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Biomedical subjects

A Ben Jebria

Publications and source records attributed to A Ben Jebria.

At least 19 recordsLinked to original sources
PubMed1991

High frequency jet ventilation and upper tracheal stenosis: a model study.

A chest-lung model, consisting of a human laryngo-tracheo-bronchial tree cast (4 or 5 bronchial generations) tightly enclosed in a 100 l rigid box was used to assess the potential efficiency of high frequency jet ventilation in patients with upper tracheal stenosis. The elasticity of the air in the box stimulated normal adult chest-lung compliance. Diaphragms (0.5 or 1.5 cm thick) were inserted into the upper trachea to simulate stenoses of 0.7, 1, 1.5 and 1.75 cm inner diameter. A rigid injector-catheter (5 mm outside diameter) was directed in the axis of the trachea with its tip 2.5 cm beneath the stenosis. The end inspiratory alveolar pressure (PA), the end expiratory pressure (PEEP) and the tidal volume (VT) were measured at a rate of 100/min and 30% inspiratory to total periods ratio. Entrained flow, Vem, measured at the start of air insufflation, was compared to that calculated (Vec) from a simple model. For a given setting of the ventilator, PEEP, PA--PEEP and VT were approximately linearly related to the difference in diameters of stenosis and injector. While PEEP decreased, both PA--PEEP and VT increased with increasing diameter of stenosis. When the diameter of the stenosis was higher than 1.5 cm no changes in PA--PEEP and VT were observed, owing to the narrowest section of the larynx. Vec was always higher than Vem. The thickness of the stenosis did not affect the results, and the diameter of the stenosis appeared to be the main factor affecting the ventilatory parameters under our experimental conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

High-Frequency Jet Ventilation↗
PubMed1988

Effect of dry warm air on respiratory water loss in children with exercise-induced asthma.

The variation in respiratory water loss (RWL) over time, expressed as the mass of water vapor lost per liter (body temperature and pressure, saturated) of ventilation (MH2O), was investigated in two groups: (1) children with exercise-induced asthma; and (2) healthy children. Children were matched for age and sex and went without medication for at least 12 hours before each experiment. The children breathed dry warm air (TI = 28.4 degrees C +/- 0.3 degree C) for 15 minutes while bicycling at constant and moderate work load (50 W). The MH2O was measured by collecting and weighing the expired water vapor (1) at rest breathing in warm conditions of inspired gas (control values), (2) every five minutes during exercise while breathing dry warm air, and (3) four minutes after the end of exercise. Pulmonary function tests were performed before and six minutes after exercise. The results were abnormal only in children with exercise-induced asthma. During exercise, RWL significantly fell (compared to control value) at the tenth and 15th minute in both groups. Whereas normal subjects recovered their initial values for MH2O four minutes after stopping exercise, asthmatic children still had a reduction in respiratory water loss. During exercise, MH2O decreased a little more in healthy than in asthmatic children. The decrease in MH2O in both groups suggests that the means to fully humidify expired gas are overwhelmed by thermal stress. The lack of increase in MH2O in asthmatic children on stopping exercise suggests that the airway mucosa is unable to produce enough water vapor and is thus dehydrated and probably hyperosmotic.

Asthma↗
PubMed1987

Effect of resident gas density on CO2 elimination during high-frequency oscillation: a model study.

In order to throw more light on the mechanisms governing the efficiency of intrapulmonary gas mixing during high-frequency oscillatory ventilation, an experimental, and theoretical, study was carried out on a model casting of the airways of a human lung that closely resembled the respiratory tract. The experiments were carried out under various conditions during high-frequency oscillation (HFO), by using alveolor resident gas mixtures of different densities. The efficiency of gas mixing was assessed by measuring the time constants of the CO2 alveolar washout which were compared to those obtained from simulations on a theoretical model based on a turbulent diffusional resistance concept. Our results showed that the decay in CO2 concentration was highly dependent on both frequency (f) and tidal volume (VT). Tidal volume was found to have a greater effect on efficiency of gas mixing than frequency. Moreover, even though there were statistically significant differences in the time courses of CO2 washout between N2 and He, N2 and SF6 or between He and SF6, this could not imply that gas mixing was limited by diffusion. Agreement between the experimental time constants of CO2 elimination during HFO and the predicted mixing time constants was satisfactory. It is concluded that turbulent augmented diffusion is the main factor responsible for effective gas transport during high-frequency oscillatory ventilation.

