* unable to achieve a prone position for mobility reasons, * unable to achieve a prone position due to agitation whatever the cause, * with clinical occlusive syndrome in order to limit the risk of inhalation, * having a contraindication to the use of the esophageal catheter, * having signs of respiratory distress or disturbance of consciousness requiring intubation within the next hours, * having hypercapnia indicating the use of non-invasive ventilation (pao2\> 50 mmhg), * having severe hypoxemia defined by pao2 / fio2 \<100mmhg, * ongoing pregnancy or breastfeeding, * subject to a measure for the protection of justice.

* unable to achieve a prone position for mobility reasons, * unable to achieve a prone position due to agitation whatever the cause, * with clinical occlusive syndrome in order to limit the risk of inhalation, * having a contraindication to the use of the esophageal catheter, * having signs of respiratory distress or disturbance of consciousness requiring intubation within the next hours, * having hypercapnia indicating the use of non-invasive ventilation (pao2\> 50 mmhg), * having severe hypoxemia defined by pao2 / fio2 \<100mmhg, * ongoing pregnancy or breastfeeding, * subject to a measure for the protection of justice.

- unable to achieve a prone position for mobility reasons, - unable to achieve a prone position due to agitation whatever the cause, - with clinical occlusive syndrome in order to limit the risk of inhalation, - having a contraindication to the use of the esophageal catheter, - having signs of respiratory distress or disturbance of consciousness requiring intubation within the next hours, - having hypercapnia indicating the use of non-invasive ventilation (pao2> 50 mmhg), - having severe hypoxemia defined by pao2 / fio2 <100mmhg, - ongoing pregnancy or breastfeeding, - subject to a measure for the protection of justice.

- unable to achieve a prone position for mobility reasons, - unable to achieve a prone position due to agitation whatever the cause, - with clinical occlusive syndrome in order to limit the risk of inhalation, - having a contraindication to the use of the esophageal catheter, - having signs of respiratory distress or disturbance of consciousness requiring intubation within the next hours, - having hypercapnia indicating the use of non-invasive ventilation (pao2> 50 mmhg), - having severe hypoxemia defined by pao2 / fio2 <100mmhg, - ongoing pregnancy or breastfeeding, - subject to a measure for the protection of justice.