In English. Summaries in Estonian
Proceedings of the Estonian Academy of Sciences.
Volume 10 No. 2 June 2004
Special issue on biomedical engineering
Model-based interpretation of intramyocardial electrograms for cardiac risk assessment and surveillance; 69–81
Abstract. Intramyocardial electrograms (IEGMs), which are acquired by using implanted devices with telemetric capability, offer a challenging potential for long-term cardiac risk assessment and surveillance. However, new approaches to the interpretation of these signals are required. Therefore, a model has been developed for the simulation of excitation spreading across the myocardium and of the subsequent contraction. It allows to analyse systematically the influence of all relevant parameters including the position of the electrode for stimulation and recording. The model can be matched to individual heart geometries, for example, taken from ultrafast CT images. This approach helps to utilize the information represented in the morphology of IEGMs for permanent cardiac risk monitoring.
Interaction of low-level microwave radiation with nervous system – a quasi-thermal effect?; 82–94
Hiie Hinrikus, Jaanus Lass, and Viiu Tuulik
Abstract. This analytical review is focused on the discussion of a possible interaction mechanism of microwave radiation with the nervous system. Energy level of the non-thermal electromagnetic field effects has a lower boundary, limited by the principal physical noise, and an upper boundary, limited by the thermal effects. The thermal energy that introduces disturbances in energy distribution of ions and movement in neurones is about 10–5 eV. The electrical field of 10–10 V/cm can introduce an equivalent disturbance of the thermal equilibrium inside a cell of 10 mm radius. If caused by the low-level microwave exposure, this phenomenon may not be associated with an increase in temperature. Fluctuations, initiated by the high-frequency field in the movement of ions and membranes, affect the gating variables and nerve cell properties like the increase in temperature does. The hypothesis that microwave radiation can affect permeability of the membranes of nerve fibres or myelin sheaths was examined experimentally. A 450 MHz microwave field, with and without modulation, polarized perpendicular or parallel to the nerve axon, was applied. The measured field power density at the skin was 0.87 mW/cm2. The left and right nervus medianus motor nerve fibres of two young female subjects were under the study. The results of 20 cycles of measurements show that the low-level microwave field caused statistically significant increase in the nerve pulse propagation velocity in human motor nerve fibre. The increase was detected only for polarization, perpendicular to the nerve fibre. The 100% amplitude modulation decreased this effect. The effect may be related rather to the threshold voltage of the nerve axon than to the myelin sheath.
EEG signal in monitoring brain function in anesthesia and intensive care: a review; 95–109
Tarmo Lipping and Ville Jäntti
Abstract. During recent years EEG has become the “golden standard” by estimating depth of anesthesia of individual patients in clinical work. Several monitors, based on processing EEG and showing graphically indices of “depth of anesthesia”, have been developed and millions of patients have been monitored with these. In this paper we first shortly discuss the terminology, particularly the meaning of “depth of anesthesia”. Next, a review of different methods and algorithms used in anesthesia monitoring is presented. Finally, we summarize the methods and outline the future development of anesthesia monitoring.
Using photoplethysmographic signal for increasing the accuracy of indirect blood pressure measurement; 110–122
Abstract. Automatic and semi-automatic blood pressure meters are widespread in home health monitoring. However, their results are not accurate and reproducible enough, the reliability of self-assessment is not satisfactory and medical doctors have reservations about their results. The aim of this study was to increase the accuracy and reproducibility of the indirect, cuff-based blood pressure measurement with the help of the photoplethysmographic (PPG) signal. The additional information gained from the PPG signal during slow inflation provides more accurate results than conventional indirect methods and assures that the cuff pressure only slightly (by less than 10 mm Hg) exceeds the systolic pressure. Measuring the pulse wave transit time and its variation during slow inflation and deflation makes it possible to characterize the sympathetic control of the cardiovascular system and the rigidity of the brachial arteries. The PPG signal also indicates if the cuff is placed or inflated improperly. The method has been validated making 420 measurements on 51 subjects.
Optical and electrical methods for pulse wave transit time measurement and its correlation with arterial blood pressure; 123–136
Jaanus Lass, Kalju Meigas, Rain Kattai, Deniss Karai, Jüri Kaik, and Mart Rossmann
Abstract. This paper gives an overview of a research, which is focused on the development of a convenient method for continuous non-invasive monitoring of the arterial blood pressure (BP). The method is based on the presumption that there is a single relationship between the pulse wave propagation time in arterial system and BP. The pulse wave transit time (PWTT) measurement involves the registration of two time markers, one of which is usually based on ECG registration and another on the detection of the pulse wave in peripheral arteries. This study makes a comparative evaluation of four different methods for pulse wave detection: 1) self-mixing in a diode laser, which can detect the skin surface vibrations near the artery, 2) photoplethysmography (PPG) that reflects volumetric changes in peripheral vasculature, 3) cardiosynchronized component of bioimpedance associated with blood displacement in the vascular system, and 4) direct pressure pulsations of finger arteries, detected noninvasively by the piezoelectric transducer. Linear correlation of PWTT, obtained with different methods, with arterial blood pressure is calculated. As a result of the current study it is shown that time intervals, which are calculated from PPG waveforms, are in good correlation with systolic BP during the exercise (linear correlation coefficient r = – 0.7). At rest slightly better results are obtained by diode-laser pulse profile (LPP) sensor (r = – 0.9) as compared to PPG (r = – 0.87). The correlations of pulsatile bioimpedance (BI) sensor (r = – 0.57) and piezoelectric pulse wave (PW) transducer (r = – 0.47) based PWTT calculations with arterial non-invative BP (NIBP) were not as good as for optical parameters (PPG, LPP). The diastolic BP did not correlate well with any of the parameters representing PWTT (| r | < 0.35). It is concluded that the PPG signal is the best for PWTT measurement. It is also most easily detectable and less sensitive to motion artefacts compared to other pulse signals.
Estimation of the finger arterial pressure–volume relationship and blood pressure waveform from photoplethysmographic signals; 137–147
Abstract. A method for deriving the finger arterial pressure–volume (P–V) relationship and blood pressure waveform from two photoplethysmographic (volumetric) signals is presented. The new approach makes it possible to estimate also the beat-to-beat as well as instantaneous values of the dynamic arterial compliance of small arteries. The identification of the P–V relationship was performed applying an asymmetric tangent approximation and non-linear fitting. The results of modelling demonstrate that two finger photoplethysmographic waveforms, recorded at different cuff pressure levels, can be used for the estimation of the finger arterial pressure–volume relationship, dynamic compliance and arterial blood pressure waveform.