Interpreting pulmonary function tests

Educational aims

1. To elucidate the purpose of pulmonary function tests (PFTs).

2. To describe a pathway (algorithm) for interpreting PFTs, in a diagnostic sense, from measurements of spirometry (forced expiratory volume in 1 s (FEV1)) and forced vital capacity (FVC)), lung volume (total lung capacity (TLC)) and gas transfer and coefficient (transfer factor for the lung for carbon monoxide (TL,CO) and transfer coefficient of the lung for carbon monoxide (KCO)).

Summary PFTs are quantitative (for assessment) as well as qualitative (for diagnosis). The assessment aspect asks “are the results normal?”, “how abnormal?”, “has there been a significant change post–bronchodilator, or since the last measurement?”, “can this patient withstand a pneumonectomy?”, etc. The qualitative aspect looks at a portfolio of results (spirometry, lung volumes, gas transfer and muscle pressures) and makes a physiological diagnosis of 1) airflow obstruction: a) intrathoracic or extrathoracic, b) with or without alveolar damage; or 2) restriction: a) intrapulmonary, b) extrapulmonary — chest wall/pleura or neuromuscular. The physiological diagnosis may or may not support the provisional clinical diagnosis as given on the Pulmonary Function Request Form. Interpretation starts with the distinction between obstructive and restrictive disease, based primarily on TLC and the FEV1, the FVC and the FEV1/FVC ratio. The transfer factor and coefficient (TL,CO and KCO) add useful information regarding alveolar damage, pulmonary microvascular pathology, decreased alveolar expansion (neuromuscular disease) and discrete loss of units. A high KCO should prompt measurement of maximal inspiratory (PI,max) and expiratory (PE,max) pressures. Special tests have been developed recently to detect bronchiolar disease (multi–breath nitrogen washout with slope analysis). Exercise testing focuses more on assessment and prognosis than on diagnosis.