Correlations Between Small Airway Function,Ventilation Distribution,and Functional Exercise Capacity in COPD Patients

Background

Interest in using the nitrogen single-breath washout (N₂SBW) test to measure ventilation inhomogeneity and small airway function in COPD patients has grown in recent years. Our aim was to assess the correlation of the measures obtained by the N₂SBW test and other pulmonary function parameters with the six-minute walk distance (6MWD), the degree of dyspnea score, and health status in COPD patients.

Methods

In this cross-sectional study, 31 patients with COPD were subjected to the N₂SBW test, spirometry, whole-body plethysmography, carbon monoxide diffusing capacity measurement, the six-minute walk test, the modified Medical Research Council (mMRC) scale, and the COPD Assessment Test (CAT).

Results

We found a strong correlation between the 6MWD and the phase III slope of the nitrogen single-breath washout (Phase III slope_N2SBW) (r = ?0.796; p = 0.0001). We found moderate correlations between the 6MWD and the residual volume (RV) (r = ?0.651; p = 0.0001) and RV/total lung capacity (RV/TLC) (r = ?0.600; p = 0.0004). We also found moderate correlations between the CAT score and Phase III slope_N2SBW (r = 0.728; p = 0.0001), RV (r = 0.646; p = 0.0001) and RV/TLC (r = 0.603; p = 0.0003). There was a significant difference between the mMRC grades for the following variables: Phase III slope_N2SBW (p = 0.0001), RV (p = 0.0001), and smoking history (p = 0.008). Multivariate analysis showed that Phase III slope_N2SBW was the only independent predictor of the 6MWD (R ²  = 0.703; p = 0.0001), CAT score (R ²  = 0.586; p = 0.0001), and mMRC scale (relative risk = 1.14; p = 0.0001).

Conclusions

In patients with COPD, our findings suggest that the ventilation inhomogeneity impacts the functional exercise capacity, the degree of dyspnea, and health status.