Showing papers in "Medical & Biological Engineering & Computing in 1996"
TL;DR: It is concluded that the patient's skin should be abraded to reduce impedance, and measurements should be avoided in the first 10 min after electrode placement, to allow satisfactory images.
Abstract: A computer simulation is used to investigate the relationship between skin impedance and image artefacts in electrical impedance tomography. Sets of electrode impedance are generated with a pseudo-random distribution and used to introduce errors in boundary voltage measurements. To simplify the analysis, the non-idealities in the current injection circuit are replaced by a fixed common-mode error term. The boundary voltages are reconstructed into images and inspected. Where the simulated skin impedance remains constant between measurements, large impedances (> 2k omega) do not cause significant degradation of the image. Where the skin impedances 'drift' between measurements, a drift of 5% from a starting impedance of 100 omega is sufficient to cause significant image distortion. If the skin impedances vary randomly between measurements, they have to be less than 10 omega to allow satisfactory images. Skin impedances are typically 100-200 omega at 50 kHz on unprepared skin. These values are sufficient to cause image distortion if they drift over time. It is concluded that the patient's skin should be abraded to reduce impedance, and measurements should be avoided in the first 10 min after electrode placement.
1,976 citations
TL;DR: The collected data show that frequencies larger than 1 MHz are needed for the bio-electrical characterisation of breast tissue, and the reduced standard error of impedivity in breast tissue is about 0.1 or less.
Abstract: The impedivity of six groups of breast tissue is measured between 0.488 kHz and 1 MHz using a hand-held probe, ensuring a constant geometry factor, and a microcomputer-controlled impedance spectroscopy system. 120 spectra are collected in excised tissue samples from 64 patients undergoing breast surgery. Each spectrum consists of 12 frequency points. The mean m, the standard deviation s, and the ‘reduced standard error’ (e=s/(m N)) of the magnitude and the phase angle of the impedivity are calculated at each frequency for all groups of tissues. The variability at low frequency (f<10 kHz) is attributed to the dispersion in measurement errors. This contributed to the choice of 32 kHz as the lower limit of measurement frequency in the constructed electrical impedance tomograph. The collected data also show that frequencies larger than 1 MHz are needed for the bio-electrical characterisation of breast tissue. In the frequency range used in electrical impedance tomography the reduced standard error of impedivity in breast tissue is about 0.1 or less. The lowest dispersions are observed in the adipose tissue, carcinoma and fibro-adenoma.
253 citations
TL;DR: The simultaneous analysis of several breath trace gases during a single exhalation is clearly demonstrated, and thus different elution times for isoprene and methanol along the respiratory tract are observed.
Abstract: The selected ion flow tube (SIFT) technique for trace gas analysis of air and breath is based on soft chemical ionisation of the trace gases to the exclusion of the major air and breath gases, in fast-flowing inert carrier gas, exploiting the ion-molecule reactions that occur between the trace gases and the pre selected precursor ions (H3O+, NO+ and O2+). The physics and ion chemistry involved in the SIFT technique are described, as are the kinetics of the ion-molecule reactions that are exploited to quantitatively analyse the trace gases. Fast on-line data-acquisition hardware and software have been developed to analyse the mass spectra obtained, from which partial pressures of the trace gases down to about 10 parts per billion can be measured. The time response of the instrument is 20 ms, allowing the profiles of the trace gas concentrations on breath to be obtained during a normal breathing cycle. Pilot results obtained with this SIFT technique include detection and quantification of the most abundant breath trace gases, analysis of cigarette smoke, detection of gases present on smokers' breath and accurate measurement of the partial pressures of NH3, NO and NO2 in air. The simultaneous analysis of several breath trace gases during a single exhalation is clearly demonstrated, and thus different elution times for isoprene and methanol along the respiratory tract are observed. This technique has great potential in many clinical and biological disciplines, and in health and safety monitoring.
200 citations
TL;DR: It is shown that at the high frequencies generally encountered in EIT the capacitive properties of the electrode interface, and especially those of the skin, are of primary importance.
