Showing papers in "Medical & Biological Engineering & Computing in 1985"

Relationships between short-term blood-pressure fluctuations and heart-rate variability in resting subjects. I: A spectral analysis approach.

Abstract: A method to attribute the short-term variability of blood pressure and heart rate of resting subjects to their various causes, using spectral techniques, is presented. Power spectra and cross-spectra are calculated for beat-to-beat values of R-R interval and blood pressure from subjects who were seated in a comfortable chair. Interval values as well as systolic, mean and pulse pressures show variations linked to respiration and to the so-called 10 s rhythm. The diastolic pressure values are scarcely influenced by respiration in the normal respiratory range (0·20–0·35 Hz), but do show 10 s variability. Relationships between pressure and interval variability which indicate that the 10 s variability in systolic pressure leads the interval variation by two to three beats become manifest in cross-spectra; however, no such lag is found between the respiration-linked variations in systolic pressure and intervals. It is argued that the technique presented provides a critical test for models of the fast regulation of the cardiovascular system.

Selective activation of muscles using peripheral nerve electrodes

Abstract: The feasibility of two methods for selectively activating muscles with peripheral nerve electrodes has been investigated. One method for achieving selectivity is to place a cuff electrode around the nerves to each group of synergistic muscles to be stimulated. A second method is to stimulate through pairs of electrodes selected from a multielectrode array placed around a common nerve trunk. Both methods have been tesed in experiments conducted on four dogs. It was shown that the first method, cuff electrodes placed on individual motor branches, is an effective technique for selective activation, Thresholds of motor bibres lying outside of, but adjacent to, cuff electrodes are much greater than the stimulus amplitudes required to maximally stimulate motor fibres contained within the cuff electrode. Good results were obtained with a multielectrode array in two animals, but results, were poor in a third dog. Electrode position and contact with the nerve were found to be important factors in achieving good selectivity.

Relationships between short-term blood-pressure fluctuations and heart-rate variability in resting subjects. II: A simple model.

Abstract: A simple model of the beat-to-beat properties of the cardiovascular system is used to interpret the results of spectral analysis of blood-pressure and interval data. The model consists of two equations, one representing the fast regulation of interval by the systolic pressure (baroreflex), the other one representing a Windkessel approximation of the systemic arterial system. The model, when applied to interval and blood-pressure data from resting subjects, explains the lack of respiratory variability in the diastolic pressure values. The baroreflex equation seems to describe the data only in the region of respiratory frequencies. The shape of the phase spectrum of systolic pressures against intervals is modelled by difference equations, but no physiological interpretation of these equations is given.

Spectrum of a series of point events, generated by the integral pulse frequency modulation model.

Abstract: The paper deals with the relationship between the spectra of the input signal and the output signal of the integral pulse frequency modulation (IPFM) model. The IPFM model is a physiologically attractive device for the conversion of a continuous input signal into an output signal, consisting of a series of events (e.g. nerve spikes, heart beats). Two different spectra are used in the analysis of a series of events; the interval spectrum and the spectrum of counts. The latter spectrum is known analytically for the event series belonging to a sinusoidal input signal. An approximation to the interval spectrum of this series is presented. Using data from a simulated IPFM model, it is shown that, for an input signal consisting of the sum of two sinusoids, terms at sum and difference frequencies appear in the interval spectrum but not in the spectrum of counts. However, the spectrum of counts is contaminated by sidebands of the mean repetition frequency. It is concluded that in general the spectral properties of the input signal cannot be recovered fully from the interval spectrum, nor from the spectrum of counts, the more so as physiological series of events will seldom be generated by an ideal IPFM model.

Fundamental criteria underlying the efficacy and safety of defibrillating current waveforms

Abstract: The efficacy (threshold average current density) and safety factor (overdose current density for a 50 per cent decrease in myocardial contractility) were determined for the rectangular, trapezoidal and damped sine wave defibrillating current waveforms using a new blood-perfused, isolated, working canine heart in an isoresistive and isotonic volume conductor. This preparation permits the achievement of defibrillation with the heart in as uniform a current density field as possible and eliminates myocardial deterioration due to the traditional loss of coronary perfusion during ventricular fibrillation. Since the heart is isolated, it is not subject to any neural influences. It was found that, for a given pulse duration, the threshold average current density (efficacy) values were very similar among the three current waveforms. It was also found that the safety factors for the three current waveforms ranged from 3·2 to 5·3, the conventional 4–6 ms damped sine wave having a safety factor of 5·0±0·9 (1SD).

Sources of the thoracic cardiogenic electrical impedance signal as determined by a model.

