Showing papers in "Medical & Biological Engineering & Computing in 1973"

The specific resistance of blood at body temperature

Abstract: The specific resistance of human, canine, bovine, and equine blood with a haematocrit range extending from 0 to 70% and at 37°C was measured at 25 kHz. The data obtained were subjected to various curve-fitting procedures; in all cases, the resistivity was found to increase with increasing haematocrit.

Influence of central and peripheral changes on the hydraulic input impedance of the systemic arterial tree

Abstract: The input impedance of the systemic arterial tree of the dog has been computed by Fourier analysis. It was shown that a distance between pressure and flow transducers of less than 2 cm results in appreciable errors which manifest themselves mainly in the phase of the input impedance. The input impedance for controls, occlusions at various locations in the aorta, and an increase and decrease of peripheral resistance were studied. For the same experiments, the total arterial compliance was calculated from the peripheral resistance of the diastolic aortic-pressure curve. The characterstic impedance of the ascending aorta was also estimated. The impedance in the control situation may be modelled by means of a 3-element Windkessel consisting of a peripheral resistance and (total) arterial compliance, together with a resistance equal to the characteristic impedance of the aorta. The occlusions of the aorta show that blockage at (and beyond) the trifurcation do not result in a detectable change in input impedance, except for a slight increase of the peripheral resistance. The more proximal an aortic occlusion, the more effect it has on the pattern of the input impedance. When the aorta is occluded at the diphragm, or higher, the single (uniform) tube appears to be a much better model than the Windkessel. Occlusion of one or both carotid arteries increases the mean pressure; consequently not only the peripheral resistance increases but also the total arterial compliance decreases. The Windkessel with increased peripheral resitance and decreased compliance is again a good model. After a sudden release of occlusion of the aorta, the arterial system has a low peripheral resistance and may also be modelled by the Windkessel.

A stable ultraminiature catheter-tip pressure transducer.

Abstract: THE DYNAMIc-response problems associated with liquidfilled catheters connected to external pressure transducers for cardiovascular studies are well known. Relief from these problems has been sought through various forms of catheter-tip transducers, the most promising of which have been those utilising semiconductor strain gauges. The primary drawbacks of the semiconductor units have been their fragility, limited life and their lack of resistance stability, which precluded long-term in vivo calibration. Recently, in response to needs in this laboratory for the continuous recording of the aortic-pressure pulse wave for the assessment of cardiac output, a semiconductorstrain-gauge catheter-tip pressure transducer has been developed which overcomes these problems. The transducer has such high sensitivity that, connected into a simple d.c. bridge circuit, it can provide stable high-level output without the use of a carrier amplifier. In addition, the transducer is temperature-stable and insensitive to flow.

Physical characteristics and mathematical modelling of the pulsed ultrasonic flowmeter.

Abstract: A pulsed ultrasonic Doppler flowmeter for detailed measurements of velocity profiles in man is described. The device projects a beam of ultrasound in bursts of 0·4 μs duration, at 5 MHz, into the flow; the back-scattered signals are processed to produce a signal corresponding to the mean velocity over a small region of the flowing stream. The size and shape of this ‘sample volume’ determines the flowmeter sensitivity and accuracy. The velocity profile obtained from this instrument can be shown to be a weighted average of the ultrasonic intensity and the flowfield velocity over the sample volume, and is mathematically described by a convolution integral. A method of probing the ultrasonic beam and describing its characteristics in mathematical terms was developed. Using this model with a system whose velocity profile is parabolic, the pulsed ultrasonic Doppler-flowmeter output could be predicted via the convolution integral. Theoretical flowmeter-output curves were generated from the mathematical model by a digital-computer simulation and verified through experimental profiles in steady laminar flow. The modelling technique is sufficiently general for flowmeter output to be predicted for any general flow-velocity profile in steady or pulsetile flow.

Statistics of the myoelectric signal from monopolar and bipolar electrodes

Abstract: With the aid of some simplifying assumptions, a model is proposed that describes certain statistics of the myoelectric signal detected with a bipolar electrode. It is shown that these statistics are determined by the statistics of the monopolar signal and the spacing of the bipolar electrode pair. In particular, the dependence of the bipolar-signal bandwidth on electrode spacing is derived. The experimental results agree reasonably well with those predicted by the model.

