title: Carbon dioxide elimination and oxygen consumption in mechanically ventilated children.
abstract: 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.
authors.affiliation: Division of Critical Care Medicine, Department of Anesthesiology, Perioperative and Pain Medicine Harvard Medical School, Boston, Massachusetts. craig.smallwood@childrens.harvard.edu.
authors.forename: Craig D
authors.initials: CD
authors.lastname: Smallwood
authors.affiliation: Division of Critical Care Medicine, Department of Anesthesiology, Perioperative and Pain Medicine Harvard Medical School, Boston, Massachusetts.
authors.forename: Brian K
authors.initials: BK
authors.lastname: Walsh
authors.affiliation: Department of Gastroenterology, Hepatology and Nutrition, Boston Children's Hospital, Boston, Massachusetts.
authors.forename: Lori J
authors.initials: LJ
authors.lastname: Bechard
authors.affiliation: Division of Critical Care Medicine, Department of Anesthesiology, Perioperative and Pain Medicine Harvard Medical School, Boston, Massachusetts.
authors.forename: Nilesh M
authors.initials: NM
authors.lastname: Mehta
mesh_terms: D000367:Age Factors; D001827:Body Height; D001835:Body Weight; D002153:Calorimetry, Indirect; D019296:Capnography; D002648:Child; D002675:Child, Preschool; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D010101:Oxygen Consumption / Q000502:physiology*; D011659:Pulmonary Gas Exchange; D012016:Reference Values; D012121:Respiration, Artificial / Q000706:statistics & numerical data*; D012680:Sensitivity and Specificity
keywords: V̇CO2; carbon dioxide elimination; children; mechanical ventilation; oxygen consumption; pediatric; steady state; volumetric capnography