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Biomedical subjects

W J Tompkins

Publications and source records attributed to W J Tompkins.

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

Neural-network-based adaptive matched filtering for QRS detection.

We have developed an adaptive matched filtering algorithm based upon an artificial neural network (ANN) for QRS detection. We use an ANN adaptive whitening filter to model the lower frequencies of the ECG which are inherently nonlinear and nonstationary. The residual signal which contains mostly higher frequency QRS complex energy is then passed through a linear matched filter to detect the location of the QRS complex. We developed an algorithm to adaptively update the matched filter template from the detected QRS complex in the ECG signal itself so that the template can be customized to an individual subject. This ANN whitening filter is very effective at removing the time-varying, nonlinear noise characteristic of ECG signals. Using this novel approach, the detection rate for a very noisy patient record in the MIT/BIH arrhythmia database is 99.5%, which compares favorably to the 97.5% obtained using a linear adaptive whitening filter and the 96.5% achieved with a bandpass filtering method.

Algorithms↗
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

Compression of the ambulatory ECG by average beat subtraction and residual differencing.

We implemented a method for compression of the abulatory ECG that includes average beat subtraction and first differencing of residual data. Our previous investigations indicated that this method is superior to other compression methods with respect to data rate as a function mean-squared-error distortion. Based on previous results we selected a sample rate of 100 samples per second and a quantization step size of 35 microV. These selections allow storage of 24 h of two-channel ECG data in 4 Mbytes of memory with a minimum rms distortion. For this sample rate and quantization level, we show that estimation of beat location and quantizer location can significantly affect compression performance. Improved compression resulted when beats were located with a temporal resolution of 5 ms and coarse quantization was performed in the compression loop. For the 24-h MIT/BIH arrhythmia database our compression algorithm coded a single-channel of ECG data with an average data rate of 174 bits per second.

Algorithms↗
PubMed1991

Theoretical and experimental rate distortion performance in compression of ambulatory ECG's.

We compare ECG data compression algorithms based on signal entropy for a given mean-square-error (MSE) compression distortion. By defining the distortion in terms of the MSE and assuming the ECG signal to be a Gaussian process we are able to estimate theoretical rate distortion bounds from average ECG power spectra. These rate distortion bounds give estimates of the minimum bits per second (bps) required for storage of ECG data with a given MSE regardless of compression method. From average power spectra of the MIT/BIH arrhythmia database we have estimated rate distortion bounds for ambulatory ECG data, both before and after average beat subtraction. These rate distortion estimates indicate that, regardless of distortion, average beat subtraction reduces the theoretical minimum data rate required for ECG storage by approximately 100 bits per second (bps). Our estimates also indicate that practical ambulatory recording requires a compression distortion on the order of 11 microV rms. We have compared the performance of common ECG compression algorithms on data from the MIT/BIH database. We sampled and quantized the data to give distortion levels of 2, 5, 8, 11, and 14 microV rms. These results indicate that, when sample rates and quantization levels are chosen for optimal rate distortion performance, minimum data rates can be achieved by average beat subtraction followed by first differencing of the residual signal. Achievable data rates approximate our theoretical estimates at low distortion levels and are within 60 bps at higher distortion levels.

Algorithms↗
PubMed1991

Electrotactile and vibrotactile displays for sensory substitution systems.

Sensory substitution systems provide their users with environmental information through a human sensory channel (eye, ear, or skin) different from that normally used, or with the information processed in some useful way. We review the methods used to present visual, auditory, and modified tactile information to the skin. First, we discuss present and potential future applications of sensory substitution, including tactile vision substitution (TVS), tactile auditory substitution, and remote tactile sensing or feedback (teletouch). Next, we review the relevant sensory physiology of the skin, including both the mechanisms of normal touch and the mechanisms and sensations associated with electrical stimulation of the skin using surface electrodes (electrotactile (also called electrocutaneous) stimulation). We briefly summarize the information-processing ability of the tactile sense and its relevance to sensory substitution. Finally, we discuss the limitations of current tactile display technologies and suggest areas requiring further research for sensory substitution systems to become more practical.

Artificial Intelligence↗
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↗
PubMed1990

An umbilical data-acquisition system for measuring pressures between the foot and shoe.

