The study was a single-center, prospective, randomized, open-label, crossover simulation trial performed from February 1, 2023 to February 10, 2023 in Peking Union Medical College Hospital, a tertiary general teaching hospital in Beijing, China. The study was approved by the institutional review board of Peking Union Medical College Hospital, Beijing, China (reference number: K2562) on 10/11/2022 and registered in clinicaltrials.gov (identifier: NCT05689125) on 18/01/2023. Written informed consent was obtained from each participant. This article adheres to Consolidated Standards of Reporting Trials (CONSORT) guidelines.

We enrolled anesthesia residents in postgraduate year one to three in the Department of Anesthesiology, Peking Union Medical College Hospital in February 2023. Our residents had accumulated a minimum of five months of experience in endotracheal intubation, comprising approximately 200 intubations with an average of two intubations per day, prior to the commencement of the study. We excluded the residents who refused to participate and who failed the screening test (as detailed below). A research assistant recruited the residents and obtained informed consent.

We conducted a screening test and excluded the ones who failed the screening test to minimize the difference in residents’ competence of different ETI approaches. The residents performed ETI using bougie first, preloaded bougie, ETT with stylet, and ETT without stylet approaches with video laryngoscopy on a manikin. An attending anesthesiologist assisted them and assessed their performance. The device, intubation approaches, and performance assessment in the screening test were identical to those employed in the formal test (as detailed in “interventions” and “outcomes” below). The sole difference lay in the absence of chest compressions on the manikin during the screening test. A failure of the screening test was defined as being unable to ventilate the manikin within one minute after the insertion of a video laryngoscope blade into the mouth using any one of the four approaches.

Each of the residents who passed the screening test performed ETIs using the four approaches in a randomized sequence generated by an epidemiologist without knowledge of participants inclusion or allocation using R (version 4.2.1, R Foundation for Statistical Computing, Vienna, Austria, 2022). After the resident passed the screening test and signed the informed consent, he/she was informed of the procedure sequence based on the predetermined random sequence.

ETIs were performed in a simulated chest compression scenario on a full-size adult manikin (“Airway Larry” with CPR Metrix & iPad®, CPR Savers & First Aid Supply, Arizona, USA) positioned supine on an operating table. Two research assistants alternately performed continuous chest compressions at a rate of 100 to 120 compressions per minute and a depth of 5 to 6 cm, which were continuously monitored by the sensor kit of the manikin. There is no interruption of chest compressions during ETI. The video laryngoscope (UEscope®, Zhejiang, China) used was featured with an angle-adjustable monitor and a blade angled at 40°. ETT (inner diameter 7.0 mm, Covidien Shiley™, Colorado, USA) cuffs were lubricated before intubation. An assistant provided necessary assistance during intubation. An attending anesthesiologist observed the performance without providing instructions. The residents had a five-minute break between each ETI. All the ETIs were recorded by a camera.

In the bougie first approach, the operator placed a bougie (15 Fr, SunMed®, Michigan, USA) into the trachea to a depth of about 25 cm (Fig. 1A). An assistant threaded an ETT over the bougie and helped secure the free end (Fig. 1B). While maintaining glottic visualization, the operator advanced the ETT over the bougie into the trachea and the assistant withdrew the bougie. In the preloaded bougie approach, the ETT was loaded onto a bougie prior to laryngoscopy. The operator placed the bougie into the trachea while holding the ETT in the intubating hand (Fig. 1C) to a depth of about 25 cm. Advancement of the ETT and withdrawal of the bougie were the same as that in the bougie first approach (Fig. 1D). In the ETT with stylet approach, a stylet (10 Fr, TuoRen™, Henan, China) was inserted into the ETT to shape a “hockey stick” bend at the cuff of 30° to 40°. Once the operator placed the ETT tip through the vocal cord, an assistant withdrew the stylet, while the operator continued to advance the ETT to an appropriate depth (Fig. 1E and F). In the ETT without stylet approach, the operator performed ETI without a bougie or a stylet in ETT (Fig. 1G and H).

