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Biomedical subjects

L A Geddes

Publications and source records attributed to L A Geddes.

At least 91 records · Page 5Linked to original sources
PubMed1989

Ability of the Lapicque and Blair strength-duration curves to fit experimentally obtained data from the dog heart.

The objective of this study was to determine the ability of the empirical Lapicque and theoretically derived Blair expressions for excitation to fit experimentally obtained threshold current values to evoke a ventricular extrasystole using rectangular-wave stimuli applied to the dog heart. The data points were fitted to both expressions and the ability of each to predict the measured values was determined. The Levenberg-Marquardt (L-M) algorithm was used to fit the Lapicque and Blair expressions. The Lapicque data were also fitted to the linear charge-duration expression of Weiss (W). It was found that the ratio of the predicted to measured current was slightly different from one 0.95 (L-M) and 1.06 (W) for the Lapicque and 0.92 (L-M) for the Blair expression. Thus, there appears to be little difference between the ability of the expressions to fit the same experimentally obtained data. The L-M Lapicque fit is best for the short durations range; the Weiss-Lapicque fit overestimates in the short duration range and underestimates near chronaxie. The L-M Blair fit is best for the short duration range and poor for the durations near the membrane time constant.

Algorithms↗
PubMed1989

The safety factor for electroventilation measured by production of cardiac ectopy in the anesthetized dog.

The safety factor of electroventilation (ie, the ratio of the current required to produce an ectopic beat to the current required to produce an inspired volume of 225 ml, which is approximately twice tidal volume) was determined in 12 pentobarbital-anesthetized dogs using transthoracic electrodes positioned at the optimal electroventilation site. The optimal stimulation site for electroventilation was first determined using hand-held, stimulating electrodes. Then electrodes, 4.1 cm in diameter, were sutured bilaterally to the optimal stimulation site. The relationship between inspired volume and stimulus intensity was determined using a 0.8-s burst of stimuli (60/s) with a pulse duration of 0.1 ms. Using the same electrodes, the threshold current for producing ectopic beats was determined for single pulses ranging from 0.1 to 10 ms duration. In all dogs, the current required to produce an ectopic beat increased greatly as the pulse duration decreased. At 0.1 ms, the safety factor for electroventilation was calculated to be 25.8.

Anesthesia↗
PubMed1988

Measurement of pulse-wave velocity using a beat-sampling technique.

The relationship between pulse-wave velocity (PWV) and blood pressure was investigated using a new method in which multiple pulse-wave velocities were determined within each blood-pressure pulse. The technique employed measuring pressure at two sites within the aorta and measuring multiple time differences between each pair of pressure waves. Blood pressure was manipulated with drugs. The technique of obtaining multiple PWVs within a beat dramatically reduced the variability of the data in the linear and nonlinear region of PWV versus pressure relationship.

Algorithms↗
PubMed1988

Electroventilation in the baboon.

The optimum sites for chest-surface electrodes and the relationship between the inspired volume of air and stimulus intensity were determined in six pentobarbital-anesthetized baboons. The optimum stimulation sites were identified using hand-held, trans-chest stimulating electrodes. Thereafter, conducting adhesive electrodes 4 cm in diameter were placed over each of the optimal sites. The relationship between inspired volume and stimulus intensity was determined with the forelimbs restrained and unrestrained using an 800-msec burst of 60/sec stimuli with a pulse duration of 20 microseconds. Two optimum stimulation sites were identified, one in the anterior axillary region, and another just lateral to the xiphoid process. The maximum volume of inspired air was obtained when the stimulating electrodes were placed at the anterior axillary sites. With the forelimbs restrained or unrestrained, an increased stimulus intensity resulted in an inspired volume well in excess of the spontaneous tidal volume of the animal. These studies indicate that, with chest-surface electrodes at the optimal locations, electroventilation can produce inspired volumes greater than spontaneous tidal volume in anesthetized baboons. With electrodes in the xiphoid region a smaller maximum inspired volume was obtained because of the stimulation of expiratory (abdominal) muscles.

Animals↗
PubMed1988

Cardiac output versus pacing rate at rest and with exercise in dogs with AV block.

