Homero Nogueira Guimarães

Investigation of determinism in heart rate variability

The article searches for the possible presence of determinism in heart rate variability (HRV) signals by using a new approach based on NARMA (nonlinear autoregressive moving average) modeling and free-run prediction. Thirty-three 256-point HRV time series obtained from Wistar rats submitted to different autonomic blockade protocols are considered, and a collection of surrogate data sets are generated from each one of them. These surrogate sequences are assumed to be nondeterministic and therefore they may not be predictable. The original HRV time series and related surrogates are submitted to NARMA modeling and prediction. Special attention has been paid to the problem of stationarity. The results consistently show that the surrogate data sets cannot be predicted better than the trivial predictor-the mean-while most of the HRV control sequences are predictable to a certain degree. This suggests that the normal HRV signals have a deterministic signature. The HRV time series derived from the autonomic blockade segments of the experimental protocols do not show the same predictability performance, albeit the physiological interpretation is not obvious. These results have important implications to the methodology of HRV analysis, indicating that techniques from nonlinear dynamics and deterministic chaos may be applied to elicit more information about the autonomic modulation of the cardiovascular activity. (c) 2000 American Institute of Physics.

Prolonged cardioprotective effect of pyridostigmine encapsulated in liposomes

AIMS The purpose of the present work was to investigate the ability of pyridostigmine encapsulated in long-circulating liposomes, to protect against ECG (electrocardiogram) alterations induced by sympathetic stimulation in rats. MAIN METHODS The encapsulation of pyridostigmine was carried out by freeze-thaw and extrusion. Blood pressure and ECG (limb lead II) were monitored in anaesthetized male Wistar rats. The formulation containing pyridostigmine was intravenously administrated in 0.1, 0.3 and 1.0mg/kg doses, and sympathetic stimulation was conducted by administration of 1 or 3 microg of noradrenaline (NA) after 1, 2, 4 or 6h. The obtained cardiovascular parameters were compared to animals that received intravenous injection of pyridostigmine in free form or saline. KEY FINDINGS After saline, NA induced a significant increase in QT interval (22.3% after 3.0 microg). Previous administration of free pyridostigmine significantly prevented the increase of QT interval after sympathetic stimulation and the most prominent effect was observed after 1h for the dose of 0.3mg/kg (6.8% after 3.0 microg of NA) and was no longer observed after 2h of the treatment. On the other hand, the maximum effect of pyridostigmine in liposomal formulation preventing QT interval increase was observed 2h after treatment (9.7% after 3.0 microg of NA) and was still present until 6h when 1mg/kg was previous administrated. SIGNIFICANCE The results of the present study, beyond to confirm the cardioprotective action of pyridostigmine, suggest that liposomal pyridostigmine may be a potential therapeutic alternative to prevent cardiovascular disturbances resulting from sympathetic hyperactivity.

Reduced cardiovascular alterations of tartar emetic administered in long-circulating liposomes in rats.

Trivalent antimonial drugs, including tartar emetic (TA), are known to induce important cardiotoxicity observed by electrocardiographic abnormalities. Liposome encapsulation was found to reduce the overall acute toxicity of TA. The present work investigated the cardiovascular parameters alterations of rats submitted to the treatment with free and encapsulated TA in long-circulating liposomes. Liposomes were made using lipids DSPC, DSPE-PEG and cholesterol. The cardiovascular signals, electrocardiogram (ECG) and arterial blood pressure (AP), were recorded from anaesthetized Wistar rats after intravenous (IV) administration of a single specially high dose (17 mg/kg) of TA in liposomes and in free form. The IV administration of TA solution caused significant increase of QT interval of ECG and significant reduction of AP when compared to the control group. These alterations were not observed when liposomes TA were administered and the profile of ECG and AP data was quite similar to the control groups. In conclusion, a liposomal formulation of TA showed a reduced cardiotoxic profile for TA when compared to the free form.

