 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. 