Inert gas mixing in the upper and central airways of man

The dose-response relationship in older men exposed to ozone

Nine male subjects (56-71 years) were exposed to filtered air (FA) or 0.45 ppm O3 with inhaled dose altered by varying ventilation volume (VE = 25, 40, or 55 L/min (BTPS)) to evaluate pulmonary function dose-responses in older men. The subjects walked on a motor-driven treadmill and rested in alternating 20-min periods during the 2-h exposures. Functional residual capacity and maximal voluntary ventilation (MVV) were measured pre- and postexposure. Forced expiratory volume in 1 s (FEV1.0) and forced expiratory flow rate between 25% and 75% of vital capacity (FEF25-75%) were calculated from FVC. Residual volume (RV) was calculated from FRC. VE was measured during the last 4 min of each exercise period, and the electrocardiogram was monitored throughout each exposure. There were no significant changes in any measured parameter with FA exposure. All three O3 exposures induced small, statistically significant decrements in FVC and FEV1.0, but no significant changes in FEF25-75%. There were no significant differences among responses to the three O3 exposures. The decrements in measured pulmonary functions were smaller than typically observed in young males exposed to similar inhaled doses of O3. The data suggest possible saturation of the pulmonary response mechanism at a low degree of functional impairment in older men. Reflex-mediated impairment of inspiration, with concomitant reduction in all dependent pulmonary functions has been suggested as the mechanism of pulmonary dysfunction with O3 exposure in young adults. The results suggest that this reflex may be less active in older adults.

Noninvasive recovery of acinar anatomic information from CO2 expirograms

A numerical single path model of respiratory gas exchange with distributed alveolar gas sources was used to estimate the anatomical changes in small peripheral airways such as occur in chronic obstructive pulmonary diseases (COPD). A previous sensitivity analysis of the single path model showed that decreasing total acinar airway cross-sectional area by an area reduction factor, R, results in computed gas expirograms with Phase III steepening similar to that observed in COPD patients. From experimental steady state CO2 washout data recorded from six healthy subjects and six COPD patients, optimized area reduction factors for the single path model were found that characterize peripheral airway anatomy for each subject. Area reduction factors were then combined with measured functional residual capacity data to calculate the normalized peripheral airspace diameters in a given subject, relative to the airspace diameters in the generations of an idealized standard lung. Mean area reduction factors for the patient subgroup were 63% of those for the healthy subgroup, which is related to the gas transport limitation observed in disease. Mean airspace sizes for the patient subgroup were 235% of the healthy subgroup, which characterizes the increase in size and reduction in number of peripheral airspaces due to tissue erosion in emphysema. From these results, the air-phase diffusive conductance in COPD patients was calculated to be 32% of the mean value in the healthy subjects. These findings correlated well with standard pulmonary function test data for the patients and yield the recovery of acinar airway information from gas washout by combining the single path model with experimental measurements.

Modelling steady state pulmonary elimination of He, SF6 and CO2: effect of morphometry

We studied the influence of acinar morphometry on the shape of simulated expirograms computed from a single path convection-diffusion model that includes a source term for gas evolution from the blood (Scherer et al., J. Appl. Physiol. 64: 1022-1029, 1988). Acinar structure was obtained from published data of 3 different lung morphometries. The simulations were performed over a range of tidal volumes (VT) and breathing frequencies (f) comparable to those observed in a previously reported human study. Airways dead space (VDaw) increased with VT in all the morphometric models tested and in the experimental data. The increase in VDaw with VT was inversely related to the diffusivity of the evolving gas and to the rate of increase in airway cross-section of the most mouthward (proximal) alveolated generations of the models. Normalized phase III slope for all the gases decreased with increasing VT in all the models as was previously reported for healthy human subjects. In the model simulations, the greatest sensitivity of phase III slope to VT was seen with the least diffusible gas using the airway morphometry with the smallest cross-sectional areas in the proximal alveolated generations. We conclude that both VDaw and phase III slope of an evolving gas are sensitive to the geometry of the proximal acinar airways and that this is manifest by their dependence on tidal volume, breathing frequency, molecular diffusivity and alveolar/blood source emission rate. The model simulations indicate that heterogeneity of gas washout is not required to explain the magnitude of the phase III slope in healthy human subjects.