Introduction

Endotracheal intubation is an important skill for maintaining airway patency in the emergency department (ED). While intubation is one of the most frequently performed procedures in the ED, emergency intubation is associated with a high incidence of complications [1]. An observational study showed that 1.8% of emergency intubations had dental complications [2]. Additionally, several complications (e.g. bradycardia, cardiopulmonary arrest) are associated with the excess force applied on the oral structures [3–5]. In this context, intubation with a lower force applied on the oral structure is an important part of intubation skills to prevent complications in the ED.

In developed countries, 37%-50% of intubation procedures were performed by junior residents [6, 7]. Therefore, assessment of intubation skills in novice physicians should be warranted for patient safety and improving intubation outcomes. Although multiple studies have focused on the success of intubation during residency training [8, 9], there is a lack of research on whether the residency program and clinical training improve other important markers of intubation skills, such as force on the oral structures during intubation. The reasons of the scarce data include the difficulty of quantification of the force on oral structures and potential harm of on-site assessment of intubation skills in the clinical settings. Nevertheless, recent newly developed high-fidelity simulators with assessment systems enabled us to quantify the force applied on oral structures [10, 11].

In this context, we examined the differences in force applied on the oral structures (maxillary incisors and the tongue) during intubation performed by novice physicians between prior to clinical training and after completion of one-year clinical training.

Methods

Results

Of 109 novice physicians (ie, PGY-1 physicians) identified at the beginning of our study, 19 were excluded due to incomplete data, and leaving 90 novice physicians were eligible for the analysis. Participant characteristics are shown in Table 1. Prior to clinical training, the overall median number of intubation attempts was 2 (interquartile range [IQR], 1–3), with a median of 0 (IQR, 0–1) with AWS application. After completion of the one-year clinical training, the median number of intubation attempts increased to 5 (IQR, 2–30), not including this trial.

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Table 1. Baseline characteristics of participants.

https://doi.org/10.1371/journal.pone.0188224.t001

When using DL, compared to prior, significantly less force was applied on the maxillary incisors during intubation in normal airway scenario after completion of one-year clinical training (median force [IQR]: 28.0 N [17.3–42.8 N] and 19.5 N [0–27.8 N], p < 0.001; Table 2). This association persisted in the difficult airway scenario (40.5 N [21.3–106.8 N] and 28.0 N [16.0–55.0 N], p = 0.002). With regard to the force applied on the tongue, compared to prior, significantly less force was applied on the tongue during intubation using DL in the normal scenario after completion of one-year clinical training (normal airway: 11.1 N [5.9–17.3 N] and 8.4 N [0.5–15.8 N], p = 0.004). Similarly, this association persisted in the difficult airway scenario (11.9 N [7.4–17.2 N] and 6.3 N [2.2–14.5 N], p = 0.001). In both scenarios, the median numbers of intubation attempts were 1 (both, IQR, 1–1).

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Table 2. Comparison of force applied on oral structures and time to intubation, according the intubation devices and airway scenarios.

https://doi.org/10.1371/journal.pone.0188224.t002

When using AWS, compared to prior, the force applied on the maxillary incisors during intubation in the normal airway scenario after completion of one-year clinical training was significantly less (22.0 N [15.0–32.8 N] and 0 N [0.0–18.0 N]; p <0.001; Table 2). This association remained in the difficult airway scenario (26.0 N [19.3–37.0 N] and 16.0 N (0.0–21.8 N); p < 0.001). In both scenarios, the force applied on the tongue were decreased after completion of one-year clinical training but not significant. In both scenarios, the median number of intubation attempts was also 1 (IQR, 1–1).

The median times to intubation completion were improved across all strata. When using DL, compared to prior, median times to intubation completion after one-year clinical training were significantly less in both airway scenarios, (normal airway: 43.5 s [37.8–74.8 s] and 34.5 s [27.0–45.9 s]; difficult airway: 51.9 s [38.5–72.2 s] and 34.8s [28.5s–43.7s], respectively; both, p < 0.001). Likewise, when using AWS, compared to prior, median times to intubation completion were significantly less after completion of one-year clinical training in both airway scenarios (normal airway: 58.5 s [39.5–88.1 s] and 28.1 s [24.2–37.2 s]; difficult airway: 72.4 s [49.0–120.2 s] and 30.5 s [25.3–36.5 s], respectively; both, p < 0.001, both; Table 2).

