Effects of PEEP on regional ventilation-perfusion mismatch in the acute respiratory distress syndrome
Authors: Pavlovsky Bertrand , Pesenti Antonio , Spinelli Elena , Scaramuzzo Gaetano , Marongiu Ines , Tagliabue Paola , Spadaro Savino , Grasselli Giacomo , Mercat Alain , Mauri Tommaso 
Published: July 2022

Abstract
Purpose
In the acute respiratory distress syndrome (ARDS), decreasing Ventilation-Perfusion $$\left( {{{\dot{V}} \mathord{\left/ {\vphantom {{\dot{V}} {\dot{Q}}}} \right. \kern-\nulldelimiterspace} {\dot{Q}}}} \right)$$

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mismatch might enhance lung protection. We investigated the regional effects of higher Positive End Expiratory Pressure (PEEP) on $${{\dot{V}} \mathord{\left/ {\vphantom {{\dot{V}} {\dot{Q}}}} \right. \kern-\nulldelimiterspace} {\dot{Q}}}$$

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mismatch and their correlation with recruitability. We aimed to verify whether PEEP improves regional $${{\dot{V}} \mathord{\left/ {\vphantom {{\dot{V}} {\dot{Q}}}} \right. \kern-\nulldelimiterspace} {\dot{Q}}}$$

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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. $${{\dot{V}} \mathord{\left/ {\vphantom {{\dot{V}} {\dot{Q}}}} \right. \kern-\nulldelimiterspace} {\dot{Q}}}$$

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mismatch was assessed by Electrical Impedance Tomography at each PEEP. Percentage of ventilation and perfusion reaching different ranges of $${{\dot{V}} \mathord{\left/ {\vphantom {{\dot{V}} {\dot{Q}}}} \right. \kern-\nulldelimiterspace} {\dot{Q}}}$$

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ratios were analyzed in 3 gravitational lung regions, leading to precise assessment of their distribution throughout different $${{\dot{V}} \mathord{\left/ {\vphantom {{\dot{V}} {\dot{Q}}}} \right. \kern-\nulldelimiterspace} {\dot{Q}}}$$

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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 $${{\dot{V}} \mathord{\left/ {\vphantom {{\dot{V}} {\dot{Q}}}} \right. \kern-\nulldelimiterspace} {\dot{Q}}}$$

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compartment (p = 0.018) increased, while it decreased in the high $${{\dot{V}} \mathord{\left/ {\vphantom {{\dot{V}} {\dot{Q}}}} \right. \kern-\nulldelimiterspace} {\dot{Q}}}$$

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one (p = 0.023). In the middle region, at PEEP 15 cmH2O, ventilation and perfusion to the low $${{\dot{V}} \mathord{\left/ {\vphantom {{\dot{V}} {\dot{Q}}}} \right. \kern-\nulldelimiterspace} {\dot{Q}}}$$

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compartment decreased (p = 0.006 and p = 0.011) and perfusion to normal $${{\dot{V}} \mathord{\left/ {\vphantom {{\dot{V}} {\dot{Q}}}} \right. \kern-\nulldelimiterspace} {\dot{Q}}}$$

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increased (p = 0.003). In the dependent lung, the percentage of blood flowing through the non-ventilated compartment decreased (p = 0.041). Regional $${{\dot{V}} \mathord{\left/ {\vphantom {{\dot{V}} {\dot{Q}}}} \right. \kern-\nulldelimiterspace} {\dot{Q}}}$$

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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 $${{\dot{V}} \mathord{\left/ {\vphantom {{\dot{V}} {\dot{Q}}}} \right. \kern-\nulldelimiterspace} {\dot{Q}}}$$

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mismatch.