Carbon Dioxide↗
PubMed1987

Simulations of steady quaternary gas diffusion between alveolar and blood compartments.

Fick's law is usually employed to analyze diffusion phenomena in the lung. However, this law is only strictly applicable to two component mixtures. When there are more than two gases in the mixture. Stefan's equations are more appropriate. Under physiological conditions during respiration, at least three gases are involved (O2, CO2, N2). Furthermore, helium is often added in studies on the effect of density on pulmonary mixing. Features of quaternary gas diffusion (O2, CO2, N2, He), and computer simulations of physiologically relevant conditions are presented. The results of the simulations were found to be in agreement with results from studies on normal and disease lungs.

Computer Simulation↗
PubMed1987

Steady pressure-flow relationship in a cast of the upper and central human airways.

Pressure drops across the upper (larynx) and central airways of a human lung cast were measured at steady state inspiratory and expiratory flows. Air, He-O2 and SF6-O2 gas mixtures were used at tracheal Reynolds' numbers ranging from 145 to 30,000. The pressure-flow characteristics of the model were analysed using standard pressure-flow diagrams and Moody plots. We found that the asymmetry between inspiratory and expiratory resistances, observed in the central airways (larynx excluded), was markedly reduced in the presence of the larynx. However, static pressure differences were greater across the entire model of the upper and central airways than across the model of the five generations of the tracheo-bronchial tree (without larynx) at the same flow-rates. In addition, our results showed that the presence of the larynx tended to reduce the zone of fully developed laminar flow in the Moody diagram with the higher density gas, while extending the zone of turbulent flow even for the low density gas at low Reynold's numbers.

Airway Resistance↗
PubMed1987

Effect of breathing dry warm air on respiratory water loss at rest and during exercise.

The changes in respiratory water loss with time, expressed as the mass of water vapour lost per liter BTPS of ventilation (MH2O), and expired temperature (TE), used to calculate the relative humidity (ERH), were investigated in ten normal subjects while breathing warm dry air by mouth (PIH2O = 0 kPa; TI = 30 degrees C): at rest for a period of 35 min; during 15 min light muscular exercise (50 W); at increasing work load from 50 to 100 W between the 5th and 10th min of the exercise. The data collected were compared to those obtained in room air conditions (PIH2O = 0.68-1.3 kPa) and under conditions with slightly heated inspired air (TI = 28-30 degrees C). At rest, when breathing dry warm air MH2O and ERH fell during the first 15 min, while they recovered their initial values during the last 20 min. In contrast no differences in MH2O or ERH were observed when breathing ambient warm air. At constant and moderate work load for 15 min, the respiratory water loss fell significantly (compared to the 5th min) at the 10th and the 15th min when breathing warm dry air. The added hyperpnea which was obtained by increasing work load from 50 to 100 W between the 5th and 10th min of exercise did not further reduce MH2O and ERH. The transient fall in MH2O and ERH, which lasted at least 15 min either at rest or during muscular exercise, suggested that the mechanism underlying humidification of expired gas is overwhelmed by thermal stress. Since the upper airways mucosa is unable to saturate expired gas, this also suggested that the mucosa is dehydrated and probably hyperosmotic. The progressive recovery in MH2O and ERH after 15 min of warm dry air breathing at rest, suggest operation of a slow adaptive mechanism.

Female↗
PubMed1985

Stochastic simulation of Taylor's dispersion in the airways.

A stochastic simulation was devised in order to obtain a more correct solution of the phenomenon of convection combined with axial and radial diffusion, which is also called Taylor's dispersion, as it could occur in the pulmonary tract. The fit with Aris' moments which can be deemed as a reference since they are obtained analytically without approximation, was quite good. On the other hand, Taylor's solution usually led to large discrepancies with these moments. Taylor's stipulation that his solution be used only under certain conditions was therefore confirmed. This solution is not applicable in the lungs.

Diffusion↗
PubMed1985

A mechanical approach to the longitudinal dispersion of gas flowing in human airways.

The aim of this work is to contribute to elucidating the mechanism underlying gas mixing in the human pulmonary airways. For this purpose, a particular attempt is made to analyse the fluid mechanical aspects of gaseous dispersion using bolus response methods. The experiments were performed on five normal subjects by injection of 10 cm3 bolus of He, Ar and SF6 into the latter part of the inspired airstream, in such a way that the whole bolus entered the inspiratory flow and was recovered during the following expiration. The results, presented in a logarithmic plot of dimensionless variance (dispersion of the output bolus) against the Peclet number, show that gaseous dispersion is only slightly dependent on the nature of the tracer gas but is strongly related to flow velocity. This is in agreement with the theory of turbulent or disturbed dispersion; however, it seems that Taylor laminar dispersion does not play a significant role in the airways.

