title: Long-range correlations of serial FEV1 measurements in emphysematous patients and normal subjects.
abstract: In obstructive lung disease the annual change in lung function is usually 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 measurements are autocorrelated. The purpose of this study was to describe the correlation structure in time series of FEV1 measurements. Nineteen patients with severe alpha1-antitrypsin deficiency (phenotype PiZ) and moderate to severe emphysema and two subjects with normal lungs were followed for several years with daily self-administered spirometry. FEV1 measurements fulfilling standard criteria were detrended, and the autocorrelation profile and the power spectrum were calculated. On average the subjects were followed for >3 yr and performed >1,000 acceptable spirometries. The autocorrelation of FEV1 measurements in the emphysematous patients was approximately 0.35 for short intervals and decreased almost exponentially with a half time of 38 days. Between 3 and 4 mo, the autocorrelation function became negative. It reached a minimum of -0.1 at approximately 8 mo and then increased toward zero over the following 12 mo. The autocorrelation function in the two normal subjects showed a similar pattern, but with a faster decay toward zero. In 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 frequencies. We conclude that serial spirometric measurements show long-range correlations. The practical implication is that FEV1 need not be measured more often than once every 3 mo in studies of the long-term trends in lung function.
authors.affiliation: Department of Respiratory Medicine, The Rigshospital, DK-2200 Copenhagen N. ADi@RH.DK
authors.forename: A
authors.initials: A
authors.lastname: Dirksen
authors.forename: N H
authors.initials: NH
authors.lastname: Holstein-Rathlou
authors.forename: F
authors.initials: F
authors.lastname: Madsen
authors.forename: L T
authors.initials: LT
authors.lastname: Skovgaard
authors.forename: C S
authors.initials: CS
authors.lastname: Ulrik
authors.forename: T
authors.initials: T
authors.lastname: Heckscher
authors.forename: A
authors.initials: A
authors.lastname: Kok-Jensen
mesh_terms: D000328:Adult; D000465:Algorithms; D005260:Female; D005541:Forced Expiratory Volume / Q000502:physiology*; D005583:Fourier Analysis; D006801:Humans; D008297:Male; D008875:Middle Aged; D010641:Phenotype; D011656:Pulmonary Emphysema / Q000503:physiopathology*; D013147:Spirometry; D019896:alpha 1-Antitrypsin Deficiency / Q000503:physiopathology