Bias Flow Research Articles

Alveolar pressures and lung volumes during high-frequency oscillatory ventilation in dogs.

A constant-volume plethysmograph was used to measure functional residual capacity (FRC) in anesthetized dogs before, during, and after high-frequency oscillatory ventilation (HFO). The phrenic nerves were electrically stimulated to provide the necessary respiratory effort. In addition to FRC, mean alveolar pressure (PA) was obtained from the pressure axis of the static pressure/volume (P/V) curve at FRC. In eight dogs, the first min of HFO at 22 ml, 20 Hz, and a 4 L/min bias flow of air increased FRC by an average of 63.6% above the apneic value, where it remained for the subsequent 15 min. PA values were significantly higher than mean airway pressures (Paw) measured in the endotracheal tube, indicating higher pressure in the terminal lung units. In five other dogs, FRC, Paw, and mean pleural pressure increased as HFO was increased from 15 to 30 Hz with 1 and 4 L/min bias flows of air. We speculate that these increases may account for both the impairment of cardiovascular function and improvements in oxygen tensions often seen with HFO.

CO 2 elimination as predicted by augmented dispersion and convection in an asymmetrical dog airway model

We have developed a computer model of the dog lung based upon an asymmetrically branching network of tubes to describe gas exchange during high-frequency oscillations. Impedances to oscillatory flows were calculated for each airway segment and used to determine flow distributions in all airway generations. Gas exchange was assumed to occur by convective and augmented dispersive mechanisms. Also included in the model were features commonly found in animal studies including the effects of bias flow of fresh gas at the airway opening and equipment dead-space volume. The magnitude of CO 2 elimination predicted by the model closely resembled actual experimental data. Specifically, it predicted that CO 2 elimination is proportional to frequency to the 0.82 power and tidal volume to the 1.25 power. It also indicated that the bias flow rate and equipment dead-space volume influence gas exchange characteristics and thus these variables should be considered when comparing data from different studies.

Effect of bias flow rate on gas transport during high-frequency oscillatory ventilation

Ventilatory support with low tidal volume, high-frequency oscillatory ventaillation (HFOV)_usually uses a bias flow system to provide fresh gas. Although the bias flow rates (V̇ bf) used previously have varied widely among experimental configurations,the precise role of the bias flow in HFOV-mediated gas transport has not been defined. We assessed the effect of bias flow rate on gas transport during HFOV by measuring CO 2 removal rate (Ṁ CO 2 ) in anesthetized, paralyzed dogs, using a wide rang of bias flow rates (0.7–28.9 L·min −1). When a fixed tidal volume of 40 ml was applied at HFOV frequencies of 2–12 Hz, Ṁ CO 2 concentration difference. Thus, when V̇ bf was reduced below a value which resulted in a substantial increase in bias flow CO 2 concentration, Ṁ CO 2 was reduced. These findings are consistent with a simple framework in which the relative magnitudes of the resistances to gas transport of the airways and of the bias flow (1/V̇ bf) determine the contribution of the bias flow rate to overall gas transport during HFOV. This relationship may be employed to assess the intra-airway contribution to HFOV-mediated gas transport at any bias flow rate, and may therefore allow comparison of results from experiments utilizing various bias flow rates.

Intra-airway gas mixing during high-frequency ventilation

We examined the intra-airway gas transport mediated by high-frequency oscillations (HFO) in 10 nonintubated healthy volunteers using a method based on comparisons of single-breath N2-washout curves obtained after various durations of breath hold or high-frequency oscillations. With a mathematical analysis based on Fick's law of diffusion we computed the local transport parameter, effective diffusivity, during oscillations of frequency 2-24 Hz and tidal volume 10-120 ml and during breath hold alone. Local effective diffusivity increased with both oscillatory frequency and tidal volume at all levels in the tracheobronchial tree; the enhancing effect of tidal volume on local effective diffusivity was more pronounced than that of frequency so that effective diffusivity was greater with larger tidal volume at fixed frequency-tidal volume product (f . VT). The greatest enhancement of gas mixing within the lung during HFO (over breath hold) was seen in the central airways. In previous studies examining CO2 removal rate during HFO (J. Clin. Invest. 68: 1475, 1981), we found that CO2 output was also greater with larger tidal volume at fixed f . VT, and we attributed this to an end constraint imposed by a fresh gas bias flow. Results of the current study, performed without a bias flow, indicate that bias flow end constraint does not solely account for the observed dependence of CO2 output on frequency and tidal volume.

