Showing papers in "Medical & Biological Engineering & Computing in 1971"

The impedance of stainless-steel electrodes.

Abstract: The resistive and capacitive properties of stainless-steel electrodes in contact with saline solutions of various concentrations were investigated over a frequency range extending from 20 to 10 kHz by using a variable-length conductivity cell. With a low current density, the series-equivalent resistance and capacitance of a single electrode-electrolyte interface were found to vary almost inversely as the square root of frequency. In the frequency range studied, it was found that the reactance was very nearly equal to the resistance and both varied almost inversely as the square root of the frequency, verifying Warburg's postulate. When current density was increased, resistance decreased and capacitance increased at all frequencies studied; the change in each was most apparent in the low frequency region. As a consequence, it can be stated that the impedance of an electrode-electrolyte interface decreases with increasing current density.

Mutual inductance of air-cored coils: effect on design of radio-frequency coupled implants.

Abstract: Values of mutual inductance between two circular conductors lying in parallel planes have been computed. The results are presented in graphical form and their relevance to the design of radio-frequency coupled implant systems is mentioned.

Hydrodynamics of male micturition. I. Theory of steady flow through elastic-walled tubes.

Abstract: A one-dimensional theory of steady flow through a tube with non-uniform elastic properties is developed. The importance of the speed of pressure waves on the tube (the sonic speed) is pointed out, and the characteristics and stability of subsonic and supersonic flow are examined. Subsonic flow is always stable. Stable supersonic flow is sometimes possible, but latent instability may lead to a localized transition (a hydraulic jump) to subsonic flow, associated with energy dissipation through turbulence.

The fatigue properties of human cortical bone

Abstract: Rotating cantileverfatigue tests were performed on specimens extracted longitudinally from the cortices of human femora. The usual inverse relationship between stress amplitude and fatigue life was found; the median life was 1·3×104 cycles at a stress amplitude of 12,200 1bf/in2 (84·1 MN/m2) and 4·6×106 cycles at 6750 1bf/in2 (46·6 MN/m2). Arguments are advanced which suggest that these results are compatible with what is known of the stresses applied in life and the clinical occurrence of fatigue fractures.

Recursive digital filters for biological signals

Abstract: Digital filters achieve their frequency-selective properties by operating on the values of a sampled-data signal. After outlining an important design method for such filters, two classes of recursive digital filter of particular value for the processing of biological signals are described in some detail. These are applied to the recovery of an ECG waveform from wide- and narrowband contaminating noise.

Engineering and physiological considerations of direct capacitor-discharge ventricular defibrillation.

Abstract: The amount of energy, current and charge per gram of heart were determined on a series of 36 dogs. The data, which are examined on the basis of current duration (time constant), show that the three criteria are different. The optimum duration range using energy as a criterion is 0·03–3 ms. Minimum peak current was required with a 4 ms duration capacitor discharge pulse. There was no optimum duration for minimum charge. It was found that although short-duration current (0·02–3·0 ms) produced defibrillation, the high current required impaired the contractility of the ventricles. From this study, the optimum range of capacitor-discharge current for defibrillation is 0·3–4 ms. Estimates are presented on the amounts of energy, current and charge to defibrillate the ventricles of large animals.

Theoretical determination of the blood's relative oxygen saturation in vivo

Abstract: The diffusion equations for randomly directed photons are applied to the problem of thein vivo determination of the oxygen relative saturation in blood. A model of the living tissue is presented. The transmitted and backscattered light fluxes are calculated for various skin properties, volume occupied by the blood and the blood relative oxygen saturation. Normalization functions are introduced and analyzed for sensitivities to changes in several parameters. The photodiffusion theoretical treatment is suggested for calculations which will lead to optimal normalization techniques and transducer design for thein vivo measurements of the oxygen relative saturation in the blood.

Fluid mechanics of nutritive sucking behaviour: the suckling infant's oral apparatus analysed as a hydraulic pump.

Abstract: The suckling infant's feeding behaviour is analogous to the functioning of a pulsating pump, and may be studied through application of the physical principles for analysing macroscopic flow systems. An apparatus has been developed to measure pressure and flow during nutritive sucking. A theoretical analysis of the flow characteristics of this instrument provides several parameters whose values may be used to estimate the quantity of energy expended by the infant in generating pressure pulses and obtaining nutrient reinforcement. The analysis was tested by an experimental study of the dynamic performance of the instrument in which the infant was replaced by a calibrated source of steady pressure and pressure pulses. The findings confirm the theoretical prediction that within the range of acceptable experimental error the instrument will provide nutrient reinforcement proportional to the time-pressure curve of the sucking pulse. This method of measuring sucking permits the study of constitutional differences, as well as maturational and environmental influences upon feeding behaviour, and facilitates the investigation of the process of behavioural acquisition in the newborn. Within this context, learning during the newborn period may be conceptualized in terms of a gradual improvement in the efficiency of energy expenditure during operant discrimination schedules of nutrient reinforcement. The infant optimizes his work output and minimizes his effort by learning to suck only when the discriminant signal informs him that reinforcement is available.

