Our study demonstrated that neck extension during advancing a tracheal tube from the nasal cavity into the oropharyngeal space could assist in smooth passage of the tube. For successful nasotracheal intubation, some previous studies have focused on tube impingement and solutions [12-15]. However, these previous studies mentioned impingement at the hypopharyngeal and laryngeal space but not at the naso/oro-pharyngeal space in fiberoptic intubation. For example, the tube can be impinged at the arytenoid cartilage, vocal cord, epiglottis, or esophageal inlet in fiberoptic nasotracheal intubation, which can be solved by counter-clockwise tube rotation after withdrawal the tube 2-3 cm . Also, if the block occurs due to small-sized nostril before advancing the tube, clinicians can easily realize and solve the problem with changing the nostril side or tube size. Even though there are not these two situations, clinicians commonly encounter resistance in the process of advancing the tube, when it reaches at the posterior wall of the nasopharynx . Our study presented the impingement of the tube in the naso/oro-pharyngeal space and the solution for this issue. Our results showed that the straight distance from the midpoint of the nares to the posterior wall of nasopharynx was very similar to the inserted tube length when the tube was blocked during advancement.
We hypothesized that the angle between the posterior wall of the nasopharynx and nasal floor was about 90 degrees. Although we could not find out any reference about the angle, the mean of the angle and SD was 100.3 ± 7.8 degree when we measured the angle in a sagittal view of preoperative computed tomography images of 39 subjects among all participants in our study. Wrinkles in the posterior nasopharyngeal wall might be a possible cause of blockage because the wall is covered by lymphoid tissue that often undergoes hypertrophy (adenoid) during the transition period to puberty . There are some folds such as salpingopharyngeal fold, salpingopalatine fold, or torus tubarius .
For blockage in tube passing from the nasal cavity to the oropharynx, clinicians usually try re-advancing 2-3 times, which can increase the possibility of nasal bleeding. In extreme cases, the tube might perforate the posterior wall [18-22]. Therefore, some experienced clinicians gently rotate the shaft of the inserted tube in the nasal cavity or extend the patient’s neck while advancing the tube like as our study protocol.
We supposed that neck extension could lead to the traction of naso/oro-pharyngeal soft tissue, as shown in Figure 1A and 1B. That is, we hypothesized that the soft tissue could be tightened from Figure 1B to Figure 1A, which could make the angle between the posterior wall of the nasopharynx and nasal floor more obtuse than neutral position (about 100 degrees in 39 subjects of our study). Finally, this extension can force the tube tip to slide more smoothly across the surface of the posterior wall of the nasopharynx toward the oral cavity. Additionally, we hypothesized that these series of processes helped to spread the wrinkles of the posterior pharyngeal wall, which can lead to smooth passage of the tube.
However, these hypotheses were not verified in our study. Nevertheless, we identified that the neck extension could increase the angle between the nasal floor and the posterior wall of the pharynx when we observe the cervical spine lateral view with the patient’s neck flexed/neutral/extended in 3 of our study subjects. The angle changed 103.2-107.8-116.8, 84.6-92.4-102.4, and 89.6-93.8-99.3 degrees respectively in them. Also, we identified soft tissue such as folds of the posterior nasopharyngeal wall of some patients widened and slightly straightening by neck extension when we observed the posterior wall of the nasopharynx with otolaryngologists using a rigid endoscope in clinical situation of endoscopic sinus surgeries.
Neck extension can lead to the alignment of the three axes, including the oral axis, pharyngeal axis, and tracheal axis . Alignment provides physicians the best view of the glottic opening with a laryngoscope for tracheal intubation. Therefore, neck extension is a very familiar maneuver for clinicians in tracheal intubation. Moreover, this maneuver is very easy to perform and is acceptable for most patients except for those with cervical spine injury . Therefore, this maneuver can easily reduce the spent on nasotracheal intubation and improve patient safety.
We evaluated the tube passing pathway to verify the results of our previous study . Unfortunately, our previous study had a small sample size by mistake. Therefore, it had lower power than originally planned. In the present study, we initially inserted the tracheal tube via a nostril with a nasal tip lifted. As a result, 72.3% of tubes passed the lower pathway in the nasal cavity. These results were similar to our previous data (78%) in the nasal tip lifting group . Therefore, the results of our present study supported the results of our previous study.
Tube passage from the nasal cavity to the oropharynx was successfully achieved within the first two attempts in the majority of our study subjects. However, tube passage was attempted four times in 4 patients and five times in 1 patient in our study regardless of the group. We thought that the advancement of the inserted tube should be tried 2-3 times to minimize mucosal injury. According to Lim et al., the Levin tube is useful for guiding a tracheal tube for nasotracheal intubation . Therefore, use of it should be considered after 2-3 times failed tube advancements.
In our study, the time was about 10 seconds on average for passing the tube into the oropharynx in the neck extension group. Considering a simple process of tube passing into the oropharynx from initiation of tube insertion via a nostril, 10 seconds may be considered a rather long time. To prevent any injury during nasotracheal intubation, clinicians usually perform thermosoftening and local vasoconstriction like as our practice in the study. Also, gentle advancement of the tube through the nasal pathway and naso/oro-pharyngeal junctional space must be important to minimize injury. However, clinicians tend to be tempted to apply a little more force for the advancement of it into the oral cavity during nasotracheal intubation. Additional force may be effective to shorten the required time for tube passing. However, that can cause mucosal injury in some cases. We focused the minimal mucosal injury and emphasized using minimal force to advance the tube in this trial. Therefore, we needed 10 seconds for tube passing in the extension group. Although the total of 16 patients experienced nasal bleeding, the severity of it was ‘blood-tinged’ for all of them in our study. They did not need any specific treatment for nasal bleeding. Also, gentle force could affect the success rate of tube passing in the first two attempts. If we had used additional force in tube passing, the success rate would have been higher than our results in both groups.
Our study had many limitations. First, our study had a small sample size. During the literature search, we found no previous studies focusing on our hypothesis. Therefore, we initially designed the present study as a pilot study because we could not calculate adequate sample size. However, when we calculated the sample size based on the hypothesized results of this trial, the required sample size was 64 (32 for each group) with 80% power at the 0.05 significance level. We assumed that the success rate for smooth tube passage into the oropharynx in the first two attempts would be increased by 30% with neck extension compared with 60% with the neutral neck position based on our data (60.6%). Therefore, according to our sample size calculation, we decided that the trial should be terminated without increasing the sample size. Second, our study did not overcome the influence of confounding covariates from personal difference in terms of anatomy. If we performed this randomized controlled study with larger sample size or planned a randomized crossover design study for our interest, we could have minimized the confounding effect. However, we did this study with small sample size, and we could not perform a crossover design study due to ethical reason. If we designed this study with a crossover manner, we had to retry to pass the tube with an alternative method (neck extension or neutral) for a patient after pulling back the tube even though the tube passed successfully into the oropharynx with the first maneuver. Third, we could not thoroughly blind the study protocol to intubation performers because it was difficult to blind anesthesiologists for our study design, including tracheal intubation. Although the outcomes of this study such as success rate were objective variables, there still might be bias from that. However, we believed that intubation performers tried to do their best to pass the tube smoothly from the nasal cavity to oropharynx in all cases of the two groups. Finally, we could not found the exact reason why the neck extension could be helpful for smooth advancing the tube from the nasal cavity to oropharynx. We just conjectured the angle could become slightly widen by neck extension from cervical spine radiologic series of only three subjects. And, we just observed soft tissue of the posterior nasopharyngeal wall became widen and slightly straightening by neck extension in some patients. Therefore, further study should be necessary to investigate our hypotheses.