Showing papers in "Medical & Biological Engineering & Computing in 1990"

High-quality recording of bioelectric events. Part 1. Interference reduction, theory and practice.

Abstract: In the first part of the review the various mechanisms that could be the cause of interference in bioelectric recordings are considered. It is demonstrated that the performance of a good amplifier can be seriously degraded in its functioning if the whole measurement situation is not taken into account. Several techniques used to reduce interference, of which guarding and driven right leg circuits are the most important, are analysed. In the second part of the review some examples of the application of the theory in practical situations are presented. The instrumentation amplifier circuit normally used in bioelectric recordings is improved for measurements under difficult circumstances. Another application is a low-cost 64-channel amplifier for multichannel ECG recordings. The third application is a device that can be added to bioelectric measurement systems and will provide a major reduction in interference.

Technique for the evaluation of derivatives from noisy biomechanical displacement data using a model-based bandwidth-selection procedure.

Abstract: Smoothing and differentiation of noisy signals are common problems whenever it is difficult or impossible to obtain derivatives by direct measurement. In biomechanics body displacements are frequently assessed and these measurements are affected by noise. To avoid high-frequency noise magnification, data filtering before differentiation is needed. In the approach reported here an autoregressive model is fitted to the signal. This allows the evaluation of the filter bandwidth and the extrapolation of the data. The extrapolation also reduces edge effects. Low-pass filtering is performed in the frequency domain by a linear phase FIR filter and differentiation is performed in the frequency domain. The reported results illustrate the accuracy of the algorithm and its speed (mainly due to the use of the FFT algorithm). Automatic bandwidth selection also guarantees the homogeneity of the results.

Biomedical signal processing (in four parts). Part 1. Time-domain methods.

Abstract: This is the first of a series of four tutorial papers on biomedical signal processing. It provides an introduction to terminology and basic ideas for testing for randomness and trend, and for the determination of basic signal properties in the time domain, given the uncertainties associated with the estimation process. Techniques outlined in the paper are: the coherent average, cross-correlation and covariance, autocorrelation and phase-shift averaging.

Computer-controlled wheelchair ergometer

Abstract: A new wheelchair ergometer has been designed in which a combination of realistic simulation of wheelchair propulsion--with adjustable parameters for rolling resistance, air drag, wind speed and slope--and force measurement has been realised. The static solution enables the measurement of physiological and kinesiological parameters. All data from force transducers in seat and backrest, torque transducers in the wheels and force transducers in the wheelframes as well as the acquired speed are sampled in a data-acquisition system. An offline curve processor allows the acquired data to be processed with standard or custom-programmed routines. Preliminary results have been added and are discussed.

Electrical properties of metallic electrodes

Abstract: The series equivalent resistance R and capacitance C of metal/saline electrode/electrolyte interfaces were measured as a function of frequency (100 Hz–20k Hz) and current density (0·25 to 1000 A m−2) for eight typical electrode metals. For each of the metals tested, R decreased and C increased as the current density was increased above a critical value (with the exception of silver and MP35N at frequencies above 1 kHz for which R increased and C decreased slightly). With the exception of copper, the current density linearity limit (for 10 per cent decrease in R or 10 per cent increase in C) increased with increasing frequency and, in most cases, the current density linearity limit for 10 per cent increase in C was slightly less than that for 10 per cent decrease in R. Among the metals tested, copper and aluminium had the lowest current carrying capability and rhodium had the highest current-carrying capability. The current carrying capabilities of 316 SS, platinum, silver and MP35N, were intermediate and similar. With increasing current density, an increase in the electrode/electrolyte capacitance was the most sensitive indicator of the current-carrying linearity limit.

Comparison of SVD methods to extract the foetal electrocardiogram from cutaneous electrode signals.

Abstract: The paper presents and compares three methods making use of the singular value decomposition (SVD) of a matrix to extract the foetal electrocardiogram (FECG) from cutaneously recorded electrode signals. The first method constructs a set of orthogonal foetal signals (the so-called principal foetal signals) from the recordings, but needs electrode positions far from the foetal heart, in addition to the abdominal electrodes that pick up a mixture of maternal and foetal electrocardiogram. An online adaptive algorithm has been developed such that a real-time implementation becomes feasible. The second method is a new online approach to a technique presented by van Oosterom. Although this method has some important drawbacks and is suboptimal as far as foetal signal-to-noise ratio is concerned, it is still very useful when only a foetal trigger is required, as the signal obtained is not a complete FECG. Finally, a third method is proposed, based on the generalised SVD and interpreted with the new concept of oriented signal-to-signal ratio. An online version is also presented for this method and some results are shown.

