Electrical Impedance Tomography-Guided Identification of the Optimal Lateral Position in Postoperative ARDS

This prospective, randomized crossover physiological study evaluates the effects of lateral positioning (left lateral position and right lateral position) versus the supine position on ventilation-perfusion (V/Q) matching in adult postoperative abdominal surgery participants with acute respiratory distress syndrome (ARDS). Bedside electrical impedance tomography (EIT) will be used to quantify regional ventilation and perfusion (perfusion derived from an intravenous tracer bolus administered during a brief breath-hold) and to calculate global "normal V/Q" (normal V/Q, %). Oxygenation, respiratory mechanics (when applicable), and hemodynamics will be recorded concurrently. Feasibility and safety of the positioning protocol will also be assessed.

Inclusion Criteria: * Age ≥18 years * Postoperative abdominal surgery patients admitted to the ICU with ARDS (Berlin definition) * Considered able to tolerate protocolized position changes by the treating team * EIT belt placement and protocol procedures feasible * Informed consent obtained from the patient or legally authorized representative Exclusion Criteria: * Contraindication to lateral positioning (e.g., unstable spine, uncontrolled bleeding, open abdomen, high-risk surgical wound condition) * Severe hemodynamic instability or other conditions making participation unsafe * High risk of airway/vascular line/drain dislodgement not manageable * Pregnancy * Refusal of informed consent * Any other condition deemed inappropriate by investigators