Seven patients were classified as "PaO 2 responders" and six were classified as "PaO 2 nonresponders" according to PaO 2 /FiO 2 ratio changes. No differences in VD alv /V T ratios or PaCO 2 or Pplat alterations during PP were observed between groups (Table 3 and Figure 1 ), whereas Crs increased more in the responders (Table 3) . Seven patients were also classified as "PaCO 2 responders" and six as "PaCO 2 nonresponders" according to the PaCO 2 changes. However, when compared with the PaO 2 /FiO 2 classification, four patients were classified differently. As shown in Table 4 and Figure 2 , VD alv /V T , PaO 2 /FiO 2 , PaCO 2 , Pplat and Crs were significantly more altered in responders than in nonresponders. As shown in Figure 3 , we found no correlation between changes in VD alv /V T and changes in PaO 2 /FiO 2 (P = 0.95), whereas we found a negative correlation between changes in VD alv /V T and changes in Crs (r = 0.29, P = 0.03).