The mean depths at which the palate and tongue could be cleaned without nausea were 6.75 ± 1.07 cm and 6.92 ± 1.11 cm, respectively, and the two were positively correlated (t = 0.730, P < 0.001), suggesting that, in clinical practice, the same cleaning depth can be used for both the tongue and palate of an individual patient. Further univariate analyses showed that sex was the main factor related to the tolerable depth of oral cleaning, with males able to tolerate deeper oral care, which may be attributable to innate differences in structural depths between the sexes [17, 18]. Similarly, Mimgu Park et al. reported that males had a longer depth of the gag reflex, which is consistent with our results ; the reason may be the longer maxillary arch size in males . Therefore, men can tolerate deeper oral care. Hence, our study recommends that when performing oral care for adult males, the cleaning depth can be suitably increased.
Height was another factor that we expected to influence the tolerable cleaning depth; however, we did not detect any significant differences in the tolerable cleaning depths for either palate or tongue between height categories, and this factor failed to enter the regression equation, suggesting that the difference in cleaning depth between individuals of different heights was less than expected. Therefore, based on our study, we do not recommend that cleaning depth be changed according to patient height.
As shown in Tables 2 and 3, the association between BMI and cleaning depth was higher than expected, with BMI being a factor that significantly influenced tolerable cleaning depth. There is evidence that obese individuals have a greater risk of regurgitation and pulmonary aspiration than underweight patients . Hence, based on our results, we recommend that the depth of cleaning of the tongue and palate should be increased for overweight/obese patients.
To reduce measurement errors, we tested tolerable cleaning depths for the tongue and palate three times. Unexpectedly, we found that tolerable cleaning depths for the tongue and palate gradually and significantly increased with the order of measurement (first to third), which could be related to increased tolerance of gagging reflexes in response to multiple stimuli. This finding suggests that multiple stimulation training could be used to increase tolerance depth in the clinic, particularly for patients with an overactive gag reflex. In addition, previous studies have proposed several useful methods for overcoming gag reflexes, including earplugs , relaxation, and distraction . Moreover, some scholars have suggested that the ideal instruments for measuring the gag reflex should include the use of different materials and be applied with variable intensities, durations, and positions of stimuli . One study used a standard disposable saliva ejector, with a stopper of heavy body addition silicone impression putty, as a device to measure the gag reflex depth of their participants . Considering that the most common type of oral care equipment used in China is forceps with cotton balls , we used them for the measurement of cleaning depth in this investigation.
Our study compared the extent of nausea caused by coronal and sagittal cleaning (Fig. 1). The results show that the extent of nausea caused by sagittal cleaning was significantly higher than that caused by coronal cleaning, which is consistent with our clinical experience. This may be because, for coronal cleaning, the deep oropharynx is only accessed once, where sagittal cleaning requires repeated insertions into the deep oropharynx, and it is possible that depth is not adequately controlled. However, these differences were only significant among female participants and those with normal BMI, likely due to the sex ratio and BMI range of our participants; the proportions of female participants and those with normal BMI were 85% and 77%, respectively. Overall, based on our study, we recommend that the tongue and palate should be cleaned coronally.
When asked about types of discomfort other than nausea, subjects mentioned itching 134 times, with one research subject saying, "The wool of the cotton ball passed through my oral mucosa, and it is truly itchy!" Another research subject said, "The cotton wool on the cotton ball hangs in the mouth, leaving so much fibre in my mouth." These results demonstrate that cotton balls were not as comfortable as expected; thus, new materials and tools should be used to replace this approach. Gauze pads are widely used in Israel  and have been proven to help nurses implement more effective and gentle oral care . In addition, a foam swab specifically designed for cleaning the tongue and palate has been reported in America; however, its cleaning effects have yet to be verified .
The second most commonly reported discomfort, following itching, was saltiness, which was mentioned 41 times. This indicates that the salinity of the 0.9% NaCl solution exceeds that of people’s daily diet and causes discomfort. Physiological saline has been recommended in textbooks for many years as a common oral care solution and is believed to contribute to oral cleansing and sterilization; however, there is scarce evidence to support its efficacy. With regard to safety, saline has no negative effects on patient oral mucosa ; therefore, the use of saline for oral care warrants further exploration.
In addition, a few participants mentioned coldness as a discomfort. This suggests that oral care at room temperature can be tolerated by most people; however, there are also some subjects sensitive to temperature. Some researchers in China have tried heated oral care solutions to improve comfort for patients during oral care, and they suggested that a specific temperature range could be selected by the patient according to their daily habits .
The strengths of this study include exploring the depth and direction of oral cleaning, which has not been researched before, providing valuable information for nurses to provide oral care using more scientific cleaning depth and direction.
Participants recruited in this study were healthy, and their average age was only 19.63 years. Due to the crucial roles of age and health in the gag reflex, the results of this study may not be representative of the overall situation for clinical patients. Further study is required to confirm these results in the clinic.