Sputum rheology in stable and exacerbated chronic obstructive pulmonary disease

<b>Background:</b> - Sputum is a mucus solution containing biological material, which is increased in volume and purulence during an acute exacerbation of COPD (AECOPD). Rheology deals with the deformation and flow of matter. We assessed sputum rheology during stable state and AECOPD. <b>Methods:</b> -&nbsp;Spontaneous sputa were collected from COPD patients during stable state, at AECOPD and 21 days after an exacerbation (AECOPD-fu). Dynamic rheology i.e. the elastic modulus G’, viscous modulus G’’, vector sum of viscosity and elasticity G*, the damping ration tanδ, yield strain γc and yield stress τc, were analyzed using a cone-and-plate rheometer (Rheomuco, Rheonova France). <b>Results:</b> -&nbsp;A total of 106 patients with 156 stable, 48 AECOPD and 30 AECOPD-fu visits were included. The mean age at baseline was 68±0.8 years and 70% were males. There was no significant difference in G’ (p=0.368), G’’ (p=0.305), G* (p=0.710), tanδ (p=0.117) or τc (p=0.877) but there was a significant difference in γc (p=0.022) between stable state (median=28.0), AECOPD (median=26.6) and AECOPD-fu (median=30.6). During stable state there was a negative association between pre-bronchodilator FEV1 (L) and FEV1 %predicted, BMI and G’ and τc. FVC and 6MWT were not associated with any of the rheological parameters. Sputum analysed with conventional microbiological methods were positive for bacteria in 66/161 (41%) cases. There was a difference in yc between positive and negative cases during AECOPD (p=0.006). <b>Conclusions:</b> -&nbsp;The non-linear property yield strain γc decreased from stable state to AECOPD and increased during AECOPD-fu. There is a negative association between sputum rheology and FEV1 and BMI during stable COPD but not during AECOPD.