Abstract: A three-dimensional electrical model was developed to study the origin of ΔZ due to cardiac activity recorded from band electrodes around the neck and lower thorax. Volume changes were simulated with resistivity changes in the lungs, large arteries, large veins and atria, ventricles, small arteries and veins and the skeletal muscle for a typical 80 ml ventricular stroke volume. The results showed the contributions to ΔZ to be 61 per cent from the lungs, 23 per cent from the large arteries and 13 per cent from the skeletal muscle. The ΔZ signal was most sensitive to skeletal muscle volume change The results indicate that the ΔZ signal has many origins which could cause significant error in calculated cardiac function it all the regions do not change in the normal related pattern.

Tissue stimulation: theoretical considerations and practical applications.

Abstract: The three electrical characteristics of a stimulus (current, charge and energy) are related and can be predicted from the nature of the tissue stimulated. The older empirically derived Weiss-Lapicque concepts of stimulation are compared with those predicted by membrane theory. It is shown that, from the strength/duration curve for charge, it is possible to determine the membrane time constant of the tissue being stimulated. Two practical cases of cardiac muscle stimulation (pacing and defibrillation) were chosen to illustrate that although the goals are different in these two cases, the common denominator is excitation of cardiac muscle and that the membrane time constants so determined are similar to direct-heart and transchest electrode locations.

Mathematical model of the human renal system.

Abstract: A mathematical model is presented which provides an overall description of the renal/body fluid system, comprising an interconnected set of physiologically based representations of the relevant subsystems of the human organism. The model is used to test a number of hypotheses relating to the dynamics and control of the human renal system, including dynamics of ADH clearance, glomerular tubular balance and the control of the rate of release of ADH. Results are presented establishing the validity of the model in a number of empirical tests.

Optimal conditions for the measurement of muscle fibre conduction velocity using surface electrode arrays.

Abstract: Three factors influencing the accuracy of the muscle fibre conduction velocity estimates were studied. They were the recording position, the interelectrode distance and the misalignment of the electrode with respect to the muscle fibres. Experiments were performed on the biceps muscle of a normal subject. Bipolar recordings were made simultaneously from a linear electrode array consisting of 13 contacts. The accuracy of the conduction velocity was evaluated by the cross-correlation method. When the recording site was close to the innervation zone, the peak values of the cross-correlation function decreased and the estimates of the time delays became unreliable. High correlations were obtained when the interelectrode distance was small. Furthermore, the misalignment between electrode axis and muscle fibre axis led to overestimation of muscle fibre conduction velocity, but this effect was less if wider electrodes were used.

Simplified electrode array for impedance cardiography.

Abstract: Impedance cardiography has enjoyed widespread interest owing to the promise of noninvasive monitoring of cardiac function. In clinical practice, one factor limiting its use has been the need to use circumferential electrodes. These electrodes can be inconvenient to apply, may interfere with chest tubes or intravenous lines, and can cause additional apprehension in critically ill patients. The paper describes a more convenient electrode array. It uses four disposable electrocardiographic electrodes, two on the base of the neck and two on the lower left, anterolateral surface of the thorax. Simultaneous impedance derivative recordings were made with this ‘spot’ electrode array and with the conventional ‘band’ electrode array, at rest and after exercise on ten normal volunteers. On a given individual, the two signals had shapes which were remarkably similar and relative heights which correlated well, exhibiting correlation coefficients (r′) between 0·77 and 0·98. This new array should facilitate impedance cardiographic measurements in the clinical setting.

Evaluation of the fan method of adaptive sampling on human electrocardiograms

Abstract: The fan is a method of adaptive sampling that selects samples from electrocardiograms more rapidly during periods of rapid waveform change and more slowly otherwise. One attribute of the fan is the guarantee that the original waveform can be reconstructed within tolerance e. Many questions about the particulars of the fan's performance on human ECGs have been undocumented, e.g. what e choice leads to good quality recording, how does the choice of e affect visual quality, and what average sampling rates occur? The paper provides answers to these and other questions. It is based on retrospective analysis of 20700 human ECG waveforms from subjects of all ages. The results show, for example, that e=10μV leads to high quality waveforms sampled at an average rate of 266 samples s−1 with maximum errors only 1/24th the maximum errors using uniform sampling at 250 samples s−1, and that e=30μV leads to waveforms showing all deflections at an average rate of 45 samples s−1 with maximum errors only 1/57th of the maximum errors from uniform sampling at 45 samples s−1.

'Objective' algorithm for maximum frequency estimation in Doppler spectral analysers.

