Showing papers in "Medical & Biological Engineering & Computing in 1979"

Noninvasive automatic determination of mean arterial pressure

Abstract: A new instrument for the indirect noninvasive measurement of mean arterial pressure (m.a.p.) has been constructed and evaluated in man. The instrument does not require an external microphone or transducer and determines m.a.p. rather than systolic and diastolic pressure. Instead, the method employs the point of maximal oscillations as an indicator of m.a.p. The instrument automatically inflates a standard blood pressure cuff and determines the m.a.p. by measuring the cuff pressure oscillations as the cuff pressure is reduced by discrete increments. Cuff deflation in discrete increments, instead of continuously, allows the oscillation data obtained at each cuff pressure to be tested for artefacts and averaged, greatly enhancing artefact-rejection ability. The m.a.p. is selected as the lowest cuff pressure at which the oscillation amplitude is a maximum. The instrument was tested on the bicep and ankle in a series of 28 studies involving 17 human subjects with intra-arterial catheters. Averaging the mean errors from each of the 28 studies, there was an overall mean error of — 0·23 mmHg, with a standard deviation of 4·21 mmHg. The correlation coefficient was 0·98. The instrument was found to give good results in a wide variety of clinical subjects and physiologic states.

A myoelectrically-controlled prosthesis with sensory feedback

Abstract: An electrically-operated hand, controlled by myopotentials, has been fitted with strain gauges in the index finger which measure the gripping force between thumb and index finger. These strain gauges cause an electrical stimulus to be applied to the skin directly above the median nerve. This stimulus consists of a series of pulses with the pulse repetition rate increasing as the pinch force increases. Patients fitted with prostheses incorporating this feedback report an improved level of confidence when using the prosthesis.

Intramural resistivity of cardiac tissue.

Abstract: The macroscopic resistivity of canine ventricular tissue has been measuredin vivo. Using a tetrapolar intramural electrode, values have been obtained for transmural resistivity as a function of depth, phase of the cardiac cycle and of the frequency of the applied current. The time course of resistivity owing to induced local ischaemia has been recorded. Using epicardial electrodes, the amount of anisotropy of the macroscopic resistivity has been estimated. The results obtained put the transmural resistivity at about 410 Ωcm and show it to be independent of the cardiac phase, independent of frequency and independent of depth. Induced local ischaemia was found to be followed by a substantial increase in the resistivity with time. The amount of anisotropy observed epicardially shows the ratio of transverse to longitudinal resistivity to be of the order of 2. These results are discussed in relation to a cylindrical model of the underlying fibre structure.

Sequential motor unit stimulation through peripheral motor nerves in the cat.

Abstract: An electrode array is described that can be used to stimulate skeletal muscle through the peripheral motor nerves. This electrode array differs from the sleeve electrode that is used conventionally in that the motor units are stimulated sequentially, a mode of stimulation which closely mimics the normal asynchronous firing pattern of motor units that occurs during voluntary activity. Using this stimulation technique, tetanic contractions can be developed by muscle at low stimulation voltages and within physiological stimulation frequencies. Furthermore, unlike synchronous electrical stimulation, during sequential stimulation, submaximal tensions can be maintained for sustained periods of time. The applications of this electrode array to rehabilitation medicine are discussed

Problems of impedance cardiography.

Abstract: Various electrical-impedance methods have been proposed for the measurement of haemodynamic parameters noninvasively. For example, an impedance method is proposed for the measurement of cardiac output from the skin surface and is called impedance cardiography. However, there are many problems in the application of the impedance technique for the noninvasive measurement of haemodynamic parameters. One of them is the complicated structure of organs in the body. Another is the anomalous electrical properties of tissues. For these reasons, it is very difficult to obtain exact information about haemodynamics from the waveforms measured by the impedance cardiograph, in spite of the fact that the waveforms include much useful information. We can get useful information, if the relationships between the waveforms and the biomedical events in the subject are understood. In this paper, the relationships between the waveforms and the circulation of the blood are discussed theoretically and experimentally. The effects of the complicated structure of the organs and the anomalous electrical properties of tissues upon the waveforms are also discussed both theoretically and experimentally. For the analysis of the waveforms, it is very important to discuss (a) equipotential lines and potential distribution on the thorax, (b) the impedance or admittance between a pair of electrodes, (c) impedance or admittance changes due to blood circulation, (d) waveforms due to the pulsatile blood flow and (e) the effects of the complicated structure of organs upon the waveforms. From our results, it is concluded that: (a) the waveforms measured by an impedance cardiograph are affected not only by the change of blood volume in the aorta but also by that in various organs such as the heart and lungs. (b) the impedances measured by a pair of small electrodes separated by a short distance give useful information about the region in the vicinity of the electrodes (c) much valuable information will be available, if the relationships between the measurement results and the pulsatile blood flow are understood, and (d) by choosing an appropriate electrode arrangement, we may get much useful information about the blood circulation in the organs we want to investigate

