Acute respiratory distress syndrome (ARDS) in its moderate to severe forms is associated with high mortality. Mechanical ventilation (MV) remains the cornerstone of ARDS management but carries a significant risk of ventilator-induced lung injury (VILI). Positive end-expiratory pressure (PEEP), a fundamental component of MV, is widely utilized in clinical practice; however, optimal PEEP selection for patients with moderate to severe ARDS remains a complex and unresolved challenge. Electrical impedance tomography (EIT), a bedside imaging modality that evaluates regional ventilation distribution, offers a means of individualizing PEEP settings in mechanically ventilated patients. By balancing the competing risks of alveolar overdistension and collapse, EIT facilitates precision in PEEP titration. This study compares the impact of EIT-guided PEEP selection versus the conventional low FiO2-PEEP table on blood oxygenation and pulmonary mechanics.

This is a prospective, single-center, randomized controlled trial designed to evaluate the effects of PEEP settings guided by electrical impedance tomography (EIT) on clinical outcomes in patients with moderate to severe ARDS undergoing lung-protective ventilation. Adult intubated patients with moderate to severe ARDS will be enrolled in the study.

Patients in the intervention group will receive PEEP titrated using EIT with a stepwise decremental PEEP trial, while those in the control group will have PEEP set based on the FiO2-PEEP table. Other ventilator parameters will be managed according to the ARDSnet protocol.

The primary outcomes are blood oxygenation and pulmonary mechanics. Secondary outcomes include 28-day mortality, ventilator-free and shock-free days at 28 days, length of ICU and hospital stay, rate of successful weaning, the proportion requiring rescue therapies, complications, respiratory variables, and the Sequential Organ Failure Assessment (SOFA) score.

Before initiating recruitment maneuver, all patients were placed on mechanical ventilation set according to the ARDS network strategy for 10 minutes, ensuring SpO2 of 88-95%, PaO2 of 55-80 mmHg, and a mean arterial pressure (MAP) ≥ 65 mmHg.

• Patients received sedation and neuromuscular blockade to achieve full synchronization with the ventilator (RASS ≤ -3).

• The mode was set to Pressure-Controlled Ventilation (PCV): ΔP = 15 cmH2O, which was maintained constant during the recruitment maneuver, while FiO2, PEEP, and respiratory rate were kept as previously set.

• PEEP was incrementally increased by 5 cmH2O every minute: from 10 to 15, to 20, and finally to 25 cmH2O, with a maximum pressure limit of 40 cmH2O.

• Following the recruitment maneuver at the final PEEP level, the process of identifying the optimal PEEP was initiated.

• Then, PEEP will be set to 20 cmH2O and gradually decreased by 2 cmH2O every 30 seconds until it reached 6 cmH2O or SpO2 dropped to ≤ 80%.

• Selection of optimal PEEP: The optimal PEEP was defined as the intersection point between the alveolar overdistension and collapse curves as measured by the EIT system.

  • Termination Criteria: If persistent hypotension (mean arterial pressure decrease > 15 mmHg) or sustained hypoxemia (SpO2 < 85% for at least 1 minute) occurs during the recruitment maneuver maneuver, the procedure is terminated, and ventilator settings are returned to the parameters used prior to the recruitment maneuver.

Low FiO2 - PEEP table In the control group, patients set PEEP by low FiO2-PEEP table based on ARDSnet protocol.

Inclusion Criteria:

• Age ≥ 18 years.
• Intubated moderate and severe ARDS according to the New Global Definition (PaO2/FiO2 ≤200 mmHg).
• Used continuous sedation with or without paralysis.

Exclusion Criteria:

• Presence of pneumothorax that is either undrained or newly occurred.
• Unstable hemodynamics with a mean arterial pressure < 60 mmHg and unresponsive to resuscitation measures, and/or heart rate < 60 bpm.
• Contraindications for EIT (pacemakers, automatic external defibrillators, cases of chest trauma or recent chest surgery limiting EIT belt application).
• Pregnancy.
• Severe neuromuscular disease.