The research protocol received ethics approval from the Health Research Ethics Board at the University of Alberta (REB#Pro00100356) as of April 2020, and written informed consent was obtained from all participants.
The study design was a randomized crossover mannequin-based study utilizing several simulated scenarios with and without an aerosol box.
Theoretically, the design of the study was a non-inferiority trial.8 The null hypothesis of a non-inferiority study is that there is a difference in time to intubation with and without the box. If the null hypothesis is rejected, it would indicate non-inferiority. Nevertheless, at the time of study implementation, there was no available empirical evaluation of the use of the aerosol box. Without prior evidence, it was not possible to calculate a delta value to set a margin for non-inferiority. As it was not appropriate to make an ad hoc determination of a potential value for delta, statistically the present study is treated as a superiority trial with no assumptions about directionality.9
The study was conducted at our local simulation centre on the University of Alberta campus. Our aerosol box is 45.7 cm tall, 61.0 cm long, and 35.6 cm deep. The holes used by the operator are 20.0 cm in diameter, with the silicone rubber reducing the size to 10.0 cm. There are also two side holes that are 10.0 cm in diameter, which allows cables to pass into the box. The box is open-ended towards the mannequin’s feet. There are slits on the side to allow cables to pass into the box, but no specific holes for the airway assistant to maneuver in. The mannequin used for our study is the SimMan 3G mannequin (Laerdal Medical Canada Ltd, Toronto, ON, Canada), which has pre-set functions to alter the difficulty of the airway, along with simulated capnography monitoring (Fig. 1).
Each participant was allowed three practice intubations on the mannequin without the aerosol box to familiarize with the mannequin’s normal airway anatomy. The participants could adjust the height of the stretcher and position the mannequin. The participants donned airborne and droplet personal protective equipment (PPE) in adherence to local institutional guidelines, which included a face shield, N95 masks, sterile gown, and gloves. Expired N95 masks were used to conserve PPE. All intubations were conducted using a GlideScope® (Verathon, Inc., Bothell, WA, USA) with a hyper-angulated disposable size 3 blade, and a rigid GlideRite® stylet. Each participant intubated four different airway scenarios with and without the aerosol box; these scenarios were normal airway, pharyngeal obstruction, cervical spine rigidity, and tongue edema. The participant’s order of scenarios was randomized using the online software, Research Randomizer,10 and the participants were blinded to the order to minimize learner bias (Fig. 2).
An anesthesiology resident, who acted as an airway assistant, was blinded to the order of scenarios. The tasks of the airway assistant were standardized, which included removal of the stylet, inflation of the tracheal tube cuff, and attachment of the self-inflating bag to ventilate the mannequin. The airway assistant also provided optimization maneuvers if requested; these maneuvers included application of the backwards and upwards pressure (BURP) maneuver, re-positioning or removal of the aerosol box, and providing a bougie or a pillow for re-positioning.
Each participant was video recorded and reviewed to confirm time to intubation, number of optimization maneuvers, number of intubation attempts, and breaches of PPE. All analysis was carried out in R software, version 184.108.40.206 Analysis was conducted using the rstatix,12 psych,13 and WRS214 packages.
Our primary outcome was the impact of the aerosol box on intubation time, defined as the time the video laryngoscope blade passed the teeth until confirmation of tracheal intubation via positive end-tidal CO2 (ETCO2) waveform using the mannequin’s simulated capnography monitor upon initiation of manual ventilation.
Secondary outcomes included intubation attempts per scenario, number of optimization maneuvers required, failed intubation attempts, and PPE breaches. We defined intubation attempts as the number of times the video laryngoscope and/or tracheal tube was withdrawn and re-inserted past the teeth. Optimization maneuvers included re-positioning of the mannequin, requesting for BURP, use of a bougie stylet, changing the position of the aerosol box, or removing the aerosol box. We also investigated the number of PPE breaches during each attempt, which was defined as exposure of the skin due to movement of participant’s gloves, and adjustment or removal of the aerosol box during the procedure. A survey was provided to the participants after the simulation.
Sample size was calculated for a two-way (box vs no box) repeated measures design with four levels (airway scenarios) using G-power version 220.127.116.11.15 As there was no prior evidence for the aerosol box, conservative estimates were set for power (0.95) and correlation between repeated measures (0.5). An alpha level of 0.05 was chosen. An effect size (0.25) was chosen based on lower estimates of effect sizes for adverse events during tracheal intubation using video and direct laryngoscopes.16 The necessary sample size was determined to be 36.
We recruited 38 participants for the study. Inclusion criteria included being either a resident physician or staff physician in anesthesiology, critical care, or emergency medicine, with at least 50 intubations using a videolaryngoscope. Participants were recruited through departmental emails, personal connections, and snowball sampling. We asked participants about their number of GlideScope® intubations prior to their participation.
Primary outcome assessment
Trials with and without the aerosol box across the four airway scenarios were analyzed using repeated measures analysis of variance (RM-ANOVA). The data were checked for outliers, influential cases, and missing data. The data were also examined for assumptions of normality and sphericity to ensure appropriateness for RM-ANOVA. Data points were considered for removal if they were extreme cases as identified by the boxplot method, > 3 times the third interquartile range, and had a Mahalanobis distance > 12.17
Secondary outcomes assessment
The effect of experience was investigated through two methods. First, correlation between years of experience and time for intubation under each condition was examined. Second, group differences between residents and staff physicians were explored. Because of sample size limitations for the subgroups, unequal cell sizes, and non-normality, residents and staff physicians were compared using the non-parametric Mann-Whitney test, and within group comparisons were made using the Wilcoxon signed rank test. Frequencies and descriptive statistics were examined for optimization maneuvers, number of intubation attempts, PPE breaches, and results from the post-simulation survey. The survey included questions on the difficulty of box use, likelihood of box use, and demographics.
Video data were missing for three participants because of unexpected technical problems; therefore, only the time to intubation recorded during the trials was used for these participants. Two raters scored all videos independently. Interrater reliability was calculated for each variable. The raters scored participants nearly identically. The intraclass correlation coefficient (ICC) for time was 0.99. The ICC for number of optimization maneuvers and intubation attempts was one, and the kappa for PPE breaches was one.