Conventional vs Neurally Adjusted Ventilatory Assist in Difficult Weaning From Mechanical Ventilation

\- Background: The NAVA is a new mode of ventilation which works from the continuous recording of the electromyogram of the diaphragm. In theory, the NAVA delivred a proportional and synchronous assistance, to respiratory efforts allowing a more adequate ventilatory support. However, no study estimates the NAVA in the weaning from mechanical ventilation. \- Purpose: The NAVA, by its more physiological aspect and a better adaptation of the patient to the ventilator with a decrease of asynchronies patients-ventilators, could decrease the duration of weaning and so decrease the duration of invasive mechanical ventilation and the morbi-mortality. The objective of this study is to compare the neurally adjusted ventilatory assist versus the usual management of weaning from mechanical ventilation, in term of duration of weaning, at the patients in period of difficult weaning from the invasive mechanical ventilation. \- Detailed description: Approximately 20 to 25 % of the patients under mechanical ventilation who have the general criteria of weaning will not pass a first SBT (Spontaneous Breathing Trial). At this moment, the management of the weaning ventilatoire are made with PSV. The NAVA is proportional assistance and depends on the intensity of the electric diaphragmatic signal (Edi). The respiratory effort of the patient is directly detected from the Edi signal. In theory, the NAVA is a proportional and synchronous respiratory. * After a first failure of a SBT, patients respecting the inclusion criteria and exlusion criteria described above are then included in the study and randomized in two groups (control group or NAVA group). * In the control group, the PSV is decreased of 2cmH2O daily or several times a day to obtain a good clinical and biologic tolerance, with a respiratory frequency between 15 and 30/min and a tidal volume at 6ml/kg. * In the NAVA group, the level of NAVA is daily adjusted to obtain Eadimax corresponding to 60 % of Eadimax of the patient during the SBT to obtain a good clinical and biologic tolerance, with a respiratory frequency between 15 and 30/min and a tidal volume at 6ml/kg. * Twice a day, the criteria of SBT are looked for. When they are present, a SBT is realized. If it is a success, the patient will be extube. If it is a failure, the patient will be reventilated in PSV or NAVA according to their group. A non-invasive ventilation preventive or of "rescue" can be administered post-extubation according to criteria. The reintubated criteria are also defined.