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 slopeN2SBW) (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 slopeN2SBW(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 slopeN2SBW (p = 0.0001), RV (p = 0.0001), and smoking history (p = 0.008). Multivariate analysis showed that Phase III slopeN2SBW was the only independent predictor of the 6MWD (R2 = 0.703; p = 0.0001), CAT score (R2 = 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.