Carbon dioxide elimination and oxygen consumption in mechanically ventilated children

BACKGROUND: Accurate measurement of carbon dioxide elimination (V_CO2) and oxygen consumption (V_O2) at the bedside may help titrate nutritional and respiratory support in mechanically ventilated patients. Continuous V_CO2 monitoring is now available with many ventilators. However, because normative data are sparsely available in the literature, we aimed to describe the range of V_CO2 and V_O2 values observed in mechanically ventilated children. We also aimed to examine the characteristics of V_CO2 values that are associated with standard steady state (5-min period when V_CO2 and V_O2 variability are < 10%). METHODS: Mechanically ventilated patients who underwent indirect calorimetry testing were eligible for inclusion, and subjects who achieved standard steady state were included. Normalized V_CO2 and V_O2 values (mL/kg/min) were modeled against subject height, and correlation coefficients were computed to quantify the goodness of fit. A steadystate definition using only V_CO2 was developed (V_CO2 variability of < 5% for a 5-min period) and tested against standard steady state using sensitivity and specificity. RESULTS: Steady-state data from 87 indirect calorimetry tests (in 70 subjects) were included. For age groups < 0.5, 0.5– 8, and > 8 y, the mean V_CO2 values were 7.6, 5.8, and 3.5 mL/kg/min. Normalized V_CO2 and V_O2 values were inversely related to subject height and age. The relationships between normalized gas exchange values and height were demonstrated by the models: V_CO2 = 115 × (height in cm)^-0.71 (R = 0.61, P < .001) and V_O2 = 130 × (height in cm)^-0.72 (R = 0.61, P < .001). Steady-state V_CO2 predicted standard steady state (sensitivity of 0.84, specificity of 1.0, P < .01). CONCLUSIONS: V_CO2 and V_O2 measurements correlated with subject height and age. Smaller and younger subjects produced larger amounts of CO₂ and consumed more O₂ per unit of body weight. The use of a 5-min period when V_CO2 varied by < 5% predicted standard steady state. Our observations may facilitate greater utility of V_CO2 at the bedside in the pediatric ICU and thereby extend the benefits of metabolic monitoring to a larger group of patients.