EFFECT OF VOLUME HISTORY ON CHANGES IN DLCOSB-3EQ WITH LUNG-VOLUME IN NORMAL SUBJECTS

The purpose of this study was to determine the relationship between the three-equation diffusing capacity for carbon monoxide (DL(co)SB-3EQ) and lung volume and to determine how this relationship was altered when maneuvers were immediately preceded by a deep breath. DL(co)SB-3EQ maneuvers were performed in nine healthy subjects either immediately after a deep breath or after tidal breathing for 10 min. The maneuvers consisted of slow inhalation of test gas from functional residual capacity to 25, 50, 75, or 100% of the inspiratory capacity and, without breath holding, slow exhalation to residual volume. After either a deep breath or tidal breathing, we found that DL(co)SB-3EQ decreased nonlinearly with decreasing lung volume. At all lung volumes, DL(co)SB-3EQ was significantly greater when measured after a deep breath than after tidal breathing. This effect increased as lung volume decreased, so that the greatest difference between DL(co)SB-3EQ after a deep breath and that after tidal breathing occurred at the lowest lung volume. We conclude that a deep breath or spontaneous sigh has a role in reestablishing the pathway for gas exchange during tidal breathing.