P122 The use of impulse oscillometry (IOS) to study fractal scaling and sample entropy in airway resistance time series in severe asthma

Introduction Severe asthma affects airway calibre and can be monitored using IOS. Sample entropy (SampEn) is a measure of complexity and is defined as the probability that sequences of patterns (template size) in time series which are initially closely related, that is, within a fraction of the standard deviation (tolerance level) of the time-series remain so within subsequent time frames. Fractal scaling is a measure of self similarity and scale invariance measured in a time-series and quantifies the memory found within as a consequence. We hypothesised that fractal scaling and SampEn will be useful in characterising severe asthma. Methods 66 GINAstage 4–5 severe asthmatics (Mean(Sem) age; 54.1(1.4), Sex M:F; 31:35, post-bronchodilator FEV 1 % predicted; 81.02 (2.7)%) and 27 Controls (Mean(Sem) age; 48.4(2.2), Sex M:F;9:18, post-bronchodilator FEV 1 % predicted; 108.2 (2.8)%) were recruited. Impulse oscillometry was performed at 5–35 Hz, with impulses triggered every 0.2 s for 150 s, at baseline and 15 min after 400 mcg inhaled salbutamol. Detrended fluctuation analysis was used to derive the fractal scaling exponent α 1 . SampEn was derived using a custom program. SampEn and α 1 were both obtained from airway resistance at 10 Hz over the 150 s time-series. Triplicate measurements of 150 s were repeated in 18 randomly selected asthmatics from our cohort after 6 months. Results SampEn was significantly increased compared to controls (Abstract P122 Table 1) and correlated significantly with exacerbation frequency from the previous 12 months (Asthma Baseline- p=0.007, rs=0.3; Post-Bronchodilator- p=0.009, rs=0.3). Fractal scaling was also found to be present in airway resistance in severe asthma (α 1 =0.94 (0.03)) and showed an inverse relationship with SampEn (p=0.0352, r=−0.4). Increased SampEn was associated with worse ACQ scores (p=0.027, rs=0.3) and lower AQLQ scores (p=0.023, rs=−0.2). SampEn measurements were repeatable (an Intra-class correlation of 0.74) in the triplicate series. In keeping with other studies, airway resistance was significantly increased in severe asthma. Conclusions SampEn a measure of complexity is (1) increased in severe asthma (2) a repeatable measure (3) associated with a lower quality of life and exacerbation frequency. The ability of this technique to monitor asthma stability and to predict future exacerbations by stochastic modelling needs to be explored.