Diurnal variation in lung function in subgroups from two Dutch populations- Consequences for longitudinal analysis

We studied circadian variation in FVC, FEV1, PEF, TLC, VC, and RV between 9 :00 A.M. and 9:00 P.M. and analyzed how this variation affected estimated l ongitudinal change. Data from 876 adults were obtained in a longitudinal su rvey of samples from two Dutch areas. Subjects participated in four surveys held at 3-yr intervals between 1975 and 1985. FVC, FEV1, PEF, and VC incre ased from 9:00 A.M. until noon and decreased afterwards. TLC was constant o ver the day, whereas RV decreased from 9:00 A.M. to noon. Average variation in FVC, FEV1 and PEF, expressed as percentages of average level, was 4.8% (200 ml), 2.8% (86 ml), and 3.1% (250 ml/s), respectively. These results ar e compatible with circadian changes in airway size. No differences in varia bility were found between men and women. Significantly larger changes betwe en 9:00 A.M. and noon were found in young adults, smokers, and those with r espiratory symptoms than in other subgroups. Adjustment for diurnal variati on reduced, albeit slightly, residual standard deviations of estimated long itudinal declines. Average diurnal change was large relative to underlying longitudinal change. Its effect on longitudinal change within an individual can therefore be large depending on age, smoking habits, symptomatology, n umber of visits, time of measurement, and difference in time between measur ements. However, when people are measured at random times during the day fo r at least three visits, or when measurements are made after 11:00 A.M., ef fects of diurnal variation in pulmonary function on estimated average longi tudinal decline are minimal.