Regional lung volumes with positive pressure inflation in erect humans

Abstract Four seated subjects attained a preselected lung volume (V O ) in two ways: voluntarily, and by relaxing against a positive airway pressure of 14–16 cm H 2 O; V O were 1.0–1.5 L below total lung capacity. Using Xe we measured the regional distribution of lung volumes at V O under both conditions. The apex to base differences in regional expansion and, by implication, regional pleural pressure differences were the same in both conditions. All regional volumes were slightly less during positive pressure inflation, implying an increase in regional TLC, presumably due to a shift of blood volume out of the thorax.