Volumetric capnography for the evaluation of chronic airways diseases

Abstract

Background: Obstructive lung diseases of different etiologies present with progressive peripheral airway involvement. The peripheral airways, known as the silent lung zone, are not adequately evaluated with conventional function tests. The principle of gas washout has been used to detect pulmonary ventilation inhomogeneity and to estimate the location of the underlying disease process. Volumetric capnography (VC) analyzes the pattern of CO2 elimination as a function of expired volume.

Objective: To measure normalized phase 3 slopes with VC in patients with non-cystic fibrosis bronchiectasis (NCB) and in bronchitic patients with chronic obstructive pulmonary disease (COPD) in order to compare the slopes obtained for the groups.

Methods: NCB and severe COPD were enrolled sequentially from an outpatient clinic (Hospital of the State University of Campinas). A control group was established for the NCB group, paired by sex and age. All subjects performed spirometry, VC, and the 6-Minute Walk Test (6MWT). Two comparisons were made: NCB group versus its control group, and NCB group versus COPD group. The project was approved by the ethical committee of the institution. Statistical tests used were Wilcoxon or Student's t-test; P<0.05 was considered to be a statistically significant difference.

Results: Concerning the NCB group (N=20) versus the control group (N=20), significant differences were found in body mass index and in several functional variables (spirometric, VC, 6MWT) with worse results observed in the NCB group. In the comparison between the COPD group (N=20) versus the NCB group, although patients with COPD had worse spirometric and 6MWT values, the capnographic variables mean phase 2 slope (Slp2), mean phase 3 slope normalized by the mean expiratory volume, or mean phase 3 slope normalized by the end-tidal CO2 concentration were similar.

Conclusion: These findings may indicate that the gas elimination curves are not sensitive enough to monitor the severity of structural abnormalities. The role of normalized phase 3 slope may be worth exploring as a more sensitive index of small airway disease, even though it may not be equally sensitive in discriminating the severity of the alterations.

Keywords: 6MWT; bronchiectasis; bronchitis; spirometry; volumetric capnography.

Figure 1

Comparison between mean values of spirometric parameters of the three groups: control, NCB, and COPD (FEV₁ and FVC shown as percentage predicted value). Notes: *P<0.0001; **P=0.01; ^†P<0.05. Abbreviations: COPD, chronic obstructive pulmonary disease; FEV₁, forced expiratory volume in the first second; FVC, forced vital capacity; NCB, non-cystic fibrosis bronchiectasis.

Figure 2

Curves of volumetric capnography of control (A), bronchiectasis (B), and COPD (C) groups (representative curves). Notes: (A) EtCO₂ =35.1 mmHg; Ve =590 mL; Slope 3=11.17 mmHg/L. (B): EtCO₂ =32.9 mmHg; Ve =502 mL; Slope 3=32.05 mmHg/L. (C) EtCO₂ =36.8 mmHg; Ve =589 mL; Slope 3=34.10 mmHg/L. Abbreviations: COPD, chronic obstructive pulmonary disease; EtCO₂, end-tidal CO₂.