In vivo dielectric spectrometer

Abstract: An instrument is described for measuring the relative permittivity and dielectric loss of body tissues and organsin vivo, over the frequency range 0·1 Hz to about 100 kHz. A voltage transient is applied by means of suitable electrodes and the resulting waveform is sampled, digitised and Fourier transformed using a microprocessor to give a spectrum in the frequency domain. The results show differences in relative permittivity and dielectric loss between different types of tissue. Much of the preliminary clinical work with this apparatus has been done using measurements on breast tumours.

Active mechanical properties of the smooth muscle of the urinary bladder

Abstract: Strips of pig bladder have been maximally stimulatedin vitro at 37°C via electrodes placed in the muscle, in order, particularly, to measure the dependence of the resulting active force on the velocity of shortening and on length changes. The active isometric force and the passive viscoelastic force are approximately, but not precisely, additive. The active isometric force, like the steady (equilibrium) passive force, is a function of the extension of the strip above its rest length, which is increased after subjection to a high passive force. The steady passive force increases quasiexponentially with this extension, of which it is therefore a measure. The active isometric force Fiso increases approximately linearly with the extension until it approaches a maximum in the region where it and the steady passive force are comparable in size. The maximum is partly obscured by rest-length changes. The dependence of the active force F on the speed of shortening of the strip has been measured in a new way, with a correction for passive viscoelastic effects. For a given strip the ratio F/Fiso is, approximately, a function of the contraction velocity only. The function is similar to that of the classical Hill equation but not identical, possibly for geometrical reasons. The results imply that a velocity parameter v*, analogous to Hill’s parameter b, is approximately constant for each strip, independent of changes of length and rest length.

Hybrid system for fast data reduction of long-term blood-pressure recordings.

Abstract: Fast analysis of long-term intra-arterial blood-pressure recordings can be approached in various ways. We describe a low cost system, consisting of a hybrid preprocessor and a minicomputer, which can either perform fast data reduction, producing simple statistical information to aid patient management, or be used as the basis of a more comprehensive signal processing facility for research purposes. The preprocessor saves computer time by detecting and measuring the systolic and diastolic points, thereby allowing recordings to be processed 25 times faster than they were recorded. Attention has been given to the special problems of calibration and artefact rejection. Data conversion accuracy was found to be within 2% on a typical recording. The system can run automatically if required to ease the task of routine blood-pressure tape processing.

Pasteless electrode for clinical use

Abstract: The design and construction of a clinically useful electrode for detecting the myoelectric signal is described. The requirements of the design were determined by a set of properties defined by clinicians and research engineers experienced in myoelectric signal detection. A novel, and seemingly useful, feature of this electrode is that the stainless-steel cans of two junction field-effect transistors are used as the recording surface while the same j.f.e.t.s. operate as the front end of a differential preamplifier. The electrode requires no paste or gel. In addition to its use as a stationary unit, it may be used as a probe to quickly and conveniently explore the myoelectric signals from many muscles.

Constant-velocity contractions in skeletal muscle by sequential stimulation of muscle efferents.

Abstract: Experiments are described that quantify the relationship between the contraction velocity, the recruitment of motor units and the electrical activity in the medial gastrocnemius muscle of the cat. The results of these experiments showed that, with differing degrees of activation, the force/velocity relationship closely followed that described by Hill. The electrical activity in the muscle was directly related to recruitment and was also linearly related to the load at any constant velocity of contraction. With progressively higher velocities of contraction, however, for any given load, the electrical activity was also increased. From the above database a computer program and the associated hardware were developed to stimulate muscle and cause it to maintain constant velocity contractions irrespective of the load imposed on the muscle. The applications of this type of stimulation to the restoration of mobility to the paralysed are discussed.

