Carbon Dioxide Elimination and Oxygen Consumption in Mechanically Ventilated Children

BACKGROUND: Accurate measurement of carbon dioxide elimination (V̇_CO2) and oxygen consumption (V̇_O2) at the bedside may help titrate nutritional and respiratory support in mechanically ventilated patients. Continuous V̇_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 V̇_CO2 and V̇_O2 values observed in mechanically ventilated children. We also aimed to examine the characteristics of V̇_CO2 values that are associated with standard steady state (5-min period when V̇_CO2 and V̇_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 V̇_CO2 and V̇_O2 values (mL/kg/min) were modeled against subject height, and correlation coefficients were computed to quantify the goodness of fit. A steady-state definition using only V̇_CO2 was developed (V̇_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 V̇_CO2 values were 7.6, 5.8, and 3.5 mL/kg/min. Normalized V̇_CO2 and V̇_O2 values were inversely related to subject height and age. The relationships between normalized gas exchange values and height were demonstrated by the models: V̇_CO2 = 115 × (height in cm)^−0.71 (R = 0.61, P < .001) and V̇_O2 = 130 × (height in cm)^−0.72 (R = 0.61, P < .001). Steady-state V̇_CO2 predicted standard steady state (sensitivity of 0.84, specificity of 1.0, P < .01).

CONCLUSIONS: V̇_CO2 and V̇_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 V̇_CO2 varied by < 5% predicted standard steady state. Our observations may facilitate greater utility of V̇_CO2 at the bedside in the pediatric ICU and thereby extend the benefits of metabolic monitoring to a larger group of patients.