BACKGROUNDThe appropriate level of positive end-expiratory pressure (PEEP) during intra-operative mechanical ventilation remains unclear.OBJECTIVEThe aim of this study was to investigate the effects of different levels of PEEP with low tidal volume (low-VT) ventilation in a steep Trendelenburg position (30°) and pneumoperitoneum on oxygenation, respiratory mechanics and ventilation distribution using electrical impedance tomography.DESIGNA randomised controlled trial.SETTINGSingle university secondary care centre, conducted from January 2017 to December 2017.PATIENTSForty female patients, aged 20 to 60 years, and of American Society of Anesthesiologists' (ASA) physical status 1 or 2, undergoing elective robotic gynaecological surgery were included.INTERVENTIONForty patients were allocated randomly to a PEEP4 (PEEP 4 cmH2O) group or a PEEP8 (PEEP 8 cmH2O) group.MAIN OUTCOME MEASURESThe primary outcomes were respiratory mechanics. The secondary outcomes included changes in ventilation distribution across the ventral and dorsal regions of interest and postoperative pulmonary complications (PPCs) using a modified clinical pulmonary infection score.RESULTSThere was no difference in PaO2 at any time point. The peak inspiratory pressure (PIP) and mean airway pressure (MPAW) of the PEEP4 group were lower than those of the PEEP8 group (P < 0.001). The oxygenation factor in the PEEP4 group was higher than that in the PEEP8 group during mechanical ventilation at all times. There was no difference in the fractional distribution of end-expiratory ventilation according to region of interest between the two groups.CONCLUSIONBoth 4 and 8 cmH2O of PEEP with low-VT ventilation can be used for robotic gynaecological surgery that requires a steep Trendelenburg position and pneumoperitoneum. However, 8 cmH2O of PEEP had no benefit over 4 cmH2O of PEEP with respect to oxygenation and improvement of dorsal regional ventilation.TRIAL REGISTRATIONThe trial was registered at the Clinical Trial Registry of Korea (KCT0002255). https://cris.nih.go.kr.