Function of implanted cathodes in electrode-induced bone growth.

Abstract: The application of microampere direct current (d.c.) to electrodes implanted in bone results in a proliferative response that has been found useful in the treatment of certain types of fracture. The mechanism of this response is unknown. In this investigation, the d.c. polarisation properties of 11 implantable cathode materials were measured,in vitro andin vivo, and then related to the conditions known to produce the osteogenic response. From these data, limiting parameters for clinical use of the various electrodes can be determined. Nitrogen purge experiments showed the dependence of the current on molecular oxygen and suggested that the products of oxygen reduction (H2O2, O2 −, OH·, OH− etc.) may mediate the proliferation of bone by appropriate cells in the region of the electrode. The presence of these intermediates, some of which are free radicals, and their possible side effects needs further study.

Accuracy of QRS detection in relation to the analysis of high-frequency components in the electrocardiogram.

Abstract: The detection of high-frequency components in the QRS complex by means of coherent signal averaging is affected by inaccuracy in the time reference. Jitter of the time reference or trigger, which is derived from the low-frequency QRS complex, will be caused by the noise in the QRS complex. A theory is developed by which the trigger jitter can be predicted from the properties of signal and noise for a system consisting of a filter and a single-level, dual-level or peak detector. The theory is applicable when the noise is additive and, under certain conditions, also when the noise is multiplicative. Using this theory the trigger jitter of a given filter-level detection system is compared with that of an optimal detection system consisting of a matched filter and peak detector. The theoretical trigger jitter of the above-mentioned detectors has been computer for e.c.g. recordings of 23 individuals, as a function of different filter settings and and with the assumption that the noise was additive. This resulted in an average trigger jitter of 0·2±0·1 ms for the optimal system, while for the peak detector and the dual level detector the jitter was slightly higher provided that the QRS complexes were symmetrical after filtering. With the effects of ventilation taken into account (multiplicative noise) it is shown that dual-level detection is considerably more accurate than single-level detection. A description of the dual-level detector is also presented.

Flexible printed-circuit probe for electrophysiology.

Abstract: Photofabrication techniques have been used to produce a nickel-iron microelectrode array on Kapton film specifically designed for biological implantation. The probe is 2·5 mm×2 mm and carries four tissue terminals, each 2 μm in width. Both spontaneous and evoked potentials have been recorded from frog sciatic nerve. Developmental possibilities for the probe are fully discussed.

Energy absorbing ability of articular cartilage during impact

Abstract: Forty-eight samples of 9 mm diameter articular cartilage and associated subchondral bone from, the tibial plateau of human knee joints were mounted in methylmethacrylate cement and subjected to controlled impact velocities. The controlled velocities provided for the testing of 21 samples at a strain rate of 500 s−1 and 27 samples at a strain rate of 1000 s−1. Data are presented on the energy absorption per unit volume of cartilage in relation to the associated stresses and strains. The data in each of the resultant nine graphs are subjected to a least squares fit to the polynomial $$Y = a_1 X + a_2 X^2 $$ The relevant statistical data for each of the strain rates and for the combined data are presented. While the total energy absorption of the articular cartilage of the knee joint at ambulatory stresses of 0·8 to 6·3 N/mm2 is calculated to be no more than 0·13 to 3·65 J, it is suggested that the compliance of the cartilage of the knee joint together with the incongruity of the tibia and femur are essential features that should be considered in the design of total knee-replacement prostheses.

Urine flow curves of healthy-males - mathematical-model of bladder and urethral function during micturition

Abstract: Previously isolated clinical and experimental observations on bladder and urethra are drawn together into a mathematical model of micturition. The urethra is treated as passively distensible, while the course of the flow rate is determined by the changing geometry of the actively contracting bladder. Numerical solution of the resulting equations, using independent clinical estimates of the majority of the parameters, shows that they can account for the observed features of the urine flow rate curves of healthy males. It appears that the series elasticity of the bladder muscle is important in maintaining a high flow rate down to low bladder volumes. A mechanical explanation of the peculiar character of micturitions from high initial bladder volumes is put forward. The bases of two clinical methods of assessing bladder function are clarified.

Evaluation of a constant-temperature hot-wire anemometer for respiratory-gas-flow measurements.

