Recent studies in rats demonstrate that prolonged (i.e., >12 hours) mechanical ventilation (MV) associated with diaphragm inactivity elicit marked decreases in diaphragm force generation. If this disorder also occurs in humans on MV, it could explain the difficulty in discontinuing MV in some patients (i.e., difficulty in weaning) as well as for some patients who can not be weaned from MV. The rat studies indicate that a combination of (a) oxidative stress, (b) increased proteolysis, and (c) atrophy occur in diaphragm myofibers of rats exposed to prolonged MV (i.e., PMV). Our preliminary observations on PMV brain-dead organ donors with an intact circulation suggest that this disorder may also occur in humans. In the proposed research, we will carry out a matched pair case control study to answer the following questions: (1) Does a similar disorder occur in humans exposed to PMV? and (2) What pathology is produced by PMV in the human diaphragm? We define a case as a brain dead organ donor who has the following attributes: (a) on MV for >12 hours but <72 hours;(b) undergoes a diaphragm biopsy immediately before cessation of MV;and (c) absence of infection. We define a matched control as a patient with a solitary pulmonary nodule (SPN) that is (a) the same age [unreadable]5 years;(b) same gender;(c) receives neuromuscular blocking agents <3 hours before intra- operative diaphragm biopsy;(d) pathology on SPN shows benign disease;and (e) absence of infection. We have previously shown that controls exhibit no diaphragm pathology. Therefore, we will test each of our hypotheses by comparing cases and controls of each matched pair. The specific hypotheses to be tested are that in comparison to controls, cases show: (a) oxidative stress manifest by increased protein carbonylation and increased protein tyrosine nitration;(b) decreased specific force and decreased calcium sensitivity of force generation in myosin heavy chain and troponin characterized single permeabilzed fibers;(c) increased proteolysis due to an up-regulation of calpain activity and ubiquitin-protein conjugation;(d) myofiber atrophy (approximately 25-30%) and (e) marked decreases in computed maximum diaphragm force generation. For each of these changes, our detailed proteomic, genomic, physiological and histological studies will provide a mechanistic interpretation. In conclusion, these studies may define a new diaphragm disorder that is quite prevalent, causes severely decreased quality of life for our patients, and also is very costly for our limited health care funds. Moreover, these studies may directly lead to therapy for this newly recognized but relatively common disease of diaphragm muscle.