Respiratory Mechanics Assessment at Different Head of the Bed Elevations in Mechanically Ventilated Patients

Patients will be sequentially positioned at 0, 10, 20, 30, and 40 degrees of head-of-bed elevation. An alveolar recruitment maneuver will be performed. For patients with body mass index ≤ 30 kg/m\^2, the maneuver will be conducted in pressure control mode, pressure control = 15 cmH2O, respiratory rate = 20 breaths per minute, and the PEEP will be increased in steps of 5 up to 30 cmH2O. For patients with body mass index \> 30, the PEEP will be increased up to 35. Then, a PEEP titration will be performed, tidal volume = 5 mL/Kg, respiratory rate = 25 breaths per minute, and the PEEP will be decreased from 24 down to 4 cmH2O in steps of 2 cmH2O with 30 seconds in each PEEP level. The PEEP titration software of Enlight 2100 will be used to determine the ideal PEEP, defined as the PEEP level with a collapse less than 5%. The alveolar recruitment maneuver will be performed again to reopen the lungs. Then, data will be collected, as with the 0 and 40-degree steps, with ideal PEEP.