Effects of PEEP on regional ventilation-perfusion mismatch in the acute respiratory distress syndrome

Purpose: In the acute respiratory distress syndrome (ARDS), decreasing Ventilation-Perfusion (V˙ / Q˙) mismatch might enhance lung protection. We investigated the regional effects of higher Positive End Expiratory Pressure (PEEP) on V˙ / Q˙ mismatch and their correlation with recruitability. We aimed to verify whether PEEP improves regional V˙ / Q˙ mismatch, and to study the underlying mechanisms. Methods: In fifteen patients with moderate and severe ARDS, two PEEP levels (5 and 15 cmH2O) were applied in random order. V˙ / Q˙ mismatch was assessed by Electrical Impedance Tomography at each PEEP. Percentage of ventilation and perfusion reaching different ranges of V˙ / Q˙ ratios were analyzed in 3 gravitational lung regions, leading to precise assessment of their distribution throughout different V˙ / Q˙ mismatch compartments. Recruitability between the two PEEP levels was measured by the recruitment-to-inflation ratio method. Results: In the non-dependent region, at higher PEEP, ventilation reaching the normal V˙ / Q˙ compartment (p = 0.018) increased, while it decreased in the high V˙ / Q˙ one (p = 0.023). In the middle region, at PEEP 15 cmH2O, ventilation and perfusion to the low V˙ / Q˙ compartment decreased (p = 0.006 and p = 0.011) and perfusion to normal V˙ / Q˙ increased (p = 0.003). In the dependent lung, the percentage of blood flowing through the non-ventilated compartment decreased (p = 0.041). Regional V˙ / Q˙ mismatch improvement was correlated to lung recruitability and changes in regional tidal volume. Conclusions: In patients with ARDS, higher PEEP optimizes the distribution of both ventilation (in the non-dependent areas) and perfusion (in the middle and dependent lung). Bedside measure of recruitability is associated with improved V˙ / Q˙ mismatch.