Biomedical subjects

G Barnas

Publications and source records attributed to G Barnas.

5 recordsLinked to original sources

PubMed1993

Low-frequency respiratory mechanics using ventilator-driven forced oscillations.

We evaluated the potential for using a fast Fourier transform (FFT) analysis applied to a standard ventilator waveform to estimate (< 2 Hz) frequency dependence of respiratory or lung resistance (R) and elastance (E). In four healthy humans we measured pressure and flow at the airway opening while applying sine wave forcing from 0.2 to 0.6 Hz at two tidal volumes (VT; 250 and 500 ml). We then applied a step inspiratory ventilator flow wave with relaxed expiration at the same VT and only 0.2 Hz. Step waveform data were also acquired from nine mechanically ventilated patients under intensive care unit conditions. Finally, we simultaneously measured total respiratory (rs), lung (L), and chest wall (cw) impedance data from two dogs (0.156-2 Hz) before and after severe pulmonary edema. Rrs and Ers were estimated by the FFT approach. Humans displayed a small frequency dependence in Rrs and Ers from 0.2 to 0.6 Hz, and both Rrs and Ers decreased at the higher VT. The spectral estimates of Rrs and Ers with the step ventilator wave were often qualitatively comparable to sine wave results below 0.6 Hz but became extremely erratic above the third harmonic. Conversely, in dogs the step wave produced reliable and stable estimates up to 2 Hz in all conditions. Nevertheless, Ecw and Ers still displayed clear and correlated oscillations with increasing frequency, whereas EL showed none. This suggests that nonlinear processes, most likely at the chest wall, contribute to periodic-like fluctuations in respiratory mechanical properties when estimated by applying FFT to a step ventilator wave. Moreover, in humans, but not dogs, a ventilator flow cycle contains insufficient signal energy beyond the third harmonic. We show that the amount of energy available at higher frequencies is largely governed by the mechanical time constant contributing to passive expiratory flow. In dogs the shorter time constant contributes to increased energy. In essence, the frequency content of the flow is subject dependent, and this is not a desirable situation for controlling the quality of the impedance spectra available from a standard ventilator wave.

Adult↗

PubMed1990

Tracheal insufflation of oxygen at low flow: capabilities and limitations.

Tracheal insufflation of oxygen (TRIO) may provide temporary oxygenation for patients or sustain life in apneic mass casualties when conventional ventilatory techniques are not available or feasible. Logistically, minimum flows of TRIO (Vmin) are desirable for field use and to reduce barotrauma should airway obstruction occur. We carried out a feasibility study to determine the efficacy of Vmin of TRIO delivered within 1 cm of the carina, in nine anesthetized and paralyzed dogs. Minimum flows of TRIO for these dogs of average weight (12 kg) was 91 mL/min. In six of the dogs Vmin TRIO was continued and provided oxygenation for an average of 1.5 h compatible with subsequent resuscitation with conventional ventilation. However, PaCO2 levels increased to mean values of 256 mm Hg in the 90 min. To determine what the effect of increased gas mixing was on gas exchange, we repeated Vmin TRIO for 10 min in six of the dogs with and without high frequency oscillations superimposed on the TRIO flow. The oscillations (60 mL at 16.3 Hz) increased carbon dioxide excretion but significantly impaired oxygenation. In completely apneic animals, TRIO at low flow delivered by cricothyroidotomy may be useful as an emergency procedure when upper airway obstruction limits the use of other airway management techniques. However, enhancement of gas mixing during low-flow TRIO impairs oxygenation, so that higher flows would be required when respiratory efforts occur.

Animals↗

PubMed1989

Afferent vagal activity during hyperthermic polypnea in the pigeon.

Respiration-modulated activity in afferent vagal fibers was recorded in 10 pigeons during euthermic breathing and thermal panting. Of these fibers, 13 were identified as intra-pulmonary chemoreceptors (IPCs), that increased discharge with diminishing lung gas PCO2, and 13 as mechanoreceptors, that increased firing with lung inflation. Two types of IPC were observed that were distinct by their firing pattern during panting. Phasic IPCs displayed phasic discharge within the respiratory cycle, even at respiratory frequencies (fresp) as high as 400 min-1. Tonic IPC fired tonically and increased their discharge as fresp increased. Several IPCs were silent during euthermic breathing, but discharged tonically as fresp increased with thermal polypnea. Discharge of neither type of IPC was consistently related to PaCO2. Discharge from mechanoreceptors was phasic with respiration, up to values of fresp as high as 350 min-1. However, the average number of impulses per breath decreased as fresp increased. We conclude that discharge from phasic intrapulmonary chemoreceptors and mechanoreceptors may contribute to setting the respiratory pattern during hyperthermic polypnea.

Animals↗

PubMed1984

Cardiovascular and blood gas responses to shivering produced by external and central cooling in the pigeon.

Cardiovascular and blood gas responses of pigeons to spinal cord cooling (35-36 degrees C) were measured at thermoneutral (28 degrees C) and low (5 degrees C) ambient temperatures. Spinal cord cooling at thermoneutral temperatures caused immediate shivering and increases in heat production (223%), heart rate (152%) and cardiac output (169%), but blood pressure and stroke volume did not change. PaCO2 and PvCO2 increased slightly during the cooling; PaO2 and CaO2 decreased slightly while PvO2 and CvO2 decreased considerably (10 Torr and 1.7 mmol . l-1, respectively), resulting in a greater a-v difference in O2 content. Ambient cooling produced responses comparable to spinal cord cooling. Simultaneous spinal cord and ambient cooling produced similar responses that were generally greater in magnitude than either kind of cooling alone. Consequently, heart rate, cardiac output and O2 extraction from the blood were all significantly, linearly related to heat production over the wide range studied. Comparisons are made between cardiovascular responses of birds to shivering and exercise in regards to the relative importance of increases in heart rate, stroke volume and blood pressure. It is suggested that exercise and shivering may effect cardiovascular responses through similar receptor mechanisms.

Animals↗

PubMed1984

Respiratory responses to shivering produced by external and central cooling in the pigeon.

Respiratory responses of pigeons to spinal cord cooling (5-6 degrees C) in neutral environment (Ta = 28 degrees C), to ambient cooling (Ta = 5 degrees C), and to simultaneous spinal cord and ambient cooling were measured. Spinal cord cooling produced shivering and a 242% increased in heat production (M); expiratory flow rate (VE) increased 216%, a result of increases in both respiratory frequency (160%) and tidal volume (140%). Increases produced by ambient cooling compared to thermoneutral controls were slightly, but not significantly, less than those during spinal cord cooling: M = 203%, VE = 199%, respiratory frequency (fR) = 146%, tidal volume (VT) = 138%. Spinal cord cooling at low ambient temperature produced greater increases in shivering, heat production and respiration compared to thermoneutral controls than either type of cooling alone: M = 337%, VE = 326%, fR = 198%, VT = 178%. The oxygen extraction from the ventilatory gas remained relatively constant among the different groups. fR, VT and VE were all significantly linearly related to M over the wide range studied. These relationships were independent of whether cooling was central or external. Respiratory changes induced by the onset and end of spinal cord cooling were immediate and closely correlated with the magnitude of shivering. It is unlikely that changes in arterial and venous blood gases during shivering effected the major portion of the respiratory response. Thus, it is suggested that a control mechanism of the respiratory center via afferents from the shivering muscles is important in increasing respiration during central or external cooling.

Animals↗