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.

Cross-over study, according to 4 preset sequences of 4 periods and randomly assigned. All patients will thus have the classic positioning technique (PP), as well as the technique under study (verticalization). For simplicity, the investigators will use the term "verticalization" even if the final positioning that is targeted is only a "pseudo-verticalization" to a 55° angle. The cross-over scheme with 4 sequences of 4 periods allows to get rid of the first order carry-over effect.

Progressive verticalization protocol in 7 steps from the strict supine position to a 55° inclination of the raising bed, according to the clinical responses of the patient with ARDS, as follows:

- Verticalization in the bed

The verticalization maneuver will take place in 7 steps of 15 minutes maximum each:

S1. The first one corresponds to the flat position (strict dorsal decubitus) S2. The second is the positioning of the patient at 25° of inclination. S3. The third is the 45° positioning S4. The fourth at 55°, which corresponds to the greatest inclination imposed to the patient.

S5. The fifth corresponds to the 45° return S6. The sixth is the return to 25° S7. Finally, the investigators reposition the patient flat back.

Examinations:

• Arterial Gazometry At the beginning, at the end and one hour after each positioning technique.
• Chest ultrasound
• Others All adverse events will be recorded in a clear and detailed manner.

Inclusion Criteria:

• Major patient > 18 years of age
• ARDS patients with PaO2/FiO2 ratio < 150 at any time within 24 hours prior to the screening visit
• Hemodynamically stable patient with mean arterial pressure (MAP) > 65 mmHg maintained without vasopressor or with norepinephrine at a dosage of less than 0.5 µg/kg/min.
• Patient having already undergone at least 1 but less than 5 PP sessions
• Patient under continuous intravenous sedation (IVSE) or not responding to simple order.
• Written consent of support person or family.

Exclusion Criteria:

• Pregnant women (positive pregnancy test during screening)
• Breastfeeding women
• Protected Majors
• Body weight greater than 198 kg (bed load limit)
• Patient with one or two lower limbs amputated at the trans tibial or upper level
• Contraindication to standing (orthopedic fracture, neurological instability with the presence of an intracranial pressure sensor or an external ventricular shunt....)
• Deep venous thrombosis of the lower limbs with curative anticoagulation for less than 48 hours
• Hemodynamic instability (MAP < 65 mm Hg) despite the use of norepinephrine at a dosage equal or greater than 0.5 µg/kg/min.
• Person under guardianship or trusteeship
• Non-beneficiary patient of a health insurance plan
• Moribund patient