All patients randomly underwent a 30 mins PSV and NAVA crossover ventilation (15 mins each) at progressive PEEPe levels. Progressive levels of PEEPe were applied stepwise in increments of 40% of baseline static PEEPi, every 30 mins, from zero to 160% of baseline static PEEPi during the protocol (0, 40%, 80%, 120% and 160% of static PEEPi, respectively). During PSV, pressure support level was set to meet a Vt 6ml/kg, I/E cycling was 30% of the maximum inspiratory peak flow, flow trigger was 1L/min, and inspired fraction of oxygen (FiO2) was set to the similar level before the study protocol. During NAVA, NAVA Level was set at 15 cmH2O / µV and a peak airway pressure limit was set in order to apply the same inspiratory pressure support at each PEEPe level during PSV. The new setting of NAVA was defined as NAVA15. Moreover the trigger was set at 0.5 µV and the I/E cycling fixed at 70% of the peak of EAdi. During the entire protocol, FiO2 and peak air way pressure were maintained the same levels at each PEEPe level between PSV and NAVA. Arterial blood gases were measured at the end of PSV or NAVA15 ventilation at each PEEPe level.