Does preprocessing change nonlinear measures of heart rate variability

This work investigated if methods used to produce a uniformly sampled heart rate variability (HRV) time series significantly change the deterministic signature underlying the dynamics of such signals and some nonlinear measures of HRV. Two methods of preprocessing were used: the convolution of inverse interval function values with a rectangular window and the cubic polynomial interpolation. The HRV time series were obtained from 33 Wistar rats submitted to autonomic blockade protocols and from 17 healthy adults. The analysis of determinism was carried out by the method of surrogate data sets and nonlinear autoregressive moving average modelling and prediction. The scaling exponents alpha, alpha(1) and alpha(2) derived from the detrended fluctuation analysis were calculated from raw HRV time series and respective preprocessed signals. It was shown that the technique of cubic interpolation of HRV time series did not significantly change any nonlinear characteristic studied in this work, while the method of convolution only affected the alpha(1) index. The results suggested that preprocessed time series may be used to study HRV in the field of nonlinear dynamics.

A New Trend-Following Indicator: Using SSA to Design Trading Rules

Singular Spectrum Analysis (SSA) is a non-parametric approach that can be used to decompose a time-series as trends, oscillations and noise. Trend-following strategies rely on the principle that financial markets move in trends for an extended period of time. Moving Averages (MAs) are the standard indicator to design such strategies. In this study, SSA is used as an alternative method to enhance trend resolution in comparison with the traditional MA. New trading rules using SSA as indicator are proposed. This paper shows that for the Down Jones Industrial Average (DJIA) and Shangai Securities Composite Index (SSCI) time-series the SSA trading rules provided, in general, better results in comparison to MA trading rules.

Improving reconstruction of time-series based in singular spectrum analysis: A segmentation approach

Abstract Singular Spectrum Analysis (SSA) is a powerful non-parametric framework to analysis and enhancement of time-series. SSA may be capable of decomposing a time-series into its meaningful components: trends, oscillations and noise. However, if the signal under analysis is non-stationary, with its spectrum spreading and varying in time, the reliability of the reconstruction is guaranteed only when many elementary matrices are used. As a consequence, the capability to discriminate dominant structures from time-series may be impaired. To circumvent this issue, a new method, called overlap-SSA (ov-SSA), is proposed for segmentation, analysis and reconstruction of long-term and/or non-stationary signals. The raw time series is divided into smaller, consecutive and overlapping segments, and standard SSA procedures are applied to each segment with the resulting series being concatenated. This variation of SSA seeks to: improve reconstruction and component separability for non-stationary time-series; enable the analysis for large datasets, avoiding the issues of concatenation of many segments; and present some benefits of the segmentation in terms of better time–frequency characterization. These advantages are illustrated in several synthetic and experimental datasets.

Sliding Mode Observer for Rotor Faults Diagnosis

Although induction motors are traditional thought to be reliable and robust, the possibility of faults is unavoidable once the machines can be exposed to different hostile environments, misoperations, and manufacturing defects. Therefore, motor monitoring incipient fault detection and diagnosis are important topics. This paper presents a method for on-line induction motor monitoring with the purpose of detecting and locating a single rotor broken bar. The method avoids any frequency analysis and observes instead the machine state with the help of the two models. The torque difference between the two models indicates a fault. The technique utilizes input signals from standard transducers. An experimental setup has been constructed to implement the new technique in on-line model

A feed-forward approach for DC cancellation in fully-differential instrumentation amplifiers

DC suppression consists of a mandatory procedure in several measurement applications. In many situations, its value is orders of magnitude above the desired information to be measured. In fully-differential instrumentation amplifiers, which present a very high common-mode rejection ratio, matched coupling capacitors are demanded for CMRR not to degrade. Stability problems can arise if active cancellation is employed. This paper presents a fully-differential instrumentation amplifier with DC suppression that employs a feed-forward scheme. A mathematical analysis of the proposed circuit is conducted in order to predict its behavior, followed by simulations and experimental measurements for analysis and validation. An excelent performance is achieved, even with standard precision components.