- contraindication for prone position ventilation: suspected high intracranial pressure, massive hemoptysis requiring an immediate surgical or interventional radiology procedure, tracheal surgery or sternotomy during the previous 15 days, serious facial trauma or facial surgery during the previous 15 days, cardiac pacemaker inserted in the last 2 days, unstable spine, femur, or pelvic fractures, single anterior chest tube with air leaks, body constitution that is incompatible with prone position ventilation (i.e. high bmi) - admitted under duress (psychiatry). - pregnant or breastfeeding. - mechanical ventilation in prone position prior to inclusion in the trial

- contraindication for prone position ventilation: suspected high intracranial pressure, massive hemoptysis requiring an immediate surgical or interventional radiology procedure, tracheal surgery or sternotomy during the previous 15 days, serious facial trauma or facial surgery during the previous 15 days, cardiac pacemaker inserted in the last 2 days, unstable spine, femur, or pelvic fractures, single anterior chest tube with air leaks, body constitution that is incompatible with prone position ventilation (i.e. high bmi) - admitted under duress (psychiatry). - pregnant or breastfeeding. - mechanical ventilation in prone position prior to inclusion in the trial