Pulmonary physiology: future directions for lung function testing in COPD

Chronic obstructive pulmonary disease (COPD) is a term that encompasses different pathological conditions having excessive airflow limitation in common. A wide body of knowledge has been accumulated over the last century explaining the mechanisms by which airway (chronic bronchitis) and parenchymal (emphysema) diseases lead to an indistinguishable spirometric abnormality. Although the definition of emphysema is anatomical, early studies showed that its presence can be inferred with good approximation from measurements of lung mechanics and gas exchange, in addition to simple spirometry. Studies using tests of ventilation distribution showed that abnormalities are present in smokers with normal spirometry, although these tests were not predictive of development of COPD. At the beginning of the third millennium, new documents and guidelines for diagnosis and treatment of COPD were developed, in which the functional diagnosis of COPD was restricted, for the sake of simplicity, to simple spirometry. In recent years, there has been a resurgence of interest in separating bronchitic from emphysematous phenotype of COPD. For this purpose, high-resolution computed tomography scanning has been added to diagnostic work-up. At the same time, methods for lung function testing have been refined and seem promising for detection of early small airways abnormalities. Among them are the forced oscillation technique and the nitrogen phase III slope analysis of the multiple-breath washout test, which may provide information on ventilation inhomogeneity. Moreover, the combined assessment of diffusing capacity for nitric oxide and carbon monoxide may be more sensitive than the latter alone for partitioning diffusive components at parenchymal level.