Scatterplots of relative ZV and ZQ within the ROI 1 to 4 for the left lung at baseline and following PEEP20 in relation to the relative ZV/ZQ ratio. ZV = ventilation-induced change in thoracic impedance; open circle. ZQ = perfusion-induced change in thoracic impedance; solid stars. Values presented as a proportion of global ZV and ZQ. Region of interest (ROI) 1 = ventral; ROI 2 = mid-ventral; ROI 3 = mid-dorsal; ROI 4 = dorsal. BL = baseline; PEEP20 = the application of 20 cmH2O of positive end-expiratory pressure.