Background:Electrical impedance tomography (EIT) is a real-time bedside monitoring tool,which can reflect dynamic regional lung ventilation.The aim of the present study was to monitor regional gas distribution in patients with acute respiratory distress syndrome (ARDS) during positive-end-expiratory pressure (PEEP) titration using EIT.Methods:Eighteen ARDS patients under mechanical ventilation in Department of Critical Care Medicine of Peking Union Medical College Hospital from January to April in 2014 were included in this prospective observational study.After recruitment maneuvers (RMs),decremental PEEP titration was performed from 20 cmH2O to 5 cmH2O in steps of 3 cmH2O every 5-10 min.Regional over-distension and recruitment were monitored with EIT.Results:After RMs,patient with arterial blood oxygen partial pressure (PaO2) + carbon dioxide partial pressure (PaCO2) ＞400 mmHg with 100％ of fractional inspired oxygen concentration were defined as RM responders.Thirteen ARDS patients was diagnosed as responders whose PaO2 + PaCO2 were higher than nonresponders (419 ± 44 mmHg vs.170 ± 73 mmHg,P ＜ 0.0001).In responders,PEEP mainly increased recruited pixels in dependent regions and over-distended pixels in nondependent regions.PEEP alleviated global inhomogeneity of tidal volume and end-expiratory lung volume.PEEP levels without significant alveolar derecruitment and over-distension were identified individually.Conclusions:After RMs,PEEP titration significantly affected regional gas distribution in lung,which could be monitored with EIT.EIT has the potential to optimize PEEP titration.