ObjectiveTo evaluate the influence of body position on the extent and distribution of experimental lung damage in an oleic acid canine model of acute respiratory distress syndrome, using mechanical ventilation with high tidal volumes and positive endexplratory pressure (PEEP).DesignProspective, randomized study.SettingExperimental animal laboratory.SubjectsTwelve anesthetized and paralyzed dogs.InterventionsNinety minutes after lung injury was induced by injection of oleic acid, 12 animals were randomized to be ventilated for 4 hrs, in either the supine (supine group, n = 6) or prone (prone group, n = 6) positions, using the same ventilatory pattern (FIO2or=to10 cm H2O, and a tidal volume that generated a peak transpulmonary pressure of 35 cm H2O when implemented in the supine position). Regardless of randomization to position, the tidal volumes, FIO2, and PEEP were kept constant and the pulmonary artery occlusion pressure was maintained between 4 and 6 mm Hg for the duration of the study.Measurements and Main ResultsAt the end of the protocol, the lungs were excised for gravimetric determination (wet/dry weight ratio) and histologic examination (histologic score). Changes over time in the static pressure-volume curve of the lungs (obtained in the supine position) were also used as end-point variables..05) different when measured in the supine position. At the end of the experiment, lung gravimetric data in the two experimental groups were not statistically different, suggesting a similar extent of edema. Histologic abnormalities, however, were less in the prone group than in the supine group (p < .01), due primarily to marked differences in extent and severity in the dependent regions of the lungs. Static lung compliance improved over time in the prone group (34 +/- 9 to 46 +/- 19 mL/cm H2O) (p = .02), but not in the supine group (34 +/- 6 to 36 +/- 6 mL/cm H2O).ConclusionsAfter oleic acid-induced lung injury, animals ventilated with high tidal volume and PEEP undergo less extensive histologic change in the prone position than in the supine position. The prone position alters the distribution of histologic abnormalities. (Crit Care Med 1997; 25:16-27)