Computer control of respiration and anaesthesia.

Abstract: Control of a number of interactive physiological variables by an online computer, operating in real time, is demonstrated. It is shown that a computer-controlled system, by monitoring values of arterial blood pressure, end-tidal CO2 percentage and inspired O2 percentage, can maintain the respiration and anaesthesia of a ventialted animal within close tolerances. Under computer control, it is possible to compensate for random disturbances of the system parameters, and changes in the desired levels of the variables can be dealt with automatically.

Magnetic stimulation of nerve tissue

Abstract: A new principle for stimulating excitable tissue has been investigated in frog nerve-muscle preparations. The nerve was placed in, a high-frequency stationary magnetic field (1 kHz to 1 MHz) and the isometric contraction of the muscle was used as a measure of the stimulatory effect. Comparisons were made with conventional electric stimulation, and studies were made of the effects of duration and frequency within each burst. A strength-duration curve was drawn up for the preparation concerned, using magnetic stimulation. The significance of the airgap geometry on the stimulatory effect was studied. A simple method of generating the magnetic field, by means of which graded contraction can be obtained in the muscle, is also presented. The method may be expected to be of interest in, therapy involving the long-term stimulation of nerves, and in studies of the basic mechanisms of excitable structures.

The use of the electrical-impedance technique for the monitoring of cardiac output and limb bloodflow during anaesthesia.

Abstract: A need has long existed during routine anaesthesia for a convenient, noninvasive method for the monitoring of stroke volume, and hence of changes in cardiac output. If knowledge of the mean arterial blood pressure is available, it becomes possible to monitor changes occurring in the left-ventricular stroke work and the total systemic resistance. Nyboer et al. (1940) proposed the use of thoracic electrical-impedance measurements for this purpose. The system used by us, however, is based on the work of Kubicek et al. (1966). The calculation method used for the stroke volume is to some extent empirical; nonetheless the values obtained compare well with those obtained by the dye-dilution technique in patients with normal hearts and lungs.

Mitral-valve mechanics—stress/strain characteristics of excised leaflets, analysis of its functional mechanics and its medical application

Abstract: The stress/strain characteristics of mitral-valve leaflets are determined. The leaflet elements exhibit the characteristic nonlinear stress/strain behaviour with a transition point; the posttransition-point modulus being orders of magnitude greater than the pretransition modulus. An element closer to the chordae tendineae has an initial modulus 4–5 times that of an element in the centre of the cusp; its transition point also occurs at a lower strain. These tests provide information on the strength characteristics of the mitral-valve leaflets.

Technique for studying the sample volume of ultrasonic Doppler devices

Abstract: The sample volume of continuous and pulsed ultrasonic Doppler velocity detectors can be examined using a small underwater jet. A test probe (line of velocity) is formed by the jet when it is seeded with silicone antifoam emulsion. A novel collection tube downstream from the nozzle prevents the spent scatters from contaminating the test tank. Studies are conducted by pointing the sound beam at the jet and moving the transducer and, in addition, for the pulsed Doppler, the range gate. The amplitude of the detected Doppler signal against range is then plotted. A method for determining the length of the sample volume from these plots is shown.

Thermal method for continuous blood-velocity measurements in large blood vessels, and cardiac-output determination

Abstract: A catheter-type flowmeter for continuous blood-velocity measurements is described. The new device is based on the thermal method: a constant amount of electrically produced heat is partly dissipated by convection into the bloodstream. The resulting equilibrium for each value of blood velocity determines the temperature of the heat-dissipating body. This temperature is accurately measured by means of thermistors. Linearisation of the hyperbolic blood-velocity thermistor-resistance relationship is performed by means of an appropriate antilogarithmic amplifier.