Abstract: The performance of a constant-temperature hot-wire anemometer for respiratory-gas-flow measurement has bee investigated. It is shown that the anemometer satisfies common demands with respect to accuracy, response time and to insensitivity to humidity and temperature variations. A calibration method is suggested that permits correction for variation of the flowmeter signal caused by changes of the gas composition. Finally, a procedure for linearisation of the flowmeter signal as a function of flow rate is presented.

Dynamic system for the measurement of electrical skin impedance.

Abstract: MEASUREMENT of the skin impedance is difficult because of its time dependency, random variation, nonreproducibility and nonlinearity. Taking these characteristics into consideration, the authors designed a dynamic system which employs a time-multiplex scheme and utilises a synchronous rectifier to measure the resistive and reactive components of the skin impedance. Since the system allows continuous impedance recordings for one point on the skin simultaneously at N different frequencies, it is possible to obtain rapidly the frequency characteristics of time varying impedance. Since the system allows continuous impedance recordings simultaneously for N points at one frequency, it is also possible to compare the impedances at different points with each other. This system can be realised in the form of a very lowcost device.

Axial mechanical properties of arterial walls and their anisotropy

Abstract: The axial deformation behaviour of arterial walls and their anisotropy were studied experimentally using abdominal aortas, common carotid arteries and femoral arteries obtained from mongrel dogs. These tubular specimens were stretched in the axial direction keeping the internal pressure at various levels. Main results obtained were: the strain rate dependency of axial mechanical behaviour is not observed in the range of 3×10−3 to 3×10−1 per second; mechanical properties of arteries in the axial direction are dependent on the internal pressure applied; in the lower stress range, arterial walls are more extensible in the circumferential direction than in the axial direction, and an apposite trend occurs in the higher stress range; mechanical properties of arterial walls in the axial direction are expressed by the constitutive equations that we proposed in a previous paper.

Laser-Doppler anemometer to study velocity fields in the vicinity of prosthetic heart valves.

Abstract: Laser-Doppler anemometry is relatively new technique which is used for measuring velocity fields. It has major applications in the field ofin vitro biofluid mechanics. The laser-Doppler anemometers have many advantages compared with the traditional hot-wire or hot-film anemometers which are still mainly used in studies of biofluid mechanics. A laser-Doppler anemometer (I.d.a) system which can be used to measurein vitro velocity and shear-stress profiles in the vicinity of prosthetic heart valves is described. Accurate velocity measurements in the vicinity of prosthetic heart valves are very scarce, and the use of I.d.a systems will facilitate acquisition of these data.

Clinical aspects of the stimulation of bone healing using electrical phenomena.

Abstract: A short overview of research, carried out before clinical trials, is given, and is followed by brief notes on some clinical aspects of various systems. A description of a method using an iron-cored electromagnet is included, along with a summary of a short series of case histories. The four techniques employed in clinical work so far are then compared in terms of their advantages and disadvantages in usage.

Electrical properties of glassy-carbon electrodes

Abstract: Some electrical properties of the glassy-carbon electrode are presented. The electrode impedance is inversely proportional to the measurement frequency. The glassy-carbon electrode has pH sensitivity. The polarisation characteristic is given. To analyse these results, it is better that the adsorption of ions on the surface of the electrode is taken into consideration. This is inferred from XPS spectra.

Ventricular defibrillation thresholds with capacitor discharge.

Abstract: By means of a selfadjusting experimental protocol based on the evaluation of the cardiac impedance, peak current thresholds were determined to defibrillate canine hearts with transventricular capacitor discharges. From 346 successful defibrillations performed on 20 mongrel dogs, an average value of 0·82 A/kg (s.d.=0·3) was found. The energy showed considerably more variability (mean of 0·22 J/kg, s.d.=0·156), suggesting that it is not the most adequate electrical parameter to describe the dose for defibrillation. From these results, a recommended dose equal to the average value plus two standard deviations (i.e. 1·42 A/kg) would appear as acceptable at an essentially 100% success level, assuming it is the first trial and that no other deterioration exists. The procedure applied in these determinations made use of a constant current 12 kHz impedance meter to estimate the load imposed on the defibrillator by the fibrillating heart. The overall average value was 25·6 Ω (s.d.=4·9) ranging from 17 to 44 Ω. The average time constant RC of the exponential discharge was 1·63 ms (s.d.=0·31) using in all cases a 63 μF capacitor. There is, of course, an error in taking the modulus of Z as equal to R and also due to the low current used in the measurements, as opposed to the high current that flows during actual defibrillation. However, it does not seem to be significant from a practical point of view. Defibrillators with a built-in Z meter and a microprocessor to automatically set the value of the discharge are, as a consequence, quite conceivable. The results given here confirm, and, in a sense, improve on those already reported in the literature.

