The transition to unassisted breathing after invasive ventilation often proves challenging. Persistent ventilator dependence predisposes patients to nosocomial complications and death and increases the economic burden of critical illness. Ventilator-dependence results from an imbalance between the load and capacity of the respiratory muscle pump. Patients who fail a trial of spontaneous breathing commonly exhibit excess respiratory loads secondary to weaning-induced pulmonary edema, atelectasis or dynamic hyperinflation. At the same time, many ventilator-dependent patients exhibit striking loss of respiratory pump capacity due to diaphragm dysfunction which predisposes to prolonged ventilator dependence. Diaphragm dysfunction is common in ventilated patients. By prolonging ventilator dependence it may be an important contributor to the poor long-term clinical and functional outcomes of survivors of critical illness. While some main risk factors for diaphragm dysfunction have been already described (diaphragm disuse induced by mechanical ventilation, sepsis, initial severity upon admission), the determinants of recovery of diaphragm dysfunction are unknown, as well it has not been elucidated whether diaphragm function can simply improve after the acute phase of ICU admission. Therefore, the goal of this study is to investigate the time course evolution of diaphragm function in patients exposed to prolonged duration of mechanical ventilation (i.e. in a weaning center) and to determine which factors are associated with an improvement of the diaphragm function leading to a safe mechanical ventilation discontinuation.

In this prospective study, all patients admitted to the weaning center of Pitié-Salpêtrière Hospital to undertake a weaning program will be closely followed up to assess twice a week their diaphragm function. Only one experimental group will be constituted. Diaphragm function will be assessed with both 1) the phrenic nerves stimulation technique and 2) diaphragm ultrasound (thickness, thickening fraction). The time course evolution of the diaphragm function over the stay will be investigated and all potential factors involved in the change in diaphragm function will be determined.

Inclusion Criteria:

• Age > 18 years old;
• Admission in weaning center;
• Tracheostomy;
• Ventilator dependence as defined by clinical intolerance criteria under spontaneous - breathing (Boles et al. ERJ 2017);
• Oral consent

Exclusion Criteria:

• Contre indication to the phrenic nerves stimulation technique (pace maker);
• Impossible liberation from the ventilator (degenerative neuromuscular diseases; high level spinal injury);
• Pregnancy;
• No insurance coverage