Inclusion Criteria:  Age > 18  Acute respiratory insufficiency defined by the three following items :  PaO2/ FiO2 <300 Respiratory rate > 25/min Oxygen support > 10L/min or High Flow nasal oxygen (HFNO) Non opposition by the patient to be included Social insurance  Exclusion Criteria:  Respiratory acidosis (pH < 7,35 PaCO2 > 45mmHg or 6kPa) Chronic respiratory disease ( COPD, bronchiectasis…) Cardiogenic Acute lung oedema Non intubation decision at randomisation Contraindication for Electro-impedance tomography-belt placement (chest trauma with rib fractures, thoracic wound, chest tube). Uncommunicative patient (Glasgow coma scale <12) Guardianship or curators for vulnerable patients.