Gas concentration profiles along airways of dog lungs during high-frequency ventilation

Longitudinal dispersion in model of central airways during high-frequency ventilation

We have measured the longitudinal dispersion of boluses of helium, acetylene and sulphur hexafluoride in a plastic model of the human airways--generations zero through six--during high frequency ventilation (HFV). HFV was maintained by a piston pump. Frequency f and tidal volume VT ranged from 2.5 to 25 Hz and from 5 to 20 ml, respectively. Boluses were injected near the entrance of the zeroth generation (trachea), and the dispersion curves were measured by mass spectrometry at the end of the sixth airway generation. The shapes of the bolus dispersion curves could be well described with Gaussian distribution functions. With the exception of the HFV-conditions with VT = 5 ml, the effective dispersion coefficient DDISP appeared to be independent of the molecular diffusion coefficient. This independency was also found by other investigators in studies with dogs and human subjects. The measured results for DDISP for different f and VT could be satisfactorily described with the empirical equation DDISP = 0.0617 f0.8VT1.38 [cm2S-1]. Application of this equation to f and VT values normally applied in man resulted in DDISP values which should be considered to be too small for maintaining eucapnic ventilation in vivo. On the basis of this result we believe that during HFV in intubated subjects gas transport by longitudinal dispersion will be limited to the instrumental dead space--the endotracheal tube inclusive--and a few generations of large bronchi.

Multiple breath washout of He and SF6 in panting dogs

Pulmonary gas transport mechanisms in panting were studied by multiple breath washout of two poorly soluble inert gases of similar solubility but different diffusivity (He and SF6). The experiments were performed in 6 chronically tracheotomized conscious dogs (mean body weight 31.0 kg) which, upon exposure to elevated room temperature, were enforced to thermal panting (mean breathing frequency 288/min). After equilibration of lung gas with 1% He and 1% SF6 followed by changeover to test gas-free air, end-tidal gas concentrations during multiple breath washout were recorded by mass spectrometry. The washout time course was analyzed into 3 exponential components. The initial fast component was considered to be in part determined by the transient response of the measuring system, whereas the intermediate and the slow component could be attributed to lung washout. The mean He/SF6 ratio of medium and slow rate constants was 1.06 and 1.13, respectively (both values differing from 1.0 at P less than 0.001). It is concluded that gas transport in dog lungs during panting was mainly determined by convection, diffusion-dependent processes being discernible but playing a minor role.

Gas mixing in dog lungs during high frequency ventilation studied by partial washout-single exhalation technique

Gas mixing was studied in 10 anesthetized paralyzed dogs during high-frequency low tidal ventilation (HFV). After simultaneous washin of ethane (1%) and washout of resident argon (0.9%) the gas inflow was switched to atmospheric air for varied time intervals leading to varied levels of C2H6 washout and Ar washin. After the stop of HFV at predetermined test gas washout/washin levels, a constant-flow exhalation by a servo ventilator was performed and expirograms of C2H6 and Ar were recorded. Measurements were performed at varied ventilation frequencies (10-40 Hz), stroke volumes (20-40 ml), lung volumes (730-830 ml), expiratory flow rates (0.1-0.01 L/sec), breath-holding prior to exhalation (0-12 sec) and test gas washout levels achieved by varying the washout time (1 to 65 sec) before onset of exhalation. The expirograms showed a close to linearly rising alveolar plateau. They were analyzed for series dead space and alveolar slope which was normalized to the initial-to-final partial pressure difference. The normalized slopes of C2H6 washout and Ar washin were averaged, whereby the effect of shrinking lung volume due to continuing CO2/O2 exchange at low R was assumed to be suppressed. The slope was little affected by changes of stroke volume, decreased slightly with increasing frequency, and decreased considerably with breath-holding or increasing lung volume. As washout progressed, the alveolar slope first increased, attained a maximum at about half-washout and thereafter decreased. The finite values of the alveolar slope indicated that intrapulmonary gas mixing during HFV was incomplete. The slopes were larger than expected from diffusion calculations on symmetrically branching lung models. The behavior of the slope at varied washout levels suggested involvement of parallel ventilation/volume inhomogeneity coupled with sequential emptying.