Abstract: Real-time spectral analysis is often used to detect the maximum frequency envelope of Doppler signals, and thus the so-called spectral broadening, which is claimed to be a sensitive indicator of arterial stenosis. However, a rational criterion for the estimation of maximum frequencies is lacking. In the paper an ‘objective’ algorithm which takes account of the specificity and the sensitivity of the procedure of maximum frequency detection is proposed. This algorithm is based on the statistical characteristics of FFT spectral estimators, and allows thresholds to be set to be used in two-step decision procedures. The proposed algorithm can be easily implemented on microcomputers and/or commercial spectral analysers. The results obtained are fairly independent of the operator’s subjective judgement and spectral analyser gain.

Effect of skin temperature on vibrotactile sensitivity

Abstract: The vibration problems relating to living bodies have so far been studied from the perspectives of engineering physiology and psychology. This study shows the relationship between vibratory sensibility and temperature in the living body. Psychological experiments were carried out by using the vibrometer of an acoustic calibration apparatus in sine, triangular and square waves. The sensibility-threshold measurements were made using 30–700 Hz sine waves, 30–300 Hz triangular and sawtooth waves, or 30–250 Hz square waves. Each of ten subjects was kept seated. The average value of the vibratory levels, varied by ascending and descending steps, was taken as that of the threshold. As the vibrometer in the apparatus used makes a noise at frequencies greater than 250 Hz it was masked from the subject by presenting him with a different noise. The threshold curve for square waves is lower by 12·3 dB than that for sine waves at about 30Hz. The threshold curve of the 26°C sine wave was lower by 10 dB than that of the 58°C sine wave vibration near 200 Hz. For example with a sine wave, at 58°C the amplitude threshold was lowest at about 270 Hz, but at −11°C at about 200 Hz. At frequency stimulation higher than 120 Hz, as the temperature of the contact point was lowered, the amplitude threshold increased and the frequency at which the threshold curve was at a minimum shifted to a lower frequency.

Quantitative weightbearing and gait evaluation of paraplegics using functional electrical stimulation

Abstract: In this study functional electrical stimulation (FES) was used to generate supported gait in paraplegic patients with traumatic upper-motor-neuron lesions, between the T5 and T12 spinal levels. Four patients have so far been treated and studied over a period of one year. The quadriceps and gluteus maximus muscles are stimulated simultaneously to achieve a supported standing position, while hip, knee and ankle flexions are achieved, alternately for each leg by stimulating the shank surface at two selected locations (flexion reflex). The stimulus used has an intensity of 120V, duration of 0·3 ms and frequency of 24 Hz. The standing and walking behaviour of patients was monitored in the gait laboratory of the Loewenstein Hospital. The amount of weightbearing on each foot during standing was established by time integration, over a standard period of time, of the reaction forces, as measured by ‘Kistler’ force platforms, on which the patient was required to stand, while taking care to support his walking aids outside the platform area. The gait of the patients was evaluated by means of an electrical contact system in which time/distance parameters of the stride were measured. Some typical results indicate a walking velocity of 10 cm s−1 and weightbearing on the feet during standing of 80 per cent of body weight. Computer processing of the data acquired was used to obtain objective evaluation of the patients' progress during their training period towards easier and more independent mobility.

Linear and nonlinear properties of platinum electrode polarisation III: Equivalence of frequency- and time-domain behaviour.

Abstract: The polarisation properties of a platinum electrode in physiological saline (0·9 per cent NaCl) were studied in the linear and nonlinear range both in the frequency and time domains (parts 1 and 2). A model which states the limits of linear behaviour in the time domain is presented, its validity tested against experimental data and its equivalence with the analogous expression in the frequency domain demonstrated.

Comparison of methods for adaptive sampling of cardiac electrograms and electrocardiograms.

Abstract: Digital sampling of cardiac electrograms and electrocardiograms is usually performed by sampling at uniform intervals with rates high enough to record the fastest signal components. Numerous redundant samples are recorded during slower deflections and baseline intervals, particularly for direct cardiac measurements that include fast Purkinje deflections. In this report, five adaptive sampling methods (voltage triggered, two-point projection, second differences, the fan and CORTES) are compared with uniform sampling for cardiac waveforms. For the electrogram, the results indicated that adaptive sampling based on the fan method might be used effectively to limit average data rates to moderate values during original data acquisition

Computerised method for pattern recognition of intestinal motility: functional significance of the spread of contractions.

