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)over dot>/<(Q)over dot>) mismatch might enhance lung protection. We investigated the regional effects of higher Positive End Expiratory Pressure (PEEP) on <(V)over dot>/<(Q)over dot> mismatch and their correlation with recruitability. We aimed to verify whether PEEP improves regional <(V)over dot>/<(Q)over dot> mismatch, and to study the underlying mechanisms.Methods: In fifteen patients with moderate and severe ARDS, two PEEP levels (5 and 15 cmH(2)O) were applied in random order. <(V)over dot>/<(Q)over dot> mismatch was assessed by Electrical Impedance Tomography at each PEEP. Percentage of ventilation and perfusion reaching different ranges of <(V)over dot>/<(Q)over dot> ratios were analyzed in 3 gravitational lung regions, leading to precise assessment of their distribution throughout different <(V)over dot>/<(Q)over dot> 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)over dot>/<(Q)over dot> compartment (p = 0.018) increased, while it decreased in the high <(V)over dot>/<(Q)over dot> one (p = 0.023). In the middle region, at PEEP 15 cmH(2)O, ventilation and perfusion to the low <(V)over dot>/<(Q)over dot> compartment decreased (p = 0.006 and p = 0.011) and perfusion to normal <(V)over dot>/<(Q)over dot> increased (p= 0.003). In the dependent lung, the percentage of blood flowing through the non-ventilated compartment decreased (p = 0.041). Regional <(V)over dot>/<(Q)over dot> 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)over dot>/<(Q)over dot> mismatch.