Detrended fluctuation analysis of peak expiratory flow and exacerbation frequency in COPD

We investigated whether exacerbation frequency in chronic obstructive pulmonary disease (COPD) was related to an exponent alpha which quantifies self-similarity in daily peak expiratory flow (PEF) and is calculated using detrended fluctuation analysis (DFA). We examined data from COPD patients who recorded an increase in respiratory symptoms and post-bronchodilator PEF on daily diary cards. We also investigated PEF data from a double-blind, placebo-controlled trial of the anti-cholinergic agent, tiotropium. In the observational study there were 308 patients with COPD (195 males; mean +/- SD age 68.3 +/- 8.4 yrs, forced expiratory volume in 1 s (FEV1) 1.12 +/- 0.46 L, FEV1 % predicted 44.5 +/- 16.4%). The mean +/- SD alpha over the first year was 0.944 +/- 0.19 and it was positively related to the frequency of exacerbations per year (p=0.009). In the clinical trial, a was lower in COPD patients randomised to tiotropium, mean +/- SD 0.87 +/- 0.21 (n=48) than on placebo, mean +/- SD 0.95 +/- 0.19 (n=52; p=0.035). Power analysis showed that fewer patients would be required for clinical studies with a as the outcome measure than exacerbation frequency. DFA shows that daily PEF in COPD has long-term correlations which are related to exacerbation frequency. Monitoring of PEF and use of a may result in smaller COPD patient sample sizes in trials.