Estimating ventilator derived energy expenditure compared to indirect calorimetry in punctual and continuous measurements

Objectives Measuring energy expenditure (EE) with indirect calorimetry (IC) is the gold standard. As availability of IC is limited, this study aims to evaluate the accuracy and agreement for estimating EE with indirect calorimetry as compared with ventilator-estimated carbon dioxide production (VCO ₂), and to assess whether continuous 24-hour VCO₂ monitoring improves accuracy over short-term measurements.Methods In this single-centre observational study conducted in a tertiary intensive care unit, mechanically ventilated adult patients receiving artificial nutrition who underwent IC as part of routine care were included. EE was calculated from ventilator-derived VCO ₂ using the modified Weir equation and compared with IC-derived EE. Agreement was assessed using repeated-measures Bland-Altman analysis and correlation analysis. Secondary analyses evaluated the impact of different assumed respiratory quotient (RQ) values and compared punctual with continuous 24-hour VCO₂ measurements.Results A total of 102 paired IC and ventilator-derived measurements were analysed in 88 patients. Mean EE by IC was 2067.7±693.4 kcal/day, while ventilator-derived EE using a standard respiratory quotient (RQ=0.86) averaged 1649.7±471.3 kcal/day, resulting in a mean bias of 403.8 kcal/day (95% CI 305 to 499 kcal/day; p<0.001). Correlation between both methods was strong (r=0.68, p<0.001). Using different fixed RQ values, the smallest bias was observed for RQ=0.80 (303 kcal/day; 95% CI 206 to 398 kcal/day), yet agreement remained insufficient. Continuous 24-hour measurements showed no significant advantage over punctual 10–16 min recordings (bias=–5.1 kcal/day; p=0.39).Conclusions Ventilator-derived calorimetry significantly underestimated EE compared with IC and demonstrated limited accuracy, irrespective of assumed RQ or measurement duration. Averaging ventilator-derived VCO ₂ over 24 hours did not improve agreement compared with the punctual measurement. While ventilator-based estimation may provide a practical alternative for determining EE in critically ill, mechanically ventilated patients, it should be viewed as a complementary tool rather than a replacement.