Background It is still controversial as to the implementation of higher positive end-expiratory pressure (PEEP) in patients with acute respiratory distress syndrome (ARDS).This study was conducted to compare the lower and higher PEEP in patients with ARDS ventilated with low tidal volume,to investigate the relationship between the recruited lung volume by higher PEEP and relevant independent variables and to provide a bedside estimate of the percentage of potentially recruitable lung by higher PEEP.Methods Twenty-four patients with ARDS were studied.A lung recruiting maneuver was performed,then each patient was ventilated with PEEP of 8 cmH2O for 4 hours and subsequently with PEEP of 16 cmH2O for 4 hours.At the end of each PEEP level period,gas exchange,hemodynamic data,lung mechanics,stress index "b" of the dynamic pressure-time curve,intrinsic PEEP and recruited volume by PEEP were measured.Results Fourteen patients were recruiters whose alveolar recruited volumes induced by PEEP 16 cmH2O were (425±65) ml and 10 patients were non-recruiters.Compared with the PEEP 8 cmH2O period,after the application of the PEEP 16 cmH2O,the PaO2/FiO2 ratio and static lung compliance both remained unchanged in non-recruiters,whereas they increased significantly in recruiters.Changes in PaO2/FiO2 and static lung compliance after PEEP increase were independently associated with the alveolar recruitment.Analyzing the relationship between recruiting maneuver (RM)-induced change in end-expiratory lung volume and the alveolar recruitment induced by PEEP,we found a notable correlation.Conclusions The results of this study indicated that the potential for alveolar recruitment might vary among the ARDS population and the higher PEEP levels should be limited to recruiters.Improving in PaO2/FiO2,static lung compliance after PEEP increase and the shape of the pressure-time curve could be helpful for PEEP application.