Intrasubject variability of maximal expiratory flow volume curve

Abstract

Analysis of airflow in the terminal portion of the maximal expiratory flow volume curve has been suggested as a useful test for the early diagnosis of chronic airways obstruction. Whether this analysis can identify early disease, and whether any subsequent action can prevent the progress of chronic airways obstruction, is unknown and will require prospective studies. As a precursor of such a study an attempt was made to establish the intrasubject variability of forced expiration tests which may be used for screening. Therefore, the expiratory flow volume curves of 5 healthy males and 5 healthy females aged 20-30 yr were measured, as this is an age group in which early detection of airways obstruction may be of value. Flow volume curves were obtained on the same day of the week for 6 wk, and on 3 separate days during this period 3 flow volume curves every hr from 9 a.m. to 6 p.m. were carried out. The data were subjected to analysis of variance to determine the variability of each measurement. Data were collected from forced expired volume in one second (FEV1), forced vital capacity (FVC), maximum expiratory flow rates at 50% and 75% of expired vital capacity, and forced expiratory time (FET). The results showed no consistent pattern of diurnal variation over the working day. The individual variation for FEV1 and FVC was considerably less than variations detected in the maximal expiratory flow rates at 50% and 75% of the expired vital capacity and FET. The results suggest that the intrasubject variation found in flow rates of the terminal portion of the maximal expiratory flow volume curve and forced expiratory time may limit the usefulness of these tests in detecting early airways obstruction. FEV1 and FVC are more reproducible tests and are therefore particularly suited for cross-sectional screening. The more sensitive the maximal expiratory flow volume curve may be, however, the more useful are these tests for long term studies in individuals when the onset of disease is sought, or for short term challenge studies requiring the most sensitive index of airway change.