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

ABSTRACT

PURPOSE: In the acute respiratory distress syndrome (ARDS), decreasing Ventilation-Perfusion [Formula see text] mismatch might enhance lung protection. We investigated the regional effects of higher Positive End Expiratory Pressure (PEEP) on [Formula see text] mismatch and their correlation with recruitability. We aimed to verify whether PEEP improves regional [Formula see text] 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. [Formula see text] mismatch was assessed by Electrical Impedance Tomography at each PEEP. Percentage of ventilation and perfusion reaching different ranges of [Formula see text] ratios were analyzed in 3 gravitational lung regions, leading to precise assessment of their distribution throughout different [Formula see text] 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 [Formula see text] compartment (p = 0.018) increased, while it decreased in the high [Formula see text] one (p = 0.023). In the middle region, at PEEP 15 cmH2O, ventilation and perfusion to the low [Formula see text] compartment decreased (p = 0.006 and p = 0.011) and perfusion to normal [Formula see text] increased (p = 0.003). In the dependent lung, the percentage of blood flowing through the non-ventilated compartment decreased (p = 0.041). Regional [Formula see text] 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 [Formula see text] mismatch.