The investigators aim is to study the specific lung elastance in ALI/ARDS patients during invasive mechanical ventilation, in supine and prone position.

In sedated and paralyzed ALI/ARDS patients, we study and compare the specific lung elastance in supine and prone position during invasive mechanical ventilation.

Inclusion Criteria:

• ALI/ARDS patients during invasive mechanical ventilation

Exclusion Criteria:

• Haemodynamic failure, pneumothorax