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Quantitative characteristics of the dynamic mechanical and gas distribution behavior in 6 normal subjects and 5 subjects with COPD were compared during tidal breathing. Transpulmonary pressure, total lung volume, flow, and N2 fraction at the mouth were measured while N2 was washed out from the lung. The washouts were performed at several frequencies and lung volumes. As an index of nonuniform mechanical behavior, we calculated the frequency variation in dynamic pulmonary compliance (CLdyn). Based on a moment analysis of the multibreath N2 washout, we calculated a mean dilution number (MDN) as an index of the inhomogeneity of alveolar gas distribution and mixing. At FRC or above FRC the CLdyn decreased much more with frequency for the COPD subjects than for the normal subjects. The MDN was also much greater in the presence of COPD. However, the frequency dependence of the MDN was small for both the normal and COPD subjects and uncorrelated with the frequency dependence of CLdyn. Because the multibreath N2 washout and the frequency dependence of CLdyn reflect different aspects of ventilation inhomogeneity, these two responses are unique.
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