Comparison Between NAVA and PSV in Neurocritical Patients

The aim of this pilot study is to compare NAVA and PSV in order to establish their influence on cranial blood flow, assessed in the mean cerebral artery (M1 tract bilaterally). Secondarily, it will be evaluated the effectiveness in guaranteeing the gas exchange and patient-ventilator interaction during each ventilation mode. Patients will be evaluated in order to establish their readiness for assisted mechanical ventilation. The following clinical criteria must be respected: * a heart rate of less than 120 bpm, blood pressure between 90 and 180 mmHg; * dopamine/dobutamine less than 5 mcg/kg/minute, norepinephrine less than 0.25 mcg/kg/min; * PaO2/FiO2 \> 150; * PEEP (positive end-expiratory pressure) minor or equal to 10 cmH2O; * Vt between 6 and 8 ml/kg; * respiratory rate less than 35 breaths per minutes and without respiratory distress signs; * blood pH between 7.35 and 7.45; Afterward, patients will be submitted to three different mechanical ventilation trials in partially assisted modes: the first one in PSV mode, the second one in NAVA mode and the third one in PSV mode again. Each trial will last 30 minutes. PSV will have PEEP set by clinician and pressure support will be set in order to obtain Vt between 6 and 8 ml/kg. NAVA will have the same PEEP as PSV and NAVA level will be set to obtain the same peak of Inspiratory pressure of PSV. The expiratory trigger will be adjusted to the 50% of the inspiratory flow peak in PSV. During NAVA, the expiratory trigger is fixed to 75% of diaphragmatic electrical activity. The inspiratory fraction of oxygen (FiO2) will be set in order to obtain a peripheral oxygen saturation of more than 94%. Ultrasound evaluation, using transcranial Doppler technique will be performed to evaluate the cerebral blood flow speed (average/systolic speed) near the point of emergency, in the middle length and at the bifurcation of M1 bilaterally, at the end of every ventilation trial. At the end of each trial, PaCO2, Ph and blood oxygenation (PaO2) will be obtained performing ABGs. Moreover, traces of flow, airways pressure (Paw) and electric diaphragmatic activity (EAdi) will be recorded and acquired through a dedicated ventilator software called NAVA tracker. This will allow evaluating the patient-ventilator interaction (trigger delay, asynchronous events, and asynchrony index).