To achieve maximum benefit from exercise (rate)-responsive pacing in subjects with sinus node dysfunction and AV block, it is necessary to determine the pacing rate (HR) which produces maximum cardiac output (CO) under specified exercise conditions. However, the CO-HR relationship for exercise has not been systematically investigated. To permit determination of the optimum HR, CO was measured at rest and with exercise for different pacing rates. Seven dogs with complete AV block and permanently implanted ventricular pacemakers were exercised on a treadmill for 5 min at each of four pacing rates (55, 76, 101, 116/min) and at two constant exercise levels (225 and 560 kg.m/min). CO was determined by impedance cardiography during the resting state preceding exercise and during a brief period (10-20 s) immediately after exercise, and was expressed as a percent of the CO determined at rest with HR = 55/min. A three-phase pattern of CO versus HR appears to exist for exercise as for rest. For exercise, starting from a low HR, CO increases markedly; a "plateau" is reached during which moderate increase in CO is achieved by increasing HR. At very rapid pacing rates, CO may actually decrease with further increase in HR. The results of this study suggest that a subject-specific optimum HR exists for each constant exercise level. Moreover, the methodology employed in the study is applicable to the identification of optimum HR for any exercise (rate)-responsive pacemaker.

Animals↗
PubMed1988

Relationship between tracheal air flow and induced changes in intrathoracic volume. A basis for calibration of pneumocardiogram.

The volume of air moved into the trachea due to ventricular activity--a record of which is the pneumocardiogram (PNCG)--is always less than true stroke volume. A new method of investigating this relationship consists of inducing a known volume change (delta V in) within the thorax and measuring the volume of air (delta V out) recorded with a pneumotachograph connected to the trachea. The range of delta V in was from 4 ml to 36 ml. A linear relationship (mean r = 0.98, p less than 0.001) between delta V out and delta V in was observed in 19 anesthetized, intubated dogs: delta V out = (f) delta V in, where f = 0.53 +/- 0.09 (mean +/- SD). The coefficient, f, ranged from 0.40 to 0.71. These results demonstrate that although f is subject-dependent, delta V out reliably reflects changes in delta V in. It is suggested that the technique of inducing a known volume change within the thorax and measuring the volume of air recovered from the trachea could be used as a method for calibrating the pneumocardiogram to obtain stroke volume.

Airway Resistance↗
PubMed1988

Electrically produced artificial ventilation.

The induction of inspiration by delivering a burst of short-duration stimuli to body-surface electrodes is over a century old. The authors have used the term electroventilation to describe the technique. Inspiration can be produced with stimuli applied to electrodes placed bilaterally at the base of the neck, over the phrenic nerve motor points. Transchest electrodes placed anterior to the axillae can be used to stimulate the long thoracic and phrenic nerves. Paraxiphoid electrodes can be used to stimulate the phrenic nerves as they enter the diaphragm. With any of these electrode placements, inspired volumes in excess of spontaneous tidal volumes can be achieved in man, baboon, and dog. Of major importance is the need to use short-duration (less than 10-microseconds) pulses in the train of stimuli that produces contraction of the inspiratory muscles. The duration of inspiration is determined by the duration of the train, which is typically 0.5-1 s. The use of short-duration pulses minimizes the risk of producing cardiac arrhythmias and results in very little sensation at the skin.

Animals↗
PubMed1988

An assessment of blood vessel vasoactivity using photoplethysmography.

The vasoactivity of the vessels of the earlobe, nailbed, finger pad, and nasal septum was investigated in 11 human subjects to determine the suitability of these sites for use in indirect oximetry. The subjects were healthy, nonsmoking volunteers in an age range of 20-32 years. Each site was illuminated with infrared radiation at 880 nm, and the reflected pulsatile signals from the tissue beds were displayed on a multichannel analog recorder. The decrease in pulse amplitude detected at each site in response to a cold-pressor test was used as the measure of vasoactivity. Among the sites investigated, the earlobe was the least vasoactive. The nasal septum, nailbed, and finger pad were similar in degrees of vessel reactivity.

Adult↗
PubMed1987

The effect of cuff pressure deflation rate on accuracy in indirect measurement of blood pressure with the auscultatory method.

