The study stopped because it was difficult to recruit the 130 expected patients

Lung recruitability is essential for optimal Positive end-expiratory pressure (PEEP) selection in ARDS patients. It is defined as the potential for the non aerated or poorly aerated lung mass to become aerated due to the increase in airway pressure. PEEP contributes to lung recruitment mostly by maintaining some amount of the end-inspiratory recruitment at the end of expiration. PEEP also stabilizes patency of the small airways and minimizes the repeated opening and closing of them during the breathing cycle, which is implicated in a further lung inflammation. The gold-standard method for assessing lung recruitability is lung CT scan. For economic and feasibility this technique cannot be used in routine. Therefore, techniques that can be used at the bedside to measure lung recruitability are very well known. The measurement of recruited lung volume (Vrec) by using pressure-volume curve generated by the ventilator is another reference method to approach lung recruitment. It can be done at the bedside. Chest-X-Ray (CXR) is an interesting option as done in routine in this setting. Furthermore, it allows quantifying aeration thanks numerical image processing and a regional approach. In a preliminary one-center study we found a significant negative correlation between the amount of Vrec and the reduction in lung density measured by digital CXR between 5 and 15 cm H2O PEEP. In present study we would like to extend this previous result on a larger number of patients in a multicenter investigation.

Inclusion Criteria:

• intubation and mechanical ventilation in the ICU
• Ramsay score 6 under sedation and analgesia
• ICU respirator implemented with pressure-volume curve device
• age equal to or greater than 18 years
• ARDS defined from the Berlin criteria
• absence of pneumothorax on the CXR before the study
• Absence of pleural effusion greater than 500 ml estimated from ultrasonography.
• no child-bearing woman
• written inform consent signed by the next of kin

Exclusion Criteria:

• Pneumothorax
• Pleural effusion greater than 500 ml estimated from ultrasonography
• Thoracic surgery in the last 3 months
• Contra-indication to CXR
• contra-indication to PEEP of 15 cm H2O
• contra-indication to PEEP of 15 or PEEP 15 mandated
• pressure-volume curve not feasible
• refusal to participate
• language barrier of the next of kin
• child-bearing woman
• person under legal protection