Prone Position in infantS/Children With Acute Respiratory Distress Syndrome

Each patient meeting inclusion criteria will be evaluated for the presence of the oxygenation criterion. After neuromuscular paralysis (or apnoeic ventilation as per PICU protocol), and endotracheal suctioning, eligible patients will be ventilated for 30 min with PEEP = 5 cmH2O in the semi-recumbent position, with a tidal volume limited to 6 mL/kg and a Plateau Pressure less than 30 cmH2O. FiO2 will be titrated to obtain and SpO2 \>92 % and \<98 %. Afterward, arterial blood gas analysis (ABG) will be performed to compute PaO2/FiO2 ratio to confirm the presence of the inclusion and the absence of exclusion criteria.Patients showing PaO2/FiO2 ≤ 200 mmHg will be enrolled. Eligible patients will undergo the following protocol: * Verify the presence of airway closure with airway opening pressure (AOP) \> PEEP5cmH2O; * PEEP will be initially set at 12 cmH2O (providing that plateau and driving pressures do not exceed 30 cmH2O and 15 cmH2O, respectively) for 40 minutes to stabilize lung volumes; afterwards, respiratory mechanics will be assessed through standard occlusions and arterial blood gases will be analyzed. Subsequently, a 4-steps decremental PEEP trial (PEEP 12 to 10 to 8 to 5 cmH2O) will be conducted. Each PEEP step will last 8 minutes, and all other ventilator settings will remain unchanged throughout the procedure. At the end of each PEEP step respiratory mechanics will be assessed by the ventilator through 1-second end-inspiratory and end-expiratory holds: plateau pressure \[Pplat\] and total PEEP \[PEEPtot\] will be measured, and driving pressure \[ΔP=Pplat-PEEPtot\] and respiratory system compliance \[Crs = VT/ΔP\] will be assessed; * End-expiratory lung impedance (EELI) will be measured by electrical impedance tomography (EIT)