Sputum elasticity: a frog in the throat?

The change in the consistency of mucus that is produced during chest diseases has often been considered to contribute to difficulty in its removal. This has stimulated interest in the flow properties (properly referred to as rheological properties) of sputum in different diseases. To this end many studies have been carried out using sputum from patients with a variety of conditioins, although in most cases workers have limited themselves to the use of techniques and instruments that are more suitable for the evaluation of simple fluids. These techniques have included the use of standard instruments , such as capillary and efflux viscometers, and also empirical techniques, such as measurement of pourability index (Keal, 1970). The information provided by such methods is limited because sputum is not a simple (viscous) fluid, such as water or glycerin. It is a complex material exhibiting viscoelastic behaviour (Davis and Dippy, 1969; Sturgess et al, 1971; Litt et al, 1977), which is a combination of liquid and solid behaviour. The typical rotational viscometer, where the experimental fluid is sheared in the gap between one stationary and one rotating surface of either two concentric cylinders or a cone and plate, provides an assessment of the liquid-like component but is incapable of evaluating the solid-like component. This explains some of the experimental difficulties and anomalous results that have been reported. A good example is the phenomenon of "rolling-up" of the sample in the gap between the measuring surfaces, which can eventually result in the expulsion of the experimental sample from the gap! This produces results that are very difficult to interpret (Tekeres and Geddes, 1971). Consequently the use of such techniques can be very misleading and therefore should be used with great caution for the evaluation of the progress of a chest condition or the in-vitro action of drugs. Materials that are viscoelastic can be evaluated properly by using techniques that impose a very small displacement on the test sample. Suitable methods include the creep test, where the sample is subjected to a small force (stress) that results in an infinitessimal displacement, and the dynamic oscillatory test in which the sample is exposed to a stress that is varied sinusoidally and is transmitted by the sample to a suitable recording device (Davis, 1973). In both methods the material is examined in a non-destructive manner. By "stretching" the material and then allowing it to recover one can obtain …