Assessment of human swallowing using in-vivo and in-vitro approaches
Texture modification is the most vital strategy to manage dysphagia: a condition in which food and beverages are difficult to swallow. Typically thickeners in the form of starch or gums are added to the food to increase the viscosity and thereby reduce the speed of bolus flow. This allows more time for the slowly responding physiology of the patients to close the entrance to the airways and allow for safe passage through the swallowing tract. The use of thickeners possesses certain limitations, e.g. that a more viscous bolus leaves more residues. There have also been suggestions on modified thickening strategies such as utilizing extensional properties of the thickened solutions.
The contribution of extensional properties of the bolus were assessed by making model Boger fluids (elastic fluid of fixed shear viscosity) and compare swallowing flow to its Newtonian counterpart (fixed shear viscosity, no elasticity), as well as to a shear thinning fluid with even higher elasticity. These fluids were evaluated in sensory and clinical studies to see if bolus elasticity plays a part in safe swallowing. A clinical study showed evidence of safer swallowing of the elastic fluids.
The hypothesis of fluid elasticity contributing to safe swallowing was further tested in an in-vitro swallowing model since clinical trials are difficult to perform. As a first step all the thickeners used in clinical studies were characterized using traditional rotational rheometry and comparing it to the more suitable Pulsed Ultrasound Velocimetry, Pressure-Drop method (PUV+PD). PUV+PD was used since the instrument geometry and measurement speed better mimics the geometry and transient nature of the swallowing process. The two methods provided similar flow curves and yield stress values indicating that PUV+PD is applicable to study the transient swallowing process in our in-vitro model, “The Gothenburg Throat”. The model fluids were analyzed together with commercial thickeners and it was noticed that gum-based thickeners are more elastic than the starch based thickeners, and have moderate yield stresses. Model fluids with non-food grade polyacrylamide were also developed as a reference to achieve much higher elasticity than the xanthan gum based ones. The results from the two studies provide the necessary base for further analysis of model fluids, commercial thickeners and fluid foods in our in-vitro swallowing model in the continuation of the project.