Tests of Lung Function: Physiological Principles and Interpretation

Gas exchange between organism and external ambient is the ultimate task of the respiratory system. Its efficiency is critically dependent on the efficiencies of ventilation and gas transport across the airspaces and lung tissues. Therefore, the knowledge of physiological principles underlying tests of lung function at different levels is basic to the understanding of the mechanisms limiting respiratory efficiency under different conditions, such as exercise and disease. The first step of lung function testing in clinical practice is spirometry, but it does not allow distinguishing the causes of airflow obstruction, i.e. airway disease versus emphysema, or establishing a diagnosis of lung restriction. Moreover, the effects of volume history and thoracic gas compressions may complicate its interpretation. Therefore, measurements of lung volumes are often necessary not only to confirm restriction in subjects with restrictive spirometric pattern but also for the assessment of lung hyperinflation. The latter may be due to either static (loss of elastic recoil) or dynamic (airflow limitation) mechanisms. The inhomogeneity of lung mechanics can be assessed by forced oscillations and/or nitrogen washout and may be more sensitive than spirometry to early obstruction of peripheral airways. The final step of lung function testing in clinical practice is the assessment of lung diffusing capacity for carbon monoxide. This test reflects the transport of gases from airspaces to blood across the alveolar-to-capillary barrier. Its interpretation is not always easy because the major resistance to carbon monoxide transfer is in the red cells rather than in the alveolar-to-capillary membrane.