title: Calculation of physiologic dead space: comparison of ventilator volumetric capnography to measurements by metabolic analyzer and volumetric CO2 monitor.
abstract: BACKGROUND
  Calculation of physiologic dead space (dead space divided by tidal volume [VD/VT]) using the Enghoff modification of the Bohr equation requires measurement of the partial pressure of mean expired CO2 (PĒCO2) by exhaled gas collection and analysis, use of a metabolic analyzer, or use of a volumetric CO2 monitor. The Dräger XL ventilator is equipped with integrated volumetric CO2 monitoring and calculates minute CO2 production (VCO2). We calculated PĒCO2 and VD/VT from ventilator derived volumetric CO2 measurements of VCO2 and compared them to metabolic analyzer and volumetric CO2 monitor measurements.
  
  
  METHODS
  A total of 67 measurements in 36 subjects recovering from acute lung injury or ARDS were compared. Thirty-one ventilator derived measurements were compared to measurements using 3 different metabolic analyzers, and 36 ventilator derived measurements were compared to measurements from a volumetric CO2 monitor.
  
  
  RESULTS
  There was a strong agreement between ventilator derived measurements and metabolic analyzer or volumetric CO2 monitor measurements of PĒCO2 and VD/VT. The correlations, bias, and precision between the ventilator and metabolic analyzer measurements for PĒCO2 were r = 0.97, r(2) = 0.93 (P < .001), bias -1.04 mm Hg, and precision ± 1.47 mm Hg. For VD/VT the correlations were r = 0.95 and r(2) = 0.91 (P < .001), and the bias and precision were 0.02 ± 0.04. The correlations between the ventilator and the volumetric CO2 monitor for PĒCO2 were r = 0.96 and r(2) = 0.92 (P < .001), and the bias and precision were -0.19 ± 1.58 mm Hg. The correlations between the ventilator and the volumetric CO2 monitor for VD/VT were r = 0.97 and r(2) = 0.95 (P < .001), and the bias and precision were 0.01 ± 0.03.
  
  
  CONCLUSIONS
  PĒCO2, and therefore VD/VT, can be accurately calculated directly from the Dräger XL ventilator volumetric capnography measurements without use of a metabolic analyzer or volumetric CO2 monitor.
authors.affiliation: Respiratory Care Services, Department of Anesthesia and Perioperative Care, San Francisco General Hospital, University of California-San Francisco, CA 94110, USA. msiobal@sfghsom.ucsf.edu
authors.forename: Mark S
authors.initials: MS
authors.lastname: Siobal
authors.forename: Hannah
authors.initials: H
authors.lastname: Ong
authors.forename: Josephine
authors.initials: J
authors.lastname: Valdes
authors.forename: Julin
authors.initials: J
authors.lastname: Tang
mesh_terms: D055371:Acute Lung Injury / Q000175:diagnosis* / Q000378:metabolism / Q000503:physiopathology; D000328:Adult; D019296:Capnography / Q000379:methods / Q000706:statistics & numerical data; D002245:Carbon Dioxide / Q000032:analysis*; D061827:Dimensional Measurement Accuracy; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic / Q000295:instrumentation / Q000379:methods / Q000706:statistics & numerical data; D015588:Observer Variation; D011659:Pulmonary Gas Exchange; D012126:Respiratory Dead Space / Q000502:physiology*; D012128:Respiratory Distress Syndrome / Q000175:diagnosis* / Q000378:metabolism / Q000503:physiopathology; D013223:Statistics as Topic; D013990:Tidal Volume
chemical_list: D002245:Carbon Dioxide