Abstract: The intestinal motility consists of local and propagative contractions. A system for determining the length of spread of intestinal contractions has been developed. The technique used for waveform recognition was based on the principle of peak detection. A computer program analysed the length of spread of contractile waves, their mean contractile force and their mean propagation velocity. The computer identified each waveform on the basis of pre-established threshold and timing criteria. A technique for fitting theoretical equations to experimental data (measured transit rate of ingesta against evaluated parameters) was used to prove the functional significance of the contractile patterns. The method provides a more detailed representation of contractile patterns than other studies based on the conventional methods of analysis.

Selectivity of electromyographic recording techniques: simulation study

Abstract: A modified line source model was used to simulate single muscle fibre action potentials and their radial decline as measured using a single fibre (SF), concentric needle (CN) and macroelectromyography (EMG) electrode. The 90 per cent attenuation distance, defined as the distance from the fibre at which the peak-to-peak amplitude of the action potential declines to 10 per cent of its value measured near the fibre, was determined. For the CN and macro electrode this distance was 1·4 and 4·1 times, respectively, the distance measured for the SF electrode Based on the decline characteristics obtained from our simulations and experimental data reported in the literature, we inferred that in a normal human biceps muscle the spike component of CN MUAPs is produced mainly by the action potentials of the closest one-six muscle fibres. This inference was used to interpret the changes in MUAP amplitude seen by clinical EMG in neuromuscular diseases. The decline of action potential amplitude with distance from the electrode was least when recordings were made by the macro-EMG electrode. The three electrodes differed in their uptake area and thus recordings made with them provided complementary information about the motor unit architecture and the way it changes in neuromuscular diseases.

Online patient-monitoring system for the simultaneous analysis of blood K+, Ca2+, Na+ and pH using a quadruple-function ChemFET integrated-circuit sensor.

Abstract: A system for the online, simultaneous analysis of bloodK +,Ca 2+,Na + andpH using a quadruple-function ChemFET integrated circuit sensor is described. The ChemFET device is mounted into a miniature flow-through cell having a very small sample volume (ca. 30 μl), which is incorporated into a sensing unit together with a pinch-tube sampling valve, a heat exchanger and a reference electrode. The sensing unit is small and robust, with an overall input deadspace small enough (ca 460 μl) to permit connection directly to an indwelling, dual-lumen cannula for continual, intermittent sampling without causing undue exanguination; the cannula is actively heparinised to prevent clot formation in the aspirated blood samples. An analyte-chopping analysis (ACA) system is used to prevent protein contamination of the ChemFET sensor, and to ensure the high precision of measurement which is necessary for clinical applications over extended periods. The system is controlled by a microcomputer, which supervises preusage calibration, logs data, and provides graphical displays of ion concentrations. Preliminary clinical results are presented which reveal significant dynamic changes inCa 2+ due to the intraoperative transfusion of citrated blood, and substantial fluctuations in the concentration profiles ofK +,Ca 2+ andpH during cardiopulmonary bypass.

Monitoring the mobility of patients in bed.

Abstract: A method of detecting and logging movements made on King's Fund hospital beds is described. The results of such mobility measurements indicate that during the first three nights after admission there is a progressive alteration in the number of movements made by elderly hospital patients. The group of patients clinically assessed as being at risk of developing pressure sores, or receiving sedatives, made a reduced number of movements.

Noninvasive measurement of beat-to-beat vascular viscoelastic properties in human fingers and forearms.

Abstract: Static and dynamic (beat-to-beat) arterial viscoelastic properties were measured noninvasively in human fingers and forearms. The volume change and pressure change were detected simultaneously by admittance plethysmography and by sphygmomanometry using the vascular unloading technique in the index and middle finger, or vice versa. These simultaneous measurements were also made on the forearms of one side and the fingers of the other side. The arterial viscoelastic properties in these segments could be successfully determined at different transmural pressure levels from 0 mm Hg to around the mean arterial pressure.

Long-term ambulatory monitoring of indirect arterial blood pressure using a volume-oscillometric method.

Abstract: A new portable instrument equipped with a microprocessor was designed for the long-term ambulatory monitoring of indirect arterial pressure in the human finger at desired intervals using a volume-oscillometric technique. All the necessary procedures such as (1) programmed control of cuff pressure, (2) detection of the systolic end-point and the point of maximum amplitude of arterial volume pulsations, (3) reading of the cuff pressures corresponding to these two points, (4) its processing and (5) recording of the systolic and mean pressure together with heart rate on a digital memory integrated circuit were performed automatically. After the monitoring, the data were reproduced and analysed by a conventional personal computer. Simultaneous comparison of the data with direct measurement, operation and evaluation of this instrument, and ambulatory monitoring were carried out. With this instrument noninvasive and accurate monitoring of arterial pressure could be made in unrestricted subjects during daily activities.

