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

Abstract

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