title: The CO₂ GAP Project--CO₂ GAP as a prognostic tool in emergency departments.
abstract: OBJECTIVE
  To determine whether CO₂ GAP [(a-ET) PCO₂] value differs consistently in patients presenting with shortness of breath to the ED requiring ventilatory support. To determine a cut-off value of CO₂ GAP, which is consistently associated with measured outcome and to compare its performance against other derived variables.
  
  
  METHODS
  This prospective observational study was conducted in ED on a convenience sample of 412 from 759 patients who underwent concurrent arterial blood gas and ETCO₂ (end-tidal CO₂) measurement. They were randomized to test sample of 312 patients and validation set of 100 patients. The primary outcome of interest was the need for ventilatory support and secondary outcomes were admission to high dependency unit or death during stay in ED. The randomly selected training set was used to select cut-points for the possible predictors; that is, CO₂ GAP, CO₂ gradient, physiologic dead space and A-a gradient. The sensitivity, specificity and predictive values of these predictors were validated in the test set of 100 patients.
  
  
  RESULTS
  Analysis of the receiver operating characteristic curves revealed the CO₂ GAP performed significantly better than the arterial-alveolar gradient in patients requiring ventilator support (area under the curve 0.950 vs 0.726). A CO₂ GAP ≥10 was associated with assisted ventilation outcomes when applied to the validation test set (100% sensitivity 70% specificity).
  
  
  CONCLUSIONS
  The CO₂ GAP [(a-ET) PCO₂] differs significantly in patients requiring assisted ventilation when presenting with shortness of breath to EDs and further research addressing the prognostic value of CO₂ GAP in this specific aspect is required.
authors.affiliation: Westmead Hospital Emergency Department, Corner Darcy and Hawkesbury Roads, Westmead, Sydney, NSW 2145, Australia. samit36@hotmail.com
authors.forename: Amith L
authors.initials: AL
authors.lastname: Shetty
authors.forename: Kevin H
authors.initials: KH
authors.lastname: Lai
authors.forename: Karen
authors.initials: K
authors.lastname: Byth
mesh_terms: D000328:Adult; D019540:Area Under Curve; D001784:Blood Gas Analysis; D019296:Capnography; D002245:Carbon Dioxide / Q000097:blood*; D004638:Emergency Treatment; D005260:Female; D006801:Humans; D007362:Intensive Care Units; D008297:Male; D008875:Middle Aged; D009517:New South Wales; D011379:Prognosis; D011446:Prospective Studies; D012372:ROC Curve; D012120:Respiration Disorders / Q000097:blood* / Q000175:diagnosis / Q000503:physiopathology*; D012121:Respiration, Artificial; D012126:Respiratory Dead Space / Q000502:physiology*; D013990:Tidal Volume
chemical_list: D002245:Carbon Dioxide