Abstract: The magnitude, mismatch and temporal variations of the electrode-gel-skin interface impedance can cause problems in electrical impedance tomography (EIT) measurement. It is shown that at the high frequencies generally encountered in EIT the capacitive properties of the electrode interface, and especially those of the skin, are of primary importance. A wide range of techniques are reviewed that could possibly be used to minimise these problems. These techniques include the use of skin preparation, penetration enhancers, temperature and electrical impulses. Although several of these techniques appear very attractive, they are not without serious potential drawbacks. A combination of some of these techniques may well hold the key to success.
157 citations
TL;DR: Basic experimental research is still needed to determine more accurate material properties and injury tolerance criteria, so that FE models can fully exercise their analytical and predictive power for the study and prevention of human head injury.
Abstract: A review is presented of the existing finite-element (FE) models for the biomechanics of human head injury. Finite element analysis can be an important tool in describing the injury biomechanics of the human head. Complex geometric and material properties pose challenges to FE modelling. Various assumptions and simplifications are made in model development that require experimental validation. More recent models incorporate anatomic details with higher precision. The cervical vertebral column and spinal cord are included. Model results have been more qualitative than quantitative owing to the lack of adequate experimental validation. Advances include transient stress distribution in the brain tissue, frequency responses, effects of boundary conditions, pressure release mechanism of the foramen magnum and the spinal cord, verification of rotation and cavitation theories of brain injury, and protective effects of helmets. These theoretical results provide a basic understanding of the internal biomechanical responses of the head under various dynamic loading conditions. Basic experimental research is still needed to determine more accurate material properties and injury tolerance criteria, so that FE models can fully exercise their analytical and predictive power for the study and prevention of human head injury.
150 citations
TL;DR: The prototype Graz BCI II, which is based on the distinction of three different types of EEG pattern, is described, and results of online and offline classification performance of four subjects are reported.
Abstract: The paper describes work on the brain-computer interface (BCI). The BCI is designed to help patients with severe motor impairment (e.g. amyotropic lateral sclerosis) to communicate with their environment through wilful modification of their EEG. To establish such a communication channel, two major prerequisites have to be fulfilled: features that reliably describe several distinctive brain states have to be available, and these features must be classified on-line, i.e. on a single-trial basis. The prototype Graz BCI II, which is based on the distinction of three different types of EEG pattern, is described, and results of online and offline classification performance of four subjects are reported. The online results suggest that, in the best case, a classification accuracy of about 60% is reached after only three training sessions. The offline results show how selection of specific frequency bands influences the classification performance in singletrial data.
145 citations
TL;DR: An Adaptive Hermite Model Estimation System (AHMES) is presented for on-line beat-to-beat estimation of the features that describe the QRS complex with the Hermite model, and an application is shown, for subsequent classification, of the AHMES in extracting theQRS features of an ECG signal with the bigeminy phenomena.
Abstract: The most characteristic wave set in ECG signals is the QRS complex. Automatic procedures to classify the QRS are very useful in the diagnosis of cardiac dysfunctions. Early detection and classification of QRS changes are important in real-time monitoring. ECG data compression is also important for storage and data transmission. An Adaptive Hermite Model Estimation System (AHMES) is presented for on-line beat-to-beat estimation of the features that describe the QRS complex with the Hermite model. The AHMES is based on the multiple-input adaptive linear combiner, using as inputs the succession of the QRS complexes and the Hermite functions, where a procedure has been incorporated to adaptively estimate a width related parameter b. The system allows an efficient real-time parameter extraction for classification and data compression. The performance of the AHMES is compared with that of direct feature estimation, studying the improvement in signal-to-noise ratio. In addition, the effect of misalignment at the QRS mark is shown to become a neglecting low-pass effect. The results allow the conditions in which the AHMES improves the direct estimate to be established. The application is shown, for subsequent classification, of the AHMES in extracting the QRS features of an ECG signal with the bigeminy phenomena. Another application is highlighted that helps wide ectopic beats detection using the width parameter b.
114 citations
TL;DR: The simulations predict that a high degree of freedom in the control of activation of dorsal spinal pathways may be obtained with the described system, which implies an easier control of paraesthesia coverage of skin areas and the possibility to correct undesired paraesthesia patterns.