Flat crossed-coil detector for blood flow measurement using nuclear magnetic resonance.

Abstract: A new flat crossed-coil nuclear magnetic resonance (n.m.r.) detector can be used to measure flood flow at many regions of the body, is introduced. It is compared, bothin vitro andin vivo, with the already proven cylindrical crossed-coil n.m.r. detector. Both detectors use a single high-field homogenous magnet. A single-sideband receiver reduces the effect of transmitter leakage into the receiver coil to allow the use of a high-level transmitter field. Preliminary application of the flat crossed-coil detector to the detection of cerebral blood flow is presented.

The calculation of forward and backward waves in the arterial system.

Abstract: To STUDY wave reflections in the arterial system, it is necessary to separate the forward and reflected components from the measured pressure and flow time histories. WESTERHOF et al. (1972) have given a method for constructing these forward and reflected waves from the Fourier analysis of the measured pressure and flow signals in the ascending aorta. A less elaborate formulation can be derived, by simplifying this procedure such that the forward and backward waves can be obtained directly from the measured pressure and flow data, if the characteristic impedance of the aorta is given or already estimated.

Misuse of computer-generated results.

Abstract: The number of statistical tests performed has increased by several orders of magnitude through the use of computers and ready-made statistical packages. The vast majority of these tests neither physically can be, nor ever are, published. Only tests which show a strong statistical correlation between some variables seem to be sorted out for publication. This practice, which can be found in applied sciences such as the social and medical sciences, is investigated and shown in many cases to lead to a considerable increase in the percentage of published false results. A new function to be performed by statistical packages is then proposed which will reduce this percentage.

Apparatus for the servocontrol of arterial oxygen tension in preterm infants.

Abstract: A description is given of apparatus that has been developed for the servocontrol of arterial oxygen tension in preterm infants with respiratory problems. By using a continuous reading from an indwelling umbilical arterial-oxygen electrode we have been able to regulate the oxygen supply to preterm infants with respiratory distress syndrome and thus prevent prolonged episodes of hypoxia and hyperoxia.

Functional electrical stimulation of the upper limb: charting the forearm surface

Abstract: Electrical stimulation is carried out by surface electrodes, placed at grid points over the forearm surface. Three orthogonal components of the isometric moment generated at the wrist joint are measured and these moment vectors are charted. The charts reveal both qualitative and quantitative information on the magnitude and direction of the resultant moment vector generated, as a function of the stimulation current level, and the electrode positioning. Clearly defined plateaux of unidirectional moment vectors are found to exist. These are analysed in greater detail.

Improved capacitive electrocardiogram electrodes for burn applications.

Abstract: An improved capacitive electrocardiogram electrode has been constructed using hybrid integrated-circuit techniques. the new configuration is compact and lightweight and provides electrocardiograms of clinical quality comparable to those obtained from conventional conducting silver/silver-chloride electrodes. The capacitive electrodes can be used successfully with nonconducting burn ointment, which ordinarily inhibits the function of conducting electrodes.

Microprocessor-controlled contrast sensitivity measurements.

Abstract: A system is described for automatic measurement of contrast sensitivity functions (c.s.f.). The measuring technique is based on the Von Bekesy tracking method. One c.s.f. can be measured in 10 min. The stimuli (sinewave gratings) are generated on a t.v. monitor screen. The stimulus parameters (spatial frequency, contrast and field size) are controlled by a microprocessor. The results are plotted in an easily interpretable from, giving the c.s.f. of both eyes together with the radio between the contrast sensitivity of the two eyes. Clinical measurements on amblyopic patients are described by way of illustration of the application of the system.

Method for routine clinical assessment of knee-joint forces

Abstract: It is important to be able to assess the loads on the knee joints of arthritic patients during wakling. We have adapted previously reported methods of gait analysis so that they can be used in routine clinical examinations. The system described in this paper is in regular use investigating patients before and after knee-joint surgery. In a four-month period data was collected from over 60 patients with a wide range of knee abnormalities.