Comparison of Breathing Pattern During Neurally Adjusted Ventilatory Assist Ventilation and Pressure Support Ventilation

Electrical activity of the diaphragm will be obtained through a naso-gastric tube with multiple array of electrodes placed at its distal end (Eadi catheter® , Maquet Critical Care, Sweden). Subject will be ventilated with a mouthpiece interface. NAVA will be compared to Pressure Support using the same ventilator. Once the pattern of breathing will be stabilized (10 minutes) in each condition (see arm) the following parameters will be recorded during 5 minutes and compared : tidal volume, inspiratory time, expiratory time, transcutaneous CO2 pressure, diaphragmatic Emg activity, delay between the inspiratory activity initiation and positive pressure initiation, delay between the end of the inspiratory effort and the end of pressurisation.