Abstract: A new approach to spinal cord stimulation is presented, by which several serious problems of conventional methods can be solved. A transverse tripolar electrode with a dual-channel voltage stimulator is evaluated theoretically by means of a volume conductor model, combined with nerve fibre models. The simulations predict that a high degree of freedom in the control of activation of dorsal spinal pathways may be obtained with the described system. This implies an easier control of paraesthesia coverage of skin areas and the possibility to correct undesired paraesthesia patterns, caused by lead migration, tissue growth, or anatomical asymmetries, for example, without surgical intervention. It will also be possible to preferentially activate either dorsal column or dorsal root fibres, which has some important clinical advantages. Compared to conventional stimulation systems, the new system has a relatively high current drain.
87 citations
TL;DR: The paper focuses on the most important application problems commonly encountered in spectral analysis of short-term recordings of cardiovascular variability signals (CVSs), critically analysing the different approaches to these problems presented in the literature and suggesting practical solutions based on sound theoretical and empirical considerations.
Abstract: The paper focuses on the most important application problems commonly encountered in spectral analysis of short-term (less than 10 min) recordings of cardiovascular variability signals (CVSs), critically analysing the different approaches to these problems presented in the literature and suggesting practical solutions based on sound theoretical and empirical considerations The Blackman-Tukey (BT) and Burg methods have been selected as the most representative of classical and AR spectral estimators, respectively For realistic simulations, ‘synthetic’ CVSs are generated as AR processes whose parameters are estimated on corresponding time series of normal, post-myocardial infarction and congestive heart failure subjects The problem of resolution of spectral estimates is addressed, and an empirical method is proposed for model order selection in AR estimation The issue of the understandability and interpretability of spectral shapes is discussed The problem of non-stationarity and removing trends is dealt with The important issue of identification and estimation of spectral components is discussed, and the main advantages and drawbacks of spectral decomposition algorithms are critically evaluated
73 citations
TL;DR: Findings indicate that the arterial baroreflex modulates both fast and slow spectral components of BP and PI; homogeneously enhances PI fluctuations at all frequencies; produces differentiated effects on BP fluctuations along the frequency axis; and at low frequencies exerts the buffering action on BP through strategies which reduce the BP-PI linear link.
Abstract: Sino-aortic denervation (SAD) is employed in cats to evaluate the baroreflex influence on blood pressure (BP) and pulse interval (PI) spectral components from 0·00008 to 0·9 Hz as assessed by FFT wide-band spectra and their 1/f modelling; and the linear coupling between BP and PI and between systolic and diastolic BP as assessed by coherence analysis. Specific procedures have been developed to obtain an effective smoothing of spectra and coherence functions. SAD induced an increase in BP powers from 0·03 to 0·0006 Hz and a power reduction of most of the remaining BP components; a reduction of PI powers at all frequencies; marked deviations of BP spectra from the 1/f trend; a reduction of the coherence between BP and PI from 0·12 to 0·5 Hz and a coherence enhancement at lower frequencies. These findings indicate that the arterial baroreflex modulates both fast and slow spectral components of BP and PI; homogeneously enhances PI fluctuations at all frequencies; produces differentiated effects on BP fluctuations along the frequency axis; and at low frequencies exerts the buffering action on BP through strategies which reduce the BP-PI linear link.
65 citations
TL;DR: The EFE algorithm has been evaluated using both simulated and real motor unit firing time data, and has been found to provide accurate and unbiased mean and standard deviation estimates, even when up to 70% of the IPI data are incorrect.
Abstract: An error-filtered estimation (EFE) algorithm for estimating the mean and standard deviation of a set of time intervals between consecutive motor unit firing times (inter-pulse intervals (IPIs)) is described. As the input IPI data are filtered and only valid IPIs are used to estimate mean and standard deviation values, the EFE algorithm provides accurate estimates even when the data defining the train of motor unit firing times are only partially complete or have several erroneous firing times. The algorithm has been evaluated using both simulated and real motor unit firing time data, and has been found to provide accurate and unbiased mean and standard deviation estimates, even when up to 70% of the IPI data are incorrect.
TL;DR: It is concluded that the coherence increase seen with nose, reference data is due to an indirecteeffect of mu rhythm desynchronisation, rather than any increase in synchrony of the mu rhythms themselves.