The importance of cuff deflation rate in the auscultatory method of measuring blood pressure was investigated using a computer-based model. To determine the relationship between the cuff deflation rate and the measurement error, two cuff deflation protocols were used, one based on heart rate (mm Hg per heartbeat), the other on a constant rate (mm Hg per second). The different deflation protocols and rates were tested using a constant blood pressure of 120/80 mm Hg and heart rates ranging from 40 to 120 beats/min. It was confirmed that a cuff deflation rate that is time based will introduce larger errors at low heart rates. Using heart rate as a basis for cuff deflation rate yields a constant error that is independent of heart rate. The currently used standard of 3 mm Hg/s could result in a maximum error of 2.5 mm Hg in both systolic and diastolic pressures at a heart rate of 72 beats/min. The maximum systolic and diastolic errors increase to more than 4 mm Hg at 40 beats/min. A deflation rate of 2 mm Hg/beat, however, yields a maximum error of 2 mm Hg for both systolic and diastolic pressures, independent of heart rate. A cuff deflation rate based on heart rate is recommended to help minimize changes in measurement error when measuring blood pressure if a wide range of heart rates will be encountered.

Auscultation↗
PubMed1987

Optimal stimulus duration for extracranial cortical stimulation.

Motor cortex stimulation is a new testing area in which either electrical or magnetic fields are used to produce transcranial stimulation of the underlying brain. The resulting signals, recorded from the spinal cord, peripheral nerves, or muscles, are monitored by electromyographic and evoked potential techniques. This report considers the issues in effective stimulation with minimal noxious sensation. Short duration pulses are optimal for electrical fields, and electrodeless stimulation with magnetic fields also makes use of short duration pulses.

Action Potentials↗
PubMed1987

Motor evoked potentials in the dog: effects of global ischemia on spinal cord and peripheral nerve signals.

Motor evoked potentials (MEPs) in cats, rats, and humans have been reported. They appear promising as a test of central nervous system function, and they are sensitive not only to mechanical injury but also to ischemia. In mechanical trauma, the peripheral nerve response is much more sensitive to damage than the cord response, with a lower threshold and an earlier disappearance. We are reporting that the MEP can also be produced in the dog and that, under conditions of cardiac arrest induced by fibrillation, the peripheral nerve response disappears first at about 30 seconds and then the spinal cord response disappears at about 10 to 13 minutes. The late disappearance of the spinal cord response raises serious questions about its role as an adequate injury monitor. The most useful warning feature of the spinal cord response is an increase in amplitude during the critical first 2 minutes of arrest. Latency changes in the cord and peripheral nerve response did not seem as useful as amplitude changes in terms of providing adequate detection of injury. We also evaluated the peripheral nerve signals to determine whether they are partially volume-conducted weak muscle responses, and evidence substantiates their nonmuscle origin.

Animals↗
PubMed1987

Analysis of the current-density distribution from a tapered, gelled-pad external cardiac pacing electrode.

We have designed a high-impedance (5000 omega-cm), tapered, gelled-pad, external cardiac pacing electrode that limits the migration of charges to the perimeter of a circular electrode and produces a more uniform current-density distribution than external cardiac pacing electrodes in clinical use. A computer simulation was developed that uses cylindrical coordinates to analyze the current-density distribution at the interface between the electrode and human tissue. Our computer simulation analyzed 32 different electrodes, and the results showed that the gelled-pad thickness, the gelled-pad taper, and the radius of the conducting disk were not significant parameters in determining the current-density distributions for low-resistivity electrodes. Those parameters were, however, significant for high-resistivity electrodes. We defined the optimum resistivity as that at which the tapered, gelled-pad electrode produces the most uniform current-density distribution and delivers the most current to human tissue. When evaluating electrodes at the optimum resistivity, we determined that the peak current density of the tapered, gelled-pad electrode was 50% lower than that of the clinically available electrodes, while delivering 58% more current to the human tissue.

Computer Simulation↗
PubMed1987

Comparison of the efficacy of defibrillation with the damped sine and constant-tilt current waveforms in the intact animal.

The efficacy of defibrillation using the damped sine and constant-tile (60%) truncated exponential waveforms was determined in each of nine dogs. Two measures of efficacy were used to compare the two waveforms: 1) threshold defibrillation current and 2) percent successful defibrillation. For both measures of efficacy, shock strength was expressed in terms of delivered energy. Mean threshold energy was 0.98 J/kg for the damped sine wave and it was 1.24 J/kg for the truncated exponential waveform. Percent successful defibrillation versus energy/kg curves were constructed for each of the waveforms and were found to be essentially the same. Percent successful defibrillation increased with increasing shock intensity. For 50% success, the energy for the damped sine wave was 1.16 J/kg; for the truncated exponential wave, the corresponding value was 1.15 J/kg. A shock of threshold intensity successfully defibrillated in approximately 50% of the defibrillation attempts, i.e., defibrillation threshold corresponds to about 50% successful defibrillation.

Animals↗