Background: Data on the influence of age and body mass index (BMI) on energy metabolism of the critically ill are heterogeneous . Due to the increasingly aging critically ill population, investigation on age- and BMI-specific energy metabolism is relevant .
Methods: A total of 394 indirect calorimetry measurements were conducted on 348 critically ill adult medical patients, including 46 repeat measurements after 3.6 ± 4.3 days . Measured resting energy expenditure (MREE) was compared for age groups, BMI, and gender . Predicted energy expenditure (PEE) using the Penn State, Swinamer, and Ireton-Jones equations and the ACCP recommendations was also compared with MREE .
Results: The patients were 65.6 ± 14.5 years old . Their mean Acute Physiology and Chronic Health Evaluation II score was 27.6 ± 7.8 . Mean BMI was 27.8 ± 8.4 kg/m2, and 25.6% were obese . MREE adjusted for ideal body weight decreased with increasing age, while it increased with increasing BMI . Age, BMI, and gender are independent determinants of MREE after adjusting for clinical factors (R2 = 0.34). All four prediction equations showed a proportional bias, with the Penn State equation performing acceptably . In 46 patients with repeat indirect calorimetry, there was no significant difference between the first and second MREE (p = 0.62).
Conclusions: Age, BMI, and gender are independent determinants of resting energy expenditure in critically ill adults. Variations between measured and predicted energy expenditure are considerable . Should prediction equations be used, their performance in the specific population should be taken into consideration . Repeat indirect calorimetry may not always be necessary . However, this may depend on the length of stay and the extent of stress.