Comparative Effects on Diaphragmatic Electrical Activity and Respiratory Pattern of Various Levels of Assistance

Inclusion Criteria: \- Patients admitted to the ICU, intubated for acute respiratory failure, and ventilated in PS mode will be eligible for inclusion in the study. Exclusion Criteria: * Severe hypoxemia requiring an FIO2 \> 0.5 * hemodynamic instability (defined as the need for high doses of catecholamines namely ≥ 0.5 μg/kg min- and/or high need in volume resuscitation namely more than two litres of crystalloids in two hours-) * oesophageal problem such as hiatal hernia, oesophageal varicosities, oesophageal surgery; * active upper gastro-intestinal bleeding; * known coagulation disorder; * any other contraindication to the insertion of a naso-gastric tube; * poor short term prognosis (death expected within the next seven days) * age \< 18 years old.