Comparative study of intra-airway gas transport by alternative modes of ventilation

The effectiveness of three alternative modes of ventilation [high-frequency ventilation (HFV), constant-flow ventilation (CFV), and high-frequency external vibration ventilation (HFVV)] was compared. Local intra-airway gas transport was measured with catheters placed in the distal trachea and in bronchi located 5.5, 9, and 11 cm from the carina. A new bolus dispersion method was devised to measure the local effective diffusivities (Deff) induced by these modes of ventilation and by cardiogenic oscillations relative to molecular diffusivity (Dmol). Mixing induced by cardiogenic oscillations was 7 +/- 2- to 26 +/- 4-fold greater than by molecular diffusion alone. Intra-airway transport by CFV, applied at three flow rates (0.3, 1.0, and 3.0 l.min-1.kg-1), was most effective in the trachea but fell sharply in the more peripheral airways. Local transport by HFVV, at a frequency of 22 Hz and a vertical amplitude of 0.4 cm, was most effective in the periphery (Deff = 793 x Dmol), whereas the effectiveness of transport by HFV, applied with 10 and 20 ml at 22 Hz, was evenly distributed. Doubling the HFV oscillatory volume caused a 4.5 +/- 2.7-fold increase in Deff/Dmol. Combining HFVV with CFV at 0.3 l.min-1.kg-1 induced transport rates that were 187- to 2,034-fold greater than by molecular diffusion alone in the bronchi and a higher relative transport (due to convection) in the trachea. We conclude that the combination of HFVV with low-flow CFV provides a high rate of intra-airway transport with minimal mechanical perturbations to the pulmonary system.

Interaction between airway lining fluid forces and parenchymal tethering during pulmonary airway reopening

In this study, our goal is to identify the interaction between airway lining fluid viscous and surface forces and parenchymal tethering forces during pulmonary airway reopening. The type of closure we modeled occurs when the airway walls and surrounding parenchyma collapse and are held in apposition by the lining fluid. We mimicked this system with a polyethylene tube coated with a Newtonian lining fluid supported by open-cell foam. Reopening occurs when a finger of air travels through the collapsed region. We measured the airway pressure (Paw) required to open the airway at a constant velocity (U). Increasing the foam stiffness (K), lining fluid viscosity (mu), and surface tension (gamma) results in an increase in Paw. Furthermore, increasing the downstream suction pressure (Pds), through tethering, causes an equivalent reduction in Paw. The upstream radius is the primary length scale, and fluid forces are represented by the capillary number: Ca = microU/gamma. On the basis of these results, we predicted the likelihood that tethering would begin to reopen collapsed airways in various disease states. This analysis showed that the ratio of tethering to fluid forces determines airway patency, which is defined as follows: lambda = PTrans/(gamma/R), where PTrans = Paw-Pds and R is airway radius. Finally, lung volume-dependent surface tension appears to be necessary to stabilize the lung.

An experimental model investigation of the opening of a collapsed untethered pulmonary airway

We developed an essentially two-dimensional planar benchtop model of an untethered collapsed airway to investigate the influence of fluid properties (viscosity, mu and surface tension, gamma) and the structural characteristics (effective diameter, D, longitudinal tension, T, and fluid film thickness, H) on airway reopening. This simplified model was used to quantify the relationship between wall deformation and meniscus curvature during reopening. We measured the pressure (P) required to move the meniscus at a constant velocity (U), and found the dimensionless post-startup pressure (PD/gamma) increased monotonically with the capillary number (Ca = microU/gamma). Startup pressures depend on the fluid viscosity and piston acceleration, and may significantly increase reopening pressures. Consistently stable steady-state pressures existed when Ca > 0.5. D was the most dominant structural characteristics, which caused an increase in the post-startup pressure (P) for a decrease in D. An increase in H caused a slight decrease in the reopening pressure, but a spatial variation in H resulted in only a transient increase in pressure. T did not significantly affect the reopening pressure. From our planar two-dimensional experiments an effective yield pressure of 3.69 gamma/D was extrapolated from the steady-state pressures. Based on these results, we predicted airway pressures and reopening times for axisymmetrically collapsed airways under various disease states. These predictions indicate that increasing surface tension (as occurs in Respiratory Distress Syndrome) increases the yield pressure necessary to reopen the airways, and increasing viscosity (as in cystic fibrosis) increases the time to reopen once the yield pressure has been exceeded.

Influences of parenchymal tethering on the reopening of closed pulmonary airways

We investigated the influence of parenchymal tethering on the reopening of collapsed pulmonary airways. Reopening experiments were performed with freshly excised canine lobes placed in a vacuum chamber with pleural pressure (Ppl) set by vacuum pressure. Noncartilaginous 2- to 3-mm airways were collapsed by suction and remained collapsed on subsequent atmospheric pressure equalization. The airway was reopened by constant-flow insufflation, and peak pressure (Ppeak) needed to reopen the collapsed airway was measured. Yield pressure needed to begin axial meniscus motion decreased markedly at Ppl = -7.5 cmH2O, indicating a possible change in airway-meniscus configuration from compliant collapse to meniscus occlusion, thus promoting onset of reopening. Two distinct types of reopening behavior were observed: unstable low-frequency fluttering phenomenon characteristic of small magnitudes of Ppl in which airway tended to recollapse after being reopened and stable reopening phenomena at larger magnitudes of Ppl in which airway remained patent after it was reopened. Stable reopening was always observed at Ppl < or = -7.0 cmH2O. Effective transmural pressure (=Ppeak - Ppl) required to reopen airway and subsequent postreopening airway pressure, reflecting airway and collateral resistance, decreased with increasing magnitudes of Ppl due to increased influence of parenchymal tethering. However, at Ppl < -8.0 cmH2O, an increase in lung volume did not result in a reduction of effective transmural pressure, possibly indicating full airway distension and influence of airway wall hoop stress.

