PubMed Health⌕ Search

Biomedical subjects

A Bezerianos

Publications and source records attributed to A Bezerianos.

30 records · Page 2Linked to original sources
PubMed1997

Cardiac electrical instability of the arrhythmogenic tissue substrate assessed by means of wavelet transform.

The unstable activation wavefront from the tissue responsible for the production of ventricular tachycardia (VT) gives rise to beat-variable signals components that are eluded during the averaging step of high resolution ECG (HRECG). We used a mathematical model of small variations in a largely repetitive waveform to evaluate the beat-to-beat variance of the HRECG signal. The ability of the Morlet Wavelet Transform to discriminate the different factors responsible for the beat-variability (the alignment error and the beat-variable signal component) has been assessed on simulated signals. The performance evaluation on real ECG signals from normal subjects and patients with a documented history of ventricular tachycardia showed that the dynamics of the arrhythmogenic substrate as revealed by wavelet transform offers a significant improvement in ventricular tachycardia risk assessment.

Computer Simulation↗
PubMed1997

The performance and reliability of wavelet denoising for Doppler ultrasound fetal heart rate signal preprocessing.

The present paper deals with the performance and the reliability of a Wavelet Denoising method for Doppler ultrasound Fetal Heart Rate (FHR) recordings. It displays strong evidence that the denoising process extracts the actual noise components. The analysis is approached with three methods. First, the power spectrum of the denoised FHR displays more clearly an 1/fa scaling law, i.e. the characteristic of fractal time series. Second, the rescaled scale analysis technique reveals a Hurst exponent at the range of 0.7-0.8 that corresponds to a long memory persistent process. Moreover, the variance of the Hurst exponent across time scales is smaller at the denoised signal. Third, a chaotic attractor reconstructed with the embedding dimension technique becomes evident at the denoised signals, while it is completely obscured at the unfiltered ones.

Artifacts↗
PubMed1997

A study of pharmacological vs. pathological autonomic nervous system blockade in humans, using heart rate chaotic dynamics.

Pathological blocking of the Autonomous Nervous System (ANS) is diagnosed for patients having Autonomic Neuropathy passing a well-defined set of criteria. In this work, It is shown that standard analysis can be complemented by a study of the chaotic dynamics of the Heart Rate (HR), over a period of 12 min in the resting position. It was found that patients who suffering from ANS blockade, typically exhibit a smaller degree of fractality and complexity of the chaotic attractor reconstructed from the time series of the HR signal. These dynamical measures are more evident in normal human subjects that have been subjected to pharmacological ANS blockade.

Algorithms↗
PubMed1997

Strategy for the pursuit of spiral waves in excitable media.

Spiral waves are though to be the underlying mechanism of re-entrant ventricular and atrial tachycardias. In such cases, one is generally interested in eliminating spiral wave activity from the medium. In this paper, solve a cubic FitzHugh-Nagumo system of PDEs is solved in two dimensions with initial conditions such that a spiral wave is formed at the center of a rectangular grid. Then the effect of a spatially-localized step-like periodic forcing placed at different positions around the spiral tip is studied. Due to this forcing, the tip begins to drift away from the perturbation in a direction that depends on their relative location. By shifting successively the location of the perturbation relative to that of the tip strategy is developed which it is possible to pursuit the spiral wave away from the center of the grid accelerating its drift with every shift of the perturbation.

Atrioventricular Node↗
PubMed1995

Data dependent weighted averages for recording of evoked potential signals.

Two new filters are proposed to replace conventional on-line artifact rejection routines. They are based on an algorithm that computes a weight for each trial according to its similarity to the rest. The robustness of the two filters and their capacity to reduce recording time significantly were verified experimentally.

Algorithms↗
PubMed1995

Latency measurement improvement of P100 complex in visual evoked potentials by FMH filters.

The ensemble average of Pattern Shift Visual Evoked Potentials (PSVEP) signals is seriously affected by random latency variations encountered in each individual sweep which is modeled as a continuous signal with linear segments and well-shaped triangular peaks. This effect is causing the smoothed peaks of the averaged PSVEP waveforms. It is our objective to restore the degraded peaks and provide accurate information about their exact location. The method used is based on nonlinear filtering of the FIR-Median Hybrid (FMH) type and is recommended as a postfiltering process to the well-known averaging methods of recovering PSVEP signals from noise by time-locking to stimuli. The new technique, tested in signals from clinical observations, has proven very promising.

