The effect of pulmonary edema on gas trapping in excised rat lungs

The object of this study was to determine how pulmonary edema affects the volume of gas trapped in excised lungs as they are slowly ventilated. In this study trapped gas is defined as the volume of gas that cannot be removed from the lungs with a negative transpulmonary pressure of -5 cm H2O (Frazer and Weber, 1976). Experimental pulmonary edema was produced by ventilating rats using high inspiratory positive pressure breathing (HIPPB) as described by Webb and Tierney (1974). The amount of gas trapped in both edematous and control lungs was then compared as the lungs were inflated-deflated under identical conditions in 6 different 4 cycle sequences. During each sequence the lungs were deflated to an end expiratory pressure of either +6, +4, +3, +2, 0.0, or -5 cm H2O. It was found, that gas became trapped at more positive values of end expiratory pressure in lungs having pulmonary edema than in control lungs. These results were interpreted as evidence that the airways close sooner, at more positive transpulmonary pressures, in edematous lungs than in control lungs.