Abstract: The impact is reported of different EEG derivation types on short-term changes in the inter-hemispheric coherence between the left and right sensorimotor areas, during the planning and execution of right index finger movements. Data are recorded during an event-related paradigm in which cued index finger movements are made. Event-related coherence analysis is then applied to the monopolar (nose reference) data, as well as source derivation. The results show that inter-hemispheric coherence between sensorimotor areas is dependent on the EEG derivation type. An increase in coherence during movement is found with nose reference and bipolar data, whereas for local average reference and source derivations, low inter-hemispheric coherence is observed, with no change in the coherence during movement. It is concluded that the coherence increase seen with nose, reference data is due to an indirecteeffect of mu rhythm desynchronisation, rather than any increase in synchrony of the mu rhythms themselves. Local average reference and source derivations better reflect the activity of the underlying cortical structures (the mu generating networks), and coherence analysis using these derivations shows that the mu rhythms of left and right hemispheres are not coherent.
TL;DR: A technique utilising copolymer piezoelectric film has allowed the manufacture of biaxial in-shoe transducers capable of simultaneously measuring two orthogonal shear forces.
Abstract: The Kent shear system is introduced and preliminary clinical results are presented. A technique utilising copolymer piezoelectric film has allowed the manufacture of biaxial in-shoe transducers capable of simultaneously measuring two orthogonal shear forces. Bipedal measurements are carried out inside everyday footwear over multiple footsteps.
TL;DR: The finite-element method in combination with the Biot-Savart law is described to compute the magnetic field distribution generated by a dipolar source within a homogeneous volume conductor of an arbitrary shape.
Abstract: The finite-element method in combination with the Biot-Savart law is described to compute the magnetic field distribution generated by a dipolar source within a homogeneous volume conductor of an arbitrary shape. In order to calculate the three independent components of the magnetic field outside the volume conductor by means of the Biot-Savart law, the distribution of the current throughout the medium is required. A finite-element mesh is constructed using four-node tetrahedral elements. The potential in each node is computed numerically by the finite-element method using the proper continuity conditions across the boundaries. The gradient of the potential denotes the current density within an element. The components of the magnetic induction are calculated by numerical integration, applying the current density within the tetrahedrons. Simulations are carried out to assess the numerical accuracy for a homogeneous spherical volume conductor. Errors of 3% can be obtained with a locally refined spherical mesh containing about 1000 nodes, for dipoles at any depth and any orientation. A homogeneous realistically shaped model with the shape of the inside of the skull is obtained from magnetic resonance images.
TL;DR: A two-dimensional model of the normal foot skeleton, which includes cartilages and ligaments, is used in this analysis of stresses during three quasi-static walking phases and demonstrates that both the shape of the foot and the type of muscle paralysis contribute to the development of high stresses in different regions of theFoot.
Abstract: A two-dimensional model of the normal foot skeleton, which includes cartilages and ligaments, is used in this analysis of stresses during three quasi-static walking phases: heel-strike, mid-stance and push-off. It is found that in all the walking phases the maximum values of principal stresses occur in the dorsal anterior region of the talus, whereas the highest stress occurs in the push-off phase. The model is used for the simulation of muscle paralysis and its effect on the distribution of principal stresses. Subsequently, the model is used to analyse stresses in the deformed feet of three leprosy patients with complete paralysis of certain muscles. The results demonstrate that both the shape of the foot and the type of muscle paralysis contribute to the development of high stresses in different regions of the foot. These high stresses in regions with reduced mechanical strength could be one of the important factors in the process of tarsal disintegration in leprosy.
TL;DR: A chamber for indirect calorimetry has been constructed that utilises previously published general equations for the calculation of respiration, and it is concluded that this chamber is useful not only for traditional 24 h energy expenditure measurements, but also for experiments requiring rapid responses.
