Fig. 2
Effect of PEEP on areas with and without flow limitation. In the upper part an area without flow limitation with 4 cmH2O of auto-PEEP and a flow-limited area with auto-PEEP of 10 cmH2O at ZEEP are presented. Hypothesizing that these two areas evenly contribute to the expired volume, the average auto-PEEP of this model is 7 cmH2O. When PEEP of 6 cmH2O (about 80 % of auto-PEEP) is applied to the whole respiratory system (lower part of the figure), the part of the lung without flow limitation will increase its end-expiratory pressure by the same amount of the applied PEEP, then increasing total PEEP to 10 cmH2O, without any change in auto-PEEP. On the contrary, the flow-limited region is not expected to be further hyperinflated by a PEEP lower than its total PEEP, with the result that total PEEP does not change and auto-PEEP decreases. The average result of PEEP application on the whole lung will be a rise in total PEEP from 7 to 10 cmH2O: The two parts react to PEEP as either flow-limited or non-flow-limited areas, and the overall observed response to PEEP is intermediate between them. PEEP positive end-expiratory pressure; PEEP tot total PEEP; ZEEP zero end-expiratory pressure