INFLUENCE OF PRONE POSITION ON THE EXTENT AND DISTRIBUTION OF LUNG INJURY IN A HIGH TIDAL VOLUME OLEIC-ACID MODEL OF ACUTE RESPIRATORY-DISTRESS SYNDROME

Objective: To evaluate the influence of body position on the extent an d distribution of experimental lung damage in an oleic acid canine mod el of acute respiratory distress syndrome, using mechanical ventilatio n with high tidal volumes and positive end-expiratory pressure (PEEP), Design: Prospective, randomized study. Setting: Experimental animal l aboratory, Subjects: Twelve anesthetized and paralyzed dogs, Intervent ions: Ninety minutes after lung injury was induced by injection of ole ic acid, 12 animals were randomized to be ventilated for 4 hrs, in eit her the supine (supine group, n = 6) or prone (prone group, n = 6) pos itions, using the same ventilatory pattern (FIO2 0.6, PEEP greater tha n or equal to 10 cm H2O, and a tidal volume that generated a peak tran spulmonary pressure of 35 cm H2O when implemented in the supine positi on), Regardless of randomization to position, the tidal volumes, FIO2, and PEEP were kept constant and the pulmonary artery occlusion pressu re was maintained between 4 and 6 mm Hg for the duration of the study, Measurements and Main Results: At 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 st atic pressure-volume curve of the lungs (obtained in the supine positi on) were also used as endpoint variables, At baseline, hemodynamic and respiratory variables did not differ between groups, Just before rand omization to position (90 mins after oleic acid injection), both group s presented similar lung static pressure volume curves, Pulmonary arte ry occlusion pressure (4.3 +/- 1.9 vs, 4.8 +/- 1.3 mm Hg [supine vs, p rone group]), cardiac output (4.1 +/- 0.4 vs, 5,2 +/- 1.3 L/min [supin e vs, prone group]), and venous admixture (36.7 +/- 20.7% vs, 28.3 +/- 19.40% [supine vs, prone group]) were also not significantly (p > .05 ) different when measured in the supine position, At the end of the ex periment, lung gravimetric data in the two experimental groups were no t statistically different, suggesting a similar extent of edema, Histo logic 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 compli ance improved over time in the prone group (34 +/- 9 to 46 +/- 19 mL/c m H2O) (p = .02), but not in the supine group (34 +/- 6 to 36 +/- 6 mL /cm H2O), Conclusions: After oleic acid induced lung injury, animals v entilated with high tidal volume and PEEP undergo less extensive histo logic change in the prone position than in the supine position. The pr one position alters the distribution of histologic abnormalities.