Single-Breath Washout Tests to Assess Small Airway Disease in COPD.

BACKGROUND

Current functional assessments do not allow a reliable assessment of small airways, which are 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.

METHODS

We performed a cross-sectional study in 65 patients with moderate to severe COPD. Phase III slope of nitrogen (SIII) and double tracer gas (SIII) 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.

RESULTS

Ventilation inhomogeneity as measured by SIII and SIII was increased in patients with COPD compared with healthy subjects (P < .001 and P < .001, respectively). SIII was associated with FEV predicted, residual volume (RV)/total lung capacity (TLC) and diffusing capacity of the lung for carbon monoxide (Dlco) (all P < .001). Furthermore, SIII was related to dyspnea, exercise-induced desaturation, and exercise capacity (P = .001, P < .001, and P = .047, respectively). SIII was associated with TLC, Dlco, and cough (P < .001, P = .001, and P = .009, respectively). In multivariate regression models, we demonstrated that these associations are largely independent of FEV and mostly stronger than associations with FEV. In contrast, FEV was superior in predicting emphysema severity.

CONCLUSIONS

SIII and SIII, two fast and clinically applicable measures of small airway disease, reflect different physiological and clinical aspects of COPD, largely independent of spirometry.

TRIAL REGISTRY

ISRCTN99586989, Ethics committee Beider Basel (approval number 295/07).