Metabolic activity, k3, was the predominant contributing factor to the reduction in 18F-FDG uptake by protective (vs. injurious) ventilation and was significantly reduced in dependent lung regions in the protective ventilation group. Given that the cells taking up 18F-FDG are predominantly neutrophils in this model [7,8,12,30,41], the lower k3 value mainly reflects lower metabolic activity of lung-infiltrating the neutrophils during protective ventilation with higher PEEP and lower VT.This predominant role of k3 was confirmed by the finding of lower Ki with protective vs. injurious ventilation for a similar range of regional neutrophil quantities in both groups, measured independently and using direct histological methods. Moreover, we found lower k3 values per neutrophil in dependent regions of the protective (vs.

injurious) ventilation group (Figure (Figure6).6). This novel finding leads us to speculate that a reduction in lung neutrophil activity could be a mechanism by which protective ventilation improves outcomes of ALI and ARDS patients [3,38]. This result also suggests that quantification of cell activation (k3) allows for characterization of differences in the type and severity of ALI, even when inflammatory cell numbers are similar. This observation is compatible with the reported relationship between k3 and severity of disease in cancer research, where cell activity was indicated as a marker of severity [14,16,17], and suggests that k3 could be a sensitive tool with which to monitor noninvasively early changes in lung inflammation in ALI and ARDS.

The observed dissociation between cell numbers and metabolic activity suggested by the current study in an experimental model of ALI paves the way for the development of new methods for quantifying the effects of mechanical or pharmacological interventions in ALI and ARDS.Factors associated with modulation of lung inflammation by a protective ventilation strategyDifferent factors could explain the reduction in neutrophil activation associated with protective ventilation. First, there are those related to regional lung mechanics. Given that the same Pplat was used in Entinostat all animals, group differences in 18F-FDG uptake should be due to different PEEP volume and VT. Because significant differences in 18F-FDG uptake were evident in the dependent poorly aerated lung regions, low-volume lung injury is a likely factor. Such injury is related to processes such as repetitive opening and closing of distal airways and alveoli [57,58], concentration of regional mechanical forces [59] and propagation of air in fluid-filled airways [60]. In fact, this low-volume effect associated with smaller PEEP levels would be magnified by the concomitant increase in VT [61].