Characterization of lung geometry from the single-breath nitrogen washout test

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.

A reappraisal of boundary conditions assumed in pulmonary gas transport models

Solutions of the classic pulmonary gas transport equation are presented in which a true 'no-flux' boundary condition is specified throughout the respiratory cycle. For the particular models studied it is demonstrated that diffusive mixing is incomplete at end expiration, and that such stratified inhomogeneities give rise to a realistic alveolar plateau for a simulated N2 washout test. The reasons for the disparity of the present findings with those obtained by contemporary workers are explained by critically examining the boundary conditions conventionally assumed at the alveolar wall.