A model of extracellular waveshape of the gastric electrical activity.

Abstract: A new mathematical model to simulate the extracellular ECA (electrical control activity) of the human stomach by computer is proposed in the paper. In this model the ECA is generated by the electrical potential produced by an annular band polarised by electric dipoles and moving distally from the mid-corpus to the terminal antrum. The model is consistent with the function of the stomach and reproduces all the morphological characteristics (waveform, amplitude, duration, frequency) of the gastric ECA as obtained by experimentsin vivo. It is easy to simulate by computer.

Versatile microcomputer-based system for the capture, storage and processing of spectrum-analysed Doppler ultrasound blood flow signals.

Abstract: The evaluation of any method of analysis of Doppler ultrasound blood flow signals is involved and time consuming because of the considerable time necessary to investigate a statistically significant representative population of arteriopathic blood flow waveforms. To overcome these problems we have developed a microcomputer-based system for the capture, storage and processing of spectrum-analysed Doppler ultrasound blood flow signals. This system allows the collection and storage on floppy disk of waveforms from many sites in a large population of arteriopaths and their later analysis using any desired method. Having thus created on disk a suitable population of arteriopathic waveforms the evaluation of any method of waveform analysis, whether existing or new, is a much more convenient and far less time-consuming process. The system described is extremely versatile, for example in addition to the collection of data for postprocessing the system is also used for the real-time analysis of blood flow waveforms.

Theoretical assessment of the effect of flow maldistributions on the mass transfer efficiency of artificial organs

Abstract: A theoretical formula for assessing numerically the extraction ratio of any lineal transport device, applicable in the presence of any type of parallel flow maldistributions, is presented Two examples of maldistribution, continuous and discrete, are described and calculations are made of their effect on extraction ratio for two general cases and a range of parameters of interest in haemodialysis Under favourable circumstances surprisingly large amounts of maldistribution may be tolerated

Assessment and optimisation of dip-coating procedure for the preparation of electroenzymic glucose transducers.

Abstract: The details of construction and the performance characteristics of a dipcoated electroenzymic glucose transducer comprising an H2O2 electrode coated with a layer of glucose oxidase encapsulated in cellulose acetate and overlaid with a layer of polyurethane are presented. The steady-state current increases when the glucose oxidase and cellulose acetate concentrations of the dip-coating solutions are increased, but high cellulose acetate concentrations yield thick and mechanically unstable membranes. A compromise between current yield and mechanical stability can, however, be achieved by employing glucose oxidase and cellulose acetate concentrations of 200 mg ml−1 and 2–5 g 100 ml−1, respectively. A polyurethane concentration of 6 g 100ml−1 is optimal both in terms of the current yield and the linearity of response. The relationship between steady-state current and D-glucose concentration is approximately linear over the concentration range 0·5 to 11·5 mm, and if correction is made for deviations from linearity at higher glucose concentrations, concentrations in excess of 20 mM can readily be quantified. The steady-state current is pH and temperature dependent, but the dependencies are relatively small in the physiological range. The mean rate of decrease of the glucose current during long-term operation of the optimised transducer is 0·83 per cent of the initial current per hour at 37°C. The hydrated electrodes perform satisfactorily after storage for more than two weeks at room temperature. the transducers have a mean response time [t 90%] of 50 s or less.

Simulation of fibre density in single-fibre electromyography and its relationship to macro-EMG

Abstract: To elucidate the relationship between various morphological motor unit parameters and electrophysiological findings obtained with single-fibre EMG (fibre density, FD) and macro-EMG, computer simulations were performed. FD and macro-MUP amplitude varied in parallel when fibre loss, fibre loss and corresponding motor unit territory shrinkage or fibre atrophy were simulated. When fibre loss and compensating reinnervation were produced the FD increased whereas the amplitude of the macro-MUP was unchanged. A discrepancy between the relative changes in these two EMG parameters seems to indicate uneven fibre distribution within the motor unit territory.

Impulse propagation in rubber-tube analogues of arterial stenoses and aneurysms.

Abstract: Analogues of arterial stenoses and aneurysms were constructed from latex tubing containing inserts of various lengths, and with diameters and elastic properties that differed from those of the surrounding tube. A pressure impulse (duration <10ms). was generated at one end of the tube and its transmission and reflection were monitored at various points within the system using a catheter-tip manometer. The complex waveforms produced by multiple reflections from either end of the insert were analysed and compared with those generated by a numerical model in which the reflection sites were regarded as isolated junctions between two tubes of infinite length. There was close agreement between the synthesised and measured waveforms.