Jozef Pucik

Dependence of the RF field absorption on the human body dimensions

In this paper, the dependence of radio-frequency (RF) field absorption on human body dimensions is investigated. The resonance properties of an ellipsoidal homogeneous human model have been investigated in frequency range from 10 MHz to 500 MHz with assistance of CST Microwave Studio program environment. In the article, the simulation of RF field absorption in the ellipsoidal body model has been performed for three various body heights, as well as three widths for a given body height.

Spectral Properties of ECG Signal

Spectral analysis belongs to widely used method for biomedical data exploration. In order to interpret spectrum correctly, prior knowledge how various phenomena affect the spectrum of analyzed signal is desirable. Spectrum of ECG signal is affected by waveform component shapes, their time positions within cardiac cycle as well as by regularity of heart period. This paper deals with effect of heart rate variability on ECG signal spectrum. Three representations are proposed to model variable heart timing in the case of normal sinus rhythm (pulse process with time jitter, random inter-pulse interval and integral pulse frequency modulation model). Properties of corresponding spectra are theoretically studied and compared with real-world signals from PhysioBank data archive.

Spectrum of counts computation for HRV analysis

Heart rate variability (HRV) analysis provides noninvasive way for assessment of neural activity that control cardiovascular system. Contributions of sympathetic and parasympathetic autonomous nervous system that modulate instantaneous heart rate are studied by means of spectral analysis. The objective is to recover modulating signal spectrum from beat occurrence times. In this paper, properties of so called spectrum of counts with respect to artifact arising in practical application of this method are described. Specifically, spectral leakage and beat time detection precision are discussed. Phenomena are analyzed on theoretical basis of integral pulse frequency modulation (IPFM). Computer simulations and recommendations for artifact rejection are presented.

Digital Group Delay Equalizer

This paper deals with problem of digital group delay equalizer for various types of digital filters. Proposed method of equalization is evaluated for a set of digital allpass filters. Whole system is analyzed in Matlab environment. Genetic algorithm has been used to design group delay equalizer. In this paper, the method of genetic algorithm is applied to group delay correction of digital IIR bandstop filters. Three examples are presented to demonstrate efficiency of the proposed method. This genetic algorithm method is suitable also for basic types of digital filters with nonlinear phase frequency response.

The critical radius of stimulation electrode related to heart activation

This article examines the dependence of stimulation current threshold on stimulation electrode radius and other cardiac tissue parameters. The dependence was evaluated on a homogeneous slab model of the heart wall, with local membrane properties determined by modified FitzHugh-Nagumo model. Simulation results reveal that stimulation current density threshold depends proportionally on diffusivity related to tissue conductivity and on membrane excitation threshold. It is also inversely proportional to stimulation electrode radius. The critical radius of the stimulation electrode for the given model parameters estimated to be 0.8 mm was compared with simulation results. Stimulation electrodes with smaller radius are ineffective in eliciting excitation in the cardiac tissue.

Continued fraction expansion of irrational transfer functions for simulation of physical systems

Transfer function of a linear continuous-time system with lumped parameters is a rational function. Transfer function of real systems, however, cannot always be written in the rational function form. It is the case for the systems with distributed parameters, transfer functions for modeling physical properties of materials, and systems for a colored noise generation such as the noise with power spectral density (PSD) falling off at 10 dB per decade of frequency. A time domain simulation of mentioned systems demands an approximation with a finite order system. In this paper, we consider the types of systems, the transfer functions of which are expressed with a square root. An expansion of the transfer function to the continued fraction, a system structure, and state space equations are derived. The stability of the system is shown and an example of the simulation is presented.

Assessment of Visual Reliance in Balance Control: An Inexpensive Extension of the Static Posturography

Ability of humans to maintain balance in an upright stance and during movement activities is one of the most natural skills affecting everyday life. This ability progressively deteriorates with increasing age, and balance impairment, often aggravated by age-related diseases, can result in falls that adversely impact the quality of life. Falls represent serious problems of health concern associated with aging. Many investigators, involved in different science disciplines such as medicine, engineering, psychology, and sport, have been attracted by a research of the human upright stance. In a clinical practice, stabilometry based on the force plate is the most widely available procedure used to evaluate the balance. In this paper, we have proposed a low-cost extension of the conventional stabilometry by the multimedia technology that allows identifying potentially disturbing effects of visual sensory information. Due to the proposed extension, a stabilometric assessment in terms of line integral of center of pressure (COP) during moving scene stimuli shows higher discrimination power between young healthy and elderly subjects with supposed stronger visual reliance.

Observability of Spectral Components beyond Nyquist Limit in Nonuniformly Sampled Signals

Identification of a signal component with the frequency exceeding the Nyquist limit is a challenging problem in signal theory as well as in some specific applications areas like astronomy and biosciences. A consequence of the well-known sampling theorem for a uniformly sampled signal is that the spectral component above the Nyquist limit is aliased into lower frequency range, making a distinction between true and aliased components impossible. The nonuniform sampling, however, offers a possibility to reduce aliased components and uncover information above the Nyquist limit. In this paper, we provide a theoretical analysis of the aliased components reduction in the nonparametric periodogram for two sampling schemes: the random sampling pattern and the sampling pattern generated by the integral pulse frequency modulation (IPFM), the latter widely accepted as the heart rate timing model. A general formula that relates the variance of timing deviations from a regular scheme with the aliased component suppression is proposed. The derived relation is illustrated by Lomb-Scargle periodograms applied on simulated data. Presented experimental data consisting of the respiration signal derived from the electrocardiogram and the heart rate signal also support possibility to detect frequencies above the Nyquist limit in the condition known as the cardiac aliasing.