EBQ:End-Tidal CO2 PaCO2 correlation

Clinical Question

Can end tidal CO2 be used as a surrogate for PaCO2 in critically Ill Patients?

Pro Argument

The greatest correlation of etCO2 to PaCO2 is in hemodynamically stable patients and isolated TBI

Since etCO2 is dependent on both perfusion and and dead space it may underestimate the PaCO2[1]

Lee 2009 Journal of Trauma[2]

  • Prospective observational study
  • Included 66 adults at single center with GCS <9 after any traumatic injury in the Emergency Dept
  • Patients were mechanically ventilated with PetCO2 and PaCO2 obtained simultaneously
  • Median difference of PaCO2 etCO2 was 3.6 mm Hg with 77.3% concordance
  • Differences of greater than 5mm occurred in patents with hypotension, acedemic and lactate > 7 mm/L
Conclusion
An acceptable correlation except hypotensive and severely acedemic

Warner 2009 Journal of Trauma[3]

  • Prospective observational study
  • Included adult patients with TBI regardless of other injuries, however, critical patients were excluded if they required immediate OR intervention
  • Concurrent PaCO2 measurement with etCO2 of patients who remained in the ED.
  • Not all had repeat PaCO2 measurements to correlated trends of convergence or divergence
  • Correlation of R=.27 between PaCO2
  • Only 53% of TBI patients had a difference of < 5mm Hg between PaCO2 and etCO2
  • Only 36% in severe abdominal trauma and 29% in severe chest trauma had an acceptable difference of <5 mm Hg.
Conclusion
An unnacceptable correlation especially in abdominal and chest trauma

Yosefy 2004 Emerg Med Journal[4]

  • Prospective semi-blind ED study of 73 adultpatients with respiratory distress
  • Non trauma patients
  • Correlation coefficient of 0.792 with etCO2 and PaCO2 with young patients having less correlation
Conclusion
An acceptable correlation exists in non trauma patients with respiratory distress

Con Argument

  • etCO2 will differ the most from PaCO2 in patients with multi system trauma especially those with chest wall and abdominal trauma
  • EtCO2 may be a reflection of perfusion rather than ventilation status. [3]


Sources

  1. Whitesell R, Asiddao C, Gollman D, et al. Relationship between arterial and peak expired carbon dioxide pressure during anesthesia and factors influencing the difference. Anesth Analg 1981;60:508–12
  2. Lee S-W, Hong Y-S, Han C, et al. Concordance of End-Tidal Carbon Dioxide and Arterial Carbon Dioxide in Severe Traumatic Brain injury. J Trauma. 2009;67(3):526–530. doi:10.1097/TA.0b013e3181866432.
  3. 3.0 3.1 Warner KJ, Cuschieri J, Garland B, et al. The Utility of Early End-Tidal Capnography in Monitoring Ventilation Status After Severe Injury. J Trauma. 2009;66(1):26–31. doi:10.1097/TA.0b013e3181957a25.
  4. Yosefy C. End tidal carbon dioxide as a predictor of the arterial PCO2 in the emergency department setting. Emerg Med J. 2004;21(5):557–559. doi:10.1136/emj.2003.005819.