Introduction Changes in end-expiratory lung volume (a dagger EELV) in response to changes in PEEP (a dagger PEEP) have not been reported in mechanically ventilated patients with ARDS. The purpose of this study was to determine the utility of measurements of a dagger EELV in determining optimal PEEP in ARDS patients.Methods Nine patients with ARDS were prospectively recruited. a dagger EELV was measured using magnetometers during serial decremental PEEP trials. Changes in PaO2 (a dagger PaO2) were simultaneously measured. Static respiratory system compliance (C-RS), a dagger PaO2/a dagger PEEP, and a dagger EELV/a dagger PEEP were calculated at each level of PEEP.Results For the group, a dagger EELV decreased by 1.09 +/- A 0.13 L (mean +/- A SD) as PEEP was reduced from 20 to 0 cm H2O with the greatest changes in a dagger EELV occurring over the mid range of the decremental PEEP curve. Optimal values for C-RS, a dagger EELV/a dagger PEEP, and a dagger PaO2/a dagger PEEP could be identified for each patient and occurred at PEEP levels ranging from 10 to 17.5 cm H2O. There was a significant correlation (r = 0.712, p = 0.047) between a dagger PaO2/a dagger PEEP and a dagger EELV/a dagger PEEP.Conclusions a dagger EELV can be measured from a decremental PEEP curve. Since a dagger EELV is highly correlated with a dagger PaO2, measures of a dagger PaO2/a dagger PEEP may provide a surrogate for measures of a dagger EELV/a dagger PEEP. Combining measures of a dagger EELV/a dagger PEEP with measures of C-RS may provide a novel means of determining optimal PEEP in patients with ARDS.