Utility of relating pulmonary airflow measures to total lung capacity

Among healthy never-smokers, forced expiratory volume in one second (FEV1) and maximal voluntary ventilation (40 breaths per minute, MVV40) divided by total lung capacity (TLC) showed less interindividual variability than FEV1 and MVV40, whereas the interindividual variability of other flow variables increased after division by TLC. FEV1, MVV40 and peak expiratory flow (PEF) divided by TLC had higher Student's t-values than FEV1, MVV40 or PEF in comparisons between 224 smokers and 232 never-smokers or between 137 asthmatic patients and 456 reference subjects. Thus these airflow variables corrected for lung size appear to be generally useful indicators of airflow limitation. In these comparisons, the variables were adjusted for body size. All t-test comparisons were based on an equal number of tests. Forced expiratory flow at 70% of TLC (MEF70%TLC) and the area of the expiratory part of the flow-volume curve had the highest t-values when comparing airflow before and after bronchodilatation in 70 asthmatic subjects, except in subjects with more advanced bronchoconstriction in whom MVV40 ranked highest. For the computation of MEF70%TLC, measured TLC could be substituted with predicted TLC with little loss of discriminatory capacity. Reference equations for forced expiratory flow at different TLC fractions and for FEV1/TLC are given.