Bioelectrical impedance plethysmographic analysis of body composition in critically injured and healthy subjects

Background: Determination of body composition during critical illness is complex because of various patient-related and technical factors. Bioelectrical impedance is a promising technique for the analysis of body composition; however, its clinical utility in critically injured patients is unknown. Objective: The purpose of this study was to compare bioelectrical impedance with metabolic activity in healthy and critically injured patients. If bioelectrical impedance accurately determines body composition during critical illness the slope between body-composition variables and oxygen consumption would be the same in critically injured and healthy subjects. Design: There is a strong linear relation between body composition and metabolic activity. In the present study, body composition (fat-free mass and body cell mass) was determined by using bioelectrical impedance and resting metabolic activity (metabolic rate and oxygen consumption) by using gas exchange analysis in a group of healthy and critically injured subjects. The relation between these variables was compared by using linear regression to a similar relation established by hydrostatic weighing in a large historical control group. Results: The slope of the line relating fat-free mass to resting metabolic rate was the same in the healthy and critically ill groups (P = 0.62) and each was similar to the slope of the line for the control group. However, in 37% of the critically injured group, overhydration contributed to an increase in fat-free mass, disturbing the relation with resting metabolic rate. The slope of the line relating body cell mass to oxygen consumption in our healthy and critically ill groups was almost identical. Conclusion: These results support the use of bioelectrical impedance to determine body cell mass in healthy and critically ill subjects.