Purpose: The aim of our study was to evaluate volumetric capnography (VCap) in the differentiation between chronic obstructive pulmonary disease (COPD) patients and normal subjects.
Patients and methods: Thirty-nine healthy male volunteers and 60 male COPD patients were enrolled. Regression equations between VCap indices and age, weight, height, and tidal volume in healthy volunteers were established by stepwise regression analysis. Predicted normal values of VCap indices in COPD patients were calculated. A paired t test was used to compare the difference between observed and predicted values for VCap indices in COPD patients. Receiver operating characteristic (ROC) curve analysis was used to evaluate the power of each VCap index alone in differentiating COPD patients and normal subjects. The power of the combination of VCap indices was assessed by discriminant analysis.
Results: All regression equations were significant (P < 0.01) as were the differences between the observed and predicted normal VCap indices in COPD patients (P < 0.001). ROC curve analysis showed that the volume between 25 and 50% of F CO2et (Vm25-50), slope of Phase II (dC2/dV), and slope of Phase III (dC3/dV) were valuable predictors. Nearly all (90.9%) subjects were correctly classified by discriminant analysis.
Conclusion: Vm25-50, dC2/dV, or dC3/dV alone are valuable for differentiating COPD patients and normal subjects, but more powerful are the combinations of Vm25-50, dC2/dV, and dC3/dV, the ratio of dC2/dV to dC3/dV (SR23), dead space according to the Bohr method (VDB), and dead space according to the Wolff and Brunner methods (PIE).