Abstract: A chamber for indirect calorimetry has been constructed that utilises previously published general equations for the calculation of respiration. Owing to the large size of the chamber, the changes in gas concentration caused by a subject are very small. Therefore, algorithms are developed for noise suppression and trend identification. Using the exact solution of the equations for steady state, each gas concentration is fitted by a least square method to two connected exponential segments, of variable length, for the preceding 30 min period. Independently of the location of the join between the two segments, the gas concentration and its time derivative are evaluated at -15 min. This process is repeated, and its results are presented once every minute. As proven by gas injection tests, this procedure gives an instantaneous response to a single change in respiration, a correct averaging of repeated changes in respiration with periods of less than 15 min and noise suppression. It is concluded that this chamber is useful not only for traditional 24 h energy expenditure measurements, but also for experiments requiring rapid responses.
TL;DR: In this paper, an adaptive algorithm is described that groups motor unit action potentials (MUAPs), detected in a composite EMG signal during signal decomposition, and creates partial motor unit actionspotential trains (PUAPTs).
Abstract: An adaptive algorithm is described that groups motor unit action potentials (MUAPs), detected in a composite EMG signal during signal decomposition, and creates partial motor unit action potential trains (MUAPTs). Data-driven MUAP shape and motor unit firing-pattern based criteria are used to form the clusters. An algorithm for estimating MUAPT temporal parameters, which provides accurate estimates even for partially defined trains, is used to obtain firing-pattern information. No a priori knowledge is required regarding the number of clusters or the distribution of their template shapes. The clustering algorithm when applied to real concentric-needle detected MUAP data provides accurate and useful clustering results. Compared to a classical leader-based algorithm, it provides more robust performance, is better able to estimate the true number of motor units represented in a set of detected MUAPs, and obtains more complete and accurate MUAPTs.
TL;DR: Three-dimensional finite-element analysis is carried out to investigate the influence of the partitioning membranes of the brain and the neck in head injury analysis through free-vibration analysis and transient analysis.
Abstract: A head injury model consisting of the skull, the CSF, the brain and its partitioning membranes and the neck region is simulated by considering its near actual geometry. Three-dimensional finite-element analysis is carried out to investigate the influence of the partitioning membranes of the brain and the neck in head injury analysis through free-vibration analysis and transient analysis. In free-vibration analysis, the first five modal frequencies are calculated, and in transient analysis intracranial pressure and maximum shear stress in the brain are determined for a given occipital impact load.
TL;DR: A new algorithm to resolve superimposed motor unit action potentials (MUAPs) is described, which uses a reduced search space and is based on the peel off approach, and demonstrates robust performance across the signals tested.
Abstract: A new algorithm to resolve superimposed motor unit action potentials (MUAPs) is described, which uses a reduced search space and is based on the peel off approach. Knowledge specific to the problem domain, such as temporal relationships between and within motor unit action potential trains and MUAP energy information, is used to reduce the search space of motor units, possibly contributing to a superposition. The algorithm is tested using real electromyographic signals, and it demonstrates robust performance across the signals tested. For the signals tested, the average total resolution rate is 94%, the average correct resolution rate is 99.2% and the average error rate is 0.85%.
TL;DR: The extent to which the generated image is caused by the changed resistivity distribution in the cross-section under investigation on the one hand, or by the changes of the peripheral geometry on the other hand, e.g. during respiration is investigated.
Abstract: WITH REGARD to dynamic imaging in electrical impedance tomography (EIT), theoretical deduction and practical measurements show that it is necessary to keep the peripheral geometry of the cross-section of the object under constant investigation during the reference and main measurements (a (WEBSTER 1990a, b HEIMBACK, 1990). This necessary condition, however, cannot generally be provided for in medical examination procedures. There are two reasons for this predicament; first it is almost impossible to keep the patient immobile during the measurement procedure as this is only possible with anaesthetised patients or patients with stabilised posture. Secondly, the peripheral geometry is susceptible to changes during the examination due to respiration, for example (ADLER et al., 1994). This leads to the question of the extent to which the generated image is caused by the changed resistivity distribution in the cross-section under investigation on the one hand, or by the changes of the peripheral geometry on the other hand, e.g. during respiration. corresponding radial force on the shell of the elastic tank near electrodes 5 and 13 (Fig. 1). Axis dl of the ellipse is situated in the direction of the force applied; axis d2 is perpendicular to dl. The relative changes Dx in da are
TL;DR: The electrochemical noise from dry NASICON-based surface electrodes and pregelled Ag−AgCl electrodes is evaluated in saline solutions and on the skin and the contribution of residual EMG signal of low amplitude is suggested for electrodes with large surface area.
