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Biomedical subjects

J G Webster

Publications and source records attributed to J G Webster.

At least 19 recordsLinked to original sources
PubMed1992

Skin impedance measurements using simple and compound electrodes.

We have studied the effect of the electrode configuration on the measurement of body impedance and found that the electrode configuration greatly affects the impedance measurement using the four-electrode method. We studied the characteristics of the compound electrode and found that the compound electrode provides the four-electrode method in a compact form. A new method of measuring the skin impedance using simple electrodes at low frequencies was developed. At high frequencies where the effect of internal tissue impedance is not negligible, we used the compensation method using compound electrodes, because they measure the voltage right under the skin. At 50 kHz, we measured the real part of the skin impedance of less than 80 omega on the thorax. We propose a simple instrument which can measure accurate skin impedance at various frequencies.

Electric Conductivity↗
PubMed1992

External finger forces in submaximal five-finger static pinch prehension.

Small conductive polymer force sensors were attached to the distal phalangeal pads for measuring individual finger forces exerted during submaximal static pinch. A linear force summing strain gauge dynamometer for measuring resultant five-finger pinch force was grasped vertically using a neutral wrist posture. Individual finger forces were measured at fixed total pinch force levels of 10%, 20%, and 30% of maximum voluntary exertion using pinch spans of 45 mm and 65 mm. Total pinch force and individual finger forces were also measured while similarly grasping the dynamometer and supporting fixed weights for 1.0 kg, 1.5 kg, and 2.0 kg loads using pinch spans of 45 mm and 65 mm. The index and middle fingers exerted more than 3 N greater average force than the ring and small fingers for the fixed total pinch force task. No significant individual finger force differences were observed at the 10% maximum voluntary exertion level, however both the index and middle fingers exerted more than 5 N greater force than the ring and small fingers at the 30% maximum voluntary exertion level. The average contribution of the index, middle, ring, and small fingers were 33%, 33%, 17%, and 15%, respectfully, for the fixed total pinch force task. As exertion level increased from 10% to 30%, the contribution of the middle finger was not constant increasing from 25% to 38%. Total pinch force increased from 15 N to 30 N when the load weight increased from 1.0 kg to 2.0 kg.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1992

Maximal dynamic range electrotactile stimulation waveforms.

A new method to measure the dynamic range of electrotactile (electrocutaneous) stimulation uses both steepest ascent (gradient) and one-variable-at-a-time methods to determine the waveform variables that maximize the subjective magnitude (intensity) of the electrotactile percept at the maximal current without discomfort for balanced-biphasic pulse bursts presented at a 15-Hz rate. The magnitude at the maximal current without discomfort is maximized by the following waveform (range tested in parentheses): number of pulses/burst = 6 (1-20), pulse repetition rate within a burst = 350 Hz (200-1500), and phase width = 150 microseconds (40-350). The interphase interval (separation between positive and negative phases in a biphasic pulse) does not affect dynamic range from 0-500 microseconds. The number of pulses/burst has a large effect on the perceived dynamic range when this is measured using a subjective-magnitude-based algorithm, whereas it has little effect on the traditional dynamic range measure, i.e., (maximal current without discomfort)/(sensation threshold current). The perceived stimulus magnitude at the maximal current without discomfort is approximately twice as strong with 6 pulses/burst as it is with 1 pulse/burst (a frequently-used waveform).

Bias↗
PubMed1992

Measuring lung resistivity using electrical impedance tomography.

We propose the use of electrical impedance tomography (EIT) imaging techniques in the measurement of lung resistivity for detection and monitoring of apnea and edema. In EIT, we inject currents into a subject using multiple electrodes and measure boundary voltages to reconstruct a cross-sectional image of internal resistivity distribution. We found that a simplified, therefore fast, version of the impedance imaging method can be used for detection and monitoring of apnea and edema. We have showed the feasibility of this method through computer simulations and human experiments. We speculate that the EIT imaging technique will be more reliable than the current impedance apnea monitoring method, since we are monitoring the change of internal lung resistivity. However, more study is required to verify that this method performs better in the presence of motion artifact than the conventional two-electrode impedance apnea monitoring method. Future work should include experiments which carefully simulate different kinds of motion artifacts.

Adult↗
PubMed1992

The electrode system in impedance-based ventilation measurement.

In this paper, we determined which electrode types, sizes, and locations were best suited for impedance-based ventilation measurement. Optimal electrodes provide high signal-to-(motion) artifact ratio (SAR) and reliability by meeting the following criteria: 1) low baseline impedance, 2) high adhesion, 3) good physical stability, 4) large effective area, 5) thin with high flexibility. We compared 14 electrodes from two main groups: adhesive-gel and conductive rubber electrodes. Adhesive-gel electrodes are easy to apply, make good body contact, and do not slip during the course of an experiment. We found that higher SAR's are obtained when electrode area is increased by connecting several small electrodes together rather than by using a single electrode with a larger area. The peak SAR is achieved when two electrode arrays (area = 70 cm2) are centered at the 8th intercostal spaces on opposite midaxillary lines. To determine the optimal electrode locations, we placed 32 electrodes on the trunk and recorded impedance between 171 electrode combinations on ten normal adult subjects. Based on these data, we conclude that the SAR's are highest when one electrode is placed on the midpoint between the left and right second intercostal spaces on the sternum and the other electrode is placed in the opposite position on the back.

Adult↗
PubMed1992

A portable insole plantar pressure measurement system.

