Rheological analysis of sputum from patients with chronic bronchial diseases

Abstract

Bronchial diseases are characterised by the weak efficiency of mucus transport through the lower airways, leading in some cases to the muco-obstruction of bronchi. It has been hypothesised that this loss of clearance results from alterations in the mucus rheology, which are reflected in sputum samples collected from patients, making sputum rheology a possible biophysical marker of these diseases and their evolution. However, previous rheological studies have focused on quasi-static viscoelastic (linear storage and loss moduli) properties only, which are not representative of the mucus mobilisation within the respiratory tract. In this paper, we extend this approach further, by analysing both quasi-static and some dynamic (flow point) properties, to assess their usability and relative performance in characterising several chronic bronchial diseases (asthma, chronic obstructive pulmonary disease, and cystic fibrosis) and distinguishing them from healthy subjects. We demonstrate that pathologies influence substantially the linear and flow properties. Linear moduli are weakly condition-specific and even though the corresponding ranges overlap, distinct levels can be identified. This directly relates to the specific mucus structure in each case. In contrast, the flow point is found to strongly increase in muco-obstructive diseases, which may reflect the complete failure of mucociliary clearance causing episodic obstructions. These results suggest that the analysis of quasi-static and dynamic regimes in sputum rheology is in fact useful as these regimes provide complementary markers of chronic bronchial diseases.

Figure 1

Sputum rheological response to strain. Evolution of the storage ($G^{\prime },▴$) and loss ($G^{\prime \prime },▾$) moduli, and the damping ratio ($tan\delta =G^{\prime }/G^{\prime \prime },\bullet$) with the applied strain, obtained during a strain sweep in oscillatory mode (frequency 0.6 Hz). Open symbols: values obtained in frequency sweeps at 5% and 10% strain, respectively.

Figure 2

Temporal stability of sputum rheology. Evolution of the linear storage modulus, $G_{5\%}^{\prime }$, and critical stress, ${\sigma }_c$, between consecutive replicates and between consecutive visits, for individual COPD (top) and CF patients (bottom). Each connected group of symbols corresponds to a single sputum sample; symbols are associated to individual patients. The horizontal bar in each replicate is the geometric mean. The variation factors are the ratios of replicates $n+1$ and n taken individually, and of replicated values in each visit (bar: mean variation factor). The shaded area delimits variations (increase or reduction) by a factor smaller than two.

Figure 3

Effect of induction on sputum rheology. Left: Evolution of the linear storage modulus, $G_{5\%}^{\prime }$, damping ratio, $tan{\delta }_{5\%}$, and critical stress, ${\sigma }_c$, before ($t=0$, spontaneous expectoration) and after nebulisation of 4.5% HSS, for individual COPD patients. Right: Same rheological parameters, comparing spontaneous expectoration to 4.5% HSS nebulisation (10 min) and to 2.5 mL rhDNase in CF patients. Each connected group of symbols corresponds to consecutive sputum samples collected from the same individual patient; each symbol is a mean over consecutive replicates when available. Horizontal bars: geometric ($G^{\prime },{\sigma }_c$) or arithmetic ($tan\delta$) mean over the corresponding populations. *$p<0.05$; **$p<0.01$.

Figure 4

Rheological levels in several bronchial conditions. Top: probability plots of the values of $G_{5\%}^{\prime }$, $tan{\delta }_{5\%}$ and ${\sigma }_c$ measured from subjects with the four conditions (healthy, $\bullet$; asthma, $▴$; COPD, $⧫$; CF, $▾$). Each symbol corresponds to the per-patient average value after HSS nebulisation; solid line is a fit with Eq. (1). Bottom: expected values and standard deviations ($X_0\pm \mathrm{\Sigma }$) of these rheological parameters, compared by condition, as a function of the corresponding relative FEV${}_1$ (mean ± standard deviation). Filled and open symbols depict HSS-induced and spontaneous sputa, respectively. **$p<0.01$; ***$p<0.001$.