Figure 5a. Percentage of alveolar expansion along cephalocaudal axis. Zero line indicates cutoff between alveolar expansion and contraction. (a) In prone group, alveolar expansion was observed in caudal regions and was most prominent in lung regions below diaphragmatic cupola. In upper lung, with 5-6 cm above the diaphragm as a turning point, alveolar contraction occurred. (b) In supine group, alveolar expansion induced by low PEEP (3 cm H2O) occurred predominantly in cephalic lung regions. Alveolar contraction was not observed. DD = dome of diaphragm.