Abstract: The electrochemical noise from dry NASICON-based surface electrodes and pregelled Ag-AgCl electrodes is evaluated in saline solutions and on the skin. The electrochemical noise from the electrode/electrolyte interface is found to be negligible (less than 1 microV peak to peak). On the skin, the noise level is highly dependent on the patient. At high frequencies, the skin/electrode interface noise is equal to 'thermal noise' and can be related to the real part of the skin/electrode impedance. At low frequencies (F < 100 Hz), excess noise is observed that varies as f-2. It is tentatively ascribed to a non-stationary process or noise of electrochemical origin due to the ionic nature of the skin. The contribution of residual EMG signal of low amplitude (5 microV peak to peak) is suggested for electrodes with large surface area.
TL;DR: An application in combination with a bionsensor for the analysis of glucose is described, but the device also allows monitoring of other subcutaneous constituents such as ions, metabolites, drugs and their metabolites.
Abstract: A light, portable sampling device for the continuous collection and storage of subcutaneous fluid is described. It consists of a hollow-fibre probe for ultrafiltration, a long tube for sample storage and a vacuum tube as driving force. Introduction of a restriction in the flow path allows a constant pulse-free flow as low as 100 nl min-1. The contents of the storage tube reflect the metabolic events that have happened during the 24 h collection period. The ultrafiltrate fluid collected from the subcutaneous tissue is free from large molecules and can be continuously analysed. An application in combination with a bionsensor for the analysis of glucose is described, but the device also allows monitoring of other subcutaneous constituents such as ions, metabolites, drugs and their metabolites.
TL;DR: A major obstacle encountered during analysis of the diaphragmatic EMG signal is electrocardiogram (ECG) contamination, which causes an increase in the power content of the signal and a distortion of its frequency content.
Abstract: THE DIAPHRAGMATIC electromyogram (EMG) conveys important information regarding the respiratory control mechanisms. For example, the intensity of the EMG signal, measured by variables such as the mean rectified EMG (MREMG) and power (PWR), is a good indicator of the magnitude of the neural drive to the diaphragm (EVANICH et aL, 1977; LYNNEDAVIES, 1979). The frequency content of the signal, measured by variables such as the zero crossing frequency (ZC), median frequency (MF), mean power frequency (MPF) and expected zero crossing frequency (E[ZC]), conveys information regarding changes in the activation patterns of the diaphragmatic motor units (ScHWEITZER et al., 1979; HAGG, 1991; 1992). A major obstacle encountered during analysis of the diaphragmatic EMG signal is electrocardiogram (ECG) contamination. This causes an increase in the power content of the signal and a distortion of its frequency content (SCHWEITZER e t al., 1979). Early techniques employed to reduce the level of ECG contamination include amplitude clipping and highpass filtering, neither of which have proved effective (SCHWEITZER et al., 1979; MUELLER and LOURENqO, 1968). At present, the method most frequently used is the gating technique (SCHWEITZER et al., 1979; CHOUKROUN et aL, 1990; LANTZu et aL, 1991; SHARP et aL, 1993), which involves the removal of a section of the EMG signal centred around each QRS
TL;DR: To develop computer techniques for extracting quantitative information from digital electrogastrograms, and to evaluate quantitatively EGG recordings from healthy volunteers, frequency is found to be the only EGG parameter that shows quantitative consistency and merit.
Abstract: Cutaneous recordings of gastric electrical activity (electrogastrography (EGG)) could become a valuable non-invasive tool for recognising gastric electrical abnormalities. Although signals obtained with internally implanted electrodes deliver quantitative information, this technique cannot be used for diagnostic purposes because of its invasive nature. On the other hand, the objectivity of electrogastrography is still in question. The aims of this work are to develop computer techniques for extracting quantitative information from digital electrogastrograms, and to evaluate quantitatively EGG recordings from healthy volunteers. The dynamics of all four EGG parameters are studied: amplitude, frequency, time shift between different channels, and waveform. Four separate two-dimensional computer plots are developed using specially designed digital signal-processing procedures. Each parameter is evaluated in a study of 20 healthy volunteers. Frequency is found to be the only EGG parameter that shows quantitative consistency and merit.
