Mode NAVA spontaneous breathing select a NAVA level that gives peak pressure about 9-10 cm H2O assist keep NAVA level for 15 minutes, do ZAM breaths every 3 minutes (make sure not to disturb the patient). Insp Hold after every ZAM.~Increase NAVA level by 50% and repeat the protocol when patient's breathing has stabilized (only one increase).Do ZAM breaths every 3 minutes (make sure not to disturb the patient). Insp Hold after every ZAMAt the end of protocol (minute 15) do one end-inspiratory hold-maneuver and one end-expiratory occlusion.~Mode N- PSV spontaneous breathing :PS level of 8-10 cmH2O, keep PS level for 15 minutes, do ZAM every 3 minutes (make sure not to disturb patient). Inspiratory Hold after every ZAM。At the end of protocol (minute 15) do one end-inspiratory hold-maneuver and one end-expiratory occlusion.Increase PS by 50% and repeat protocol when patient's breathing has stabilized (only 1 increase). Do ZAM every 3 minutes (make sure not to disturb patient). Inspiratory Hold after every ZAM.At the end of protocol (minute 15) do one end-inspiratory hold-maneuver and one end-expiratory occlusion.~Make record of RR, Vt, Flow, Ti during both periods.~Mode VCV no spontaneous breathing：Set Vt, Ti and RR to match the breathing pattern observed during NAVA~Mode PCV no spontaneous breathing-do ZAM after each PS level：Increase PSV level from 8 to 20 in steps of 1 cmH2O every 2 minutes.
Stable patient with ARF spontaneously breathing light sedation (not too awake but with stable breathing pattern) and EAdi >8 uV. Four modes of mechanical ventilation were performed under different support level for the measuring of EAdi, air way pressure and other parameters.