Expirograms of O2, CO2 and intravenously infused C2H2 and Freon-22 during panting in dogs

To study pulmonary gas transport in panting, expirograms of several inert and respiratory gases were simultaneously measured in panting dogs. The experiments were performed on 5 conscious dogs (mean body weight 34.4 kg) provided with a chronic tracheostomy. Panting at a mean frequency of 312/min (5.2 Hz) was induced by elevated room temperature (mean 28.1 degrees C). Isotonic saline equilibrated with 50% acetylene and 50% Freon-22 was infused intravenously at a constant rate (4 ml/min). Fractional concentrations in the tracheostomy tube were measured by a respiratory mass spectrometer, using a special sampling system designed for quasi-continuous analysis of rapidly changing gas concentrations. Air flow was monitored by an ultrasonic transit-time flowmeter. A tracing of expired gas concentrations versus expired volume showed no alveolar plateau, displaying a steep increase of Freon-22, acetylene and CO2 (decrease of O2) up to the onset of inspiration. The small but statistically highly significant differences between the expirograms of CO2 and O2, and of Freon-22 and acetylene, could be qualitatively explained by ventilation-perfusion inequalities with sequential emptying, by Taylor dispersion and by reversible solution in airway mucosa in the course of the respiratory cycle.

Augmentation of CO2 elimination during high frequency oscillation by removing the bias tube--an in vitro study

In clinical applications of high frequency oscillation (HFO), sufficient CO2 elimination (VCO2) may represent a problem mainly at higher oscillation frequencies. With the intention of examining how to increase VCO2 a modified bias flow system was investigated in vitro with wash-out experiments. In bias flow systems, long tubes have been used in order to minimize the loss of oscillatory volume; however, a distinct increase of VCO2 was achieved in the present study by removing the bias tube. This improvement occurred over the whole frequency range of 2-60 Hz, although the oscillatory volume, effectively delivered to the lungs was smaller with the HFO circuit without bias tube (HFO-BT) as compared to the arrangement with bias tube (HFO + BT). A long bias tube flattens the CO2 concentration gradient from the alveoli to the atmosphere. Removing the bias tube results in a steeper CO2 concentration gradient and in a correspondingly enhanced VCO2. Furthermore, the large oscillatory volume at the exit of the bias flow system in HFO-BT supports VCO2 as an additional wash-out mechanism. Based upon longitudinal tracer gas concentration measurements between the alveoli and the atmosphere during HFO16,17, an increase of gas transport up to 20% can be expected for in vivo applications by removing the bias tube.

Helium and SF6 washout from dog lungs during high-frequency ventilation

Simultaneous washout of He and SF6 was studied in anesthetized paralyzed dogs (mean body weight 19 kg) subjected to high-frequency ventilation at varying frequencies (10-40 Hz), stroke volumes (20-40 ml), lung volumes (0.8-1.2 L) and fresh gas flow rates (7-13 L/min). The washout curves could be analyzed into three exponential components for both test gases. The rate constants of the intermediate and slow components were slightly but significantly higher for He than for SF6 while the fast component was the same for the two test gases. The data were analyzed on the basis of a series lung model with a dead space compartment and two serially arranged alveolar compartments. The He/SF6 ratio of the effective conductances for gas transfer between the alveolar compartments averaged 1.15 +/- 0.08 (SD). Since this ratio is much closer to unity, predicted for convective transport, than 6 to 7, predicted for diffusive transport, it is concluded that during high-frequency ventilation gas transport in peripheral airways occurs by both convection and diffusion, convection being quantitatively more important.