TL;DR: It is deduced that external uterine EMG is a good image of the genuine uterine electrical activity, and can be related to an average cellular electrical activity.
Abstract: The objective is to analyse internal and external recordings of uterine EMG in order to reveal common features and to assess the relationship between electrical activity and intra-uterine pressure modification. Three monkeys participated in the study, one as a reference and the others for data. EMGs are recorded simultaneously, internally by unipolar wire electrodes and externally by bipolar Ag/AgCl electrodes. Intra-uterine pressure is recorded as a mechanical index. Except for delay measurements, parameters are derived from spectral analysis and relationships between recordings are assessed by studying the coherence. Spectral analysis exhibits two basic activities in the analysed frequency band, and frequency limits are defined as relevant parameters for electrical activity description. Parameter values do not depend on the internal electrode location. Internal and external EMGs present a similar spectral shape, despite differences in electrode configuration and tissue filtering. It is deduced that external uterine EMG is a good image of the genuine uterine electrical activity. To some extent, it can be related to an average cellular electrical activity.
TL;DR: From the results of simultaneous measurement of the subject's posture changes, the effect of posture change on blood pressure, e.g. baroreceptor-cardiac reflex, is also clearly demonstrated.
Abstract: A portable instrument, based on a volume-compensation technique, is designed for ambulatory monitoring of indirect beat-to-beat blood pressure (BP) in the superficial temporal artery. The instrument consists of a small disc-type cuff and a portable unit carried by the subject. Several components are integrated in the cuff for applying counter-pressure to the artery, i.e. a reflectance-type photo-plethysmographic sensor for arterial volume detection, a pressure sensor for cuff pressure Pc measurement and a nozzle flapper-type electro-pneumatic convertor for controlling Pc. The portable unit includes volume servo control circuitry and a microprocessor-based signal-processing and recording unit. This automatically performs all the necessary measurement procedures and stores into a memory IC element the processed systolic, mean and diastolic blood pressure data, together with pulse intervals on a beat-to-beat basis from the servo-controlled Pc (indirectly measured BP waveform). With this instrument, momentary changes in BP during ambulatory situations such as bicycle ergometer exercise and daily activities including motorway driving are successfully recorded. From the results of simultaneous measurement of the subject's posture changes, the effect of posture change on blood pressure, e.g. baroreceptor-cardiac reflex, is also clearly demonstrated.
TL;DR: A variety of adaptive filters for this purpose have been proposed, but they require an adaptation period after each interference or abrupt signal change and are not so effective in cases of extra-systoles and arrhythmias.
Abstract: THE FILTERING of power-line interference from the electrocardiogram (ECG) is a very important procedure in any modem electrocardiograph. The requirements for this type of procedure are generally well known and have been very well formulated by Cramer et al. (eRASER et al. , 1987). Despite the high common mode rejection ratio of the input amplifiers of the electrocardiographs, recordings are often contaminated by mains frequency interferences, usually phase-shifted with respect to the mains voltage. This event is due to parasitic currents flowing through the patient's body and the connecting cables, and to the difference in the electrode impedances, converting the common mode interference voltages into unwanted differential signals (HUHTA and WEBSTER, 1973; TOWE, 1981). The use of conventional filters affects the signal components surrounding the mains frequency. A variety of adaptive filters for this purpose have been proposed (FtJRNO and TOMPKINS, 1983; AHLSTROM and TOMPK1NS, 1985; FERRARA and WIDROW, 1982), but they require an adaptation period after each interference or abrupt signal change and are not so effective in cases of extra-systoles and arrhythmias. Another problem is the disturbance of the adaptation process by the ECG signal itself, as can be seen in the work of Thakor and Zhu (THAKOR and ZHU, 1991) where, after each QRS complex, a burst of interference reappears.
TL;DR: It is shown that the amplitude modulated (AM) sinusoidal signal model, which is a special case of the time-dependent AR/ARMA model, can have the periodicity property, and the model can exhibit a burst-like feature very well, when the modulating signal is an exponential function.
