Tracheal intubation with TVT in CL grade IIb and III limited the failure rate to 5.5%. Even though we included novice residents, their failure rate reached only 6.9%. According to the manikin study , when they applied cervical collars to the manikins, all of their CL grades were II and III, and the intubation success rate reached 100% with TVT compared to 84% with direct laryngoscopy. These results imply that endotracheal intubation with TVT under assist of direct laryngoscopy provides easier intubation conditions and a higher success rate. In fact, we preformed “J” shaped TVT with a stylet, rather than a traditional hockey stick shape (Fig. 2). Endotracheal tube insertion over a slightly rounded angle is easy compared to an acutely angled “hockey stick” shape. Furthermore, a slightly upward movement of the tube tip while inserting allows one to make precise movements toward the vocal cords.
The median number of acceptable performances was 36 and ranged from 26 to 48. In consideration of the experiences of novice persons, not many intubation experiences were required to reach the smaller failure rate of 10% compared to other literatures[18, 19]. It was not a similar situation with this trial, only 60% of novice trainees were fulfilled the 20% acceptable failure rate within the mean 45 attempts . In other systematic review , to reach at least 90% success rate within 2 attempts, more than 50 cases of intubation experiences were required with a direct laryngoscope. Considering we simulated moderate grade of CL classification, our results indicate that TVT could provide another excellent intubating method when anesthesiologists encounter an unexpectedly difficult intubation situation. With a rigid video stylet, successful intubation rate at first attempt was reported as 73.3% in patients with Mallampati grade I and II . A semi-rigid video stylet, the Clarus video system is another type of video stylet which allows a custom angle to be made by the user. The overall successful intubation rate using this device was reported to be 95.7% in patients with Mallampati grade I to III . In our study including only CL grade IIb and III, we achieved an overall successful intubation rate of 94.5% and reached nearly 93% even in novice trainees. These promising results could be evidence of an alternative method for overcoming difficult intubation situations.
Most experiments using TVT in cases of difficult intubation were performed with manikins [5, 11, 17]. Manikin studies in airway management have been advocated due to lack of real-life settings . A clinical trial was mandatory to evaluate the efficacy of tracheal intubation with TVT. Difficult intubation is a situation which is closely related to airway anatomy. Using a classic laryngoscope, the visual field of the naked eye is significantly affected by structures from the teeth to the glottic area. These include the large and thick tongue, elongated floppy epiglottis, limited angle between oral, pharyngeal, and laryngeal axis, and so on. Because subtle variations in these structures could create different intubation conditions, preparation for difficult intubation should be personalized. In addition, most general anesthetics decrease upper airway tone, therefore the anticipated pathway for intubation should be mapped prior to the attempt. Most commercial video stylets are supplied with a fixed angle. A fixed angle with a rigid stylet could provide superior handling ability during tracheal intubation [23, 23]. Tracheal intubation with these is possible without a midline approach. On the other hand, the most flexible intubating stylet is an intubating bronchoscope. Because of its flexibility, the user can manipulate the bronchoscope advancement toward where one wants to. However, it is hard to find adequate anatomical landmarks because the bronchoscope cannot make its own way due to its flexibility, especially when the patient is fully relaxed. The airway of a relaxed patient is almost totally obstructed by the base of the tongue. The midline should be kept during bronchoscope advancement to easily find the glottic area . Keeping midline during bronchoscope insertion is a simple principle but not easy to achieve; bronchoscope-guided intubation requires delicate training programs and various assisting devices to establish acceptable performance even in standard intubation situations [26–28]. Predicting and making an adequate angle of the stylet almost depends on personal experience; it is not difficult to perform. It is required only expecting the pathway of an endotracheal tube according to the surface anatomy from teeth to thyroid cartilage (Fig. 3). During the study period, even a novice trainee could do this within a few attempts. Customizing the endotracheal angle is the most important feature of TVT. This may make handling during an intubation procedure easier compared to a flexible fiberoptic guided intubation. Glottic area approaching could also be enhanced by a customized stylet angle. Facilitation by making a customized stylet angle could also be possible in a semi-flexible video stylet.
Using a laryngoscope to make enough space and to identify the target glottis area enhanced TVT handling in anesthetized patients. It made it possible for us to find the pathway to the glottic area easily through relaxed structures. The only thing to do was inserting the tip of TVT under the epiglottis. By doing this, high CL grade with the naked eye became low CL grade in the view of the TVT (Fig. 6). Customized stylet shape and laryngoscope assist could maximize the likelihood of successful intubation with less effort and less seeking movement which is a cause of intubation-related trauma.
