Intrinsic PEEP was measured during a four-second end-expiratory occlusion period. Pplat was measured during a 0.5-second end-inspiratory pause. Respiratory system compliance (Crs) was calculated as Crs = V T / (Pplat -PEEP total ). Responders to PP were defined in two different ways: (1) an increase in PaO 2 /FiO 2 ratio > 20 mmHg after 15 hours of PP or (2) a decrease in PaCO 2 > 2 mmHg after 15 hours of PP.