Prolonged mechanical ventilation (MV) results in reduced diaphrag-matic maximal force production and diaphragmatic atrophy. To investigate the mechanisms responsible for MV-induced diaphrag-matic atrophy, we tested the hypothesis that controlled MV results in oxidation of diaphragmatic proteins and increased diaphrag-matic proteolysis due to elevated protease activity. Further, we postulated that MV would result in atrophy of all diaphragmatic muscle fiber types. Mechanically ventilated animals were anesthe-tized, tracheostomized, and ventilated with 21% O2 for 18 hours. MV resulted in a decrease (p <0.05) in diaphragmatic myofibrillar protein and the cross-sectional area of all muscle fiber types (i.e., I, IIa, IId/x, and IIb). Further, MV promoted an increase (p <.05) in diaphragmatic protein degradation along with elevated (p <0.05) calpain and 20S proteasome activity. Finally, MV was also associated with a rise (p <.05) in both protein oxidation and lipid peroxidation. These data support the hypothesis that MV is associated with atrophy of all diaphragmatic fiber types, increased diaphragmatic protease activity, and augmented diaphragmatic ox-idative stress
