Evidence of sequential opening and closing of lung units during inflation-deflation of excised rat lungs.

In this study we propose a descriptive model of the events occurring in an excised lung during an inflation-deflation cycle. The model was developed by observing changes in small pressure-volume loops superimposed on quasistatic pressure volume curves. It was found that the shape of the small loops during lung inflation was a function of the previous end-expiratory pressure. These experimental results could most easily be explained by a model of the lung in which individual lung units open sequentially as the lung is inflated. During sequential recruitment, individual lung units open quickly to a volume determined by the transpulmonary pressure. The units then homogeneously increase and decrease in size according to pressure-volume curves similar to the deflation curve of the entire lung. Once lung units have been recruited, they remain open until the lung has been deflated to end-expiratory pressures below 3-4 cm H2O. Reducing the end-expiratory pressure to lower values causes additional derecruitment of lung units until a transpulmonary pressure of 0.0 to -1.0 cm H2O has been reached.