LUNG-TISSUE DISTORTION IN RESPONSE TO METHACHOLINE IN RATS - EFFECT OF LUNG-VOLUME

We have previously shown that, after induced constriction, there is su bstantial distortion of the parenchymal tissues. In this study we inve stigated whether differences in parenchymal morphology occurred when t he lung was constricted at different lung volumes. We measured trachea l pressure and calculated lung resistance in mechanically ventilated r ats under control conditions and after aerosol administration of metha choline (256 mg/ml) at different positive end-expiratory pressures (PE EP = 7, 5, and 3 cmH(2)O). The lungs were frozen with liquid nitrogen at different PEEP levels and processed via freeze substitution. We mea sured airway constriction as the ratio of the airway lumen to the idea lly relaxed area, alveolar size by measuring the mean linear intercept between alveolar walls, and tissue distortion as the standard deviati on of the number of intercepts in the 40 fields sampled per lung. The increase in lung resistance was largest at PEEP = 3 cmH(2)O. Whereas a irway constriction decreased and alveolar size increased at higher lun g volume (PEEP = 7 cmH(2)O), tissue distortion did not change. These d ata suggest that parenchymal distortion after induced constriction was not attributable to airway narrowing and/or closure.