To analyze plantar pressures during activities of daily living, one needs a fully portable system capable of measuring many steps over extended periods. This paper presents an inexpensive, reliable, portable plantar pressure acquisition system which we have developed. It allows the long-term recording (up to 2 hours) of pressure-time data from 14 pressure sensors within insoles. The sensor chosen is an inexpensive, conductive polymer sensor that is only 0.25 mm thick yet able to withstand sudden overloads. The portable, battery-powered, microprocessor-based data acquisition system has a memory space of 480 kbytes for data storage. It can collect pressure data from 14 insole sensors at a 20 Hz sample frequency for 5 seconds every minute over a 2-hour period. It enables the long-term measurement of plantar pressures during normal activities in a natural unrestricted environment. The design and development of this portable insole plantar pressure measurement system is described.

Foot↗
PubMed1991

A conductive polymer sensor for measuring external finger forces.

This paper describes the construction and use of a durable and thin force sensor that can be attached to the palmar surface of the fingers and hands for studying the biomechanics of grasp and for use in hand injury rehabilitation. These force sensors were constructed using a modified commercially available conductive polymer pressure sensing element and installing an epoxy dome for directing applied forces through a 12 mm diameter active sensing area. The installation of an epoxy dome was effective for making the sensors insensitive to contact surfaces varying from 25 to 1100 mm2 and a 16 mm radius surface curved convex towards the finger. The completed sensors were only 1.8 mm thick and capable of being taped to the distal phalangeal finger pads. They were calibrated on the hand by pinching a strain gage dynamometer. The useful range was between 0 and 30 N with an accuracy of 1 N for both static loading and normal dynamic grasp activities. The sensor time constant was 0.54 ms for a step force input. Because of varying offset voltages every time the sensors were attached, these sensors should be calibrated on the hand before each use. The sensors were used for measuring finger forces during controlled pinching and lifting tasks, and during ordinary grasping activities, such as picking up a book or a box, where the useful force range and response for these sensors were adequate.

Biomechanical Phenomena↗
PubMed1991

A microprocessor-based data-acquisition system for measuring plantar pressures from ambulatory subjects.

We have developed a portable microprocessor-based data-acquisition system to measure discrete plantar pressures within the shoe from ambulatory subjects. The system offers improved accuracy, repeatability, portability, and flexibility not available in current commercial systems. It consists of 14 conductive polymer pressure sensors, 14 analog amplifiers, an 8-bit analog-to-digital converter, a microprocessor, 120 kbytes of memory space, and a parallel I/O interface. Seven pressure sensors are embedded within each insole and located at the posterior heel, anterior heel, the four metatarsal heads, and hallux of each foot. The system is capable of continuously sampling 14 channels of pressure data for 7 min at a 20-Hz sample rate. The recorded data are downloaded into a microcomputer for further processing, analysis, and display. Foot pressures have been acquired from a sensate subject during multiple walking trials.

Diagnosis, Computer-Assisted↗
PubMed1991

A 16-channel 8-parameter waveform electrotactile stimulation system.

We have developed a general-purpose electrotactile (electrocutaneous) stimulation system as a research tool for studying psychophysiological performance associated with various stimulation waveforms. An experimenter-defined command file specifies the stimulation current and waveform of each of the 16 channels. The system provides burst onset delay of 0-20 ms, phase current of 0-50 mA, interphase interval of 0-1000 microseconds, number of pulses per burst from 1-100, pulse repetition rate of 0.1-25 kHz, phase width of 2-1000 microseconds, and functionally-monophasic pulses (with zero dc current) or balanced-biphasic pulses (with equal positive and negative phases). The system automatically delivers the desired stimulation, prompts the subject for responses, and then logs subject responses. Key features of the system are 1) very flexible choice of bursts of pulsatile waveforms, 2) real-time control of all of the waveform parameters as mathematical functions of external analog inputs, and 3) high-performance electrode-driver circuitry.

Electric Stimulation↗
PubMed1979

Heart rate recorder.

Explore the source record for details and available documents.

Electronics, Medical↗
PubMed1979

Human pulmonary resistance: effect of frequency and gas physical properties.

We have investigated a new technique that could be specific for detecting small airway diseases. We measured resistance in nine subjects, evaluating the effect of breathing 80% He-20%O2, air, and 80% SF6-20% O2 at different flow rates (0.25--1.01 l/s) and frequencies (4--12 Hz). To test the sensitivity and specificity of this new technique, we used intravenous histamine infusion to cause peripheral constriction. During histamine infusion six of nine subjects showed the following changes: 1) dynamic compliance decreased; 2) subjects developed frequency dependence of compliance, and 3) there was no significant change in pulmonary resistance or Rrs (respiratory resistance) at 4 Hz. These data suggested a time-constant discrepancy in the periphery due to histamine infusion. These six subjects also developed frequency dependence of resistance (delta Rrs from 4 to 12 Hz), which was significant when breathing air (-22 +/- 6%) and maximal when we used He-O2 (-32 +/- 3%). We conclude that He-O2 can improve the specificity of frequency dependence of Rrs for detecting events occurring in the peripheral airways.

Adult↗
PubMed1978

Pressure-flow relationships and pathological changes during renal preservation.

The renal pedicle of one kidney from each of four dogs was ligated for one hour. The contralateral kidney served as a control. Both kidneys were removed and perfused using the "Belzer" technique. Pressure-flow relationships were determined and biopsy samples taken. The vasculature was then injected with silicone rubber. Perfusion resistance, vascular filling with silicone rubber and observations made by electron microscopy were compared.

Animals↗