Airway reopening pressure in isolated rat lungs

In a previous modeling study, we predicted that the yield pressure for airway reopening (Pyield) should depend on airway fluid surface tension (gamma) and airway radius (R), according to the relationship Pyield = 8.3 gamma/R. To test this prediction, we studied tantalum bronchograms of isolated perfused rat lungs from three rats by using microfocal X-ray imaging. Thirty-two airways with diameters ranging from 300 to 2,400 microns were recorded as the airways were collapsed and reinflated. Airway pressure was reduced transiently to -40 cmH2O to produce airway closure. Airway pressure was then slowly increased from 0 to 25 cmH2O. In each airway, the observed diameter remained constant until a Pyield was reached; at this pressure, airways were seen to "pop" open, allowing clear identification of airway reopening pressure. When Pyield was plotted against diameter at maximum inflation, the experimental data were in approximate agreement with predictions of Pyield made assuming a gamma of 35 dyn/cm. The close correspondence of the measured values with these predictions suggests that surfactant is present in these airways and facilitates airway reopening.

Construction and uses of a concentric catheter for gas sampling in lung airways

A catheter for intra-airway sampling of gas concentrations was constructed from concentric polyethylene tubes. The internal tube (0.58 mm ID, 0.91 mm OD) was connected to a gas analyzer while the external tube (1.20 mm ID, 1.75 mm OD) was constantly flushed by air or a calibration gas, except during sampling. Injection and sampling dead spaces were 0.35 and 0.28 ml, respectively. Delay at 4-ml/min sampling rate was 4.0 +/- 0.2 s. The 0-90% step response to a sudden change in gas composition was 0.24 s when connected to a mass spectrometer. This catheter was used to assess tracer gas dispersion during oscillatory flow (1-20 Hz) in a straight long tube. Local concentrations measured through the catheter, after a small bolus of tracer gas was injected through the external tube, compared favorably with direct measurements through needles inserted via the tube wall and with theoretical predictions. The catheter was also used to measure intra-airway gas concentrations in dog airways during spontaneous breathing, conventional mechanical ventilation, high-frequency ventilation, high-frequency vibration ventilation, and constant-flow ventilation. It ws placed by a fiber-optic bronchoscope and used to measure local quasi-steady concentrations of CO2 and local dispersion with the bolus method. The occurrence of catheter clogging with secretions was substantially reduced with flow through the external tube. Transmitting a calibration gas through the external tube facilitated in situ recalibration of the gas analyzer without removing the catheter. The use of this catheter improved the efficiency and accuracy of measurements of gas concentrations inside lung airways.

Gas dispersion in volume-cycled tube flow. II. Tracer bolus experiments

We present a new method for rapid measurement of local gas dispersion in volume-cycled tube flow. After a small bolus of tracer gas (argon) was injected into the oscillating flow, the time-averaged effective diffusion coefficient (mean value of Deff/D) for axial transport of a tracer gas is evaluated from local argon concentration measurements taken by a mass spectrometer. Two methods are presented for the evaluation of mean value of Deff/D from the concentration measurements: one uses all the sampled data, and the other uses only the local peaks of the concentration. Experiments were conducted in two tubes (radius = 0.85 or 1.0 cm) over a range of frequencies (0.42 less than or equal to f less than or equal to 8.5 Hz) and tidal volumes (7 less than or equal to VT less than or equal to 48 ml). The experimental results show very good agreement with the theoretical predictions of Elad et al. (J. Appl. Physiol. 72: 312-320, 1992). In the absence of oscillations (static fluid), the resulting mean value of Deff/D converges to that of molecular diffusion. We also show that concentration data may be acquired at any radial or axial position, not necessarily at the tracer gas injection point, and the resulting mean value of Deff/D is independent of the spatial position of the sampling catheter. This method is of similar accuracy and is substantially faster than previous methods for measuring gas dispersion in oscillatory flows. The rapidity of these measurements may permit this method to be used for the in vivo assessment of gas transport properties within the pulmonary system.

Effects of surface tension and viscosity on airway reopening

We studied airway opening in a benchtop model intended to mimic bronchial walls held in apposition by airway lining fluid. We measured the relationship between the airway opening velocity (U) and the applied airway opening pressure in thin-walled polyethylene tubes of different radii (R) using lining fluids of different surface tensions (gamma) and viscosities (mu). Axial wall tension (T) was applied to modify the apparent wall compliance characteristics, and the lining film thickness (H) was varied. Increasing mu or gamma or decreasing R or T led to an increase in the airway opening pressures. The effect of H depended on T: when T was small, opening pressures increased slightly as H was decreased; when T was large, opening pressure was independent of H. Using dimensional analysis, we found that the relative importance of viscous and surface tension forces depends on the capillary number (Ca = microU/gamma). When Ca is small, the opening pressure is approximately 8 gamma/R and acts as an apparent "yield pressure" that must be exceeded before airway opening can begin. When Ca is large (Ca greater than 0.5), viscous forces add appreciably to the overall opening pressures. Based on these results, predictions of airway opening times suggest that airway closure can persist through a considerable portion of inspiration when lining fluid viscosity or surface tension are elevated.