Acetylcysteine, Adult, Antioxidants, Bilirubin, Bronchoalveolar Lavage Fluid, Cardiac Output, Cysteine, Double-Blind Method, Free Radical Scavengers, Glutathione, Humans, Middle Aged, Prospective Studies, Pyrrolidonecarboxylic Acid, Respiration, Artificial, Respiratory Distress Syndrome, Thiazoles, Thiazolidines, Time Factors