Characteristics of a particle velocity hydrophone based on hot‐film anemometry

It is desirable to measure particle velocity in an acoustic field in addition to pressure. The heat transfer inherent in hot‐film anemometry is mostly a function of particle motion. The relation between particle velocity and voltage output in a harmonic hydroacoustic field appears different from that in a nonperiodic flow (King's law). For horizontal motion, the output is proportional to the velocity squared, with a frequency twice that of the acoustic field. If the motion is vertical, it modulates the steady free‐convection flow off the film, and a linear relation ensues with the same frequency as the field. Experimental results were obtained by means of a horizontally driven open trough for the first case and a vertical inertial calibrator for the second. It was attempted to correlate the output, expressed as a Nusselt number amplitude, to the relevant dimensionless numbers: particle displacement relative to sensor diameter, Stokes, Fourier, and Grashof numbers. Preliminary experiments show that an imposed bias flow improves the sensitivity and directional response of the sensor [Work supported by Office of Naval Research.]

Pulmonary gas exchange during high-frequency ventilation in dogs

Steady-state gas exhange was studied in anesthetized paralyzed dogs subjected to high-frequency ventilation (HFB) to determine the effects of oscillation frequency (f 10–40 Hx) and stroke volume (Vs 20–40 ml) on gas exchange efficiency. Alveolar gas fractions were measured by rapid withdrawal of lung gas while oscillations and the bias flow circuit were interrupted. Alveolar ventilation (V̇ A) calculated from steady-state CO 2 elimination and F A co 2 was found to exhibit a frequency-dependent maximum. Similarly, Pa CO 2 increased and Pa O 2 decreased again when f exceeded a critical level. V̇ A was 13% of total ventilation (f · Vs) at 10 Hz (40 ml Vs) decreasing to about 4% at 40 Hz. Blood — gas differences for O 2 (P A - Pa) were about 6 torr and close to zero for CO 2 (Pa - P A). Lung volume assessed from He wash-out was increased with f and Vs. O 2 uptake and CO 2 elimination were significantly enhanced by HFV as a function of Vs. Because the effective V T was essentially unknown, the apparent gas exchange inefficiency at high f and Vs values may be due to the functional characteristics of the apparatus rather than a general feature of HFV.

Gas exchange during high-frequency ventilation of the chicken.

Recent studies have shown that high-frequency ventilation (HFV) at 1-30 Hz is capable of maintaining adequate gas exchange in humans and dogs even when tidal volumes are substantially less than dead space. We evaluated the effectiveness of HFV in roosters by comparing CO2 elimination during various frequencies and tidal volumes of HFV with CO2 elimination during conventional mechanical ventilation. Sinusoidal oscillations were applied at the tracheal cannula. A bias flow provided fresh gas at the top of the tracheal cannula. Three conclusions emerge from the data. 1) HFV enhances gas transport in the chicken as it does in mammals. 2) At low oscillatory flows (amplitude X frequency) CO2 elimination depends on both frequency and tidal volume, whereas at higher flows CO2 elimination depends more strongly on tidal volume. The flow at which this transition occurs is relatively lower than in humans and much lower than in dogs. 3) HFV at volumes below dead space is usually not capable of maintaining adequate gas exchange in the chicken in contrast to results in dogs and humans.

1720 YENTILATORY EFFICIENCY OF NEONATES

The relationship of pulmonary minute volume to the prevailing oxygen consumption (V1/VO2), the convection requirement (CR) is a useful index of the overall efficiency of ventilation. This study was undertaken to determine the CR of healthy infants and the relationship of CR to the size and age of the infant. Continuous concurrent measurements of V1(bias flow pneumotachometry)and VO2 (open circuit indirect calorimetry) were made for 100 to 150 min. in 31 healthy infants, wt. 650 to 3380 grams, postconceptual age (PA)30 to 42 weeks. Spontaneous changes in V1 and VO2 followed changes in activity and feeding. Data were divided into 3 min. intervals and linear regression analysis of mean V1 vs mean VO2 and first differences, (ΔV1 vs ΔVO2)were performed following time series correction for lags. All infants showed significant positive correlations(V1 vs VO2 r=.79, mean slope=46, ΔV1 vs ΔVO2 r=.64, mean slope=37.2). Mean regression analysis of the relationship between the slope of individual V1 vs VO2 regression lines(S) & the infants BW,age,PA,GA & study wt. demonstrated significant negative correlations between S & several indices of the size & age of the infant. The V1 associated with VO2=8.5 ml/min.kg for individual infants was also negatively correlated with size and wt. We conclude that smaller & younger infants are at a relative disadvantage because of high CR,ie.they require a larger V1 for a given VO2 & relatively greater increases in V1 to meet increases in VO2. Measurement of CR should prove useful as a pulmonary function test in neonates with & without respiratory diseases.

Ventilation by high-frequency oscillation.

