Background
In spontaneously breathing patients, the differences between arterial PaCO2 and end-tidal CO2 (EtCO2) and the influence of bronchial obstruction have not clearly established.
Material and Method
This was a prospective observational study. Patients (n=120) were classified according to spirometric criteria into groups with normal, mild, moderate, and severe obstruction. Arterial blood gases and capnography were performed in two ways: with a tidal volume (Vt) and a non-forced maximal expiration maneuver. Pearson correlation coefficients (r) between PaCO2 and capnographic values were determined for the entire cohort and the subgroups. A concordance study was performed with Bland-Altman analysis.
Results
Comparison of PaCO2 and EtCO2 measured at Vt showed a significant correlation (r=0.722, p<0.01) for the entire cohort, but with a significant mean infra-estimation: P(a-et)CO2=5.2+/-4.4 mmHg, p<0.05. Analysis of subgroups: At Vt, P(a-et)CO2 was 1.7+/-2.9 mmHg (p=ns) in patients with normal spirometry, being maximal in the group with greater obstruction (8.2+/-5.6 mmHg, p<0.05). At maximal expiration, the comparison between PaCO2 and EtCO2 showed a significant correlation (r=0.88, p<0.001), but a significant lack of concordance for the entire cohort (P(a-et)CO2=-4.8+/-4 mmHg, p<0.05) and subgroups. Finally, comparison of PaCO2 and mean EtCO2 values showed a significant correlation (r=0.74, p<0.001) and concordance (P(a-et)CO2= 0.2+/-3.3 mmHg, p=ns) for the entire cohort and subgroups.
Conclusions
Capnographic results at Vt are accurate predictors of true PaCO2 only in patients without bronchial obstruction. The maneuvers of slow maximal expiration overestimate PaCO2 in all groups. The best concordance was obtained comparing PaCO2 with mean EtCO2.

Keywords: Respiration, Tidal Volume, Pulmonary Disease, Chronic Obstructive - physiopathology, Prospective Studies, Partial Pressure, Exhalation, Statistics as Topic, Carbon Dioxide - blood, Capnography - methods, Blood Gas Analysis