Selective diaphragm muscle weakness after contractile inactivity during thoracic surgery

RATIONALE:: Postoperative pulmonary complications are significant contributors to morbidity in patients who have undergone upper abdominal, thoracic, or cardiac surgery. The pathophysiology of these complications might involve postoperative inspiratory muscle weakness. The nature of postoperative inspiratory muscle weakness is unknown. OBJECTIVE:: To investigate the effect of surgery on the functioning of the diaphragm, the main muscle of inspiration. METHODS:: Serial biopsies from the diaphragm and the latissimus dorsi muscle were obtained from 6 patients during thoracotomy for resection of a tumor in the right lung. Biopsies were taken as soon as the diaphragm had been exposed (t0) and again after 2 hours (t2). The contractile performance of demembranated muscle fibers, as well as fiber morphology and markers for proteolysis, was determined. RESULTS:: In all patients, the force-generating capacity of diaphragm muscle fibers at t2 was significantly reduced (∼35%) compared with that at t0, with a more pronounced force loss in type 2 fibers compared with type 1 fibers. Diaphragm weakness was not part of a generalized muscle weakness as contractile performance of latissimus dorsi fibers was preserved at t2. Diaphragm fiber size and myofibrillar structure were not different at t2 compared with t0, but myosin heavy chain type 2 was significantly reduced at t2 and MuRF-1 mRNA and protein levels were elevated at t2. CONCLUSIONS:: Only 2 hours of thoracic surgery causes marked, and selective, diaphragm muscle fiber weakness. Copyright C