the relation between detrended fluctuation analysis and power spectral density analysis for stochastic processes

Stochastic fractal signals can be characterized by the Hurst coefficient H, which is related to the exponents of various power-law statistics characteristic of these processes. Two techniques widely used to estimate H are spectral analysis and detrended fluctuation analysis (DFA). This paper examines the analytical link between these two measures and shows that they are related through an integral transform. Numerical simulations confirm this relationship for ideal synthesized fractal signals. Their performance as estimators of H is compared based on a mean square error criterion and found to be similar. DFA measures are derived for physiological signals of heartbeat R-R intervals through the integral transform of a spectral density estimate. These agree with directly calculated DFA estimates, indicating that the relationship holds for signals with nonideal fractal properties. It is concluded that DFA and spectral measures provide equivalent characterizations of stochastic signals with long-term correlation.

Journal Article↗
PubMed2006

[Application of spectral analysis of the heart rhythm variability for estimation of the autonomic nervous system state in patients with ulcer disease].

In 57 patients with ulcer disease the spectral analysis of the heart rhythm variability (HRV) was conducted for estimation of the vegetative nervous system (VNS) state. In 66.7% of patients there was noted prevalence of the parasympathetic part of VNS tone, in 14.0% - sympathetic one. Dystonic activity of various parts of VNS had accompanied by raising of definite mediators (markers) of VNS: acetylcholine (r = 0.75), adrenaline and noradrenaline (r = 0.25), complicated course of ulcer disease. Sympathicotonia was associated with ulcerative hemorrhage (r = 0.5), vagotonia with perforation of the ulcer (r = 0.75). So, choosing the surgical intervention volume in patients with the ulcer disease it is expedient to apply the method of spectral analysis of HRV for estimation of the VNS state.

Adult↗
PubMed1999

Broad-band spectral analysis of 24 h continuous finger blood pressure: comparison with intra-arterial recordings.

The present study compares the spectral characteristics of 24-h blood pressure variability estimated invasively at the brachial artery level with those estimated by measurement of blood pressure at the finger artery using the non-invasive Portapres device. Broad-band spectra (from 3x10(-5) to 0.5 Hz) were derived from both finger and intra-brachial pressures recorded simultaneously for 24 h in eight normotensive and twelve hypertensive ambulant subjects. At frequencies lower than 0.07 Hz, higher spectral estimates were obtained by Portapres than by intra-brachial measurements. The maximum overestimation occurred in systolic pressure at around 10(-2) Hz, where the amplitude of the oscillations was two times greater when measured by Portapres. A less pronounced overestimation was found for diastolic pressures. The maximum overestimation was greater during daytime than during night-time. At around 0.1 Hz, invasive and non-invasive spectra were similar. At the respiratory frequencies (0.15-0.50 Hz), the power spectra were overestimated by Portapres during daytime, and underestimated at night. These results provide reference information for the correct interpretation of Portapres data in the estimation of 24-h blood pressure spectral power.

Adult↗
PubMed2001

Estimation of leaf nitrogen content from spectral characteristics of rice canopy.

Ground-based remotely sensed reflectance spectra of hyperspectral resolution were monitored during the growing period of rice under various nitrogen application rates. It was found that reflectance spectrum of rice canopy changed in both wavelength and reflectance as the plants developed. Fifteen characteristic wavebands were identified from the apparent peaks and valleys of spectral reflectance curves, in accordance with the results of the first-order differentiation, measured over the growing season of rice. The bandwidths and center wavelengths of these characteristic wavebands were different among nitrogen treatments. The simplified features by connecting these 15 characteristic wavelengths may be considered as spectral signatures of rice canopy, but spectral signatures varied with developmental age and nitrogen application rates. Among these characteristic wavebands, the changes of the wavelength in band 11 showed a positive linear relationship with application rates of nitrogen fertilizer, while it was a negative linear relationship in band 5. Mean reflectance of wavelengths in bands 1, 2, 3, 5, 11, and 15 was significantly correlated with application rates. Reflectance of these six wavelengths changed nonlinearly after transplanting and could be used in combination to distinguish rice plants subjected to different nitrogen application rates. From the correlation analyses, there are a variety of correlation coefficients for spectral reflectance to leaf nitrogen content in the range of 350-2400 nm. Reflectance of most wavelengths exhibited an inverse correlation with leaf nitrogen content, with the largest negative value (r = -0.581) located at about 1376 nm. Changes in reflectance at 1376 nm to leaf nitrogen content during the growing period were closely related and were best fitted to a nonlinear function. This relationship may be used to estimate and to monitor nitrogen content of rice leaves during rice growth. Reflectance of red light minimum and near-infrared peak and leaf nitrogen content were correlated nonlinearly.

