V/Q Matching Variations With PEEP in ARDS According to Compliance-based Phenotypes (France)

Effect of PEEP on Ventilartion/Perfusion Ratios According to Different Phenotypes in Patients With ARDS (France)

This study aim to compare the effect of Positive End Expiratory Pressure (PEEP) on ventilation/perfusion mismatch in two phenotypes of patients with moderate-to-severe Acute Respiratory Distress Syndrome (ARDS), characterized by their respiratory system elastance (Ers). Ventilation/perfusion mismatch will be assessed by Electrical Impedance Tomography (EIT).

Acute Respiratory Distress Syndrome (ARDS) is characterized by hypoxemia caused by inflammatory lung injury. Recent studies showed an important variability in ARDS phenotypes . The recent COVID-19 crisis highlighted the presence of ARDS patients with severe hypoxemia and normal respiratory system compliance, and retrospective series confirmed the existence of this atypical ARDS pattern also in non-COVID etiologies.

The dissociation between mechanics and hypoxemia may be related to a specific diversity in the pattern of ventilation-perfusion matching (V/Q matching) among patients with normal compliance and ARDS. In patients with low compliance, V/Q mismatch may be characterized by right-to-left shunt, secondary to collapse of the gravity-dependent regions; while, in patients with normal compliance, V/Q mismatch may show a redistribution of blood flow to hypo-ventilated lung areas by larger dead space and impaired hypoxic vasoconstriction.

These differences may also influence the response to PEEP in terms of gas exchange and lung protection . It may also explain the failure for high PEEP levels to improve significantly mortality in the global ARDS population (i.e., with patients' selection only based on hypoxemia).

Electrical Impedance Tomography (EIT) is a device allowing to assess ventilation and perfusion distribution. Thus, EIT can be used to evaluate global and regional V/Q matching, and could be used to understand mechanisms of hypoxemia, especially in patients with normal mechanics and ARDS.

The aim of this study is to assess V/Q matching according to these different ARDS phenotypes, and to evaluate the effects of PEEP in each one.

Inclusion criteria:

• intubated patients with moderate and severe ARDS (Berlin definition, PaO2/FiO2 ≤200 mmHg at PEEP 5 cmH2O)
• undergoing deep sedation
• on controlled mechanical ventilation
• between 24 hours and 5 days after intubation.

Exclusion criteria:

• age <18 years old; pregnancy
• patient undergoing legal protection
• contra-indications to EIT (e. g. severe chest trauma or wounds)
• pneumothorax; patient undergoing ECMO
• patient with BMI ≥35 kg/m2
• hemodynamic instability with MAP <60 mmHg despite vasopressors.