We have developed an umbilical data-acquisition system for measuring pressures between the foot and shoe during walking. It consists of pressure sensors in the insoles of shoes, amplifier circuits, umbilical cables, an analog-to-digital converter, and a graphics display card in an IBM PC for real-time data collection and display. The applied pressure on a sensor decreases its resistance, which causes the output voltage of the amplifier circuit to increase. We attach seven sensors to the surface of each insole of a pair of extra-depth shoes and calibrate all the sensors in the insole before and after each test using a load cell as a reference. The IBM PC samples the outputs from the sensor and the load cell and stores a piecewise linear lookup table for use in compensation for the nonlinearity of the sensor. On the PC's graphics display, two programs provide displays of foot pressures as real-time bar graphs or as analog pressure versus time curves.

Analog-Digital Conversion↗
PubMed1989

Development of a three-channel, 24-h ambulatory esophageal pressure monitor.

We have developed a three-channel ambulatory esophageal pressure monitor and tested it with a series of 24-h studies. The monitor is a battery-operated, microprocessor-based device that measures pressures from three transducers positioned in the esophagus, stores the data in its memory, and transfers the data to an IBM PC computer system at the end of the recording period. Programs on the PC then analyze the data and identify contractile events, categorizing them according to specific parameters. Other programs display the pressure waveforms on the PC and allow visual inspection of the entire recordings or, alternatively, of particular events of interest. The system detects contractile abnormalities in patients with intermittent, noncardiac chest pains. We tested the system on ten normal subjects and found a relatively high incidence of what are usually considered "abnormal" contractions.

Esophagus↗
PubMed1989

A capacitance pressure sensor using a phase-locked loop.

We are using a Hercules (model #F4-4R, 100 psi) pressure sensor to measure the pressure between the foot and shoe. An interface circuit converts the capacitance change into voltage. Over the pressure range from 0 to 1300 kPa, the capacitance changes from 275 to 580 pF. A 555 timer circuit converts the capacitance into a frequency range from 30 to 63 kHz. A phase-locked loop (PLL) converts this frequency to voltage from 0 to 5 V, which is then filtered using a first-order, low-pass filter, having a corner frequency of 20 Hz to reduce the ripple to 10 mV. The sensor's hysteresis is about 8 percent at 40 degrees Celsius (C) and 12 percent at 20 degrees C. The sensor has a maximal nonlinearity of 8 percent and a worst-case nonrepeatibility of 7 percent. Its temperature coefficient is -0.147 percent per degree C. Its spatial sensitivity decreases nonlinearly from 1 to 0.17 from the center towards the periphery. The sensitivity of the system is 2.77 mV/kPa and the temperature drift is +0.53 percent per degree C. We monitor the pressure at 7 locations under each foot (the rear and the front heel, great toe, and 4 of the 5 metatarsal heads). A portable data-acquisition system permits continuous monitoring for 7 minutes. Test results for pressure distribution for normal walk and run are presented. Results are useful when studying normal and abnormal gait, and for possibly providing feedback (sensory substitution) to diabetic patients with insensate feet in order to help them dynamically adjust pressure distribution under their feet.

Biomechanical Phenomena↗
PubMed1988

A regularised electrical impedance tomography reconstruction algorithm.

The Newton-Raphson algorithm is an effective reconstruction method. However, it exhibits degraded performance for ill-conditioned problems, especially in the presence of measurement error. This paper uses the regularisation method to improve the system's conditioning, which stabilises the reconstruction method. It shows that the [[ p'' ]] penalty form yields superior images, as compared to the [[ p ]] penalty form.

Algorithms↗
PubMed1986

Microprocessor-based weight shift monitors for paraplegic patients.

The prevention of pressure sores for patients with spinal cord injuries includes pressure relief through the patient shifting position and, thus, weight. We have designed a microprocessor-based device to mount on the seat of a conventional wheelchair. This device has applications in hospital rehabilitation programs for improving patient care and in research to study the cause and prevention of pressure sores. It can use audible alarms, which can be disarmed, to train patients to regularly shift weight. The device stores in its memory data on the interval between and the duration of weight shifts. The contents of this memory can be transmitted to a laboratory microcomputer system and displayed in tabular, graphical, or histogram formats. This device is more versatile than previous timer-based systems, not only because its timer and alarm parameters easily can be reprogrammed, but especially because it can store and play back detailed data on the patient's performance. The device is powered by rechargeable NiCad batteries, which can last several days without recharging.

Body Weight↗