Agriculture↗
PubMed2000

Automated discrimination between atrial fibrillation and atrial flutter in the resting 12-lead electrocardiogram.

Computerized time-domain analysis of the QRST-subtracted 12-lead electrocardiogram (ECG) has been used successfully to determine several atrial activity patterns. These time-domain methods are particularly useful for low-frequency signals such as those originating at the sinus node. However, high frequency atrial fibrillation (AFIB) and atrial flutter (AFL) waves can be better estimated by using spectral methods. In this study, we investigated the use of spectral entropy (SE) and spectral peak detection to distinguish fibrillatory from flutter activity in the QRST-subtracted ECG. In a set of 4,172 cardiologist-overread ECGs, a computerized ECG analysis program (12SL MAC-Rhythm, GE-Marquette Medical Systems, Milwaukee, WI) detected 270 AFIB rhythms and 100 AFL rhythms. Compared to the cardiologist's reading, the AFIB versus AFL miss-classification error was 5.6%. The Fourier Transform was used to estimate the power spectral density of the QRST-subtracted ECG data. Individual lead spectra were then averaged and SE was computed for each of the ECGs originally called AFIB or AFL by the computer program. Additional criteria that included SE, spectral peak frequencies, and time-domain measures of atrial activity were then applied to discriminate between the 2 rhythms. Employing these criteria resulted in a decrease of miss-classification error to 2.5%.

Algorithms↗
PubMed1996

Estimation of respiratory frequency from autoregressive spectral analysis of heart period.

Variability in heart period is modulated by respiration. Using an autoregressive spectral analytic approach on the heart period time series produces a components analysis. The central frequency of the heart period spectral component in the frequency of respiration (HFFreq) has been suggested as an index of respiratory frequency. The present experiment measured heart period variability (HPV) using an autoregressive algorithm and respiratory frequency using a mercury strain gauge (SGResp) to assess the relationship between the respiratory frequency indexed by the two methods. Twenty human participants were studied during resting baseline, relaxation, and mental effort conditions. Within- and between-person Pearson correlations and t-tests were used to assess the concordance between HFFreq and SGResp. Both within-person and between-person correlations indicated good concordance among the two methods. Moreover, t-tests, corrected for Type I error, indicated no significant differences between the respiratory frequency estimates derived from the two methods. The resolution of the estimates was approximately 1 breath per minute. These results suggest that the HFFreq may be a useful index of respiratory frequency.

Electrocardiography↗
PubMed1998

Power spectral density of unevenly sampled data by least-square analysis: performance and application to heart rate signals.

This work studies the frequency behavior of a least-square method to estimate the power spectral density of unevenly sampled signals. When the uneven sampling can be modeled as uniform sampling plus a stationary random deviation, this spectrum results in a periodic repetition of the original continuous time spectrum at the mean Nyquist frequency, with a low-pass effect affecting upper frequency bands that depends on the sampling dispersion. If the dispersion is small compared with the mean sampling period, the estimation at the base band is unbiased with practically no dispersion. When uneven sampling is modeled by a deterministic sinusoidal variation respect to the uniform sampling the obtained results are in agreement with those obtained for small random deviation. This approximation is usually well satisfied in signals like heart rate (HR) series. The theoretically predicted performance has been tested and corroborated with simulated and real HR signals. The Lomb method has been compared with the classical power spectral density (PSD) estimators that include resampling to get uniform sampling. We have found that the Lomb method avoids the major problem of classical methods: the low-pass effect of the resampling. Also only frequencies up to the mean Nyquist frequency should be considered (lower than 0.5 Hz if the HR is lower than 60 bpm). We conclude that for PSD estimation of unevenly sampled signals the Lomb method is more suitable than fast Fourier transform or autoregressive estimate with linear or cubic interpolation. In extreme situations (low-HR or high-frequency components) the Lomb estimate still introduces high-frequency contamination that suggest further studies of superior performance interpolators. In the case of HR signals we have also marked the convenience of selecting a stationary heart rate period to carry out a heart rate variability analysis.

Cardiac Pacing, Artificial↗
PubMed1978

On the quantitative characterization of human body sway in experiments with long-term performance.

