Effects of prone position and positive end-expiratory pressure on lung perfusion and ventilation*


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Abstract

Objectives:Prone positioning is frequently used during acute respiratory distress syndrome. However, mechanisms by which it improves oxygenation are poorly understood, as well as its interaction with positive end-expiratory pressure. This study was conducted to decipher the respective effects of positive end-expiratory pressure and posture during lung injury on regional lung ventilation, perfusion and recruitment assessed by positron emission tomography.Design:Experimental study.Setting:Research laboratory of a university hospital.Subjects:Six female piglets.Interventions:After oleic acid-induced lung injury, all animals were studied in supine and prone position at both positive end-expiratory pressure 0 and positive end-expiratory pressure 10 cm H2O.Measurements and Main Results:In each experimental condition, regional lung perfusion and ventilation were assessed with positron emission tomograph using intravenous 15O-labeled water and inhaled nitrogen-13. Nonaerated lung weight was assessed with positron emission tomograph, and alveolar recruitment was defined as the difference of nonaerated lung weight between conditions. Positive end-expiratory pressure was associated with significant alveolar recruitment (130 ± 85 and 65 ± 29 g of lung in supine and prone position, respectively [p < 0.05 vs. 0]), whereas recruitment induced by posture was not statistically significant (77 ± 97 g with positive end-expiratory pressure 0 and 13 ± 19 g with positive end-expiratory pressure 10 [p > 0.05 vs. 0]). Regardless the posture, positive end-expiratory pressure redistributed both perfusion and ventilation toward dependent regions. Recruitment by positive end-expiratory pressure was restricted to dorsal regions in supine position, but extended diffusely along the ventral-to-dorsal dimension in prone position. Prone position was associated with recruitment in dorsal regions with concomitant derecruitment in ventral regions, magnitude of this being reduced by positive end-expiratory pressure. Prone position redistributed ventilation toward dorsal and ventral regions at positive end-expiratory pressure 0 and positive end-expiratory pressure, respectively. Finally, prone position redistributed perfusion toward ventral regions, to an extent amplified by positive end-expiratory pressure.Conclusions:Positive end-expiratory pressure and posture act synergistically by redistributing lung regional perfusion toward ventral regions, but have antagonistic effects on regional ventilation.

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