NONINVASIVE RECOVERY OF ACINAR ANATOMIC INFORMATION FROM CO2 EXPIROGRAMS

A numerical single path model of respiratory gas exchange with distrib uted alveolar gas sources was used to estimate the anatomical changes in small peripheral airways such as occur in chronic obstructive pulmo nary diseases (COPD). A previous sensitivity analysis of the single pa th model showed that decreasing total acinar airway cross-sectional ar ea 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 heal thy subjects and six COPD patients, optimized area reduction factors f or the single path model were found that characterize peripheral airwa y anatomy for each subject. Area reduction factors were then combined with measured functional residual capacity data to calculate the norma lized peripheral airspace diameters in a given subject, relative to th e airspace diameters in the generations of an idealized standard lung. Mean area reduction factors for the patient subgroup were 63% of thos e for the healthy subgroup, which is related to the gas transport limi tation observed in disease. Mean airspace sizes for the patient subgro up 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 con ductance in COPD patients was calculated to be 32% of the mean value i n the healthy subjects. These findings correlated well with standard p ulmonary function test data for the patients and yield the recovery of acinar airway information from gas washout by combining the single pa th model with experimental measurements.