Journal of Applied Physiology AJP: Heart and Circulatory Physiology
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J Appl Physiol 85: 259-265, 1998;
8750-7587/98 $5.00
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Vol. 85, Issue 1, 259-265, July 1998

Long-range correlations of serial FEV1 measurements in emphysematous patients and normal subjects

Asger Dirksen1, Niels-Henrik Holstein-Rathlou2, Flemming Madsen1, Lene T. Skovgaard3, Charlotte S. Ulrik1, Thomas Heckscher4, and Axel Kok-Jensen1

1 Department of Respiratory Medicine, The Rigshospital, DK-2200 Copenhagen N; Departments of 2 Medical Physiology and 3 Biostatistics, The Panum Institute, University of Copenhagen, DK-2200 Copenhagen N; and 4 Department of Respiratory Medicine, Bispebjerg Hospital, DK-2400 Copenhagen NV, Denmark

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 alpha 1-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 ~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 ~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.

spirometry; periodicity; Fourier analysis; power spectrum


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