Introduction: This study compares different parameters derived from electrical impedance tomography (EIT) data to define 'best' positive end-expiratory pressure (PEEP) during a decremental PEEP trial in mechanically-ventilated patients. 'Best' PEEP is regarded as minimal lung collapse and overdistention in order to prevent ventilator-induced lung injury.Methods: A decremental PEEP trial (from 15 to 0 cm H2O PEEP in 4 steps) was performed in 12 post-cardiac surgery patients on the ICU. At each PEEP step, EIT measurements were performed and from this data the following were calculated: tidal impedance variation (TIV), regional compliance, ventilation surface area (VSA), center of ventilation (COV), regional ventilation delay (RVD index), global inhomogeneity (GI index), and intratidal gas distribution. From the latter parameter we developed the ITV index as a new homogeneity parameter. The EIT parameters were compared with dynamic compliance and the PaO2/FiO2 ratio.Results: Dynamic compliance and the PaO2/FiO2 ratio had the highest value at 10 and 15 cm H2O PEEP, respectively. TIV, regional compliance and VSA had a maximum value at 5 cm H2O PEEP for the non-dependent lung region and a maximal value at 15 cm H2O PEEP for the dependent lung region. GI index showed the lowest value at 10 cm H2O PEEP, whereas for COV and the RVD index this was at 15 cm H2O PEEP. The intratidal gas distribution showed an equal contribution of both lung regions at a specific PEEP level in each patient.Conclusion: In post-cardiac surgery patients, the ITV index was comparable with dynamic compliance to indicate 'best' PEEP. The ITV index can visualize the PEEP level at which ventilation of the non-dependent region is diminished, indicating overdistention. Additional studies should test whether application of this specific PEEP level leads to better outcome and also confirm these results in patients with acute respiratory distress syndrome.