Two different conditions of standing were examined in young male subjects with an intact vestibular system by means of stabilography. Low-frequency sways were eliminated by means of a digital high-pass filter. The parameters of the histograms of amplitudes, the autocorrelation functions, and the power-spectral density were estimated. The quantitative parameters of the stabilograms are compared for the two standing positions, both for the frontal and sagittal planes. Quotients of estimates of variance components calculated on the basis of analysis of variance are stated as measures of reliability for the parameters of histograms of amplitudes and power-spectral density in defined frequency bands. The reliability of the standard deviations of amplitudes was much better than the reliability of skewness and excess. The reliability of spectral density differed for frequency ranges investigated. Generally, the quotients of estimates of variance components were higher for the spectral density in the frequency bands of sagittal stabilograms than in those of frontal ones. Performance significantly affected several parameters of histograms and the distributions of power-spectral density. Standard deviation of amplitudes and power-spectral density proved to be suitable for the quantitative characterization of stabilograms.

Behavior↗
PubMed2002

Experimental approach for testing the uncoupling between cardiovascular variability series.

In cardiovascular variability analysis, the significance of the coupling between two series is commonly assessed by defining a zero level on the magnitude-squared coherence (MSC). Although the use of the conventional value of 0.5 does not consider the dependence of MSC estimates on the analysis parameters, a theoretical threshold Tt is available only for the weighted covariance (WC) estimator. In this study, an experimental threshold for zero coherence Te was derived by a statistical test from the sampling distribution of MSC estimated on completely uncoupled time series. MSC was estimated by the WC method (Parzen window, spectral bandwidth B = 0.015, 0.02, 0.025, 0.03 Hz) and by the parametric autoregressive (AR) method (model order M= 4, 8, 12, 16), on time series with length L = 180, 300, 420, 540 s. Te decreased with increasing B and L and with decreasing M (range: 0.11-0.54 for WC estimator, 0.06-0.46 for AR estimator). Values for the typical parameter settings of WC and AR estimation (B = 0.025 Hz; M = 8; L = 300 s) were, respectively, 0.24 and 0.17. Moreover, Tt was always higher (range: 0.12-0.65) and the results were less dependable than those for Te in defining the zero level of MSC. Thus, with the proposed method, the hypothesis of uncoupling is rejected by accounting for the parameters that affect the confidence of spectral and cross-spectral estimates. The broad applicability of this approach should favour its introduction for assessing the significance of the coupling between cardiovascular variability series.

Electrocardiography↗
PubMed2004

Adiponectin concentrations in sera from patients with type 2 diabetes are negatively associated with sympathovagal balance as evaluated by power spectral analysis of heart rate variation.

OBJECTIVE: To investigate whether cardiac autonomic activity, particularly sympathovagal balance as estimated by power spectral analysis (PSA) of heart rate variation (HRV), is associated with serum adiponectin concentrations in patients with type 2 diabetes. RESEARCH DESIGN AND METHODS: We studied 105 patients with type 2 diabetes (51 women and 54 men). Serum adiponectin concentrations were measured by sandwich enzyme-linked immunosorbent assay. HRV was determined automatically every 5 min over 24 h using Holter electrocardiographic recording. PSA of R-R intervals was performed using fast Fourier transformation. Low-frequency (both sympathetic and parasympathetic activities), high- frequency (pure parasympathetic activity), and the ratio of low-frequency-to-high-frequency power (LF-to-HF ratio), an index of sympathovagal balance, were used as indexes of cardiac autonomic activity. RESULTS: We found no significant correlation between serum adiponectin and low-frequency or high-frequency power in patients with diabetes. Serum adiponectin concentration correlated negatively with the 24-h LF-to-HF ratio (r = -0.343, P = 0.0009) and creatinine clearance (r = -0.411, P < 0.0001). Serum adiponectin concentrations were significantly higher in patients with overt albuminuria than in those with normoalbuminuria or microalbuminuria. In multivariate analysis controlling for sex, BMI, glycemic control, lipid profile, and renal function, serum adiponectin concentration showed an independent negative association with 24-h LF-to-HF ratio (beta = -0.332, P = 0.020). Furthermore, sex, HDL cholesterol, and renal function retained significant influence on the serum adiponectin concentration in patients with diabetes. CONCLUSIONS: Sympathovagal balance favoring relative sympathetic activation was associated with low serum concentrations of adiponectin in patients with type 2 diabetes.

