Prospective, physiological study to systematically assess the effects of high positive end-expiratory pressure (PEEP) with and without inspiratory synchronization in patients with moderate to severe acute respiratory distress syndrome (ARDS) exhibiting intense inspiratory effort while on assisted ventilation.

This prospective, physiological, randomized, cross-over study will be conducted in the 20-bed general ICU of the Fondazione Policlinico Universitario A. Gemelli IRCCS, Roma, Italy. Patients with ARDS will be screened for eligibility. ARDS will be defined according to the Berlin Definition.

Intubated patients fulfilling ARDS criteria, undergoing assisted ventilation as per clinical decision and exhibiting intense inspiratory effort (occlusion pressure equal or greater than 13 cmH2O) will be enrolled.

Patients will be placed in a 30°-45° head up position during all study phases. According to our institution protocol, pressure support setting will be selected by the attending physician to target a predefined tidal volume range of 6-8 mL/kg of predicted body weight (PBW) (female PBW [kg] = 45.5 + 0.91 [cm of height - 152.4]; male PBW [kg] = 50 + 0.91 [cm of height - 152.4]).The lowest pressure support level allowed will be 7 cmH2O, even if Vt <8 ml/Kg PBW is not achieved.

In each patient, four different ventilator settings will be applied in a random order. Each step will last 45 minutes.

Inclusion Criteria:

• (a) ARDS according to the Berlin definition (ratio of arterial partial pressure of oxygen to fraction of inspired oxygen (PaO2/FiO2) ≤ 300 mmHg);
• (b) assisted ventilation, as prescribed by the attending physician
• (c) intense inspiratory effort, defined as a negative deflection in the airway pressure equal or greater than 13 cmH2O in the first breath recorded during an end-expiratory occlusion

Exclusion Criteria:

Pregnancy, clinically documented barotrauma, contraindication to electrical impedance tomography (EIT) use (e.g., presence of pacemaker or automatic implantable cardioverter defibrillator), impossibility to place the EIT belt in the right position (e.g., presence of surgical wounds dressing), and any contraindication to the insertion of a nasogastric tube (eg, recent upper-gastrointestinal surgery, esophageal varices), chest tubes with active air leaks, fever.

Individual patient data will be made available upon a reasonable request