Measuring characteristics of the displacement cardiograph

Abstract: The displacement cardiograph (d.c.g.) is a noncontacting electromagnetic transducer that can be used to measure movement of tissue. In normal operation a sensing probe is placed near the moving tissue, and this produces an output voltage. The d.c.g. detects movement by means of the electromagnetic field it generates. Since this field penetrates into tissue, two sources of output exist: changes in the air-gap distance between the tissue surface and the probe, and movements occurring within the tissue itself. The dominant source of signal proved to be movement of the tissue surface. A quantitative relationship exists between instrument output and surface movement only when the movement is primarily one-dimensional. Where surface movement is small or absent, instrument response is due to movements within the tissue. The significance of these results is discussed in relation to measurement of heart movements in the closed and open chest.

A mathematical model of left-ventricular function

Abstract: A model of the left ventricle is developed which characterises ventricular elastance as a function of end-diastolic volume and time. The heart model is terminated in a modifiedWindkessel load, and accurately simulates the mechanical response of the heart to various various pre- and after-load conditions.

Arterial haemodynamic parameters derived from noninvasively recorded pulsewaves, using parameter estimation

Abstract: A parameter-estimation technique has been developed to derive haemodynamic parameters from pulsewaves, at a number of positions along a peripheral artery. The technique is entirely noninvasive, and uses pulsewaves recorded simultaneously from the brachial and radial arteries, which are fed into an analogue-computer model of the arm arteries. As the pulsewave amplitudes are uncalibrated, a novel technique was developed for comparing the model pulses with the pulsations of the actual artery. A least-squares error criterion was used in preference to an absolute-value criterion. As pressure but no flow waveforms were used, the impedance level, and thus the arterial cross-sections, in the model are, at first sight, undetermined. Linking the Poiseuille resistance of each segment of artery to its blood-mass parameter via the length of the arterial segment nevertheless gives unique per-segment values for radius. Parameters estimated with an acceptable result include the arterial internal radius, arterial compliance and the magnitude of the amplification effect of the pulsewave. Values for peripheral resistance are consistently understimated, owing to the use of an arterial model lacking viscous wall damping. Although the model parameter values are not exact, for various reasons explained in the text, they may still be useful for comparison between normal and abnormal conditions.

Electrical sizing and counting of platelets in whole blood.

Abstract: Hydrodynamic focusing with a jet close to the orifice of a Coulter counter is used as a detector for particle sizing, with a substantial increase in resolution and size range. To maintain these improvements during analogue-digital conversion, zero crossover triggering for sample and hold and a pulse-shape dependent inhibition before address transfer to the memory of a multichannel pulse-height analyser have been performed. Results are given showing size distributions of polystyrene latex particles, fixed red cells and living red cells with platelets from whole blood.

Left ventricular output derived from the time-derivative and phase velocities of the aortic pressure wave.

Abstract: A method of deriving aortic flow from pressures measured in the proximal aorta has been tested. The Womersley equation was used with the pressure gradient substituted by the time derivative of the pressure and the measured phase velocity c′ of each harmonic component. Reflected waves alter the value of c′ and are a source of error when a constant value of c is used, as in previous methods in which dP/dt has been used.

Computer acquisition of multiunit nerve-spike signals.

Abstract: Two methods are described for the computer acquisition of multichannel, multiunit nervespike sequences. Both methods retain the spike peak-amplitude information, so that separation of the component single-unit populations can be carried out by the computer using this information. The first method uses analogue peak tracking to determine the spike amplitude, while in the second method the spike shape is reconstituted from the continuously sampled signal, and the peak amplitude is determined digitally. The performance and implementation requirements of the two systems are compared.

Dry electrodes and holder for electro.oculography

Abstract: The temporal decrease in electrode-subject resistance was measured for dry silver electrodes placed bitemporally to measure the horizontal component of the electro-oculogram. It was found that the electrode-subject resistance decreased exponentially with time. The average time constant for ten determinations was 3·26 min. After about 20 min there was no further reduction in electrode-subject resistance. On average, the ratio of the final to the intial resistance was approximately 0·1.

The Morton-Keele pump—a hydraulically activated pulsatile pump for use in extracorporeal circulation

Abstract: A new pulsatile pump is described, and the results of bench tests and animal experiments are presented. The arterial blood pressure and flow waveforms generated by the pump resemble those produced by the canine heart. The present pump is satisfactory for dog perfusions, and should, with minor modifications, be suitable for use in human surgery.

Simple impedance pneumograph and volume integrator.

