Viscosity and Non‐Newtonian Features of Thickened Fluids Used for Dysphagia Therapy

Abstract

Thickening agents based primarily on granulated maize starch are widely used in the care of patients with swallowing difficulties, increasing viscosity of consumed fluids. This slows bolus flow during swallowing, allowing airway protection to be more properly engaged. Thickened fluids have been shown to exhibit time-varying behavior and are non-Newtonian, complicating assessment of fluid thickness, potentially compromising efficacy of therapy. This work aimed to quantify the flow properties of fluids produced with commercial thickeners at shear rates representative of slow tipping in a beaker to fast swallowing. Results were presented as indices calculated using a power-law model representing apparent viscosity (consistency index) and non-Newtonian nature of flow (flow behavior index). Immediately following mixing, 3 fluid thicknesses showed distinct consistency indices and decreasing flow behavior index with increasing thickener concentration. An increase in consistency index over 30 min was observed, but only for samples that were repeatedly sheared during acquisition. Three-hour measurements showed changes in consistency index across fluids with the largest being a 25% rise from initial value. This may have implications for efficacy of treatment, as fluids are not always consumed immediately upon mixing. Flow behavior indices were comparable across thickeners exhibiting similar rises over time. The indices were a more complete method of quantifying flow properties compared with single viscosity measurements, allowing an increased depth of analysis. The non-Newtonian nature of fluids perhaps renders them particularly suitable for use as dysphagia therapies, and such analysis may allow the possibility of altering these properties to optimize therapeutic efficacy to be explored. Practical Application: Effective treatment of swallowing disorders relies upon the appropriate choice and subsequent reproduction of drinks thickened to one of a number of predetermined levels. Currently there are no agreed methods of measuring the thickness of these drinks in use and the specifications are subjective, relying on descriptions such as "syrup" thick. This research aims to further understanding of the flow properties of thickened drinks and bring a quantified measure of thickness closer to being a practical reality.

The Effect of Saliva on the Viscosity of Thickened Drinks

“…However, this reduction in viscosity was not readily detected in the subjective assessment of the fluid [23]. This may be attributable to the non-Newtonian flow behaviour of thickened drinks [24] whereby the apparent viscosity at low flow rates (such as tipping in a cup) is different to the viscosity at higher flow rates (we tested at 40 s -1 , which is thought to be representative of the higher flow rates experienced during a swallow [24]). We hypothesise that amylase acted to break down the mechanical structure of the maize starch component of the mixture, resulting in water thickened by the gum components alone.…”

“…This continual thickening has been documented as an inherent property of thickeners [16], and we hypothesise that the thickening was exaggerated slightly as a result of the repeated stirring [24].…”

Commercial thickeners used by patients with dysphagia: Rheological and structural behaviour in different food matrices

“…Modified maize starch (pre-gelatinised) and natural gums (such as xanthan gum or galactomannans) are the polysaccharides normally used (Cichero, 2013;O'Leary, Hanson, & Smith, 2010;Sopade, Liang, Halley, Cichero, & Ward, 2007). Physical treatment of starch makes it able to form a paste even in cold liquids, so it swells and behaves as an instant thickener (National Dysphagia Diet Task Force, 2002).…”

A review of gastrointestinal physiology and the mechanisms underlying the health benefits of dietary fiber: Matching an effective fiber with specific patient needs

“…Highly viscous soluble fiber can be used to thicken and form a cohesive gel-matrix in a concentration-dependent manner when hydrated, increasing bolus viscosity and cohesion and facilitating passage of an intact bolus. This reduces the risk of aspiration [4,5] .…”