Discussion

In this multi-center longitudinal study, we found that, by using DL, novice physicians intubated with a less force on the maxillary incisors and the tongue after completion of one-year clinical training. We also found that, by using AWS, less force was applied on the maxillary incisors during intubation after completion of one-year clinical training, while the reduced force applied on the tongue was not statistically significant. These findings were consistent in both normal and difficult scenarios. Time to intubation was significantly shorter after completion of one-year clinical training across all strata. To our knowledge, this is the first study that demonstrates the improved force applied on the oral structures–an important part of intubation skills–after completion of one-year clinical training.

Our findings are consistent with the previous studies that reported the increasing experience is likely to have lower rate of intubation-related complications [16, 17]. In the previous large multicenter study of 4,094 patients, the intubation-related adverse event rate was 17% in PGY 1, 13% in PGY 2, and 10% in PGY 3 [16]. Additionally, in the field of critical care medicine, a previous study has shown that skill acquisition by residents can reduce the incidence of complications related to intubation [18]. Our observed findings corroborate with these prior findings and extend it by demonstrating the reduced force applied on the oral structures after completion of one-year clinical training.

Although the use of force applied on the oral structures are surrogates of intubation-related complications, there is a linkage between the force applied on the oral structures and intubation-related complications [19]. For example, excess force on the maxillary incisors increases the risk of dental injury—a frequent adverse event of intubation, and is the most common claim against intubators [20]. According to the prior study, force on the maxillary incisors greater than 150 N could be a risk factor for dental injury [19]. In the current study, 25% of novice physicians applied force of ≥100 N to maxillary incisors prior to the clinical training in this study; however, the force was significantly reduced after completion of one-year training. Therefore, our findings suggest that clinical training may reduce the incidence of dental trauma in the clinical settings.

Another linkage between the force applied on the oral structures and intubation-related complications is increased sympathetic activities due to the force applied oral structures (e.g., tongue). The increased sympathetic activities may cause hemodynamic responses, such as hypertension, tachycardia, and arrhythmia [17, 21, 22]. The plasma concentration of catecholamine increases in response to excess intubation stimulation [23]. Therefore, intubation should be performed with minimal force to reduce the number of adverse events. In this study, the force applied on the tongue decreased after one-year of clinical training. Thus, the clinical training of residents might reduce the risk of hemodynamic changes during intubation attempts.

When an intubator used AWS, the force applied on the tongue were decreased but not statistically significant due to the relatively small differences and the limited statistical power. Because AWS allows indirect visualization of the vocal cords and enables intubation without upward lifting force required to expose the glottis, intubators can intubate with less force on the tongue compared to intubation with DL, even if the intubator is a novice physician. Indeed, in the current study, the force applied on the tongue by using AWS (e.g., 1.1 N in the normal scenario) was relatively lower compared to those by using DL (e.g., 11.1 N in the normal scenario) at prior to clinical training. This is consistent with a prior literature that reported the association between the use of AWS and less force applied on the tongue compared to use of DL [10].

Our findings suggest that evaluating the intubation skills using a simulation-based assessment system may be useful to reduce the force applied on the tongue or incisors during intubation resulting in a decrease of intubation-related complications and improvement of quality of emergency care. More importantly, the use of simulator for the evaluation is harmless and repeatable. Although there are no well-defined “minimal” force applied on the oral structures to prevent complications, our results may suggest that novice physicians should intubate with less than 20 N for maxillary incisors and 10 N for tongue based on the force after completions of one-year training.

Conclusions

In this multi-center longitudinal study, among novice physicians, we found that the force applied on the oral structures were significantly improved after completion of one-year clinical training, suggesting that preferable force is ≤20 N for maxillary incisors and ≤10 N for tongue. In addition, the time to intubation was also improved after completion of one-year clinical training. These findings support the effectiveness of current residency program and clinical training to improve intubation skills. Furthermore, our observations also indicate that the assessment of intra-oral pressure monitoring using high fidelity simulators prior to intubation in the clinical setting is beneficial for resident physicians and students to potentially decrease dental and hemodynamic complications.