The waveshape of action potentials recorded with different types of electromyographic needles

Abstract: The paper is concerned with a theoretical comparison of the wave-shapes of action potentials recorded by different types of needle electrodes as used in clinical electromyography.

Hydrodynamics of male micturition. II. Measurements of stream parameters and urethral elasticity.

Abstract: Measurements on a normal male of the radius and speed of the urine stream, and the relation between them, are described. The tendency of the radius to increase with increasing stream speed is evidence that the urethral walls are elastic. The theoretical connection between the stream characteristics and urethral elasticity is developed and used to deduce the elastic properties of the urethra at the point which controls the stream (the distal constriction). Direct measurements of the pressure in the resting urethra of the same subject, using catheters of different radii, give further information on the elastic properties of the urethra between the external meatus and the external sphincter. The flow and pressure measurements together show that in this subject the stream is controlled about 3 cm from the external meatus. At this point the urethra behaves like an elastic walled tube of unstretched radius (0·16±0·02) cm, subject to an external pressure of (18±3) cm H2O, and the urethral walls have an average elastic constant per unit length of order 6×103 dyn cm−2 (6 cm H2O).

An experimental stress analysis of the neck of the femur.

Abstract: The paper is concerned with the experimental analysis of the distribution of stress across the neck of the femur (thigh bone). The case considered was that of a man supported on one leg, a position which imposes maximum static load on the femur. The frozen stress method of three dimensional photoelasticity was adopted as the means of solution. A procedure was devised for correcting the results obtained from a homogeneous photoelastic model to apply to the case of bone which was considered to be composed of two main regions, a hard shell and a softer core. The bending and axial stress components so determined show a distribution which can be predicted by an engineering analysis but the shear stress distribution is not so satisfactory.

Statistical monitoring techniques.

Abstract: The medical profession's increasing interest in monitoring, suggests that it would be appropriate to describe some of the statistical techniques used for monitoring in Operational Research and related fields. This article discusses what ‘monitoring’ means in statistical terms and describes the two most widely used statistical monitoring techniques currently available, namely the Cusum and Trigg (or smoothed error) methods. The article also discusses the possible off-line and on-line monitoring possibilities of these two methods in the medical field.

Equipment for continuous measurements of pulse wave velocities.

Abstract: Equipment that measures continously ‘foot to foot’ time intervals between two peripheral arterial pulse waves is described. To calculate the pulse wave velocity one has to know, besides this time interval, the distance between the sites of the proximal and distal pulse wave transducers. The foot of the proximal and the distal pulse waves is detected by differantiation and by marking signals generated by a zero crossig marker. These signals start and stop a time interval counter connected to a digital printer and a D/A converter. Gating circuits prevent the operation of the time interval counter by extrema of the pulse wave other than its foot. The accuracy of the differentiator, the accuracy in detecting the zero crossing of the differentiated pulse wave, and the influence of a sloping baseline on the position of the pulse foot, are discussed. In a similar fashion, it is possible to measure the time interval between theR-wave and the foot of a peripheral pulse wave. The equipment is being tested clinically. Examples of recordings are given. Cases where the equipment fails to measure, due to peculiarities in the shape of the pule wave, are pointed out.

Thin films of glass and their application to biomedical sensors.

Abstract: Thin films of glass are attractive as a means for protecting integrated circuits for use in biological systems and are especially suitable for use with biomedical sensors. They offer advantages over polymer films since they can be deposited in thin, uniform layers and can be photoengraved using conventional techniques. This paper first reviews the requirements for film quality in using thin glassy films in these applications and then reviews the techniques available for film deposition. One technique which has been found especially suitable for biomedical work is described in detail, and the characterization of the resulting films is discussed.

Steady fluid flow through veins and collapsible tubes.

Abstract: The theory of flow through elastic-walled tubes, developed previously (Griffiths, 1971a) is applied to steady flow through collapsible tubes, in particular to one model system (Conrad, 1969). The experimental observations are well accounted for. The collapse to small cross-section which sometimes occurs in this and similar systems is a sign that the mean flow velocity locally exceeds the sonic velocity (the velocity of pressure waves on the tube). The consequences for the study of venous blood flow are briefly examined.