TVT was initially designed for tracheal surveillance during endobronchial blocker insertion. After its introduction, many articles presented its usefulness in difficult intubation situations [9, 12, 29, 30]. Its ability to manage difficult intubation was reported in unexpectedly difficult bag-mask ventilation , difficult intubation caused by tracheal injury , and high CL grade . Compared to other rigid or semi-rigid intubating video stylets and the flexible intubating bronchoscope, TVT provided superior aspects in several ways. Firstly, it can deliver high flow and a large volume of oxygen during the intubation procedure. During the intubation process, oxygenation could prolong the apnea time . Some intubating stylets are also designed with oxygen insufflation through the tube during intubation. However, TVT has another small route for lens clearing lumens; their ends are pointed toward the lens surface. Oxygen through these holes enables clear sight during procedure against secretions and fog. Secondly, tube passage over the glottic area is easier because of its bevel appearance. Usual single-lumen endotracheal tubes have a leftward bevel on their tips with right-sided Murphy’s eye. This design ensures both lungs are ventilated when the endotracheal tube tip has reached the carina. In contrast, the TVT has a downward bevel and a camera on its upside tip. The midline placed TVT camera could enable inserting most of the end of the tube between the vocal cords and simultaneously enable confirming adequate depth of the endotracheal tube . With the intubating video stylet or flexible intubating bronchoscope, railroading endotracheal tube into trachea usually follows confirming the scope is in the trachea. In this step, the left-sided bevel could get caught in the glottis because this is performed in a blind manner over the scopes as a guide. In addition to several other advantages, customizing its curvature to each patient is the supreme advantage. This diminishes the number of attempts, decreasing the chance of trauma around the airway which is another cause of difficult intubations. If the first attempt fails, one can modify the curvature to the most likely shape for that patient, then the possibility of success with the next attempt can be increased.
Most clinical trials about the feasibility of various intubation devices tested intubation time as their trial objective [6, 21, 31]. They tested and reported the feasibility of intubation devices with mean intubation time. Intubation time in most previous clinical trials is defined from the insertion of intubation devices to visual confirmation of endotracheal tube insertion through the vocal cords. A visual check by an experienced person is almost exact but sometimes provides the wrong information, especially for the novice anesthesiologist. In this trial, we regarded intubation to be successful only when 3 successive end-tidal CO2 concentrations over 30 mmHg were confirmed. Also, we included this testing period in the measurement of the intubation time, which is why mean intubation time in this trial reached around 25 seconds. However, the mean difference of intubation time between novice and expert trainees was only 2.8 seconds. This small difference means that tracheal intubation with TVT in CL grade IIb and III is easy to learn and overcomes difficult intubations.
Complications during the intubation process were not so different compared to other devices in the literature [7, 32]. The incidence of sore throat after tracheal intubation for general anesthesia is reported from 30–70% . The incidence of postoperative sore throat in this trial was numerically lower (8.6% for expert, 13.6% for novice trainees) than reported results. Because we used commercially available and FDA-approved stylets, and all participating anesthesiologists exercised not to do too many manipulations near the glottic area by individual stylet shaping before intubation, there were decreased factors of mucosal injury. However, sore throat developed nearly twice as much in the patients of novice trainees as in the patients of expert trainees. This means small injuries during intubation could be developed more frequently in novice trainees, although their mean intubation time is only different by 2 seconds. A sore throat could be caused by forced movement around the glottic area or vigorous endotracheal tube advancement which can cause tracheal injury. Cuff volume and pressure also contribute to sore throat development [33–35]. We guess that the cause of sore throat in this trial was inadvertent railroading of the endotracheal tube. Expert trainees could be more skilled in smooth advancement of the endotracheal tube into the trachea.
One of the limitations of this trial is the small number of trainees. If we tested more persons as trainees, it could decrease the inter-individual variance especially in the point of individual learning ability. To overcome these effects, every trainee did attempt enough cases and the results of each trainee fulfilled the normality test, which means that the data were neither skewed nor sparse. Second, Mallampati Grade III/IV and subsequent CL grade IIb/III with cervical in-line stabilization could not perfectly guarantee the difficult intubation situation. Real-difficult intubation situation could present more limited intraoral spaces to manipulated airway devices, could be harder to reach adequate tip position of a direct laryngoscope. Because of the ethical problem and rare incidence of these situations, a similar situation produced from simulation including cervical in-line stabilization and limited direct laryngoscope manipulation could enhance the personal technique of overcoming real difficult intubation situations. Third, we excluded patients with CL grade IV. Because all trainees were using TVT for the first time in high CL grade patients, we excluded CL grade IV patients to avoid accidental airway injury and “cannot intubation” status with unaccustomed devices. Fortunately, there was no patient presenting with CL grade IV during the trial.
In conclusion, tracheal intubation with TVT in CL grade IIb and III increases the chance to successfully and easily learn an alternative method to overcome unexpectedly difficult intubation situations. Tracheal intubation using TVT did not require many cases to reach an acceptable failure rate, even for novice anesthesiologists who uneventfully reached an acceptable level not very different from that of expert anesthesiologists.