The study was conducted during the first session of PP. Our sessions routinely last 15 to 18 hours per day. Blood gas analysis, Pplat, total PEEP, end-tidal CO 2 (P etCO2 ) and mixed expired CO 2 (P ECO2 ) were recorded with the patient in the supine position, just before turning the patient to the PP, and every 3 hours in the PP until 15 hours had elapsed. Expired CO 2 was measured by a sensor positioned between the proximal end of the endotracheal tube and the Y piece of the ventilator circuit (COSMO; Novametrix, Wallingford, CT, USA). The ratio of VD/V T was calculated using the simplified Bohr equation [18] as follows: (1) VD alv /V T = 1 -P etCO2 / PaCO 2 and (2) VD physiol /V T = 1 -P ECO2 /PaCO 2 .