Abstract: LIKE MANY other natural signals, the electrocardiograph (ECG) is also a non-stationary signal (GrU~N1ER, 1983). The burst-like QRS feature contributes localised high-frequency components in the ECG signal, making it distinctly non-stationary (WALDO and CHITRAPA, 1991). Although this feature of the QRS wave has helped detection of the wave by filtering/linear prediction (FRIESEN et al., 1990), it makes the modelling of the signal very difficult. Most of the work in modelling a ECG is non-parametric in nature (GRAHAM, 1976; WOMBLE et al., 1977; JALALEDDINE et al., 1990). An attempt to represent a segment of the ECG by the impulse response of a pole-zero model was unsuccessful because of its prohibitively large order (MURTHY et al., 1979). Later, modelling a small segment (about a period) of the ECG by damped sinusoids was found to be superior to the earlier attempt. The method, however, fails to exploit the global nature (e.g. pseudo-periodicity) of the ECG signal (NIRANJAN and MURTHY, 1993). The time-dependent autoregressive (AR)/autoregressive moving average (ARMA) model is the representative of the general class of non-stationary signals (GRENIER, 1983). As such, the model can also be used for the ECG signal. However, the ECG has some distinctive features; its pseudo-periodicity, and different features of the constituent signals (P, QRS and T) representing actions of various parts of the heart (GUYTON, 1985) etc. It would be useful to know how the general timedependent AR/ARMA model is restricted by the special features of the ECG-type signals. We show that the amplitude modulated (AM) sinusoidal signal model, which is a special case of the time-dependent AR/ARMA model, can have the periodicity property, and the model can exhibit a burst-like feature very well, when the modulating signal is an exponential function. We propose that one or more AM sinusoidal signal(s) can be employed to model separately each feature of the ECG signal. The suitability of the developed model is then investigated for the ECG signal using an analysis-by-synthesis technique.
TL;DR: Impaired cardiography remains a controversial methodology as a method for monitoring stroke volume and the most frequently used spot electrode array is the 8-spot electrode array used by Bernstein (BERNSTEIN, 1986).
Abstract: THE TECHNIQUE of impedance cardiography has been suggested as a low-cost, simple, safe and non-invasive technique for monitoring cardiac function. As a means of measurement of cardiac systolic time intervals, its validity has received strong support (PETROVICK e t al., 1980; SHEPS et al., 1982; BALASUBRAMAMIAN et al., 1978; GOLLAN et al., 1978). However, as a method for monitoring stroke volume, for which it has attracted the most interest, impedance cardiography remains a controversial methodology (DONOVAN et al., 1986; GOLDSTEIN et al., 1986; MILLER and HORVATH, 1977; MUZZI et al., 1985; MOHAPATRA, 1981; PORTER and SWAIN, 1983; LAMBERTS et al., 1984). The method was introduced in clinical practice by Kubicek et al., together with his originally proposed equation and band electrode array (KUBICEK et al., 1966). However, the band electrodes were not practical for use; they are difficult to apply correctly and uncomfortable for the patient. New electrode arrays have been introduced using disposable spot electrodes. Currently, the most frequently used spot electrode array is the 8-spot electrode array used by Bernstein (BERNSTEIN, 1986). Unfortunately, Bernstein also introduced a new equation, based on Sramek's findings (SRAMEK et al., 1983), to calculate stroke volume from the impedance cardiogram at that same time. This resulted in several studies comparing the Kubicek and SramekBemstein equation to other methods to estimate stroke volume. However, apart from incidental notes, no reports have been published in which the replacement of the band electrodes by the 8-spot electrodes was evaluated. Other spot electrode arrays using only 4-spot electrodes, however, have been tested in comparison with the band electrodes (Qu et al., 1986), but the results have never appeared to be reproducible (SHERWOOD et al., 1992).
TL;DR: Three parametric modelling approaches based on the Cole-Cole model are introduced and the results of modelling the in vitro data collected from sheep are given to reach the conclusions.
Abstract: Three parametric modelling approaches based on the Cole-Cole model are introduced. Comparison between modelling only the real part and modelling both the real and imaginary parts is carried out by simulations, in which random and systematic noise are considered, respectively. The results of modelling the in vitro data collected from sheep are given to reach the conclusions.