This study will try to define the threshold of the recruitable volume (Vrec), obtained by a derecruitment maneuver, that permit to identify patients responder or not to alveolar recruitment maneuvers.

Background:

Acute respiratory distress syndrome (ARDS) is characterized by a major loss of lung volume due to alveolar flooding, atelectasis and consolidation. Alveolar recruitment maneuvers (ARM) permit to recruit non-aerated or poorly aerated lung tissue, and thus may improve gas exchange and oxygenation in patients with ARDS. However, ARM may also be harmful by overdistending the previously open lung tissue and can induce hemodynamic instability. The potential effectiveness and benefit of ARM depends on the patient's recruitability. A simple, routinely applicable bedside method to assess recruitability has been described in 2020 and is as efficient as multiple pressure-volume curves. We called it MERP for Potential for lung Recruitability Evaluation Method.

The investigators therefore propose to use this new method that determine lung recruitability as a diagnostic test to determine responder an non responder to ARM, and define a threshold.

Objectives:

The major aim is to determine the threshold of Vrec with the best sensitivity and specificity to define patient responders to alveolar recruitment maneuvers in ARDS.

The secondary objectives are :

• Evaluate the differences in morbi-mortality comparing responders and non responders to ARM
• Calculate the mean time of realization of the MERP+ARM maneuvers
• Estimate the occurence of secondary effects due to ARM (hemodynamic instability, arrhythmia, hypoxemia)

Methods:

This study will enroll at least 42 patients from the three Intensive Care Units at CHU Caen Normandy in 18 months. In all patients a MERP will be performed to determine the recruitable volume (Vrec). It consists to a brutal drop in PEEP level from 15 cmH2O to 5 cmH2O over a single breath maneuver to determine the increased end expiratory volume at the next respiratory cycle. The difference between this volume and the tidal volume represent the " measured recruitable volume " (Vmes). We can therefore calculate the predicted change in lung volume (Vcalc) by the product of respiratory system compliance at low PEEP (5 cmH20) and the change in pressure (10 cmH2O). Finally, the Vrec for each patients is the difference between the Vmes and the Vcalc.

An alveolar recruitment maneuver is then performed following the maximal alveolar recruitment method (driving pressure of 15 cmH2O, with 3 steps of PEEP 20-25-30 cmH2O) followed by a decremental PEEP procedure to determine the best PEEP. 15 minutes a posteriori, a blood gas sample permits to calculate the change in PaO2/FiO2 ratio.

Data Analysis:

The primary endpoint is to determine the threshold of Vrec that permits to identify responders to alveolar recruitment maneuvers defined by an improvement of the PaO2/FiO2 ratio > 20% 15 minutes after ARM, through ROC curve and Youden index.

Inclusion Criteria:

• Moderate to severe ARDS as per the Berlin definition (Partial pressure of oxygen (PaO2)/Fraction of inspired oxygen (FiO2) < 200 mmHg) and within 72h of onset, with a FiO2 > 60%
• Receiving assist/control mechanical ventilation with continuous sedation and neuromuscular blocker
• In supine position during the procedures and measurements
• Patients > 18 years of age

Exclusion Criteria:

• Patients < 18 years of age
• Pregnant woman
• Patient deprived of freedom or under a legal protective measure
• Hemodynamic instability defined as a mean arterial pressure < 65 mmHg or increasing doses of vasopressor drugs
• Pneumothorax, pneumomediastinum, pleural fistula, sub-cutaneous emphysema
• Chronic or recent severe cardiac dysfunction
• Patient with a PEEP > 15 cmH20 before inclusion
• Patient with no therapeutic perspective; candidate for palliative care exclusively or with end of life decision before inclusion

Moderate to severe ARDS patients as per the Berlin definition