Abstract: THE PRINCIPLE of impedance pneumography is to pass a current through the chest between two electrodes, and from the resultant voltage to determine the changes in chest impedance which occur during respiration. It has been proposed that the impedance change occurring in respiration is directly proportional to the change in volume of air contained in the thorax, a n d therefore reflects tidal volume (VALENTINUZZI et aL, 1971). While there is still discussion as to the origin of the impedance change, there is little doubt that the method is a useful means of measuring respiration and causes little discomfort to the patient. The instrument to be described was designed as a monitor of tidal volume in children with abnormal respiratory patterns and adult patients suffering from cor pulmonale. 1 rain volume is derived in addition to tidal volume, as this provides a voltage which can be used to trip an alarm when air intake falls below a set level.

Associated change of transthoracic electrical impedance with regional ventilation of the lung.

Abstract: Based on previous results which suggested that transthoracic electrical-impedance variations correspond to ventilatory air-volume changes within the lung, the capability of the impedance method to detect regional ventilatory changes was tested. Three groups of tetrapolar electrodes were attached to the right-middle and bilateral-diaphragmatic lobe areas on the thoraxes of dogs. Injecting either a single current of 50 kHz or two currents of 50 and 55 kHz simultaneously through these electrodes, impedance changes were measured simultaneously during ventilatory manipulations in either the right-diaphragmatic and mediastinal lobes or the mediastinal lobe. Individual placements of the electrodes detected an independent impedance variation, which corresponded reasonably with the air-volume fluctuation within the right-middle and bilateral-diaphragmatic lobes. No response was observed from the bilateral-diaphragmatic lobe areas on selective manipulation in the mediastinal lobe. From these results, it is clear that the method can detect regional ventilatory variations which occur within the lobe in the immediate vicinity of the electrode.

A megawatt defibrillator for transchest defibrillation of heavy subjects.

Abstract: The design and performance characteristics of a high-energy (5000J) defibrillator are described. The instrument was constructed for the purpose of obtaining strength duration curves for transchest defibrillation of very heavy subjects. To date, the heaviest subject that has been defibrillated is a 270 kg horse.

The use of a compartmental hypothesis for the estimation of cardiac output from dye-dilution curves and the analysis of radiocardiograms

Abstract: Three approximate methods for the computation of cardiac output from dye-dilution curves have been analysed using the classical Stewart-Hamilton technique as a reference in a total of 89 curves obtained from 11 dogs. The gamma-function method has a tendency to overestimate on the average (+7·1%±1·6%), with an 82% probability of falling within ±20% of the Stewart-Hamilton value. The forward-triangle and compartmental methods are approximately equivalent, the first giving a small overestimation of (0·3%±1·2%), and the second a small underestimation of (−3%±1·2%); both methods have a probability of better than 92% of being within ±20% of the Stewart-Hamilton value. The analysis of human radiocardiograms by the compartmental method was encouraging. A number N of equivalent compartments greater than 2·2 for either the right-or left-heart peaks was associated with an abnormal radiocardiogram. In eight normal subjects, N yielded an average of 1·73 for the right heart and 1·69 for the left heart. The compartmental analysis of radiocardiograms, together with a study of other parameters of the curves, would seem to provide a useful assessment of the cardio-vascular function.

Mathematical model for flow in the heated Fleisch pneumotachometer.

Abstract: A simple model is presented for description of flow in the pneumotachometer and its quantitative relationship to observed pressure drop, gas composition and viscosity, gas temperature and case temperature. The equations developed are readily applicable to online computer analysis of respiratory waveforms.

Cannon-bone fracture in the thoroughbred racehorse.

Abstract: The high incidence of lameness in thoroughbred racehorses in California led to a study which indicated a relationship between track hardness and the percentage of occurrence of lameness. One type of lameness is due to a longitudinal fracture of the foreleg cannon bone (third metacarpal). A study was made to obtain the compressive strength of cannon bonein vitro due to single and cyclical loading, and material properties of bone material in the longitudinal and transverse directions in the bone. A theory for the interaction of bone-marrow fluid with the longitudinal compression predicts a transverse tension in the bone, which, coupled with weak transverse properties, explains the longitudinal fractures that are commonly observed in the racehorse.

