High frequency percussive ventilation (HFPV) is a high frequency ventilation mode in which high frequency pulses accumulate to generate a low frequency tidal volume at a controlled airway pressure. Previous studies have suggested a protective effect of HFPV on lung injury. The goal of our study is to compare, in early ARDS patients, the effects of conventional mechanical ventilation and of high frequency percussive ventilation on alveolar recruitment and distension evaluated by CT-scan

Inclusion Criteria:

• Patient undergoing invasive ventilation for more than 24h
• ARDS patient

Exclusion Criteria:

• Pneumothorax drained or not
• Patient undergoing extracorporel mechanical oxygenation
• Patient who had a pneumonectomy partiel or total