Longitudinal changes in time domain spirogram indices and their variability

The purpose of this study was to describe longitudinal changes in time domain spirogram indices and their variability. We therefore measured forced expiratory spirograms of 326 middle-aged male employees in two asbestos-using factories four times over 5 yrs. From the original sample of 326, 225 healthy subjects, who provided reliable results for three or more surveys, were selected for analysis. The mean and standard deviation of transit time and their log-transformed values (MTT, STT, ln(MTT), and ln(STT), respectively), as well as two indices of the estimated time constant distribution (Mu and Sigma, respectively) were analysed using a longitudinal model. The longitudinally estimated annual increase of MTT, STT, ln(MTT), and ln(STT) was about three times larger than the cross-sectional estimate, whilst they were comparable in Mu and Sigma. A highly significant contribution of between-subject variability was found in all indices. This was particularly remarkable in the mean components. The between-subject variability was about eight times larger than the error variance in Mu, and three times in MTT and ln(MTT). We conclude that the longitudinal data of transit time indices should not be compared with cross-sectional reference values, and that the magnitude of error variance of these indices in longitudinal repeated measurements will be rather less than the cross-sectional counterpart reported previously.