title: Accuracy of physiologic dead space measurements in patients with acute respiratory distress syndrome using volumetric capnography: comparison with the metabolic monitor method.
abstract: BACKGROUND
  Volumetric capnography is an alternative method of measuring expired carbon dioxide partial pressure (P(eCO2)) and physiologic dead-space-to-tidal-volume ratio (V(D)/V(T)) during mechanical ventilation. In this method, P(eCO2) is measured at the Y-adapter of the ventilator circuit, thus eliminating the effects of compression volume contamination and the need to apply a correction factor. We investigated the accuracy of volumetric capnography in measuring V(D)/V(T), compared to both uncorrected and corrected measurements, using a metabolic monitor in patients with acute respiratory distress syndrome (ARDS).
  
  
  METHODS
  There were 90 measurements of V(D)/V(T) made in 23 patients with ARDS. The P(eCO2) was measured during a 5-min expired-gas collection period with a Delta-trac metabolic monitor, and was corrected for compression volume contamination using a standard formula. Simultaneous measurements of P(eCO2) and V(D)/V(T) were obtained using volumetric capnography.
  
  
  RESULTS
  V(D)/V(T) measured by volumetric capnography was strongly correlated with both the uncorrected (r2 = 0.93, p < 0.0001) and corrected (r2 = 0.89, p < 0.0001) measurements of V(D)/V(T) made using the metabolic monitor technique. Measurements of V(D)/V(T) made with volumetric capnography had a bias of 0.02 and a precision of 0.05 when compared to the V(D)/V(T) corrected for estimated compression volume contamination.
  
  
  CONCLUSIONS
  Volumetric capnography measurements of V(D)/V(T) in mechanically-ventilated patients with ARDS are as accurate as those obtained by metabolic monitor technique. .
authors.affiliation: Cardiovascular Research Institute, University of California, San Francisco, CA, USA. richkallet@earthlink.net
authors.forename: Richard H
authors.initials: RH
authors.lastname: Kallet
authors.forename: Brian M
authors.initials: BM
authors.lastname: Daniel
authors.forename: Oscar
authors.initials: O
authors.lastname: Garcia
authors.forename: Michael A
authors.initials: MA
authors.lastname: Matthay
mesh_terms: D000328:Adult; D000368:Aged; D019296:Capnography / Q000379:methods*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic / Q000379:methods; D012126:Respiratory Dead Space; D012128:Respiratory Distress Syndrome / Q000175:diagnosis* / Q000503:physiopathology; D012680:Sensitivity and Specificity