 - 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. 