Comparison of lung diffusing capacity during rebreathing and during slow exhalation

In five normal sitting subjects DLCO and Qc were measured from the disappearances of a stable isotope of carbon monoxide (C18O) and of acetylene with respect to an inert and insoluble reference gas (Helium). Measurements were made during two respiratory maneuvers: (1) during rebreathing both at functional residual capacity (FRC) and near total lung capacity (TLC); and (2) during a slow exhalation at a constant rate from TLC to FRC. Changes in gas concentration were measured at the mouth during both maneuvers with a respiratory mass spectrometer. Mean DLCO was significantly higher during rebreathing near TLC (34.6 ml . min-1 . mm Hg-1) than near FRC (28.8 ml . min-1 . mm Hg). Mean DLCO measured during slow exhalation near FRC (32.7 ml . min-1 . mm Hg) was significantly higher than DLCO measured during rebreathing over the same volume range. Measurements of Qc were not significantly different between the rebreathing and slow exhalation maneuvers. Differences in DLCO between the two methods at FRC were not due to differences in Qc.