Fig. 7

Correlation between PEEP and only ventilated/only perfused units in patients with low and high compliance. Higher PEEP was associated with higher only ventilated/only perfused units ratio) in the subgroup of patients with higher compliance (r = 0.386, p = 0.056), but not in the subgroup with lower compliance (r =  − 0.193, p = 0.351). The two subgroups were defined based on the median value of compliance of the respiratory system in the study population (lower or equal to 41 ml/cmH₂O versus higher than 41 ml/cmH₂O)