[Functional measurements of the peripheral airways in COPD]

Abstract

Introduction: COPD is characterized by airflow limitation that is not fully reversible. Changes in the structure and function of the small airways (less than 2mm diameter) are now recognized to play a major pathophysiological and mechanical role in airflow limitation in COPD. There is, therefore, a need for technology to quantify small airways disease.

Background: For the diagnosis and assessment of COPD, spirometry is the gold standard (postbronchodilator FEV(1)/FVC less than 70%). The ATS/ERS definition of an obstructive pulmonary defect contrasts with the definitions suggested by clinical guidelines, in that FEV(1) is related to VC rather than FVC and the cut-off value of this ratio is set at the 5(th) percentile of the normal distribution rather than at a fixed value of 0.7. There is also a significant association between the severity of the disease and the degree of inflammation in the small airways. Therefore, a variety of physiological tests have been proposed as non-invasive surrogate measurements of distal lung function, such as a reduction in the forced expiratory flow at 25% to 75% of forced vital capacity (FEF(25-75)). However, the reproducibility and comparability of the FEF(25-75) is limited if not adjusted for lung volume, and the relationship between FEF(25-75) and histology is unknown. It is difficult to quantify the narrowing of the small airways and physiological measurements are difficult to interpret because there is abnormal airflow in the larger airways. Therefore, it will be difficult to assess the effects of new treatments on small airway function, and it is important to develop new techniques in order to do so.

Viewpoint: The single breath nitrogen washout, with calculation of the slope (dN(2)) of the N(2) alveolar plateau, the closing volume and the closing capacity, is a more sensitive test of early lung damage in smokers than the FEV(1). The Forced Oscillation Technique (FOT) is a method for assessing respiratory mechanics non-invasively during spontaneous breathing. The more sophisticated multiple breath washout test (MBW) has the potential to locate the affected small airways anatomically in acinar and conductive lung zones through increased phase III slope indices, Sacin and Scond. In more advanced stages of smoking-induced lung disease, differential patterns of Sacin and Scond are characteristics of parenchymal destruction in addition to peripheral airways changes.

Conclusion: These functional tests are promising solutions for small airways assessment. FOT has the advantage of being a simple method, requiring only the passive cooperation of the subject.