title: Accuracy of gas exchange monitoring during noninvasive ventilation: an in vitro metabolic simulation.
abstract: BACKGROUND
  Gas exchange monitoring by indirect calorimetry (IC) during noninvasive ventilation (NIV) is desirable but currently not available. Leaks around the mask preclude reliable measurements of carbon dioxide production (VCO2) and oxygen consumption (VO2) in this population. We aimed to examine the impact of system leaks and gas flows on the accuracy of gas exchange measurements during NIV using an in vitro metabolic simulation.
  
  
  METHODS
  We examined the agreement between VCO2 and VO2 measurements by IC (using a novel canopy device) and reference values generated during an in vitro metabolic simulation of NIV at room air. The flow rate of gas sampled by the IC device (VIC) was set relative to the output flow of the ventilator (VVENT) to obtain a range of sample factors (SF = VIC/VVENT). Linear regression was used to determine the effect of SF on the accuracy of the system.
  
  
  RESULTS
  An acceptable agreement between measured and reference values was observed, with mean bias (limits of agreement) of -3.3% (-6.9% to 0.3%) and -10.6% (-14.9% to -6.4%) for VCO2 and VO2, respectively. An SF of 1.25 was associated with the highest accuracy of measurement. VO2 measurement accuracy deteriorated with system leak and at SF >1.25 and was linearly related to sample dilution by ambient air entrainment.
  
  
  CONCLUSIONS
  A novel canopy device with titration of IC sample flow in relation to the ventilator flow allowed in vitro gas exchange measurements during simulated NIV with acceptable accuracy. This model needs to be tested in clinical settings.
authors.affiliation: Department of Respiratory Care.
authors.forename: Craig D
authors.initials: CD
authors.lastname: Smallwood
authors.forename: Nilesh M
authors.initials: NM
authors.lastname: Mehta
mesh_terms: D002153:Calorimetry, Indirect; D002245:Carbon Dioxide; D006801:Humans; D008954:Models, Biological; D063087:Noninvasive Ventilation / Q000379:methods*; D010101:Oxygen Consumption; D011659:Pulmonary Gas Exchange / Q000502:physiology*
chemical_list: D002245:Carbon Dioxide
keywords: carbon dioxide elimination; gas exchange; indirect calorimetry; noninvasive ventilation; oxygen consumption