Carbon dioxide elimination and oxygen consumption in mechanically ventilated children
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Published on: January 29, 2011
Craig D Smallwood1, Brian K Walsh2, Lori J Bechard3
1Division of Critical Care Medicine, Department of Anesthesiology, Perioperative and Pain Medicine Harvard Medical School, Boston, Massachusetts. craig.smallwood@childrens.harvard.edu.
Insights
Accurate bedside measurement of carbon dioxide elimination (VCO2) and oxygen consumption (VO2) in ventilated children is now possible. These metabolic monitoring values correlate with height and age, aiding clinical decisions.
Area of Science:
- Pediatric critical care medicine
- Respiratory physiology
- Metabolic monitoring
Background:
- Accurate bedside measurement of carbon dioxide elimination (VCO2) and oxygen consumption (VO2) is crucial for titrating support in mechanically ventilated patients.
- Continuous VCO2 monitoring is available on many ventilators, but normative data in children are limited.
- This study aimed to describe VCO2 and VO2 ranges in ventilated children and identify characteristics of VCO2 associated with a standard steady state.
Purpose of the Study:
- To establish normative data for VCO2 and VO2 in mechanically ventilated children.
- To examine the relationship between VCO2 and VO2 with patient height and age.
- To validate a simplified steady-state definition for VCO2 monitoring.
Main Methods:
- Indirect calorimetry data from 87 tests in 70 mechanically ventilated children who achieved standard steady state were analyzed.
- Normalized VCO2 and VO2 (mL/kg/min) were modeled against subject height.
- A new steady-state definition using VCO2 variability (<5% over 5 min) was developed and tested against the standard definition.
Main Results:
- Mean VCO2 values varied by age group (<0.5y: 7.6, 0.5-8y: 5.8, >8y: 3.5 mL/kg/min).
- Normalized VCO2 and VO2 were inversely related to height and age, with models showing significant correlation (VCO2: R=0.61, VO2: R=0.61).
- The simplified VCO2 steady-state definition demonstrated high accuracy (sensitivity 0.84, specificity 1.0) compared to the standard definition.
Conclusions:
- Bedside VCO2 and VO2 measurements in ventilated children correlate with height and age, with smaller, younger patients exhibiting higher values.
- A 5-minute period of <5% VCO2 variability reliably predicts standard steady state.
- These findings support enhanced utility of bedside VCO2 monitoring in pediatric ICUs for improved metabolic assessment.
Background:
Accurate measurement of carbon dioxide elimination (V̇CO2 ) and oxygen consumption (V̇O2 ) at the bedside may help titrate nutritional and respiratory support in mechanically ventilated patients. Continuous V̇CO2 monitoring is now available with many ventilators. However, because normative data are sparsely available in the literature, we aimed to describe the range of V̇CO2 and V̇O2 values observed in mechanically ventilated children. We also aimed to examine the characteristics of V̇CO2 values that are associated with standard steady state (5-min period when V̇CO2 and V̇O2 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 V̇CO2 and V̇O2 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 V̇CO2 was developed (V̇CO2 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 V̇CO2 values were 7.6, 5.8, and 3.5 mL/kg/min. Normalized V̇CO2 and V̇O2 values were inversely related to subject height and age. The relationships between normalized gas exchange values and height were demonstrated by the models: V̇CO2 = 115 × (height in cm)(-0.71) (R = 0.61, P < .001) and V̇O2 = 130 × (height in cm)(-0.72) (R = 0.61, P < .001). Steady-state V̇CO2 predicted standard steady state (sensitivity of 0.84, specificity of 1.0, P < .01).
Conclusions:
V̇CO2 and V̇O2 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 V̇CO2 varied by < 5% predicted standard steady state. Our observations may facilitate greater utility of V̇CO2 at the bedside in the pediatric ICU and thereby extend the benefits of metabolic monitoring to a larger group of patients.
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