Capnometry in the Surgical ICU: An Analysis of the Arterial-to-End-Tidal Carbon Dioxide Difference

BACKGROUND: Capnometry offers a continuous measurement of the end-tidal carbon dioxide tension (PetCO2) that is thought to predict arterial carbon dioxide tension (PaCO2) and to be useful in monitoring and adjusting mechanical ventilation in the Intensive Care Unit (ICU). METHODS: We designed a study to examine the relationship between the PaCO2 and the PetCO2 in surgical ICU patients. We pooled data from our previous experiments in the ICU setting. In addition to evaluating the correlation between the PaCO2 and the PetCO2, we compared the sequential changes in PaCO2 and PetCO2 (APaCO2 and APetCO2). To be clinically useful as a predictor of PaCO2, the APetCO2 should very closely approximate the APaCO2. RESULTS: A total of 693 pairs of PaCO2 and PetCO2 measurements were included in the analysis. The PetCO2 did demonstrate a statistical correlation with the PaCO2. However, in a large number of individuals (51/80), there was a poor correlation between the two variables, and 31.3% of the APetCO2S did not predict APaCO2. CONCLUSION: Although capnometry may be useful for other reasons, we conclude from the data presented that the continuous measurement of PetCO2 cannot reliably replace measurement of PaCO2 in the ICU setting.