EBQ:End-Tidal CO2 PaCO2 correlation

Clinical Question

Can end tidal CO2(etCO2) 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 etCO2 and PaCO2 obtained simultaneously
  • Median difference of PaCO2 and 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 more a reflection of perfusion rather than ventilation status. [3]

Conclusion

  • End-tidal CO2 (ETCO2) generally correlates with PaCO2, but the gradient is variable and increases with dead space ventilation
  • In healthy patients, ETCO2 typically underestimates PaCO2 by 2-5 mmHg
  • The ETCO2-PaCO2 gradient is unreliable in critically ill patients, particularly those with V/Q mismatch, pulmonary embolism, or shock

Major Points

  • ETCO2 monitoring is a useful noninvasive tool for trending CO2 levels in intubated patients
  • Normal ETCO2 values (35-45 mmHg) do not guarantee normal PaCO2 in patients with significant dead space
  • The correlation worsens in patients with COPD, PE, severe hypotension, and other causes of V/Q mismatch
  • Serial ETCO2 trending is more useful than single measurements for clinical decision-making
  • ABG remains the gold standard for accurate PaCO2 measurement in critically ill patients

Study Design

  • Evidence review and analysis of studies examining the ETCO2-PaCO2 correlation
  • Multiple studies reviewed across ED, ICU, and anesthesia settings

Population

  • Intubated patients in various clinical settings (operating room, ICU, ED)
  • Includes both hemodynamically stable and critically ill patients

Interventions

  • Simultaneous ETCO2 and arterial blood gas measurement comparison
  • No therapeutic intervention; diagnostic accuracy assessment

Outcomes

  • In hemodynamically stable patients: ETCO2-PaCO2 gradient typically 2-5 mmHg
  • In critically ill patients: gradient is unpredictable and can exceed 10-20 mmHg
  • Correlation coefficient (r) ranges from 0.7-0.9 depending on patient population
  • ETCO2 trending reliably reflects directional changes in PaCO2

Criticisms

  • Most studies are small and conducted in controlled settings (operating room)
  • The ETCO2-PaCO2 gradient varies significantly based on pathophysiology, limiting universal application
  • Studies rarely include the sickest ED patients where the correlation is most likely to fail
  • Technical factors (sampling line issues, circuit leaks) can independently affect ETCO2 accuracy

Funding

  • Variable across reviewed studies

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.
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