A significant increase in PaO 2 /FiO 2 ratio occurred after 15 hours of PP, from 70 mmHg (51 to 77) in the supine position to 99 mmHg in the prone (83 to 139) (P < 0.0001) ( Table 2) . A significant decrease in PaCO 2 was also observed, from 58 mmHg (52 to 60) to 52 mmHg (47 to 56) (P = 0.04) ( Table 2) , with the lowest value occurring after nine hours of PP. As noted in Table 2 , Pplat was significantly reduced (P = 0.0004) and Crs improved (from 16 mL/cmH 2 O (13 to 30) to 18 mL/ cmH 2 O (15 to 30); P = 0.02). Finally, the VD alv /V T ratio was significantly reduced from 0.42 (0.35 to 0.47) to 0.40 (0.26 to 0.45), with the lowest value occurring after three hours in PP (hour 3) (0.31) ( Table 2) .