Fig. 1

Illustration of the proposed analysis method. First row dynamic images at end-inspiration (a) and end-expiration (b) are identified (averaging of five corresponding images is used to minimize noise if necessary). Status image (c) of tidal variation equals pixel impedance values at end-inspiration minus that at end-expiration. Second row lung regions containing air are identified at reference positive end-expiratory pressure (PEEP) level and arbitrary PEEP level (d, white regions end-expiration at reference PEEP; e, light gray end-expiration at arbitrary PEEP; f, gray lung regions for tidal breathing). Third row overdistended regions (i, white regions) are defined as regions that contain air at end-expiration but no air comes in or out during tidal breathing. Fourth row regions subjected to repeated alveolar collapse and reopening (l, red regions) are defined as regions that are not in the lung regions at end-expiration but are ventilated during tidal breathing. Fifth row recruited regions compared to reference PEEP level (o, blue regions) are defined as regions that contain air at end-expiration of the arbitrary PEEP level but not at that of the reference PEEP level. For the combination of the three types of regions please refer to, e.g., Fig. 2, PEEP = 9 cmH₂O