An inhomogeneous anisotropic model of the human torso for electrocardiographic studies

Abstract: A method which can account for body tissue resistivity and anisotropy in studies of the heart-lead relation has long been desired. This paper describes a twice-life-size analogue model which mimics the resistive properties of heart muscle, heart-blood cavities, great vessels, lung, liver, fat, anisotropic skeletal muscle, ribs and spine. Although designed for electrocardiographic studies, its use for impedance plethysmographic investigation is equally suitable. The exterior boundary can be employed with or without the internal analogue structure to emphasize effects specifically associated with the inhomogeneities and anisotropies of the thoracic tissues. The technique for model construction combines two established methods of field investigation; the electrolytic tank and the resistive network. A matrix of interlocking plastic rods immersed in the conducting fluid constrains the current flow so as to create the desired resistor network within the electrolytic tank. The electrical instrumentation, which consists of a pulsed bridge with a storage-oscilloscope null detector, has several advantages over conventional measurement techniques and is also described in the paper.

The measurement of intracranial echo pulsations.

Abstract: Since slight movements of the ultrasonic transducer relative to the head result in marked variations in the pattern and intensity of the ultrasonic energy within the head, it is essential that any ultrasonic technique that records variations in amplitude from one individual intracranial interface must incorporate a transducer holding device that fixes the tranducer rigidly with respect to the skull. It is obvious that such rigid fixation of the transducer is equally important if the range of the interface from the transducer is to be measured. Such a device, that has been found satisfactory in practice, is described.

The law of impedance pneumography

Abstract: The impedance coefficient in ohms/litre obtained by means of bipolar impedance pneumography exhibits an inverse relationship with body weight in kilogrammes; this relationship appears valid on an inter-species basis and is probably also true from an intra-species point of view. The relationship is linear on log-log paper and nearly hyperbolic on a linear scale. A statistical regression analysis produced the expression log(ΔZ/ΔV)=2·656–1·08 logW with a Standard Error of Estimate of 0·24 and a correlation coefficientr=−0·976, where ΔZ/ΔV is the impedance coefficient in ohms/litre andW the body weight in kilogrammes. The impedance coefficient per kilogramme, obtained as the derivative of the expression above, is given by (log 490–2·08 logW) and describes with accuracy the change in sensitivity with different weights. For low values of body weightW, the impedance coefficient is very high; for high values ofW, the impedance coefficient is very low and, beyond 1000 kg, negligible signals are to be expected. This observation was verified in the elephant.

Absolute calibration of phonocardiographic microphones and measurements of chest wall vibration.

Abstract: An absolute calibration by means of the reciprocity theorem has been applied to the calibration of contact-type phonocardiographic microphones. The methods are discussed in detail and examples of the results are presented. Human chest wall vibrations have been measured with these calibrated microphones and the results are also presented.

Fatigue fracture in the subchondral bone of the human cadaver femoral head

Abstract: (1) The strains occurring in the subchondral bone of the cadaver human femoral head were measured when the head was subjected to a load of physiological magnitude and direction. (2) The strains were tensile around the circumference of the femoral head and compressive in the plane of the applied load. (3) In the intact femoral head the magnitude of the strains was such that fatigue fracture of the subchondral bone would not be expected to occur in life. (4) When a cylindrical defect 13 mm in length and 10 mm in diameter was made in the centre of the femoral head, the measured strains in the subchondral bone invariably rose, and in some femoral heads the calculated stresses were high enough to be likely to cause fatigue fracture. (5) It is concluded that cysts developing in the femoral head in osteoarthrosis may usually result in a rise in the stresses in the subchondral bone, with a possible consequent revascularization of the overlying cartilage either via a fatigue crack or by reason of the vascular changes accompanying the healing of an incipient fatigue crack. The probability of either of these events cannot be assessed accurately with the information available, but it appears to be low.

Viscoelastic response of intervertebral disks at audiofrequencies

Abstract: The viscoelastic responses of intervertebral disks taken from the cervical regions of the spinal columns of several adult, male, mongrel dogs have been determined. The results are presented in terms of a mean complex shear compliance,Jd*=J′d−iJ″d, for the disk structures when these are subjected to small-amplitude audiofrequency vibrations (25–5000 Hz) while surrounded by normal salt solution, both at room temperatures and at 98·6°F (37.0°C) (in vitro). Values of the general compliance levels decrease with time after sacrifice, and an abrupt transition in the 100-Hz compliance-time curves at 5·5 hours after death is attributed to the completion of rigor mortis (at room temperature). Comparisons of the broad compliance-frequency disk spectra with those of five synthetic polymeric materials, including a silicone rubber, are presented and suggestions for materials suitable to be used in disk prosthesis are made. Such materials must have values of both elastic compliance (J′) and viscous compliance (J″) which are close to the corresponding living disk compliances. A brief discussion of the characteristic sharp resonance dispersions or fine structure observed in the disk spectra is also presented along with suggestions for future measurements. Finally, the idea is advanced that a composite synthetic structure with a stiff, outer ring of low compliance and a very compliant inner core might most satisfactorily mimic the viscoelastic behaviour of a normal intervertebral diskin vivo.