Adiponectin↗
PubMed1988

Ultrasound attenuation estimation using the LMSE filters and the median filter.

The ultrasound attenuation coefficient in known to increase with frequency in soft biological tissue within the diagnostic frequency range. The frequency dependent attenuation coefficient is an important parameter in clinical tissue characterization, especially in liver, and in ultrasonic quantitative B-scan imaging. But, because of the random spatial variation in the backscattered signal amplitude, the estimator variance becomes very large, making it difficult to estimate the attenuation slope from the backscattered ultrasound signal. In this paper we suggest a new ultrasound attenuation estimator based on the calibrated log spectral difference. The proposed estimation process consists of Wiener filtering, Kalman filtering, and median filtering in this order. The simulation results and experimental results with tissue equivalent (TE) phantoms show that combination of the above filterings yields a considerably reduced estimator variance compared with the conventional estimators.

Acoustics↗
PubMed1998

An estimate of fetal autonomic state by spectral analysis of human umbilical artery flow velocity waveforms.

OBJECTIVE: Determination of gestational age-related fluctuations in heart rate in the umbilical artery of the early human fetus. METHODS: Doppler velocity recordings from human umbilical artery were obtained, in a cross-sectional study design in 137 singleton pregnancies at 10-20 weeks of gestation. After exclusion criteria were applied, data on 117 normal pregnancies were available and subdivided into group I: 10-12 weeks (n = 49); group II: 13-16 weeks (n = 43); and group III: 17-20 weeks (n = 25). Blood flow velocity waveforms were reconstructed from Doppler audio signals. Variability in heart rate was calculated using Fast Fourier Transforms (FFT). Individual heart rate variability power spectra were subdivided into frequency bands. RESULTS: Fetal heart rate variability decreases at 10-20 weeks and demonstrates a shift to lower frequencies at 17-20 weeks. CONCLUSIONS: Fetal heart rate variability is related to gestational age and shows a shift to lower frequencies which may reflect autonomic functional development.

Adolescent↗
PubMed1993

An estimate of fetal autonomic state by spectral analysis of fetal heart rate fluctuations.

The assessment of the functional state of the autonomic nervous system (ANS) in real time, by means of spectral analysis of fetal heart rate variability, may serve to improve the diagnosis of pathologic conditions of importance to the perinatologist. The combination of two approaches, namely an efficient method for detecting fetal ECG from the abdominal maternal signal, followed by spectral analysis of heart rate variability, is tested as a new noninvasive tool to assess fetal viability in real time. This study demonstrates a pattern of ANS development via the spectral contents of heart rate variability. It is shown that during "quiet state," the "young" fetuses (gestational age = 23.5 +/- 1 wk) present twice as much power of heart rate fluctuations at all frequencies from 0.2 to 1.0 Hz as "mature" fetuses (gestational age = 39.75 +/- 1.5 wk). This finding is coherent with the evolution of a stable and mature ANS activity. At frequencies below 0.1 Hz, a 1/f alpha power law relationship (alpha = 0.85, r2 > 0.9) between spectral density and frequency is displayed in the two age groups. A respiratory peak has been observed in some of the short (64-s) traces we analyzed. However, no respiratory peak was ever observed in a long (256-s) trace, due to the episodic nature of the fetal breathing and immaturity of the ANS.

Algorithms↗
PubMed2002

Motor unit recruitment strategies investigated by surface EMG variables.

During isometric contractions of increasing strength, motor units (MUs) are recruited by the central nervous system in an orderly manner starting with the smallest, with muscle fibers that usually show the lowest conduction velocity (CV). Theory predicts that the higher the velocity of propagation of the action potential, the higher the power at high frequencies of the detected surface signal. These considerations suggest that the power spectral density of the surface detected electromyogram (EMG) signal may give indications about the MU recruitment process. The purpose of this paper is to investigate the potential and limitations of spectral analysis of the surface EMG signal as a technique for the investigation of muscle force control. The study is based on a simulation approach and on an experimental investigation of the properties of surface EMG signals detected from the biceps brachii during isometric linearly increasing torque contractions. Both simulation and experimental data indicate that volume conductor properties play an important role as confounding factors that may mask any relation between EMG spectral variables and estimated CV as a size principle parameter during ramp contractions. The correlation between spectral variables and CV is thus significantly lower when the MU pool is not stable than during constant-torque isometric contractions. Our results do not support the establishment of a general relationship between spectral EMG variables and torque or recruitment strategy.

Adult↗