The effect of applying a high-frequency small-volume sinusoidal oscillation at the airway was investigated in anesthetized apneic beagle dogs (mean wt 11 kg, mean VDphys 6.6 +/- 0.6 ml/kg). Oscillations generated by a piston in a cylinder were transmitter to the lungs through an uncuffed endotracheal tube (4.5 mm ID, 6.0 mm OD), which allowed a substantial leak back through the vocal cords. A bias flow of fresh gas presented inspired air to the midtracheal level. The minimum distal airway pressure (measured at the end of the endotracheal tube) was maintained between 0 and 2 cmH2O. Peak airway pressures were 4-8 cmH2O. The optimal frequency for CO2 elimination was 15 Hz. Using volumes of 1.9 ml/kg (range 1.7-2.3) at this frequency the mean PaCO2 was 33.1 +/- 0.5 Torr. In four dogs breathing 100% O2 the PaO2 was 594 +/- 9 Torr during spontaneous ventilation and 580 +/- 9 Torr after 5 h of uninterrupted oscillation. In four experiments using room air the PaO2 was 95 +/- 5 Torr during spontaneous respiration and 106 +/- 1 Torr after 5 h of oscillation. In an additional seven studies there was no difference in mean cardiac output between oscillation and conventional mechanical ventilation. This study demonstrates that high-frequency small-volume oscillations can maintain gas exchange for many hours presumably by markedly enhancing the diffusivity of gases in the lung.

On the theory of unsteady high Reynolds number flow through a circular aperture

This paper examines the theory of the unsteady motion caused by fluctuations in the driving pressure of a high Reynolds number mean flow through a circular aperture in a thin rigid plate. A theoretical model is proposed which is amenable to exact analytical treatment, and involves the shedding of vorticity from the rim of the aperture. The theory determines the dependence of the Rayleigh conductivity of the aperture on the Strouhal number, and provides quantitative estimates for the rate of dissipation of large scale ordered structures as a result of the generation of turbulence at the apertures in a perforated liner. The limit of zero Strouhal number yields a description of steady high Reynolds number flow, the contraction ratio of the emerging jet being predicted to be equal to the minimum theoretical value of ½. Application is made to the problem of sound trans­mission through a uniformly perforated screen in the presence of a low Mach number bias flow.

A simple high-performance current-switched Josephson gate

A versatile current-switched gate is described which consists of two Josephson tunnel junctions and a small resistor in a loop. The directly combined bias and control currents flow through one junction in the zero-voltage state, causing the switching. The second junction and the resistor provide isolation between input and output after switching. The switching speed of a few tens of picoseconds and the microwatt power dissipation are comparable to those of other present designs. The design is particularly simple, compact, and easy to fabricate.

Estimating Plant Flows and Inventories Using Material Balance Relations

Investigation of on-line information processing techniques for recursive least squares estimation of plant flows and inventories based on available measurements and plant material balance relations is the main topic of this paper. The use of statistical hypothesis testing to detect changes in measurement bias and unmeasured process flows are also studied.The successful application of estimation techniques in the plant requires practical methods for estimating instrumentation calibration factors and process model parameters. These factors were investigated by tests conducted through the use of an on-line minicomputer to process the flow and inventory data from a heat exchange pilot plant. Artificial measurement bias errors and process leaks were introduced to evaluate the performance of the sequential probability ratio test in detecting these faults. The test results indicate that changes in measurement bias of about 1 to 2 percent of full-scale range can be detected.

Posthyperventilation breathing: different effects of active and passive hyperventilation.

Posthyperventilation breathing: different effects of active and passive hyperventilation.

Experimental and Numerical Studies of Flush Electrostatic Probes in Hypersonic Ionized Flows: I. Experiment

Numerical studies were conducted to determine current collection characteristics of flush mounted electrostatic probes on a flat plate in ionized hypersonic flows. The effects of probe size, position, bias, and local flow properties were explored in detail. To this end, the charged particle conservation equations were solved with the two- and three-dimensional Poisson equation, and the results compared with thick sheath probe data obtained in hypersonic hock tunnel experiments. Agreement with the experimental data is good. Nomenclature diffusion coefficient Damkohler number, krL?/D0 electron charge probe current current density Mach number number density pressure probe radius Reynolds number Schmidt number temperature time flow velocity probe voltage distance from plate leading edge source term distance normal to surface of flat plate ratio of ion to electron diffusion coefficient boundary-layer thickness D Da e / J M N,n p R Re Sc T t 17, u V x vv Y, y

Switching Dynamics of Bistable Fluidic Amplifiers With Low Setbacks

The transient switching of a model bistable fluidic amplifier with low setbacks and variable configuration has been studied. The amplifier was of conventional design with straight and convex side-walls, cusped and pointed splitters, and variable control port width. Water was used as the fluid and the element was operated at different Reynolds numbers. Effect of loading, inertia of the fluid in the output legs, bias flows from the inactive control port, and geometric changes on the transient switching times were determined. The flow field was made visible by generating hydrogen bubbles in the power nozzle and at the power nozzle exit. Using visual studies of the switching process as a guide, an analysis has been developed to predict the switching times approximately.

THE USE OF HELIUM IN THE TREATMENT OF ASTHMA AND OBSTRUCTIVE LESIONS IN THE LARYNX AND TRACHEA

Excerpt INTRODUCTION The proposal of helium as a new therapeutic gas was based on the conception that its decreased specific gravity in relation to nitrogen would make a helium-oxygen mixture easie...