Kinematic-based technique for event time determination during gait.

Abstract: A kinematic-based technique for the estimation of the times at which gait events occur is presented. A kinematic-based model (KM) is defined by the trajectory of a point, which has an anatomically fixed location on the subject's body, about a time at which a measurement system defined gait event takes place. The times at which subsequent occurrences of the gait event takes place are determined by identifying the kinematic pattern that best fits the previously defined KM. The results of an experiment that used the gait patterns of a normal and a pathological walker indicate that the accuracy of the algorithm is limited by the kinematic data sampling interval and that optimal kinematic predictors of gait event times occur within the primary (sagittal) plane of motion. The technique is intended to obviate the need for multiple force plates, instrumented floors and instruments which are worn by the subject for the purpose of determining the times at which gait events occur.

Spectral analysis of heart rate variability signal and respiration in diabetic subjects

Abstract: The paper deals with methods of processing ECG and respiration signals which aim at detecting parameters whose values may be correlated to normal and diabetic subjects with or without cardiovascular autonomic neuropathy (CAN). Beatto-beat R-R duration values of the ECG and discrete series of respiration are obtained from original signals using a recognition algorithm. Power spectrum analysis (autospectra, cross-spectra and coherence via autoregressive modelling) is carried out on segments of about 200 consecutive cardiac cycles. Spectral parameters of the R-R variability signal are obtained as follows: total power, power of low-frequency (LF) and high-frequency (HF) components, power of the signal which is (or is not) coherent with respiration, in absolute or in percentage values. The experimental protocol considers 40 diabetic patients (21 of whom have diabetic neuropathy) and 14 normals in three different conditions: resting, standing and controlled respiration. The developed spectral parameters seem sensitive enough to differentiate between normal and pathological subjects. These parameters may constitute a quantitative means to be edded to the classical diabetic tests for the diagnosis of cardiovascular autonomic neuropathy.

Improved method for cardiac output determination in man using ultrasound Doppler technique

Abstract: An existing ultrasound Doppler method for measuring cardiac output has been improved and refined, partly by locating the sampling volume higher up in the aorta while still using the aortic ring size as the effective transverse flow area. The basis for using this technique is the approximately rectangular systolic velocity profile in the aortic orifice in physiologically and anatomically normal subjects, and the fact that this profile velocity is conserved as the maximum velocity in the ascending aorta for some 3 to 4 cm above the valves. This higher location of the sampling volume improves Doppler signal quality, and does not reduce the accuracy of the method, as can be confirmed in each experimental subject. Together with automatic computer-based online signal analysis, the technique employed enables us to make continuous long-term beat-to-beat measurements of cardiac output in subjects without aortic valve disease or grossly deforming disease of the aortic root.

EMG power spectrum as a measure of muscular fatigue at different levels of contraction.

Abstract: The shift in the power spectrum resulting from a 5-7 min fatigue-inducing effort followed by a 1-2 min recovery period of two elbow flexors, the biceps brachii (BB) and the brachio-radialis (BR), was assessed using two variables, the mean frequency Fm and the median or central frequency Fmd. These two variables were calculated in pre- and post-fatigue conditions and following a brief recovery, at four levels, namely 20, 40, 60 and 80 per cent of maximum voluntary contraction (MVC). These were taken from a ramped isometric effort that is from 0 to 100 per cent MVC. The EMG activity of the two flexors was recorded with bipolar surface electrodes from a group of ten volunteers. Following muscle fatigue, induced with a maintained 60 per cent MVC isometric contraction, a statistically significant (p less than 0.05) shift towards the lower frequencies was observed for both Fm and Fmd for both muscles. Following a brief recovery, a shift towards the pre-fatigue higher frequencies was statistically significant (p less than 0.05). These two synergists responded to muscle fatigue and recovery similarly, as they both demonstrated parallel shifts in power spectrum. The power spectrum is consequently a reliable measure of muscular fatigue. It is also complementary to the net articular moment results.

