Assessment of the Reproducibility of the Recruitment-to-inflation Ratio (R/I Ratio) Measurement in Patients With Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is a severe lung condition that often requires invasive mechanical ventilation in the intensive care unit. In these patients, setting the ventilator appropriately is essential to improve oxygenation while limiting ventilator-induced lung injury. One important ventilator setting is positive end-expiratory pressure (PEEP), which helps keep the lungs open. However, the optimal PEEP level may vary from one patient to another. The recruitment-to-inflation ratio (R/I ratio) is a bedside measurement used to estimate the potential for lung recruitment during a decrease in PEEP. It compares the compliance of the lung volume recruited by PEEP with the compliance of the already aerated lung. A higher R/I ratio suggests that increasing PEEP is more likely to reopen collapsed lung units, whereas a lower R/I ratio suggests limited recruitability and a higher likelihood that additional pressure would mainly distend lung areas that are already open. In clinical practice, the R/I ratio is increasingly used to guide PEEP adjustment, with the aim of improving recruitment and oxygenation while avoiding unnecessary increases in airway pressure. However, although the R/I ratio is used in routine care, there are currently no data demonstrating that this measurement is reproducible when repeated in the same patient under similar conditions. The hypothesis of this study is that the R/I ratio is reproducible when measured twice in the same patient under stable conditions, including no significant changes in ventilator settings, hemodynamic status, or ongoing treatments. This prospective, multicenter, non-interventional study will include adult ICU patients with ARDS who are receiving invasive mechanical ventilation, deep sedation, and assist-control ventilation. For each patient, airway opening pressure will be assessed, and the R/I ratio will be measured twice on the same day by a trained clinician, between 20 and 120 minutes apart, without changes in ventilator settings or treatments likely to influence the measurement. The main objective is to evaluate the within-patient reproducibility of the R/I ratio. Secondary objectives include describing changes in airway opening pressure and R/I ratio over time, assessing the reproducibility of expired tidal volume during the maneuvers, and evaluating the clinical tolerance of these ventilatory measurements. The study will include 80 patients across 4 French intensive care units. No additional intervention outside routine care will be performed. Clinical, ventilatory, and biological data already collected as part of usual care will be recorded.

Inclusion Criteria: * Adult patient (age ≥ 18 years) * Patient hospitalized in the Intensive Care Unit (ICU) for acute respiratory distress syndrome (according to the Berlin definition), requiring invasive mechanical ventilation with deep sedation and controlled mechanical ventilation mode * Patient or next of kin informed about the study and not objecting to participation Exclusion Criteria: * Undrained pneumothorax or any contraindication to a transient increase in airway pressure * Known or suspected intracranial hypertension * Patient under legal protection (e.g., guardianship) * Not affiliated with a social security system * Presence of spontaneous respiratory effort * Presence of a leak in the ventilatory circuit * Hemodynamic instability (e.g., uncontrolled shock, increasing vasopressor requirement) * Uncontrolled severe hypoxemia * Undrained pneumothorax or any contraindication to a transient increase in airway pressure * Known or suspected intracranial hypertension