Current functional assessments do not allow a reliable assessment of small airways, a major site of disease in COPD. Single-breath washout (SBW) tests are feasible and reproducible methods for evaluating small airway disease. Their relevance in COPD remains unknown.
We performed a cross-sectional study in 65 patients with moderate to severe COPD. Phase III slope of N2 (SIIIN2) and double tracer gas (SIIIDTG) SBW tests were used as a measure of ventilation inhomogeneity. The association of both markers with established physiological and clinical features of COPD was assessed.
Ventilation inhomogeneity as measured by SIIIN2 and SIIIDTG was increased in COPD patients compared to healthy subjects (p < 0.001 and p < 0.001, respectively). SIIIN2 was associated with FEV1 predicted, RV/TLC and DLCO (all p < 0.001). Furthermore, SIIIN2 was related to dyspnea, exercise-induced desaturation and exercise capacity (p = 0.001, p < 0.001 and p = 0.047). SIIIDTG was associated with TLC, DLCO and cough (p < 0.001, p = 0.001 and p = 0.009). In multivariate regression models we demonstrate that these associations are largely independent of FEV1, and mostly stronger than associations with FEV1. In contrast, FEV1 was superior in predicting emphysema severity.
SIIIN2 and SIIIDTG, two fast and clinically applicable measures of small airway disease, reflect different physiological and clinical aspects of COPD, largely independent of spirometry.