Abstract
Introduction:
Sputum has the capacity to stretch into long thin thread-like shapes, from which the measurement of its core-central diameter can in turn provide its dynamic extensional properties.Viscosity of sputum from patients with Chronic Obstructive Pulmonary Disease (COPD) changes when they have exacerbations.
Measuring viscoelastic properties of sputum may lead to biomarkers for type of infection (e.g. viral versus bacterial) or response to treatment. We wanted to see if it is feasible to objectively measure viscoelastic properties of sputum using rheological techniques.
Methods:
Spontaneous sputum was collected from 18 patients during an exacerbation of known COPD, then analysed using an AR-2000G2 rheometer (TA Instruments, UK) . Oscillatory rheology was performed between 0.01Hz and 100Hz with a strain of 2% which ensured that the sample was within the linear viscoelastic region.
Results: As a whole, the elastic modulus was greater than the viscous modulus at all frequencies which highlights that sputum has a predominantly viscoelastic solid-like behaviour. The magnitude of G', G'' (a measure of the energy stored in a material in which a deformation / shear has been imposed) varied between samples but the phase angles for all samples were in a relatively narrow range between 9 and 17 and were repeatable.
Conclusion: Rheology is a reliable method for quantifying viscoelastic properties of sputum from patients with COPD which have been successfully modelled with an approximate power law behaviour. Other studies are now comparing core rheological properties of sputum in the stable versus exacerbating state and in frequent exacerbators versus stable patients, with similar lung function.
- © 2014 ERS
