LONG-RANGE CORRELATIONS OF SERIAL FEV1 MEASUREMENTS IN EMPHYSEMATOUS PATIENTS AND NORMAL SUBJECTS

In obstructive lung disease the annual change in lung function is usua lly estimated from serial measurements of forced expiratory volume in 1 s (FEV1). Frequent measurements in each patient may not improve this estimate because data are not statistically independent; i.e., the me asurements are autocorrelated. The purpose of this study was to descri be the correlation structure in time series of FEV1 measurements. Nine teen patients with severe alpha(1)-antitrypsin deficiency (phenotype P iZ) and moderate to severe emphysema and two subjects with normal lung s were followed for several years with daily self-administered spirome try. FEV1 measurements fulfilling standard criteria were detrended, an d the autocorrelation profile and the power spectrum were calculated. On average the subjects were followed for >3 yr and performed >1,000 a cceptable spirometries. The autocorrelation of FEV1 measurements in th e emphysematous patients was similar to 0.35 for short intervals and d ecreased almost exponentially with a half time of 38 days. Between 3 a nd 4 mo, the autocorrelation function became negative. It reached a mi nimum of -0.1 at similar to 8 mo and then increased toward zero over t he following 12 mo. The autocorrelation function in the two normal sub jects showed a similar pattern, but with a faster decay toward zero. I n the patients, the power spectrum had a peak at 1 cycle/wk and showed a 1/f pattern, where f is frequency, with a slope of -0.88 at lower f requencies. We conclude that serial spirometric measurements show long -range correlations. The practical implication is that FEV1 need not b e measured more often than once every 3 mo in studies of the long-term trends in lung function.