Comparison of indirect calorimetry, the Fick method, and prediction equations in estimating the energy requirements of critically ill patients

Abstract

Background: Accurate measurement of resting energy expenditure (REE) is helpful in determining the energy needs of critically ill patients requiring nutritional support. Currently, the most accurate clinical tool used to measure REE is indirect calorimetry, which is expensive, requires trained personnel, and has significant error at higher inspired oxygen concentrations. Objective: The purpose of this study was to compare REE measured by indirect calorimetry with REE calculated by using the Fick method and prediction equations by Harris-Benedict, Ireton-Jones, Fusco, and Frankenfield. Design: REEs of 36 patients [12 men and 24 women, mean age 58 ± 22 y and mean Acute Physiology and Chronic Health Evaluation II score 22 ± 8] in a hospital intensive care unit and receiving mechanical ventilation and total parenteral nutrition (TPN) were measured for ≥15 min by using indirect calorimetry and compared with REEs calculated from a mean of 2 sets of hemodynamic measurements taken during the metabolic testing period with an oximetric pulmonary artery catheter. Results: Mean REE by indirect calorimetry was 8381 ± 1940 kJ/d and correlated poorly with the other methods tested (r² = 0.057–0.154). This correlation did not improve after adjusting for changes in respiratory quotient (r² = 0.28). Conclusions: These data do not support previous findings showing a strong correlation between REE determined by the Fick method and other prediction equations and indirect calorimetry. In critically ill patients receiving TPN, indirect calorimetry, if available, remains the most appropriate clinical tool for accurate measurement of REE.