Abstract
Objectives
We wished to determine whether dental injuries during intubation would occur more frequently when performed by inexperienced beginners. We measured the laryngoscopic force exerted on maxillary teeth of a modified manikin by experienced anesthesiologists and unexperienced medical students and estimated the injury risk.
Methods
Thirty-two anesthesiologists and 32 medical students participated in this study. Each testee performed tracheal intubation in two scenarios in a random order. In Scenario 1, the testee performed tracheal intubation for a manikin as a patient with normal dentition, in an emergency type situation. In Scenario 2, the testee performed tracheal intubation for a manikin as a patient with unstable dentition, in a routine anesthetic situation.
Results
The mean peak forces in Scenarios 1 and 2 were 6.1 and 1.1 N in the experienced testee group and 7.7 and 3.8 N in the unexperienced testee group, respectively (Scenario 2, p < 0.05). The unexperienced group applied higher forces than the experienced group in the nonemergency situation. However, the maximum force applied by the inexperienced group was 40.2 N, which is substantially lower than the maximum bite force of normal incisors (150–200 N).
Conclusion
Our results suggest that the experience levels of the laryngoscopists are not a major determinant of dental injuries in patients with healthy dentition.
Keywords
tooth injuries; laryngoscopes; manikins;
1. Introduction
Dental injuries are often associated with tracheal intubation. According to a review of anesthesia-related claims in England, the USA, and South Australia, dental injuries were the most common incidents reported, which accounted for up to a third of the claims.1 These injuries directly affect a patient in terms of discomfort and defacement and increase the risk of aspiration.
Several devices and techniques for easy and safe tracheal intubations have been developed for inexperienced medical practitioners.2, 3, 4 However, standard tracheal intubation by means of direct laryngoscopy remains the preferred technique because laryngoscope is an inexpensive and ubiquitous tool.5 Many medical residents complete an anesthesiology department rotation to learn the standard tracheal intubation technique during their residency programs.
Empirically, dental injuries during intubation are believed to occur more frequently when performed by less experienced beginners. However, few studies have investigated the forces applied during tracheal intubation by laryngoscopists with different levels of experience. Here, we measured the force applied to the maxillary incisors during direct laryngoscopy performed by experienced anesthesiologists and unexperienced medical students using a modified manikin to estimate the risk of dental injury.
2. Methods
Approval from an Ethics Committee was not required because of the use of manikins rather than patients in this study. Thirty-two anesthesiologists and 32 medical students participated in this study after their written informed consents were obtained. The anesthesiologists were either chief residents or department of anesthesiology staff. All anesthesiologists had at least 6 years of experience in anesthesia. Overall, the group had worked in anesthesia for a mean [standard deviation (SD)] period of 18 (9) years. The medical students had no experience in performing tracheal intubations for patients. Before the study, they received an anatomy lecture and theoretical background about the tracheal intubation technique.
For this study, we modified a manikin (Laerdal Airway Management Trainer) to measure the force applied to the maxillary central incisors, because these teeth have the greatest risk of tracheal intubation-related injuries.1 As shown in Fig. 1, a metal rod was set on the opposite side of the upper central incisors to push the film-type pressure sensor (FlexiForce; Tekscan, Inc., Boston, MA, USA) in the vertical direction. Response time of this sensor is less than 5 μs and its hysteresis is less than 4.5% of full scale. The metal rod was sustained by a ball bearing on the manikin’s forehead. The pressure sensor was connected to a laptop computer, and pressure data were sampled every 0.25 s using data collection software [Economical Load & Force System (ELF, Single-handle; Tekscan, Inc., Boston, MA, USA)]. Because laryngoscopic forces are affected by patient characteristics, such as height, weight, age, sex, and the presence of maxillary incisors,6, 7 we used the same manikin as the “patient” for every testee in this study.
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The timing of each intubation attempt was measured from the time the mask was removed from the manikin’s face until its lungs were inflated. After each intubation attempt, the correct placement of the tracheal tube was confirmed visually by inflating the lung. We planned to exclude failed intubation attempts from this study. Because our study did not involve patients, the intubation time was not limited by a predetermined cut-off time. In random order, each testee performed tracheal intubations in two scenarios. In Scenario 1, the testee performed a tracheal intubation for the manikin mimicking as a patient with normal dentition, in an emergency type situation, where the primary prerequisite was a quick intubation. In Scenario 2, the testee performed a tracheal intubation for the manikin as a patient with unstable dentition (severe teeth mobility), in a routine anesthetic situation where gentle intubation had priority over speed. These scenarios were explained by an instructor just before the study. Because different teeth might affect the pressure measurement system, no fragile teeth were set for Scenario 2. The participants used the same manikin in both scenarios. To be free from bias of training effects, each testee participated in this study only once. A size 3 Macintosh laryngoscope blade and handle (Welch Allyn Inc., Skaneateles Falls, NY, USA) and a size 7.0 mm tracheal tube (Rüschelit Super Safety Clear; Rüsch GmbH, Kernen, Germany) were used for all procedures.
The sample size was calculated based on a pilot study of Scenario 2. We found that 32 testees per group was required to demonstrate a 20% difference in the intubation time between groups (β = 0.2; α = 0.05). Data were analyzed using the Students t test, Students paired t test, and χ2 test. We considered p values <0.05 to be statistically significant. Data are presented as the mean (SD). Force was expressed in newtons (N).