Adaptive spectral analysis of cutaneous electrogastric signals using autoregressive moving average modelling.

Abstract: The recording of the human, gastric myoelectrical activity, by means of cutaneous electrodes is called electrogastrography (EGG). It provides a noninvasive method of studying electrogastric behaviour. The normal frequency of the gastric signal is about 0·05 Hz. However, sudden changes of its frequency have been observed and are generally considered to be related to gastric motility disorders. Thus, spectral analysis, especially online spectral analysis, can serve as a valuable tool for practical purposes. The paper presents a new method of the adaptive spectral analysis of cutaneous electrogastric signals using autoregressive moving average (ARMA) modelling. It is based on an adaptive ARMA filter and provides both time and frequency information of the signal. Its performance is investigated in comparison with the conventional FFT-based periodogram method. Its properties in tracking time-varying instantaneous frequencies are shown. Its applications to the running spectral analysis of cutaneous electrogastric signals are presented. The proposed adaptive ARMA spectral analysis method is easy to implement and is efficient in computations. The results presented in the paper show that this new method provides a better performance and is very useful for the online monitoring of cutaneous electrogastric signals.

Behaviour space of a stretch reflex model and its implications for the neural control of voluntary movement.

Abstract: A nonlinear model for the stretch reflex has recently been used to study the interactions between voluntary and reflex controls during fast, targeted movements. The present study explores the topography of a ‘behaviour space’ generated by computer simulations of this model under various combinations of values for the gain parameters and time constants in the model's feedback loops. In general, we define a behaviour space to be any set of behavioural characteristics of the simulated movement, such as movement time, peak acceleration or peak velocity. The mathematical model can therefore be viewed as an M×N dimensional map from its parameter space N to a behaviour space M. Here, a one-dimensional behaviour space is explored. This provides a method for quantitatively comparing the different control strategies that might be employed by the nervous system for integrating reflex and descending signals during fast, voluntary movements. The results indicate that an optimal strategy will employ proprioceptive feedback as a means of fine-tuning the braking and clamping activities of fast, goal-directed movements and that descending signals are primarily important for initiating the movement and for controlling reciprocal patterns of muscle activity during the end phase of the movement.

Lactate and glucose electrochemical biosensors for the evaluation of the aerobic and anaerobic threshold in runners.

Abstract: Lactate and glucose are measured in whole blood of athletes running on a treadmill by using two extracorporeal electrochemical biosensors. The lactate sensor was fixed to an endocrine artificial pancreas (Betalike) which had been used in previous extracorporeal experiments. The lactate sensor gave a signal which resulted in a well defined curve that allowed the evaluation of the aerobic as well as the anaerobic threshold. The results obtained with the glucose sensor supported the theory that muscie anaerobic glycolysis is dependent on muscle glycogen rather than on blood glucose.

Neurophysiological and technical considerations for the design of an implantable phrenic nerve stimulator.

Abstract: Sequential stimulation during one muscle contraction of several compartments of a motor nerve, using multiple-electrodes, allows individual nerve-muscle compartments to be stimulated at fairly low frequencies. This provides time for recovery even during muscle contraction. However, the whole muscle is stimulated at near to its optimum fusion frequency, which provides smooth muscle contraction. This stimulation system imitates the natural activation of skeletal muscle. The new phrenic nerve stimulator described utilises the principle of sequential motor nerve stimulation. It also incorporates a sigh function. The sigh current recruits additional axons at certain intervals and thus creates and keeps available a reserve of conditioned muscle. Clinical advantages result: the conditioning phase after the beginning of long-term phrenic nerve stimulation for electroventilation is shortened and muscle fatigue is delayed. A need of increase of gas exchange can be answered by increasing tidal volume instead of respiration rate alone.

Spectral analysis and acoustic transmission of mitral and aortic valve closure sounds in dogs. Part 2. Effects of neuromuscular blockade, sternotomy and pacemaker control, and a two-week recovery period.

