ObjectiveTo compare the indirect calorimetry (IC) measured resting energy expenditure (MREE) with adjusted Harris-Benedict formula calculating resting energy expenditure (CREE) in the mechanically ventilated surgical critically ill patients and to evaluate the relationship between the resting energy expenditure (REE) with the severity of illness. MethodsTwenty-one patients undergonging mechanical ventilation for critical illness in the intensive care unit of general surgery between August 2008 and February 2010 were included in this study. Data during the study period of nutrition support were collected for computation of the severity of critical illness by acute physiology and chronic health evaluation Ⅱ scores (APACHE Ⅱ scores) and organ dysfunction scores (Marshall scores). MREE was measured by using IC of the MedGraphics CCM/D System within the first 7 d after nutrition therapy. CREE was calculated by using the HarrisBenedict formula adjusted with correction factors for illness at the same time. According to APACHE Ⅱ scores on admission, the enrolled patients were divided into two groups: APACHEⅡ score ≥20 scores group (n=8) and APACHE Ⅱ score lt;20 scores group (n=13), and the differences between MREE and CREE of patients in two groups were determined. ResultsThe reduction of variation tendency in CREE other than MREE in the enrolled patients within the first week of nutritional support was statistical significance (Plt;0.001). The CREE of patients 〔(1 984.49±461.83) kcal/d〕 was significantly higher than the MREE 〔(1 563.88±496.93) kcal/d〕 during the first week of nutritional support (Plt;0.001). The MREE on the 0, 1, 2, and 4 d after nutrition therapy were statistically significant lower than CREE at the same time interval in these patients (Plt;0.01), and the differences at the other time points were not significant (Pgt;0.05). There was a trend towards a reduction in APACHE Ⅱ and Marshall scores within the first week of nutrition therapy that reached statistical significance (Plt;0.001). During the first week of nutrition therapy, APACHEⅡ and Marshall scores of patients in ≥20 scores group were significantly higher than those in lt;20 scores group, respectively (Plt;0.05 or Plt;0.01), and the reductions of APACHE Ⅱ scores and Marshall scores were significant in patients of two groups (Plt;0.001). A significant positive correlation was found between CREE with APACHE Ⅱ scores (r=0.656, Plt;0.001) and Marshall scores (r=0.608，Plt;0.001) in patients within the first week after nutrition support. Although no statistically significant correlation was observed between MREE and APACHEⅡ scores (r=-0.045， P=0.563), a significant positive correlation was observed between MREE and Marshall scores (r=0.263, P=0.001) within the first week after nutrition therapy. There was no correlation between MREE and CREE (r=0.064, P=0.408) in patients at the same time interval. The reduction of MREE of patients in ≥20 scores group other than in lt;20 scores group was statistically significant within the first week after nutrition therapy (P=0.034). In addition, the MREE of patients in ≥20 scores group were not significantly different from those in lt;20 scores group (Pgt;0.05), and the mean CREE was not different in two groups patients within the first week of nutritional therapy 〔(1 999.55±372.73) kcal/d vs. (1 918.39±375.27) kcal/d, P=0.887〕. CREE was significantly higher than MREE of patients in ≥20 scores group within the first week except the 3 d and 5 d after nutrition therapy (Plt;0.05), while in lt;20 scores group CREE was significantly higher than MREE in patients only within the first 3 d after nutrition therapy (Plt;0.05 or Plt;0.01). MREE and CREE of patients in ≥20 scores group were not different from those in lt;20 scores group, respectively (Pgt;0.05).