3. Results
The mean peak force results are shown in Fig. 2. In Scenario 1, there was no significant difference between the two groups in terms of the mean (SD) peak force [6.1 (8.2) N for the experienced group vs. 7.7 (9.3) N for the inexperienced group]. In Scenario 2, the mean (SD) peak force applied by the experienced anesthesiologist group [1.1 (2.1) N] was significantly lower than that of the inexperienced medical student group [3.8 (4.7) N] (p < 0.05). The mean peak forces applied in Scenario 2 were significantly lower than the forces applied in Scenario 1 by both groups (p < 0.05).
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In Scenario 1, the peak forces applied by the experienced anesthesiologist group and the inexperienced medical student group ranged from 0 to 29.8 N and from 0 to 40.2 N, respectively. Both the experienced anesthesiologist and inexperienced medical student groups had 7 testees who exerted no force on the maxillary teeth in Scenario 1. In Scenario 2, the peak forces applied by the experienced anesthesiologist group and inexperienced medical student group ranged from 0 to 7.7 N and from 0 to 16.1 N, respectively. In the experienced anesthesiologist group, 17 testees applied a peak force of 0 N in Scenario 2, whereas 8 subjects applied a peak force of 0 N in the inexperienced medical student group (p < 0.05).
As shown in Fig. 3, the experienced anesthesiologist group required a significantly shorter time for tracheal intubation than the inexperienced medical student group in both scenarios (p < 0.05). For both groups, the length of time required to perform the intubations in Scenario 1 was significantly shorter than the time required in Scenario 2 (p < 0.05). The longest intubation time of 118 s was performed by a testee in the inexperienced medical student group. Because no esophageal intubation was confirmed in any intubation attempts, the data of such misconduct were unobtainable from this study.
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4. Discussion
This is the first study to measure the forces applied to the maxillary incisors during direct laryngoscopy using a pressure sensor attached to the dental side. Because many dental injuries are triggered by close contact of the laryngoscope with the incisors, we measured the force of the contact by our manikin system. Stronger forces are assumed to increase the risk of dental injury. Our results demonstrated that the experience levels of the laryngoscopists influenced both the peak force applied and the intubation time required. However, the peak forces applied by the inexperienced medical student group were unexpectedly low.
The relationship between the experience level of laryngoscopist and the force applied during tracheal intubation is controversial. Bishop et al.8 measured the intubation force using a blade and handle instrumented with strain gauges in a patient study. They observed no significant difference between the experienced and novice intubators. Bucx et al.9 also investigated the intubation force in a manikin study, and concluded that the forces applied to the maxillary incisors were unaffected by the experience level of laryngoscopists. We speculate that our results differ from these previous studies because we used a different type of scenario (unstable dentition). Hastings et al.10 reported that laryngoscopic force depended on individual techniques, because the force variability was very small when the same practitioner performed intubation, but the peak force varied as much as 30 N between different anesthesiologists performing intubation in the same patient. Although they did not evaluate novice practitioners, our results appear to be consistent with their findings.
The peak forces measured in the present study were lower than forces measured in previous studies, especially in Scenario 2. We speculated that the mean peak force value in our study was less than 10 N because of methodological differences in the force measurement. The strain gauge used in previous studies measured forces that were applied on both the maxillary incisor teeth and the side of the tongue.8, 9, 10 This method is useful for evaluating cardiovascular stimulation by tracheal intubation. In contrast, we measured only the force applied on the side of maxillary incisors tooth side to investigate the relationship between tracheal intubation and dental injuries. If the laryngoscopic blade does not touch the maxillary teeth, then the force value was measured as 0 N using our method.
The highest peak force measured in the present study was 40.2 N, which was applied by one of the inexperienced medical students during the emergency scenario. In healthy adults, the bite force is reported to be greater than 400 N.11 Even children in nursery schools have bite forces greater than 180 N.12 Although the bite force of the incisors is less than that of molars, the complete arch of incisors has maximum bite forces ranging from 150 to 200 N.13 Bite forces on a 2-mm sliced raw carrot were reported to range from 28 N to 48 N.14 Thus, when compared with normal bite forces, the peak forces we measured were exceedingly low. If teeth are healthy, they have a low risk of injury by laryngoscopy. Previous reports have also pointed out that the risk of dental injury by tracheal intubation is small in healthy teeth.1, 15 Therefore, the main risk factors for dental trauma associated with anesthesia may include difficult intubation and/or preexistent dental problems, such as dental caries, periodontal disease, and abnormally positioned teeth.
There were two limitations in the present study. We measured only the force exerted in the axial direction, but not the transverse direction. Inexperienced medical students might use the laryngoscope as a lever and put excessive force on the maxillary teeth in the transverse direction. These effects might not be reflected in the results of this study. Another limitation is that only laryngoscopy force was evaluated. For example, many experienced anesthesiologists change their intubation approach from the standard method to the left molar approach if they recognize that the upper center incisors are fragile. Additionally, experienced anesthesiologists may prepare other intubation devices if they judge that intubation will be difficult from the patient’s facial line. The present study did not include these types of professional judgments.
In conclusion, inexperienced medical students apply higher forces on the maxillary incisors than do the experienced anesthesiologists during direct laryngoscopy. The maximum peak force applied by the inexperienced medical students was 40.2 N in the emergency scenario and 16.1 N in the unstable dentition scenario. These peak forces are much lower than the maximum bite force (150–200 N) of normal incisors. Therefore, in terms of laryngoscopic force applied to the maxillary incisors, laryngoscopists’ experience levels are not a major determinant of dental injuries in patients with healthy dentition.