Driving Pressure Variation: NAVA vs PSV

Any patient who is already on an assisted mode of ventilation and displays triggering efforts will be enrolled in the study and will be submitted to 3 ventilation trials, in PSV and NAVA ventilation modes; each trial will last 20 minutes. Every trial will be performed in a randomized order, based on random computer generated sequences. During the first trial, PSV will be set in order to obtain a Vt between 6 and 8 ml/kg;this support level will be defined as PSV100. Subsequently, the corresponding NAVA level (NAVA 100) will be determined using a dedicated ventilator function (NAVA Preview) which is able to estimate NAVA level in order to deliver an equivalent inspiratory peak pressure (Paw peak) compared to that obtained during PSV mode. Afterwards, pressure support level of assistance of PSV100 and NAVA100 will be firstly increased (PSV150 and NAVA150) during the second trial and then decreased during the third trial (PSV50 and NAVA150) by 50% from basal value. During the study period, PEEP and FiO2 will be kept equal to the values in use before patient enrollment. End-inspiration and end-expiration pauses will be performed at the end of each trial by pressing the dedicated button on the ventilator control panel. Airway pressure and flow will be recorded. Patients, as usual, clinical practice, will be sedated at different levels and this could compromise their content of consciousness. At the beginning of each trial, an endotracheal tube suction will be done. Last 5 minutes of each trial will be recorded and stored in a computer for subsequent statistical analysis. Respiratory mechanical indices (airway pressure, tidal volume, flow) and electric diaphragmatic activity will be recorded by a dedicated software called NAVA- tracker. At the end of each trial an arterial blood gas analysis (ABGs) will be performed to evaluate PaCO2, PH and blood oxygenation (PaO2). at the end of each trial, an ultrasound evaluation of diaphragm will be performed.