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.
Study Type
INTERVENTIONAL
Allocation
RANDOMIZED
Purpose
TREATMENT
Masking
NONE
Enrollment
50
Positive End Expiratory Pressure (PEEP) will be increased from 5 to 15 cmH2O.
Positive End Expiratory Pressure (PEEP) will be decreased from 15 to 5 cmH2O.
Angers Hospital
Angers, Maine et Loire, France
RECRUITINGDifference in ventilation/perfusion mismatch between PEEP 5 and 15 cmH2O according to the two studied phenotypes
Ventilation/perfusion (V/Q) mismatch will be assessed by Electrical Impedance Tomography (EIT). Mismatch is expressed in %. Comparison between phenotype with higher and lower elastance will be performed.
Time frame: immediately after each intervention
Difference in respiratory mechanics between PEEP 5 and 15 cmH2O according to the two studied phenotypes
Plateau pressure and total PEEP will be aggregated to compute driving pressure (Plateau pressure minus total PEEP, all in cmH2O). Respiratory system compliance will be computed by dividing tidal volume by driving pressure (in mL.cmH2O-1; respiratory system resistance will be computed as the inverse of compliance; all these values will be assessed and compared between the two PEEP levels. Comparison between phenotype with higher and lower elastance will be performed.
Time frame: immediately after each intervention
Difference in oxygenation between PEEP 5 and 15 cmH2O according to the two studied phenotypes
PaO2 (in mmHg) will be assessed and compared between the two PEEP levels. Comparison between phenotype with higher and lower elastance will be performed.
Time frame: immediately after each intervention
Difference in carbon clearance between PEEP 5 and 15 cmH2O according to the two studied phenotypes
PaCO2 (in mmHg) will be assessed and compared between the two PEEP levels. Ventilatory ratio (no unit) will be derived from the PaCO2 values. Comparison between phenotype with higher and lower elastance will be performed.
Time frame: immediately after each intervention
Difference in dead space measured by capnometric volumetry between PEEP 5 and 15 cmH2O according to the two studied phenotypes
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VCO2 (measured by Vcap, in mmHg) will be assessed and compared between the two PEEP levels. Comparison between phenotype with higher and lower elastance will be performed.
Time frame: immediately after each intervention
Difference in dead space measured by calorimetry between PEEP 5 and 15 cmH2O according to the two studied phenotypes
VCO2 (measured by calorimetry, in mmHg) will be assessed and compared between the two PEEP levels. Comparison between phenotype with higher and lower elastance will be performed.
Time frame: immediately after each intervention
Difference in venous oxygen saturation between PEEP 5 and 15 cmH2O according to the two studied phenotypes
SvO2 (in %) will be assessed and compared between the two PEEP levels. Comparison between phenotype with higher and lower elastance will be performed.
Time frame: immediately after each intervention
Correlation between V/Q mismatch markers and recruitability
Recruitability will be assessed between 15 and 5 cmH2O by respiratory mechanics and EIT, as the recruited volumes value (in mL). R/I ratio will be derived from these data (no unit). V/Q mismatch will be computed by EIT, and expressed in %. Correlation will be performed by linear regression.
Time frame: immediately after each intervention
Correlations between V/Q mismatch assessed by EIT and dead space markers
Dead space will be assessed by volumetric capnography (if available), venrtilatory ratio, and calorimetriy (if available). V/Q mismatch will be computed by EIT, and expressed in %. Correlation will be performed by linear regression.
Time frame: immediately after each intervention
Correlations between V.Q mismacth and overdisension and lung collapsus
Overdistension (%) and lung collapsus (%) will be assessed during the Step 3, by EIT. These two values cannot be measured separately. V/Q mismatch will be computed by EIT, and expressed in %. Correlation will be performed by linear regression.
Time frame: immediately after each intervention
Difference in stress index between PEEP 5 and 15 cmH2O according to the two studied phenotypes
stress index (no unit) will be assessed and compared between the two PEEP levels. Comparison between phenotype with higher and lower elastance will be performed.
Time frame: immediately after each intervention