Computer simulation of the pharyngeal bolus transport of Newtonian and non-Newtonian fluids

A computational fluid dynamics (CFD) programme was used to study dysphagia, a swallowing disorder, and demonstrated that the rheological properties of a liquid affect the pharyngeal transport of a food bolus. A fully coupled, Newton-Raphson solution algorithm was used in conjunction with the Backward-Euler scheme. Concomitant axial and radial movement of the fluid bolus was assumed, and the force exerted by the base of the tongue was assumed to be linear. Boundary conditions were based on data published in the clinical literature. The properties of three fluid types were modelled: water (p = 1000kg m^-3,y = 0.001Pa s), 250% w/v barium sulfate mixture (p = 1800kg m^-3 y = 0.150 Pa s), and starch-thickened beverage (power law parameters K = 2.0 Pa sⁿ, n = 0.7). Results show that when the base of the tongue pushes against the throat with the same amount of force, water is transported through the pharynx at a much higher flow rate than the barium sulphate mixture, causing parts of the water bolus to flow backwards. A typical starch-thickened beverage, which is a shear-thinning non-Newtonian fluid, undergoes much lower flow rates. Furthermore, under the same conditions, a smaller volume of the non-Newtonian bolus (2 mL compared to 20 mL of the Newtonian fluids) is passed through by the end of the swallow. Values for the time to swallow a critical bolus volume, tev, show that non-Newtonian fluids increase swallowing time more effectively than Newtonian fluids and are thus safer to swallow for patients with dysphagia. These findings suggest that non-Newtonian foods may either slow down the swallowing process or trigger the subject to swallow a smaller amount, allowing the neuromuscular system more time to shut off air passages and reduce the risk of aspiration. Based on this simple CFD modellin of the swallowing process, the effects of fluid properties on bolus transit can be predicted.