Longitudinal mixing in dog lungs during high-frequency forced flow oscillation

Longitudinal mixing in the conducting airways of eight intubated anesthetized beagles (10.8 +/- 0.9 kg) was studied at functional residual capacity in the presence of forced sinusoidal flow oscillations and in the absence of fresh air bias flow. The ranges of oscillation conditions were: frequencies, f, from 3 to 18 Hz and minute volumes, Vosc, from 50 to 150 ml/sec, corresponding to tidal volumes, Vosc/f, from 0.3 to 4.5 ml/kg body mass. Oscillations were imposed during a breath holding interval incorporated into a modified single-breath nitrogen (N2) washout maneuver. The expired N2 fraction curves were analyzed with a Fickian diffusion model by adjusting the value of a global mixing parameter, (DA2), to achieve an optimal fit of the model to the data. The mixing parameter was an increasing function of minute volume and a decreasing function of frequency, which is well represented by the equation: (DA2) = 2.72 Vosc 1.74 f-1.57 By comparison to available theory and previous measurements in physical systems, this formula implies that Taylor-type dispersion is the dominant mixing mechanism in the conducting airways. Also, the diffusion model predicted, and the data verified, the existence of a mouth-ward 'diffusion flow' during breath holding. This effect, caused by the non-uniform nature of the summed airway cross-section, is directly correlated with the value of (DA2).

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.

Intraregional gas mixing in humans during high frequency oscillations

We measured the effect of high frequency oscillation (HFO) on gas mixing in the human lung. In seven healthy, seated subjects the alveolar slope of the single breath nitrogen washout was used to assess the effects of HFO on gas mixing. A reduction in slope was interpreted as reflecting improved gas mixing within topographical lung regions. The alveolar slope was measured after 0, 10, 20 and 30 sec of control breathhold or HFO applied at the subject's TLC. We found that HFO reduced the alveolar slope more than did control breathhold. The difference in the slopes was greater with longer durations, increased stroke volume (SV) and frequency (f) of oscillations. The reduction in alveolar slope correlated better with SV than with f. We conclude that to the extend that a change in the slope in the alveolar plateau reflects intraregional mixing, this mixing is more dependent on SV than on f.

Gas exchange efficacy of high-frequency oscillatory ventilation studied by helium washout from dog lungs

The gas exchange efficacy of high-frequency oscillatory ventilation (HFOV) was assessed from an analysis of helium washout from lungs in anesthetized paralyzed supine dogs. Piston stroke volumes (Vs) were varied from 20 to 40 ml, frequencies (f), from 10 to 40 Hz and mean airway opening pressures from 2 to 10 cm H2O. The time course of washout could be described as the sum of three exponential components. Based on a series model comprising a proximal and a distal lung compartment, two component conductances, a 'distal' conductance (Gd) and a 'proximal' conductance (Gp) and an overall conductance of the lung (Gl) could be calculated. Gd, Gp and Gl increased with f up to a maximum value remaining constant or decreasing at higher f; the frequency at which the maximum occurred depending on Vs and on the diameter of the endotracheal tube (ET). With increasing Vs generally the G values increased, but decreased at higher f with the smaller ET. The insoluble inert gas washout is shown to be a useful method for assessing the ventilatory gas exchange conductance of lungs during HFOV.

Distribution and uptake of helium, carbon monoxide, and acetylene in the lungs during high frequency oscillatory ventilation

In order to obtain a better understanding of intrapulmonary gas mixing and alveolar-capillary gas transport during high frequency oscillatory ventilation (HFO), we measured insoluble gas (He) equilibration, and soluble gas (CO, C2H2) uptake in the lungs of ten anesthetized dogs during closed system HFO (i.e. no fresh gas bias flow). These gases were introduced as a bolus into the lumen of an endotracheal tube and their concentrations were subsequently measured for 20-25 sec from a catheter in the distal end of this tube. Analysis of He concentrations over time was performed using a two compartment series model to calculate a value for effective ventilation (Veff). This Veff was found to range from 0.83 to 23.8 L/min and was directly related to oscillator output (f X VT product, r = 0.77). Analysis of CO and C2H2 concentrations during HFO using a similar two-compartment model having alveolar capillary gas transport in series with Veff allowed for the calculation of pulmonary capillary blood flow (QHFO) and lung diffusing capacity (DHFO). These values for QHFO were found to be not significantly different from simultaneous thermodilution determinations of cardiac output and these values for DHFO were found to be not significantly different from single breath or rebreathing determinations of CO diffusing capacity. Moreover, QHFO and DHFO did not vary with Veff. We conclude that this two compartment in series model is a reasonable way to characterize insoluble and soluble gas behavior during HFO, that Veff is related to oscillator output, and that QHFO and DHFO are not affected by HFO over the range of Veff studied.