Biomedical subjects

D Isabey

Publications and source records attributed to D Isabey.

60 records · Page 4Linked to original sources

PubMed1984

Alveolar ventilation during high-frequency oscillation: core dead space concept.

To assess the role of direct alveolar ventilation during high-frequency ventilation, we studied convective gas mixing during high-frequency oscillation with tidal volumes close to the dead space volume in a simple physical model. A main conduit representing a large airway was connected with a rigid sphere (V = 77, 517, and 1,719 cm3) by a small circular tube (d = 0.3 and 0.5 cm; L = 5, 10, and 20 cm). The efficiency of sinusoidal oscillations (f = 5, 20, and 40 Hz) applied at one end of the main conduit was assessed from the washout of a CO2 mixture from the sphere; to flush CO2 from the main fluid line, a constant flow of air was used. The decay in CO2 concentration measured in the sphere was exponential and therefore characterized by a measured time constant (tau m). Taking the small tube volume as the ventilatory dead space (VD), an effective tidal volume (VT*) was computed from tau m and compared with the tidal volume (VT) obtained separately from the pressure variation in the sphere. The discrepancy between these two tidal volumes has been found to be uniquely dependent on the ratio VT/VD within the range of VT/VD studied (0.5-2.2). For VT/VD less than 1.2, VT* was larger than VT, indicating that the conventional concept of alveolar ventilation does not apply. From the partition of the oscillatory flow in the small tube into two regions, the core and the unsteady boundary layer, we theoretically computed the proportions of the sinusoidal flow (or tidal volume) and the dead space for each region.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1982

Steady and pulsatile flow distribution in a multiple branching network with physiological applications.

Flow rate of distribution in steady and pulsatile flow is investigated in a multiple branching network including six successive generations in the same plane. In this model, the geometry dependence of flow rate distribution has already been pointed out in steady flow (previous study) by observing the occurrence of non-uniform flow rate distribution at terminal orifices despite the symmetrical dichotomy, identical distance and cross-sectional profile of the 64 parallel pathways. In the present study, we point out two additional properties of the steady flow rate distribution. (i) The flow rate distribution is not markedly sensitive to a change in viscosity and therefore Reynolds number does not appear to be a determinant factor to modify the flow distribution. (ii) The effect of a branch obstruction on flow rate distribution is limited in space and its extension remains the same for the different Reynolds numbers tested. These properties also characterize the interdependence between the model geometry and the flow distribution in steady state. The results obtained in pulsatile flow show that the flow distribution remains similar to steady state as long as the parameter lambda = Qp/Qs (Qp = amplitude of flow oscillation, Qs = steady component of the total flow) is less than 1. When lambda is greater than 1, the flow distribution may become uniform, which means that contrary to the steady flow case, the velocity profiles become rapidly symmetrical downstream from the bifurcations. Physical explanations for this are proposed after considering the theoretical problem of pulsatile developed flow in a straight tube. In this case, the parameter lambda again plays a crucial role in the velocity distribution.

Arteries↗

PubMed1982

A model study of flow dynamics in human central airways. Part II: secondary flow velocities.

Secondary velocity components perpendicular to the tube axis were measured in a 3 : 1 scale model of the human central airways. Slanted hot-wire probes were introduced axially in order to measure the secondary velocities at about 12 points for each of the 7 stations investigated. Secondary velocities in the inspiratory direction never exceeded a mean value of 18% of the mean axial velocity. Secondary velocities in the expiratory direction reached a mean value of 21.5% of the mean axial velocity. In the inspiratory direction, two unequal eddies were formed in the left main bronchus and in the right upper lobe. Moreover, maximum velocities were observed near the wall and the decay of secondary velocities in the left main bronchus was observed. The secondary flow patterns observed in the left upper and lower lobes after the secondary bifurcation were difficult to recognize, although they seemed to be more influenced by the second bifurcation. The complexity of the flow pattern was reinforced by viscous effects acting near the wall. In the expiratory direction, only two stations in the trachea were measured; four uneven eddies seemed to have existed, with the ventral eddies appearing to be predominant. Overall, the secondary velocity magnitudes as well as the patterns of eddies were very dependent on the geometry of the model used.

Humans↗

PubMed1981

Steady and unsteady pressure-flow relationships in central airways.

We measured pressure-flow relationships in a noncompliant five-generation cast of human central airways using air, HeO2, and SF6O2 at 0, 0.25, 0.50, 1.0, and 2.0 Hz with tidal volumes of 0.25, 0.5, and 1.0 liter. When dimensionless pressure drops for steady inspiratory and expiratory flows of the various gas mixture were plotted against Reynolds' number on a log-log scale (Moody diagram), they formed two curves as fluid mechanical theory predicts. At frequencies higher than 0.25 Hz, data obtained from 1) the same gas and same stroke volume, 2) the same frequency and same stroke volume but different gases, and 3) the same gas at the same frequency but with different stroke volumes, all deviated from the steady flow curves in the Moody diagram, always tending to increase the dimensionless pressure drop. The increase was largest when instantaneous flow was near zero and was minimal at the peak flow in a given inspiration or expiration. These data led to the identification of a dimensionless parameter (epsilon) that reflects the relative importance of local acceleration (unsteadiness) to convective acceleration at any given instant during a flow cycle. A dimensional analysis then reveals that the pressure-flow relationship in a given airway system is uniquely and completely determined by a combination of three dimensionless parameters: Reynolds number (Re), Womersley number (alpha), and the new parameter (epsilon). With this set of parameters we can explain all reported apparent paradoxes as well as the present findings.

Airway Resistance↗

PubMedDate not supplied

A transducer for measuring high frequency oscillatory flow rates.

Recent systems proposed in the literature to accurately measure flow rates at high frequency, such as the wave tube technique or the loudspeaker technique, are not adequate for large amplitude oscillations. Other standard pneumotachometer systems require analytical or numerical correction procedures. In this study, we propose a low cost, rapid response flowmeter for high and low flow rates, made up of a standard screen pneumotachometer coupled with a piezoresistive differential pressure transducer of recent conception. By reference to a gas capacitance, the frequency response of this device is compared to the frequency response obtained with the same screen pneumotachometer coupled with a standard low range medical pressure transducer. Contrary to the standard device, the new device has a flat amplitude response up to 70 Hz and introduces a minimal phase delay up to 90 Hz (9 degrees), both with a 1.51.s-1 constant peak flow and a peak flow rate two times greater. The authors suggest that this improved flowmeter can be advantageously used in physiological studies requiring accurate high frequency flow measurements over short periods of time.

Pulmonary Ventilation↗

PubMedDate not supplied

[Pressure-flow relationship in the airways (author's transl)].

The perplexing features of the pressure-flow relationship in the pulmonary airways have been explained on the basis of a combined experimental-theoretical study. By systematically plotting the in vivo data on the pressure-flow relationship existing in the literature in a dimensionless manner (Moody diagram), we confirmed the results reported by Lisboa et al. [8] : contrary to the well-known theory of Jaffrin and Kesic [6], it appeared that under certain spontaneous breathing conditions Reynolds number is not the only parameter which governs the flow in the airways. We further confirmed this result using a cast of the central airways and a piston pump. The data showed that the pressure-flow relationship depended greatly on frequency, tidal volume and the physical properties of the gas used. After a systematic variation of these parameters, a dimensionless parameter epsilon, which depends importantly on local acceleration, was identified. This new parameter, together with those already identified (Re : Reynolds number; alpha : Womersley number), define the pressure-flow relationship in a given geometry during the course of a respiratory cycle. Implications of this theoretical knowledge on the measurement and interpretation of airway resistance are discussed.

Gases↗