Metabolic evaluation during weaning from mechanical ventilation using indirect calorimetry

Introduction

Indirect calorimetry (IC) can be useful in the evaluation of metabolic status from critical care patients, especially during weaning from mechanical ventilation (MV) when energy expenditure can increase. The goals of this study were to compare the energy expenditure (EE) from patients during weaning from MV, comparing pressure support ventilation (PSV) and T tube (TT) using IC, as well as to compare these findings with results calculated with Harris–Benedict equation.

Methods

Patients clinically ready to discontinue MV support were evaluated from August 2006 to January 2007. They were studied, in a random order, during PSV and TT. Measurements from EE were registered during 20 minutes in both methods. Indirect calorimetry was registered using a specific metabolic monitor (Datex-Ohmeda/M-COVX). EE was also estimated using the Harris–Benedict equation with and without an activity factor. Results are shown as the mean ± standard deviation. Statistical analysis was performed with the paired t test, Pearson's correlation coefficient and the Bland–Altman test. The significance level was P < 0.05.

Results

Forty patients were enrolled. The mean age was 56 ± 16 years, APACHE II score was 23 ± 8 and the majority of patients were male (70%). The mean EE during TT was 14.43% greater than during PSV (P < 0.001). The mean EE estimated by the Harris–Benedict equation was 1,455.05 ± 210.4 kcal/24 hours, and considering the activity factor 1,608 ± 236.14 kcal/24 hours. Both calculated values showed correlation with that measured by indirect calorimetry during PSV (r = 0.647) and TT (r = 0.539). The agreement limits comparing measured and estimated EE with the Bland–Altman analysis suggest that the Harris–Benedict equation underestimates EE during TT.

Conclusion

Comparing EE during PSV and TT, using IC, we observed that during TT there was, as expected, an increase in EE (14.43%). The results also suggest that the Harris–Benedict equation underestimates energy expenditure during TT.