ObjectiveTo characterize biochemical changes in skeletal muscle in critically ill patients.DesignSurvey of critically ill patients.SettingIntensive care unit (ICU) at a university hospital.PatientsCritically ill patients (n equals 20) subjected to trauma, surgical complications, and/or bacteremia who were treated in the ICU and showed no risk of bleeding complications were included. Reference groups of metabolically healthy volunteers and patients served as the control/reference groups.InterventionsPercutaneous muscle biopsy was obtained from both patients and healthy volunteers.Measurements and Main ResultsTotal free amino acids in skeletal muscle decreased 59% (p less than .001) and skeletal muscle glutamine concentration decreased 72% (p less than .001) in the critically ill patients. Basic amino acids decreased 49% (p less than .001). Branch-chain amino acids increased 39% (p less than .01), and aromatic amino acids increased 88% (p less than .001) in the patients.Adenosine triphosphate (ATP) was reduced by 12% (p less than .01). Total creatine concentration increased by 26% (p less than .001) due to an 80% increase in free creatine (p less than .001). The phosphorylated creatine fraction of total creatine decreased 22% (p less than .001) in the patients.Alkali-soluble protein/DNA decreased 24% (p less than .01) and fat free solid/DNA decreased 21% (p less than .01) in patients sampled on or after ICU day 5 compared with the reference group. Muscle water increased 10% due to a doubling of the extracellular water fraction.ConclusionsAlthough critically ill patients are a very heterogeneous group from a clinical point of view, there is a remarkable homogeneity in many of the biochemical parameters regardless of the severity of illness and the length of the ICU admission. The three most consistent differences were the skeletal muscle low glutamine concentration, the decrease in protein content, and the increase in extracellular water in the patients.(Crit Care Med 1996; 24:575-583)