 Neurally Adjusted Ventilatory Assist NAVA is a new mode of mechanical ventilation that is controlled by the electrical activity of the diaphragm EAdi. The EAdi is a signal that represents the patients breathing effort, and hence with NAVA, the assist being delivered is synchronized and proportional to the demands of the patient. This is a prospective physiological study of the feasibility of NAVA ventilation over 24 hours. The aim is to demonstrate that NAVA can maintain spontaneous breathing and unload the respiratory muscles during both sleep and wake cycles over a 24 hour period. 