Regional gas distributions and single-breath washout curves in head-down position.

Regional distributions of inspired 133Xe and single-breath washout curves were compared in six young healthy subjects for the upright and the head-down positions. The regional distributions of volumes (at 0, 25, 50, and 75% vital capacity, VC) and of 133Xe boluses inhaled at residual volume (RV) were inverted in the head-down position, thus behaving as if they were determined by gravity acting via the weight of the lung rather than by thoracicoabdominal shape adaptations. Nevertheless no mirror image was obtained. The vertical differences in regional distribution of the 133Xe RV bolus and of the volumes at 25% VC were increased in the head-down position, whereas the vertical difference in volumes at RV was decreased, indicating enhanced air trapping and sequential ventilation at low volumes. This was attributed to the effect of the increased pulmonary blood volume in the head-down posture. Accordingly the size of phase IV on the washout curves with the SF6-bolus as well as with the N2-resident gas method was increased in the head-down position.