Background: Indirect calorimetry (IC) is considered to be the standard method for estimating energy requirements in intensive care unit (ICU) patients. Hence, most ICU clinicians still rely on various mathematical formulas to calculate caloric requirements in their patients. We assessed whether measurements obtained by IC reached agreement with the results of such commonly used equations.
Methods: Retrospective study in consecutively hospitalized patients in a mixed medico-surgical adult ICU. Resting energy expenditure (REE) was measured by IC in all patients as a standard procedure within our routine nutritional care planning and simultaneously calculated from 10 distinct predictive equations. IC was performed with the VmaxTM Encore 29n calorimeter (VIASYS Healthcare Inc, Yorba Linda, CA). Bland-Altman plots and regression analysis were used to assess agreement between measured and calculated REE.
Results: The study included 259 critically ill patients: 161 subjects (62%) met final analysis criteria (age 63 ± 16 years; 58% males). Measured REE was 1571 ± 423.5 kcal/24 h with VO2 0.23 ± 0.06 L/min and VCO2 0.18 ± 0.05 L/min. Calculated values correlated very weakly with IC-derived measurements. Only the Swinamer equation and the Penn State 2010 reached an R² > 0.5. Widely used formulas in daily ICU practice such as the adjusted Harris Benedict, Faisy-Fagon, and ESICM '98 statement equations, reached R² values of respectively only 0.44, 0.49, and 0.41. Calculation resulted in under- as well as overestimation of REE. Global formulas reached no acceptable correlation in elderly or obese critically ill patients.
Conclusion: In critically ill adult patients, measured REE poorly correlated with calculated values, regardless what formula was used. Our findings underscore the important role of IC to adequately estimate energy requirements in this particularly frail population.