VCO₂-derived energy expenditure: do not throw the baby out with the bath water!

Excerpt

With great interest we read the retrospective study performed by Oshima et al. [1] evaluating whether VCO₂-based energy expenditure (EE-VCO₂) could be considered as an alternative to EE measured by indirect calorimetry. This study followed several prospective studies reporting good agreement between EE-VCO₂ and EE measured by indirect calorimetry [2, 3]. Indeed, in their retrospective cohort of 278 mechanically ventilated patients, the authors found a low bias of −48 kcal/day for EE-VCO₂, when calculated with a fixed respiratory quotient (RQ) of 0.85. They reported 5%-accuracy rates of 46% and 10%-accuracy rates of 77% for EE-VCO₂. This indicates that EE-VCO₂ is unreliable in some patients, likely due to extreme RQs or to high ventilator rates as observed in young children [4]. The weak spot of using EE-VCO₂ is that an RQ has to be assumed in order to derive the unknown oxygen consumption (VO₂) needed to calculate EE according to the Weir formula: EE(kcal/day) = 1.44 × (3.94 × VO₂(mL/min) + 1.11 × VCO₂(mL/min)). …