This study examined the effects of various types of thickening agents and the viscosities of thickened water samples on mucociliary transport function. The results revealed that the thickened water samples with high viscosity may take longer to be discharged via the mucociliary transport function. At the same time, there is no difference in the time of discharge between the thickened water samples with different types of thickening agents.

Aspirated objects are discharged by expectoration with coughing, which is the defensive reflex of the body, followed by ciliary transport in the trachea [14, 15, 22, 23]. However, aspirated water, which runs down the tracheal wall, is less susceptible to the wind pressure of coughing, and the mucociliary transport function is extensively involved in its discharge. It has been reported that differences in viscosity affect the ease of discharge of nasal mucus and sputum from the nasal cavity and trachea by ciliary transport [24]. Although ciliary transport may be altered by the thickening of water and the viscosity of thickened water, it has not been previously clarified. Therefore, this study examined the effects of thickening, the differences in types of thickening agents, and the viscosities of thickened water on the mucociliary transport function.

In this study, mucociliary transport function was measured using the saccharine dye test. The methods of measuring the mucociliary transport function include the observation of ciliary transport with a γ-camera using 99m technetium (99m Tc) as a marker [25]. Although this method is advantageous in that it can visually confirm the discharge by mucociliary transport, it exposes subjects to radioactive substances, and the facilities that have the necessary equipment for the measurement are limited. On the other hand, the saccharine dye test used in the present study is a screening test for the mucociliary transport function of the nasal cavity that has the characteristics of low harmfulness and high reproducibility [20]. Given the reported correlation between mucociliary transport functions in the nasal cavity and the trachea [26], this study presumed that a longer saccharin time in the nasal cavity indicates a prolonged discharge time for aspirated water by the mucociliary transport function in the trachea.

It has been previously reported that xanthan-gum-based thickening agents prevent aspiration more effectively than starch-based thickening agents [27, 28]. This may be because xanthan-gum-based thickening agents have less pharyngeal residue, resulting in the prevention of aspiration after swallowing. However, their effect on the ease of discharging aspirated foreign objects has not been elucidated, and no studies have discussed the selection of the types of thickening agents from this point of view. The results of the present study showed no significant difference in saccharin time between the two types of thickening agents. This indicates that the difference in raw materials of thickening agents does not affect the ease of discharging aspirated foreign objects by the mucociliary transport function, suggesting that xanthan-gum-based thickening agents are more advantageous in preventing aspiration pneumonia than starch-based ones.

The results of this study revealed that the ciliary transport of thickened water samples requires a longer time than that of water without a thickening agent and that a longer time is required for the ciliary transport of thickened water with higher viscosity. These results indicate that the ease of discharging aspirated foreign objects involves two factors: objects or fluids being transported and ciliary movement.

First, the shear stress of thickened water increases with increasing viscosity. Due to the increased shear stress, it requires a longer time for the thickened water itself to flow [29]. This study also suggested that the flow of thickened water is altered because of the thickening agent and the increased viscosity, requiring a longer time for ciliary transport.

Next, the ciliated epithelium of the trachea and nasal cavity has a periciliary layer between the cilia, as well as a mucous layer above it, and the two layers maintain proper hydration of the surface by supplying and accepting water [16, 30]. An increase in the concentration of the sol layer leads to the generation of partial osmotic pressure and changes the osmotic pressure of the periciliary layer. This is known to disrupt proper hydration, reducing the mucociliary transport function and prolonging the time required for removing mucus [31]. The thickened water used in the present study has a higher osmotic pressure than fresh water because of its macromolecular solvent. Therefore, an increase in the concentration of the sol layer due to the thickened water might have altered the osmotic pressure and lowered the mucociliary transport function, affecting the time required for ciliary transport.

Robbins et al. reported a higher incidence of pneumonia in patients using thickened water with high viscosity. This study contributes to clarifying the cause behind their observation. Moreover, this study found that there is a paradox in which while the use of thickened water aims to prevent aspiration [7], its discharge may be difficult when it is aspirated.

It has been reported in recent years that taking drugs with thickened water may adversely affect their absorption [32, 33]. Based on this and the results of the present study, it may be necessary to review the advantages and disadvantages of the use of thickening agents.

The saccharine dye test used in this study is a screening test for the mucociliary transport function of the nasal cavity, and it does not directly observe the ciliary transport in the trachea. Therefore, the lack of direct observation of the trachea is considered a significant limitation in the examination of the discharge from the trachea. However, a correlation between mucociliary transport functions in the nasal cavity and trachea has been established [26, 34]. In conditions like Kartagener syndrome and primary ciliary dyskinesia, characterized by ciliary transport dysfunction, clinical evidence shows simultaneous reduction or promotion of mucociliary transport function in both the nasal cavity and the trachea [17, 18]. Therefore, measuring saccharin time in the nasal cavity seems to be able to reflect the effect of thickening agents on the mucociliary transport function in the trachea.

Thickened water with higher viscosity may be more challenging to be discharged by the mucociliary transport function.

The use of thickening agents to prevent aspiration and the difficulty of discharging aspirated thickened water must be weighed against one another in patients with a high risk of aspiration and weak coughing reflex or impaired airway clearance.

While the use of thickened water aims to prevent aspiration, its discharge may be difficult when it is aspirated.