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Biomedical subjects

J G Webster

Publications and source records attributed to J G Webster.

At least 73 records · Page 4Linked to original sources
PubMed1989

The validity of a portable accelerometer for estimating energy expenditure in bicycle riding.

The purpose of the investigation was to determine the validity of a portable vertical accelerometer and a Large-Scale Integrated Motor Activity Monitor (LSI) for estimating energy expenditure in riding a bicycle at various velocities. Instrument placement was either at the knee or ankle. Energy consumption, i.e. oxygen consumption (VO2) was determined during bicycle rides after steady state metabolism was reached. Standard errors of estimate were used to express the accuracy of estimating VO2 from accelerometer or LSI recordings. The reliability of the vertical accelerometer was found to be satisfactory. The vertical accelerometer was also effective for estimating VO2 in bicycling (standard errors of estimate = 3.3 to 4.4 ml.kg-1.min-1). The accuracy of the LSI was not as good; the standard errors of estimate being = 5.9 to 8.5 ml.kg-1.min-1.

Adolescent↗
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

Three-dimensional reconstruction in electrical impedance tomography.

We have developed a three-dimensional, computer-simulated, electrical impedance tomographic imaging system using the four-electrode measurement technique. The reconstruction process finds the numerical solution of Laplace's equation by using the finite element method and the Newton-Raphson algorithm. A new diagonal electrodes measurement method provides lower error than the usual neighbouring electrodes measurement method. For a three-layer, 4 x 4 model, the number of iterations increased from 3 to 16 as the number of randomly placed, high-resistivity voxels increased from 3 to 18.

Electric Conductivity↗
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↗
PubMed1988

An impedance method for blood pressure measurement in awake rats without preheating.

The tail-cuff methods for measuring systolic blood pressure in the rat usually require preheating of the animal to obtain recordable pulse signals. To find a more sensitive method, we applied the principle of differentiated impedance (dZ/dt) to the tail-cuff measurement of systolic blood pressure. We obtained clear pulse signals from the tail in awake rats without preheating the animals, and the systolic blood pressure obtained by this method had an excellent correlation with the directly measured femoral artery pressure (correlation coefficient = 0.98). Heating the animals at 40 degrees C for 5 minutes increased systolic blood pressure by a mean of 6 mm Hg as compared with that determined at the ambient temperature of 21 to 24 degrees C. Mean systolic blood pressure in young female diabetic rats was 122 +/- 3 mm Hg, which was significantly higher than the 111 +/- 2 mm Hg of normal rats. It is concluded that the technique of electrical impedance as applied to the tail-cuff method is simple and highly sensitive and is suitable for measurement of tail systolic blood pressure in awake rats without preheating.

Animals↗
PubMed1988

Human electrogastrograms: comparison of techniques of recording.

There are different methods for recording physiologic electrical signals cutaneously. Monopolar recording has been used, by us and by others, to record the basic electrical rhythm of the stomach. In an effort to improve our ability to routinely obtain a recording of the basic electrical rhythm of the stomach, using cutaneous electrodes (electrogastrography), we have investigated different electrode recording techniques, and compared them with monopolar recording. We studied 10 healthy asymptomatic volunteers who were fasting, using each of the techniques while simultaneously monitoring a monopolar electrogastrogram. Three different recording techniques were studied: a Wilson central terminal, comparing alternative placements of the reference electrodes; a Wilson central terminal with a single placement of the reference electrodes, but alternative placements of the active exploratory electrode; and, last, a differential recording technique. We found all of the studied techniques to be superior to monopolar recording in the amount of time a recognizable three/min basic electrical rhythm could be recorded. However, the differential recording arrangement yielded an improvement in percent of recording time so significant that the electrogastrographic signal could be visually discerned to greater than 80%, compared with 37%, of the monopolar recording. Thus, the differential recording technique is far superior for recording the electrogastrogram.

Electrodes↗
PubMed1987

A comparison of impedance tomographic reconstruction algorithms.

We compared the reconstruction performance of six different impedance tomographic reconstruction algorithms in a two-dimensional computer simulation. We found the modified Newton-Raphson method applied to the finite-element method performed the best. We show that this method can also make a better one step correction in less time than any of the other methods.

Algorithms↗
PubMed1987

Some physical results from an impedance camera.

We discuss some physical results from an impedance camera. We placed agar blocks of various resistivities and sizes in saline. We showed that when the agar block was small or close in resistivity to the saline solution, we were unable to reconstruct it. We attempted to reconstruct an image of a fist, but were unable to achieve this due to current flowing in the arm. We concluded that three-dimensional analysis may be needed, along with more sensitive reconstruction algorithms and measurements.

Agar↗
PubMed1986

Human electrogastrograms. Comparison of surface and mucosal recordings.

A method to record the gastric basic electrical rhythm (BER) using skin electrodes is described. Correlation is made with mucosal suction electrodes in 20 volunteers. Successful simultaneous recording from surface electrodes and mucosal electrodes is made in 16/20 subjects (80%). An increase in amplitude of the recorded gastric BER with gastric contractions is demonstrated and discussed. Five patients with gastric motility disturbances and chronic nausea are also investigated. Four of these patients had diabetic gastroparesis. Each had preservation of the gastric BER. The rate was usually normal at three per minute, but during acute nausea episodes which occurred in three of the four, an abnormally increased rate of the BER (tachygastria) was seen. This finding was also seen in a subject with atrophic gastritis and delay of gastric emptying. The surface electrogastrogram may prove to be a useful tool for the investigation of patients with suspected gastric arrhythmias.

Abdominal Muscles↗