Artifacts↗
PubMed1994

Pattern reversal visual evoked potentials in minor head injury.

Pattern reversal visual evoked potentials (PR-VEPs) have been recorded in 50 patients with minor head injury (MHI) on days 1 and 30 after trauma and the data compared to 20 normals. None of the patients had visual complaints. The aim was to investigate a possible visual pathway affection in MHI and test the usefulness of PR-VEPs as an objective noninvasive tool in the detection of a possible subclinical affection of the visual system in MHI. P100 latency and amplitude had no significant difference compared to normals. Comparison of patient data on days 1 and 30 after trauma showed a significant latency decrease and amplitude increase on day 30, compared to day 1. These alterations were not age dependent. Our data suggest affection of the human visual pathway in MHI. PR-VEP recording seems to be a useful, objective, noninvasive tool, helping to identify possible subclinical affections of the visual pathway in MHI.

Adolescent↗
PubMed1993

A method for the determination of viscoelastic parameters of human hair in relation to its structure.

An analytical model and the corresponding experimental procedure are presented, which make it possible to separately analyze the viscous and elastic components of human hair. Moreover, a computerized experimental system is described with which a standardized quantitative determination of the most important mechanical parameters of human hair can be performed. Comparative investigations of the mechanical behavior of hair specimens obtained from the frontoparietal and occipital scalp regions of 10 healthy human subjects showed no statistically significant differences with regard to their mechanical parameters. Nevertheless, the paired analysis of the mechanical parameters of thin and thick hair specimens revealed that the values of two of these parameters are significantly higher in thin than in thick hair specimens. These findings suggest that there are mechanical parameters of human hair related to gross hair morphology, whereas others depend on hair microstructure.

Adolescent↗
PubMed1990

Monitoring of fracture healing by lateral and axial vibration analysis.

The ability to monitor the healing of bone fractures is crucially important in their treatment. The aim of the present study was to develop and validate an objective method for monitoring fracture healing based on bone vibrational response. An analytic model was formulated, with which the mechanical parameters at the fracture site could be studied in relation to both lateral and axial bone vibration. Non-uniformities in the stiffness of the bone at the fracture site can be detected since they produce shifting of the vibration and the phase spectrum and result in strong coupling between the lateral and axial vibration response spectra. The validity of the model was tested in experiments using fresh cadaver tibiae with transverse osteotomy and materials simulating fracture callus. The results of the study of vibration amplitude and phase angle and the coupling of axial and lateral vibration in these experiments confirm our analytic projection. Preliminary results of in vivo investigations using the described method are encouraging.

Adolescent↗
PubMedDate not supplied

[Adaptation of the viscoelastic behavior of bone cement to bone as a stabilizing factor in endoprostheses].

Incompatibility of viscoelastic properties is a major factor contributing to the development and enlargement of microscopic cracks between bone and bone cement. In the study reported here the viscoelastic characteristics of fresh long bones were studied in vitro and compared with those of bone cement. Using strain gauges, creep elongation was measured over time spans ranging from 10(-2) to 10(3) seconds with an applied stress of 10 MPa. On the basis of these measurements creep compliance and permanent elongation at two points on a sheep femur were calculated by means of a rheologic Kelvin model with three parameters. The possibility of obtaining an optimal bone/cement bond was studied in the light of these results and of data published in the literature on bone cement creep related to cement composition factors.

Biomechanical Phenomena↗
PubMedDate not supplied

Adaptive denoising and multiscale detection of the V wave in brainstem auditory evoked potentials.

This paper describes a wavelet-transform-based system for the V wave identification in brainstem auditory evoked potentials (BAEP). The system combines signal denoising and rule-based localization modules. The signal denoising module has the potential of effective noise reduction after signal averaging. It analyses adaptively the evolution of the wavelet transform maxima across scales. The singularities of the signal create wavelet maxima with different properties from those of the induced noise. A non-linear filtering process implemented with a neural network extracts out the noise-induced maxima. The filtered wavelet details are subsequently analysed by the rule-based localization module for the automatic identification of the V wave. In the first phase, it implements a set of statistical observations as well as heuristic criteria used by human experts in order to classify the IV-V complex. At the second phase, using a multiscale focusing algorithm, the IV and V waves are positioned on the BAEP signal. Our experiments revealed that the system provides accurate results even for signals exhibiting unclear IV-V complexes.

Auditory Threshold↗