Optimization of positive end-expiratory pressure by volumetric capnography variables in lavage-induced acute lung injury.

BACKGROUND: In the acute respiratory distress syndrome (ARDS), lung-protective ventilation strategies combine the delivery of small tidal volumes (VT) with sufficient positive end-expiratory pressure (PEEP). However, an optimal approach guiding the setting of PEEP has not been defined. Monitoring volumetric capnography is useful to detect changes in lung aeration.

OBJECTIVES: The aim of this study was to determine whether volumetric capnography may be a useful method to determine the optimal PEEP in ARDS.

METHODS: In 8 lung-lavaged piglets, PEEP was reduced from 20 to 4 cm H2O in steps of 4 cm H2O every 10 min followed by full lung recruitment. Volumetric capnography, respiratory mechanics, blood gas analysis, hemodynamic data and whole-lung computed tomography scans were obtained at each PEEP level.

RESULTS: After lung recruitment, end-expiratory lung volume progressively decreased from 1,160 ± 273 ml at PEEP 20 cm H2O to 314 ± 86 ml at PEEP 4 cm H2O. The ratio of alveolar dead space (VDalv) to alveolar VT (VTalv) and the phase III slope of volumetric capnography (SIII) reached a minimum at PEEP 16 cm H2O. At this PEEP level, overaerated lung regions were significantly reduced, nonaerated lung regions did not increase, and partial pressure of oxygen in arterial blood/fraction of inspired oxygen (P/F) and static respiratory system compliance (Crs) reached a maximum. At PEEP levels <16 cm H2O, nonaerated lung regions significantly increased, P/F and Crs deteriorated, and VDalv/VTalv and SIII began to increase.

CONCLUSIONS: In this surfactant-depleted model, PEEP at the lowest VDalv/VTalv and SIII allows an optimal balance between lung overinflation and collapse. Hence, volumetric capnography is a useful bedside approach to identify the optimal PEEP.