Programmable nanoampere constant current sources for iontophoresis

Abstract: A number of constant current sources for microiontophoretic drug application to neurones are described, and the limitations of these discussed.

A more efficient waveform for cardiac stimulation.

Abstract: This paper describes a waveform which is more efficient than the standard square pulse for use in chronic cardiac pacing. The experimental techniques are discussed and long term results obtained from animals implanted with endocardial catheters are presented.

A cardiac pacemaker model

Abstract: A cardiac pacemaker model is described based on a close approximation of the properties of cardiac pacemaker cells. It is a minimum-parameter model which has a simple relaxation oscillator circuit as its main element. The pacemaker model is analyzed in detail in order to show that its behaviour is similar to that of cardiac pacemaker cells. The main feature of the model is the possibility of controlling the time course of phase 4 depolarization, the threshold level and the maximum level of repolarization. Emphasis is placed on phenomena of acceleration and deceleration of the pacemaker model by pulsatile stimulation, and on synchronization and frequency entrainment. Finally it is noted that this particular pacemaker model is very useful for the study of interactions between cardiac pacemakers and the description of the mechanism of arrhythmias.

An encapsulated, implantable metal-oxygen cell as a long-term power source for medical and biological applications.

Abstract: This study describes the structure and performance of an encapsulated, implantable aluminium/platinum black electrochemical cell. It is covered with a silicone rubber membrane, and the anode and cathode compartments are separated by an anionic ion exchange membrane.In vitro studies and intermittentin vivo tests with rats demonstrate that a cell of 5·5 cm2 area gave a continuous power output of more than 70–100 μW at 0·9 V over 12 months. Although the results of long term implantation experiments now in progress remain to be assessed, preliminary data show that there was no leakage of waste products, and that tissue-cell reactions should not be serious enough to affect cell performance. It is considered that this cell can overcome the disadvantages of unencapsulated biogalvanic cells, and that it offers great promise as a long-termin vivo power source.

Unit activity from peripheral nerve bundles utilizing correlation techniques.

Abstract: Recording the extracellular neuroelectric activity at several sites along a nerve trunk provides information on single unit action potential shape and conduction velocity. Several correlation techniques are examined for separating the multi-unit neuroelectric activity into the activity of single nerve fibres. An instrument is described that simultaneously separates multi-unit recordings, made at two sites along a nerve trunk, into single unit activity.

Computer analysis and simulation of human gastroduodenal electrical activity.

Abstract: The electrical activity of the human duodenum and stomach has been analysed, using vector, correlation and joint interval histogram techniques, with a view to establishing the direction of conduction of this activity. It is shown that the gastric electrical activity at a frequency of 3/min is conducted down the pyloric antrum across the pyloric ring and some distance down the duodenum. Whilst the gastric activity is conducted only in the aboral direction the duodenal activity has been found to have no consistent direction of transmission. The length of small intestine has been likened by previous workers to a series of loosely linked oscillators running at frequencies which decrease with distance from the duodenum. The duodenal activity is shown to be consistent with a model consisting of tightly linked relaxation oscillators which, whilst oscillating at a common frequency, are not phase locked.

Hand prosthesis with adaptive internally powered fingers.

Abstract: A new pneumatic powered, completely adaptive, hand prosthesis has been developed. Because of the internal powering of the fingers no additional volume is necessary and the prosthesis can be adapted to patients who have lost their fingers but still possess the palm of their hand. Other features are good prehension, low weight and high operating speed. A description of the construction is given, and the measured performance is discussed.

Continuous measurement of peripheral vascular resistance and circulatory function.

Abstract: Peripheral vascular resistance, stroke volume, cardiac output and maximum acceleration were computed in ‘real-time’ using aortic pressure and flow signals processed by simple analogue circuits incorporated into a multichannel recording system. The use of the system was exemplified by records of the haemodynamic effects of methoxamine and adrenaline in anaesthetized dogs and noradrenaline, isoprenaline and ephedrine in unanaesthetized dogs. These analogue techniques are applicable to circulatory studies in man.