The Impact of Bolus Rheology on Physiological Swallowing Parameters Derived by Pharyngeal High-Resolution Manometry Impedance

Background: The shear rheology of ingested fluids influences their pharyngo-esophageal transit during deglutition. Thus, swallowed fluids elicit differing physiological responses due to their shear-thinning profile. 

Methods: Two hydrocolloid fluids, xanthan gum (XG) and sodium carboxymethylcellulose gum (CMC), were compared in 10 healthy adults (mean age 39 years). Manometry swallowing assessments were performed using an 8-French catheter. Swallows were analyzed using the Swallow Gateway web application (www.swallowgateway.com). Grouped data were analyzed by a mixed statistical model. The coefficient of determination (r²) assessed the relationship between measures and bolus viscosity (SI units, mPa.s) at shear rates of 1—1000 s⁻¹. 

Key Results: Rheology confirmed that the thickened fluids had similar viscosities at 50 s⁻¹ shear rate (XG IDDSI Level-1, 2, and 3 respectively, 74.3, 161.2, and 399.6 mPa.s vs. CMC Level-1, 2, and 3 respectively 78.0, 176.5, and 429.2 mPa.s). However, at 300 s⁻¹ shear, CMC-thickened fluids exhibited approximately double the viscosity (XG Level-1, 2, and 3 respectively 19.5, 34.4, and 84.8 mPa.s vs. CMC Level-1, 2, and 3 respectively, 41.3, 80.8, and 160.2 mPa.s). In vivo swallows of CMC, when compared to XG, showed evidence of greater flow resistance, such as increased intrabolus pressure (p < 0.01) and UES Integrated Relaxation Pressure (UESIRP, p < 0.01) and shorter UES Relaxation Time (p < 0.05) and Bolus Presence Time (p < 0.001). The apparent fluid viscosity (mPa.s) correlated most significantly with increasing UESIRP (r² 0.69 at 50 s⁻¹ and r² 0.97 at 300 s⁻¹, p < 0.05). 

Conclusion: Fluids with divergent shear viscosities demonstrated differences in pharyngeal function. These physiological responses were linked to the shear viscosity and not the IDDSI level.