Abstract: The paper describes the effects of neuromuscular blockade, sternotomy and atrio-ventricular pacing, and a two-week recovery period on the spectra and acoustic transmission of mitral M1 and aortic A2 sound components in dogs. Results indicate that neuromuscular blockade does not affect the attenuation properties of the heart/thorax acoustic system even if it modifies the intensity of M1 and the phase of the transfer function. The immediate effect of sternotomy and cardiac pacing is an important increase in the attenuation of the heart/thorax acoustic system. This increased attenuation is different for both sounds (20 dB for M1 and 11 dB for A2) and disappears after a two-week recovery period. However, the resulting controlled dog model shows slightly different acoustic characteristics than those of the normal animal model.

Artificial-reflex stimulation for FES-induced standing with minimum quadriceps force

Abstract: A control strategy is proposed to decrease quadriceps activity during standing. Modified on/off (or artificial reflex) control is used: a non-numerical or finite-state control scheme based on feedback of knee angle and angular velocity. The control strategy is evaluated in paraplegic patients in an experimental setup using transcutaneous stimulation. The stability of the system and its sensitivity to various control parameters are determined. It is concluded that the control scheme will enable reduction of muscle force independent of additional mechanical bracing or specific posture, and may result in continuous dynamic activation of muscle.

Pattern recognition in biomagnetic signals by spatio-temporal correlation and application to the localisation of propagating neuronal activity.

Abstract: A combined correlation analysis of temporal and spatial patterns in a biomagnetic multichannel recording is proposed to extract patterns of transient, randomly repetitive physiological events from biomagnetic data sets with a low signal-to-noise ratio. This method is especially sensitive in the differentiation of signals from different source propagation pathways. The recognition and subsequent averaging of spike events in the MEG of an epileptic patient by this method is shown to provide an essential improvement in the signal-to-noise ratio. The source localisation from the averaged data showed electrical activity which propagated from the primary epileptogenic focus to distant parts of the brain.

Role of the calcaneal inclination in the energy storage capacity of the human foot—a biomechanical model

Abstract: The energy storage capacity of the human foot as a passive structure depends on both its geometry and elastic properties. The present study examines theoretically the relationship between the energy stored in the longitudinal arch and the inclination of the calcaneus, when a vertical load is applied. A simple two-dimensional model was used, consisting of two inclined rigid elements, hinged at the apex of the arch and connected by a horizontal tension spring at the bottom of the arch. The spring serves as the energy-storing element of the model, representing the foot plantar ligaments. The behaviour of the model was examined using both a linear and an exponential spring. It demonstrated that the stored energy depends strongly on the calcaneal inclination. For a given vertical load, the energy storage capacity is low at both large and small inclinations and rises markedly at an intermediate value. Assuming that the incidence of stress fractures during locomotion is affected by insufficient energy attenuation of the foot, this model provides an explanation for the higher prevalence of tibial and femoral stress fractures in subjects with high or low arches.

Two-frequency impedance plethysmograph: real and imaginary parts.

Abstract: A four-channel impedance plethysmograph has been designed. Impedance signals are obtained at two frequencies by measuring both real and imaginary parts. Particular attention has been paid to the sine wave generation circuits that provide system versatility. The required phase-sensitive demodulation is achieved by means of analogue multiplexers. Results show that there are significant variations in the thoracic equivalent capacitance related to respiration and that there is an increased sensitivity to cardiac activity at low frequencies.

Results of an international survey of clinical engineering departments. Part 1. Role, functional involvement and recognition.

Abstract: Data collected from clinical engineering departments in Canada, the USA, the European Economic Community and two Nordic countries, Sweden and Finland, have led to an evaluation of their functional involvement in their healthcare institutions and of the level of recognition which they have achieved.

Analysis of excitable cell activation: relative effects of external electrical stimuli

Abstract: In the functional electrical stimulation of nerve an expression defined as the ‘activating function’ has been introduced to evaluate the propensity for a particular fibre to excite. This approach to determine resulting activation is only an approximation as it neglects the presence of the fibres on the applied field, in contrast to activity determined from a rigorous solution to the core conductor/excitable membrane equations. An alternative approach to determining relative excitability based on the induced transmembrane potential is presented, thereby allowing for current redistribution via the space constant of the target fibre. The paper critically examines the approximations made with activating functions, and concludes that as currently formulated the activating function has limitations in predicting relative excitability under a number of important conditions. In contrast, it is the induced (passive) transmembrane potential that provides a quantitatively reliable estimate of the tendency for fibres to excite.