Analogue preprocessor for the measurement by a digital computer of R-R intervals and R wave widths.

Abstract: The circuitry is presented for an analogue preprocessor to measure R-R intervals and R wave widths from an analogue e.c.g. signal in conjunction with a digital computer. The circuit will handle both positive- and negative-going R waves, and will cope with e.c.g. baseline shifts due to respiration. Examples are given of the use of the preprocessor and its associated computer programs.

A radiotelemetric method for the study of pH and fluoride-ion concentration in dental plaque and saliva.

Abstract: Telemetry has been applied to dental research for ten years in the field of tooth-contact studies. The measurement of specific ion levels in oral fluid and in dental plaque also requires telemetric investigation, but the technical problems involved have resulted in slow development of oral telemetry in this field. A suitable method is described for thein vivo telemetric recording of pH and fluoride-ion concentration from dental plaque. The miniaturised transmitter construction is described and the theory of operation explained, together with the type and fabrication of suitable transducers. The equipment available for reception of the oral signal, together with modifications where necessary, is described, and the method of conversion of the radio signal to a recordable voltage is also explained.

Stimulating electrode with low energy consumption.

Abstract: ATTEMPTS TO EXTEND the lifetime of implantable cardiac pacemakers are presently being focused on the development of power sources with higher energy densities than those of the conventionally used Zn/HgO batteries. In particular, nuclear power sources, alkali-metal batteries and still some 'biogalvanic' cells are under discussion. Little has recently been published, however, on a different and perhaps easier approach, i.e. to reduce the energy consumption of the stimulating electrode. It is known that part of the electrical energy of the stimulating pulse is lost at the electrode (GREATBATCH and CHARDACK, 1968; JARON et al., 1968; SCHALDACH, 1971). Experiments by LEWIN et al. (1967) and PARSONNET et al. (1968) provided evidence showing that it is possible to reduce the energy threshold for stimulating the heart muscle by more than a factor of 100 by using an appropriate electrode geometry. Unfortunately, their electrode has some disadvantages which have prevented its broad clinical application. We are reporting preliminary studies indicating that a modification of the concept of Lewin and Parsonnet may bring some important improvements. The Lewin-Parsonnet electrode consists of a polymer capsule filled with electrolyte and containing a metal electrode having a large surface area (stainless-steel helix). The current pulse passes through a small hole at the tip of the capsule. The purpose of this configuration is to fulfil both the need for a small effective electrode size having regard to the low energy consumption ([RNICH, 1969) and a high local current density delivered to the tissue in Such a way that polarisation losses at the metalelectrolyte interface are minimised. Although very low energy thresholds are obtained with this electrode, it has one intrinsic disadvantage: only a small tissue area surrounding the hole in the capsule is susceptible to excitation, since a sufficiently high current density is only achieved there. This makes it difficult to position the electrode, and results in an increased sensitivity to dislocation. To improve the Lewin-Parsonnet electrode, the tissue area susceptible to excitation should be increased in such a way that the advantages of a small effective electrode size delivering a high local current density and of low polarisation losses are maintained. We have therefore investigated the possibility of obtaining low energy thresholds with a capsule electrode having several holes. A first feasibility test was conducted by simply coating a commercial electrode (ELEMA, type EMT 588, diameter 2.5 mm, length 5 mm) with an isolating polymer layer and drilling three holes of 0\" 6 mm diameter, one in a central and two in a radial direction as indicated in Fig. la. The electrode therefore consists of five active sites each 0' 6 mm in diameter. The area of these sites is small compared with the surface area of the platinum in contact with the electrolyte inside the holes. Thus, the current

The usefulness of a temporal correlation technique in the assessment of human motor performance on a tracking device

Abstract: The usefulness of quantifying a human subject's tracking performance by means of a temporal correlation technique has been investigated using a pursuit tracking device. Three tracking tasks of varying degrees of difficulty were employed in order to test the technique over a range of performances. A crosspower spectral-density analysis of the results has aided in checking the validity of this method of performance assessment. Ten subjects of various ages were investigated over a period of several months. From the results it was felt that this technique provided an ibjective assessment of motor performance that was both meaningful and reproducible.