Airway Resistance↗
PubMed1982

Computer simulation of ternary diffusion in distal airways of the human lung.

Gaseous diffusion plays a fundamental role in the terminal generations of airways for respiratory physiology. It has been proposed as a prime mechanism underlying stratified inhomogeneity in the alveolar space. Nevertheless, the diffusion phenomenon in the lung has often been studied using Fick's law which is only valid for binary diffusion. Under conditions of more than two gases in a mixture, the appropriate equations for diffusion are those of Stefan. In respiration, diffusion involves at least three gases (O2, CO2 and N2), and in physiological experiments complex mixtures including heavy or light gases (SF6, He) are often added to enhance the effect of diffusion. We present in this paper the features of ternary diffusion and solve the appropriate equations for the non-steady state by a finite difference method. The simulation was performed using two models derived from the anatomical data of Weibel and Hansen-Ampaya. Moreover, four initial conditions most often encountered during current respiratory physiology tests, were used for the computations. Therefore in these four situations, O2-N2-He, O2-N2-Ar, O2-N2-SF6 and O2-N2-CO2 combinations were used. Our results showed that for each case mentioned above the oxygen acted differently in ternary diffusion owing to the specific nature of the components of each mixture. Moreover, the behaviour of each component in ternary diffusion was very different from that of binary diffusion. However, this difference may be negligible when the subject breathed normal air.

Computers↗
PubMedDate not supplied

Analytical simulation of longitudinal gas mixing in the human central airways.

In order to study gaseous mixing in the proximal respiratory airways during stationary breathing, a simple mathematical model with an analytical solution of the corresponding equation is presented. Calculations were carried out by solving the differential equation analytically according to the system response to a unit impulse combined with the convolution method. It seems that this analytical method gives similar results to those obtained by the numerical ones; however, our method is computationally simple and can provide a reasonable tool to study gas transport in the airways.

Humans↗
PubMedDate not supplied

Effect of He-O2 breathing on blood gases and ventilation during exercise in normal man.

Temporal changes in ventilation (VI) and arterial blood gases after substitution of helium (He) for nitrogen were studied in normal man during constant load exercises of 14 min duration (30 and 90 W). An abrupt switch of helium for air breathing (protocol 1; 5 subjects), or vice-versa (protocol 2; 4 subjects), was made at the 7th min. Whatever the work loads, the effect of He appeared rapidly: higher values of VI (protocol 1) were observed throughout the 7 min period of He-O2 breathing, but were only significant (p less than or equal to 0.05) during the first minute after substitution at 90 W. Reverse pattern was observed in protocol 2. Helium induced alveolar hyperventilation: sustained and significant hypocapnia (p less than or equal to 0.05) was observed during helium breathing. This effect does not seem to be a consequence of pulmonary gas exchange disturbance, in that concomitant Po2 was normal. It is suggested that He could have evoked a reflex which overrode humoral regulation. Significant increase in ventilatory CO2 responses at rest during He-O2 compared to air breathing in seven subjects (p less than or equal to 0.01) seems to confirm this hypothesis.

Acid-Base Equilibrium↗
PubMedDate not supplied

[Gaseous dispersion in the central airways of the human lung. In vivo study].

In order to estimate the contribution of either laminar or turbulent dispersion in the central airways during spontaneous breathing, the results of the computer simulation of a mathematical model were compared with those of an appropriate in vivo test performed on normal subjects. The model takes into account, along with the summed cross-section of Weibel lung model [31], both axial convection and longitudinal dispersion of helium and sulphur hexafluoride in air. The simulations were carried out by incorporating two dispersion coefficients corresponding to laminar and turbulent flow into the mathematical model. The experiments were performed on five normal subjects by injection of 10 ml bolus of helium and sulphur hexafluoride into the later part of the inspired airstream in such a way that the whole bolus entered the inspiratory flow and was recovered during the following expiration. When the experiment is simulated using the laminar dispersion coefficient, no concordance could be seen between computed and experimental data; however, there is a good concordance when the simulation is performed with the turbulent dispersion coefficient. It is concluded that Taylor laminar dispersion [24] cannot play a significant role in the human airways; however, it seems that convective gas mixing with turbulent dispersion [21] can account for most gas transport during spontaneous ventilation.

Adult↗