Endotracheal intubation approaches. A and B, bougie first; C and D, preloaded bougie; E and F, endotracheal tube with stylet; G and H, endotracheal tube without stylet

The primary outcome was success on the first attempt as a binary variable, which was defined as ventilating the manikin using an Ambu bag within one minute after the insertion of a video laryngoscope blade into the mouth with a single insertion of a bougie or an ETT into the mouth. An ETI attempt was terminated and defined as a failure when a resident tried for one minute but did not start ventilation, or when a resident felt unable to succeed and decided to give up in advance, or when a laryngoscope blade, a bougie, or an ETT was withdrawn out of the mouth after the initial insertion. The one-minute time restraint was added to represent the time pressure during CPR. If the first attempt of an approach failed, the resident would be given a second opportunity of the same approach. The outcome (success or failure) and duration of ETIs were measured based on the recorded videos of ETI procedures.

The secondary outcomes were success on two attempts, duration of the first attempt, overall intubation duration, and self-reported intubation difficulty. Success on two attempts included success on the first attempt and success on the second attempt after the failure of the first attempt. Duration of the first attempt was defined as the time elapsed between the insertion of a laryngoscope blade into the mouth and either the start of ventilation in a successful attempt or the termination of a failed attempt. Overall intubation duration was the duration of the first attempt if it succeeded, or the sum of the duration of both attempts if the first attempt failed. Self-reported intubation difficulty was a 0–10 ranking scale with 0 representing “the easiest” and 10 representing “the most difficult”.

We determined the difference of the primary outcome in each comparison using a generalized estimating equation (GEE) logistic model to account for clustering of intubations operated by the same resident. Success on the first attempt was regressed against group allocation with log linkage. The autoregressive 1 correlation structure was selected since it had the smallest quasi-likelihood information criterion indicating the best fitness. Since each resident performed all the four ETI approaches, resident characteristics (such as postgraduate year) distributed equally among different approaches and thus did not cause confounding effects. To control the type I error in the multiple comparisons among four intervention groups, a hierarchical testing strategy was predefined before accessing the data. We first compared the bougie first group and the ETT without stylet group. If a significant difference was found, we next compared the bougie first group and the ETT with stylet group. If the primary outcome differed significantly between these two groups, we finally compared the bougie first group and the preloaded bougie group. Any non-significant results in the procedures would prevent the following tests and all the results in the following comparisons were regarded as insignificant. Risk ratio (RR) estimated from the GEE logistic model was reported as effect size measure, and the hierarchical testing was conducted based on the statistical tests for the dummy categorical intervention variables from the model at a two-sided α of 0.05.

For the secondary outcomes, failure on two attempts was analyzed using a GEE logistic model. Duration of the first attempt and overall intubation duration were analyzed using a Cox regression model in which a successful intubation was defined as an event and a cluster term of residents was included to calculate robust standard errors. A hazard ratio (HR) larger than 1 indicated shorter time to achieve the successful intubation. We used a mixed-effects linear regression model to compare the self-reported intubation difficulty between groups. Model fit was checked by the Q-Q plot of residuals from the model. If the Q-Q plot indicated potential skewed distribution, the outcome variable would be transformed using log function. No multiple comparison adjustment was conducted in the analyses of secondary outcomes; hence, these findings were regarded as exploratory results.

The sample size was calculated based on the results of a pilot study that included ten residents who had completed two-month rotations in the Department of Anesthesiology in the recent six months. Among the total 40 intubations in the pilot study, success on the first attempt occurred in 9 (90%), 7 (70%), 7 (70%), and 6 (60%) intubations in the bougie first, preloaded bougie, ETT with stylet, and ETT without stylet groups, respectively, indicating the smallest absolute differences between the bougie first group and any other group was 20% (90% vs. 70%). A sample of 260 intubations (65 residents) was deemed necessary to provide 80% power to detect an absolute difference of 20% in success on the first attempt with a 2-sided α of 0.05.

The statistical analysis was carried out using R (version 4.2.1, R Foundation for Statistical Computing, Vienna, Austria, 2022) along with the packages “pwr”, “geepack”, “survival”, and “lme4”.