The Relationship between Arterial Carbon Dioxide and End Tidal Carbon Dioxide in Acute Asthma

Han, Kap Su; Park, Sang Min; Yim, Sung Ik; Choi, Sung Huk; Lee, Sung Woo; Hong, Yun Sik

Abstract

PURPOSE: We examined the relationship and concordance between end-tidal carbon dioxide (Petco2) and arterial CO2 (Paco2) in acute asthmatic patients presenting to emergency department.
METHODS
This was prospective observational cohort study of acutely ill adult asthmatic patients observed March 2005 to February 2006. Data Collected were age, sex, vital sign, treatment, simultaneous Paco2 and Petco2 value and peak expiratory flow rate (PEFR). Concordance between Paco2 and Petco2 was represented by Bland-Altman plot, using pre-specified limits of agreement of +/-6 mmHg difference and described by interclass correlation coefficient.
RESULTS
The study population consisted 51 adult asthma patients admitted during the study period. There was 92 Paco2 and Petco2 pairs. The Pearson correlation coefficient was 0.773 (p=0.000). The mean GAP (The difference Paco2 and Petco2) is 6.04mmHg. PEFR and GAP were negative correlation (R=-0.370). We obtained a Receiver operating characteristic (ROC) curve from PEFR and GAP, using a PEFR cutoff value of 30.5%. The interclass correlation coefficient between Paco2 and Petco2 was 0.652 for patients with PEFR above 30.5%, 0.362 for patients with PEFR below 30.5%, and 0.575 for patients with undetectable PEFR.
CONCLUSION
Good correlation exists between Paco2 and Petco2, butr concordance is poor (among all patients the interclass correlation coefficient was 0.508). We attribute this to ventilation-perfusion mismatch. We concluded that in severely asthmatic patients with low PEFR or mechanical ventilation, end-tidal carbon dioxide is not concordant with arterial carbon dioxide.