This study included first-year residents who started a two-month rotation at the Department of Anesthesiology between April 2017 and March 2020. The study received ethical approval for the use of an opt-out methodology for patients. Written informed consent was obtained from the first-year residents before enrolment in this study. This study was approved by the Jikei university ethics committee (approval number, 26–044 (7549))
We conducted a cluster randomized control trial because each group consisted of two to four residents who rotated every two months. Randomization was performed with a balanced randomization schedule and a predefined random number table. Each rotation resident was assigned to either the Macintosh laryngoscope (ML) group or McGRATH® MAC (Covidien Japan Inc., Tokyo, Japan) laryngoscope (MML) group. This study adheres to the applicable Consolidated Standards of Reporting Trial Statement and The Ottawa Statement on the Ethical Design and Conduct of Cluster Randomized Trials.
In the ML group, the larynx could only be viewed by residents during the actual intubation. The residents explained their field of view, and the anesthesiology faculty provided feedback based on the residents’ explanations. In the MML group, the residents were encouraged not to look at the video monitor but the larynx under direct vision, while the anesthesiology faculty viewed the monitor and provided timely feedback.
Endotracheal intubation skill retention was investigated immediately after completing the rotation at the Department of Anesthesiology, three months later, and six months later.
To standardize the evaluations, first-year residents performed the intubations on a mannequin rather than a human. We also used an endotracheal intubation device that was not used in both groups during the rotation period.
The high-fidelity simulator SimMan® 3G (Laerdal Medical Japan Co., Ltd., Tokyo, Japan) and the video laryngoscope C-MAC® (KARL STORZ Endoscopy Japan K.K., Tokyo, Japan) were selected for the evaluation of endotracheal intubation skill retention.
SimMan was used for the following reasons: 1) intubation conditions can be standardized while the laryngeal exposure varies during patient intubation, thus influencing the difficulty; and 2) an evaluation environment similar to the actual clinical setting can be created because it is a whole-body manikin with an intricate laryngeal structure. The height of the SimMan head was set at 6 cm, and the neck flexion was set at the same angle as the pillow. The SimMan airway setting was kept at the default status and did not require any adjustments. A 7.5-mm Mallinckrodt™ Hi-Lo tracheal tube (Mallinckrodt Inc., Covidien Japan Inc., Tokyo, Japan) was made to shape and standardized with an intubating stylet following a photograph of the actual size.
C-MAC was used as the intubation device because the C-MAC laryngoscope has a Macintosh-type blade and allows faculty members to verify the field of view on the monitor through a camera attached at the tip of the blade. The monitor is separated from the blade and completely shielded from the view of first-year residents.
The primary outcome was a retention of endotracheal intubation skills evaluated as endotracheal intubation time. The secondary outcome was the percentage of glottic opening (POGO) scale  scores over the observation period.
The time required for intubation was defined as when the resident touched SimMan until the tip of the endotracheal tube passed through the glottis. Intubation was considered a failure if the tube was not passed through the glottis within 60 seconds. In cases of failure, data from the second successful intubation were analyzed. In addition, the POGO scale was visually evaluated by faculty members based on the image displayed on the C-MAC monitor.
Sample size calculation was performed using G*Power version 18.104.22.168 for Macintosh (Heinrich-Heine-Universität, Düsseldorf, Germany). The parameters were based on our experience and data from past studies: α = 0.05, power of test = 0.8, effect size f = 0.25, which is estimated to be moderate, and the absence of correlation among repeated measures (Corr.≈0). Repeated-measures ANOVA was performed for statistical analysis of the primary endpoint.[8, 9] As a result, the calculated sample size was 44 subjects (22 in each group).
The number of dropouts was estimated to be 1 in each group. Moreover, the researchers of this study confirmed the inclusion and exclusion criteria and assignment. Thus, the final sample size was 46 subjects (23 in each group).
Data analysis was conducted using JMP® Pro 13.1.0 for Macintosh (SAS Institute Inc., Cary, NC, USA). The background characteristics of both groups are expressed as medians and interquartile ranges for continuous variables and as numbers and percentages for frequencies. Variables used in primary and secondary analyses are expressed as means, standard deviations, and 95% confidence intervals. A repeated-measures ANOVA was conducted for both primary and secondary analyses. The data were blinded before analysis by a trial statistician.