Spectral analysis and acoustic transmission of mitral and aortic valve closure sounds in dogs. Part 1. Modelling the heart/thorax acoustic system.

Abstract: A system model based on the simultaneous recording and analysis of the intracardiac and thoracic phonocardiograms to estimate the time-varying properties of the heart/thorax acoustic system of the dog is described. The presence of instrumental noise in the recording of intracardiac phonocardiograms is characterised, and it is demonstrated that its effect on the estimate of the transfer and coherence functions of the system can be quantified and corrected. Application of the model to study the spectral characteristics and the acoustic transmission properties of the mitral component M1 of the first heart sound and of the aortic component A2 of the second heart sound in the dog shows that the heart/thorax acoustic system acts like a bandpass filter having a higher attenuation for A2 than for M1. Between 20 and 100 Hz, the mean attenuation of M1 is 30 dB while that of A2 is 46 dB. Above 100 Hz, the attenuation slope is -12 dB per octave for M1 and -6 dB per octave for A2.

Effect of compliance mismatch on flow disturbances in a model of an arterial graft replacement.

Abstract: Flow disturbances in a model of an interposition graft in an arterial segment were measured using an ultrasound Doppler velocimeter. The effect of the degree of compliance mismatch between a stiff ‘graft’ and compliant ‘arterial’ segments was investigated. In steady flow, disturbances were detected when the compliance ratio (stiff to compliant segments) was ≤0·1 and the Reynolds number ≥2200. A recirculation zone just downstream of the distal anastomosis was observed at a Reynolds number ≥2400. Disturbances were also measured under pulsatile flow which consisted of a time-varying component superimposed on a steady flow component. The time-varying flow component was either quasiphysiological or sinusoidal in shape. The Reynolds number was 500 but the frequency parameter varied from 4·2 to 8·5. Significant disturbances were observed for conduits with compliance ratio ≤0·19. The disturbance intensity tended to increase as the compliance ratio decreased and the frequency parameter increased. The magnitude of the disturbance was also greater with the quasiphysiological than the sinusoidal input flow waveform.

In vivo chemical sensors for intensive-care monitoring.

Abstract: There is a need for rapid assessment of a patient's biochemical status during intensive care so that therapies may be optimised. Chemical sensors for key species have the potential to allow continuous in vivo monitoring, and some progress is being made with certain sensors. Gases, ions and certain catabolites such as glucose and urea may be measured with devices based on mass spectrometric, electrochemical or optical principles. The physical form, and size of sensors must be matched to the measurement site, which can include the airway, the intravascular space, tissue and the skin surface. Electrochemical sensors for measurement of O2, pH and glucose have been the most widely used to date, although fibre-optic devices are currently attracting considerable interest. Invasive sensors still suffer from the problem of poor biocompatibility, particularly devices used in arteries and veins. Noninvasive methods may be successful in certain circumstances and in some patient groups, but peripheral measurements are often significantly influenced by circulatory phenomena such as shock. Further research is required if these limitations of both invasive and noninvasive sensors are to be overcome and continuous chemical monitoring is to be established as a routine clinical technique.

Development of an eye-movement analyser possessing functions for wireless transmission and autocalibration

Abstract: An eye pointer was developed as a new line-of-sight displacement analyser. It analyses an observer's eye movements and superimposes their line of vision on a TV image which is the same as their view. A newly developed miniature colour field camera and very compact eye-movement sensors are attached to a translucent reinforced plastic goggle affording a wide view and excellent safety. A small case is provided for the eye-movement detecting circuit and the camera controller unit etc. Its weight is only 1.2 kg and one can easily participate in sports activities while wearing it. Eye-movement data are transmitted with the VHF video signal. The receiver, which analyses the eye movements and processes the TV image, is also compact and easy to carry. The calibration is automatic and completed within 1 min with a high degree of accuracy. By showing the tennis player's line of vision as an example, we were able to obtain good results with application in broadcasting programmes.

Results of an international survey of clinical engineering departments. Part 2. Budgets, staffing, resources and financial strategies.

Abstract: Data collected from clinical engineering departments in Canada, the USA, the European Economic Community and two Nordic countries, Sweden and Finland, have led to an evaluation of their budget, staffing and other resource levels. Financial strategies to support their role are proposed.