Acute Respiratory distress syndrome (ARDS) is a severe condition in which protective ventilation is a critical point in its management. Positive end expiratory pressure (PEEP) setting can be challenging for clinicians and high PEEP has been associated with better outcome in moderate and severe ARDS. Recently, recruitment to inflation ratio and airway closure have been investigated in order to help PEEP adjustment. However, ventilatory maneuvers are performed with a low level of PEEP and therefore expose to derecruitment and oxygen desaturation. So far, the risk of oxygen desaturation has not been investigated and risk factors are unknown. The aim of this study is to evaluate the prevalence of oxygen desaturation during ventilatory maneuvers at low level of PEEP in patients with moderate or severe ARDS