Acute Respiratory Distress Syndrome (ARDS) is defined according to the Berlin definition (1) as diffuse lung damage occurring in patients with a predisposing risk factor. Positioning in the prone position (PP) has been shown to decrease mortality in patients with moderate to severe ARDS. However, this technique is not without deleterious effects such as ventilator-associated pneumonia, endotracheal tube obstruction, development of pressure ulcers, and increased workload for the caregivers. There are other positioning techniques such as the upright position, which simulates a relative verticality, which allows to increase the effects of the prone position and even in some patients to improve oxygenation without the PP in the acute phase of ARDS. However, given the revolution caused by the use of PP in ARDS patients, verticalization have not been studied in more details. Today, there is a bed on the market that allows patients to be truly upright without having to transfer them to a tilt table. The investigators believe that raising ARDS patients in the acute phase is safe and feasible in routine practice. In this research protocol comparing PP and verticalization in a crossover trial design in acute ARDS patients, the investigators want to show that this technique can be safe and feasible, with the same effects on oxygenation as PP.