Effects of back-up position on tracheal intubation using a Macintosh-geometry videolaryngoscope in patients with a simulated difficult airway: A prospective randomized crossover study

Effects of back-up position on tracheal intubation using a Macintosh-geometry videolaryngoscope in patients with a simulated difficult airway: A prospective randomized crossover study | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effects of back-up position on tracheal intubation using a Macintosh-geometry videolaryngoscope in patients with a simulated difficult airway: A prospective randomized crossover study Joo Hyun Jun, Eun Hee Chun, Mi Hwa Chung, Jung Eun Kim, Kyung Mi Kim, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-1448707/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract Background: Both Macintosh-geometry and hyper-angulated videolaryngoscopes provide a better laryngeal view than conventional direct laryngoscope. However, the Macintosh-geometry videolaryngoscopes may not provide as dramatic an improvement in the laryngeal view as the hyper-angulated videolaryngoscopes because they lack the capacity to view structures around corners, so glottic exposure using the Macintosh-geometry videolaryngoscopes may be incomplete in some difficult airway. Patient positioning has an important effect on laryngeal visualization and successful tracheal intubation. However, the ideal head and neck position for videolaryngoscope has not yet been evaluated. We evaluated the effects of the back-up position on laryngeal visualization and ease of intubation using a Macintosh-geometry videolaryngoscope in patients with a simulated difficult airway. Methods: Sixty-four patients were included in this prospective, crossover, randomized controlled trial. Limited neck movement and mouth opening were simulated using a cervical collar. The laryngeal view was assessed in both neutral and back-up positions, in a randomized order, using a Macintosh-geometry videolaryngoscope, based on the percentage of glottic opening (POGO) score and modified Cormack–Lehane (MCL) grade. When scoring laryngeal visualization, external laryngeal manipulation was not permitted. The trachea was intubated only once (in the second position). The ease of intubation was assessed based on the need for optimization maneuvers, Intubation Difficulty Scale (IDS) scores, and time to intubation. Results: The mean POGO score improved significantly in the back-up position (59.4 ± 23.8%) compared with the neutral position (37.5 ± 24%) ( P <0.0001). MCL grade 1 or 2a was achieved in 56 (85.9%) and 28 (43.7%) of patients in the back-up and neutral positions, respectively ( P <0.0001). Optimization maneuvers for intubation were required in 7 (21.9%) and 17 (53.1%) patients in the back-up and neutral positions, respectively ( P <0.0001). The IDS and time to intubation did not differ significantly between positions. Conclusion: The back-up position significantly improved the laryngeal view during Macintosh-geometry videolaryngoscope-guided intubation. It also improved ease of intubation, reflected in a reduced need for optimization maneuvers. Trial registration: This trial was registered with www.clinicaltrials.gov , NCT04716218, on 20/01/2021. Laryngoscopy videolaryngoscope Macintosh blade tracheal intubation Figures Figure 1 Figure 2 Figure 3 Background Owing to the many advances that have resulted in improved laryngeal views, reduced intubation difficulty, and decreased incidence of failed intubation, videolaryngoscopes (VLs) are increasingly being used as first-line tools not only for anticipated difficult tracheal intubation but also as a substitute for direct laryngoscopy (DL), even in routine airway care [ 1 – 3 ]. These changes are being rapidly implemented due to the coronavirus disease 2019 (COVID-19) pandemic, because of the perceived risk of infection due to possible aerosolization during the intubation process [ 4 – 6 ]. There are several types of VLs, with different device designs and blade geometries. Each VL has unique features that may be advantageous under some conditions but disadvantageous under others [ 7 ]. VLs have two main blade types: hyper-angulated (HA-VL) and Macintosh-geometry (Mac-VL). The HA-VL, originally designed for use in difficult airways, provides an improved view of the larynx with minimal head and neck manipulation. However, the HA-VL also prevents direct visualization of the larynx and has the potential to make it difficult to guide the tracheal tube (TT) towards the glottis despite obtaining a good laryngeal view [ 7 ]. In comparison, Mac-VL can be used to perform both indirect and direct laryngoscopy. The TT is inserted in much the same way as in DL, with or without a stylet. Combining the benefits of DL and VL in one device, the Mac-VL can serve as a standard intubation device for both routine airway management and educational purposes [ 7 – 9 ]. However, the Mac-VL may not provide as dramatic an improvement in the laryngeal view as the HA-VL because it lacks the capacity to view structures around corners, so glottic exposure using the Mac-VL may be incomplete in unexpected difficult airways [ 10 – 12 ]. Optimal patient positioning maximizes the likelihood of successful laryngoscopy and tracheal intubation [ 13 ]. However, the ideal head and neck position for VL has not yet been evaluated. The back-up position during DL improves the laryngeal view, lessening the need for assisted maneuvers and shortening the intubation time [ 14 – 16 ]. Moreover, placing the patient in a back-up position is a simple maneuver to improve safety during tracheal intubation [ 17 ]. Magnetic resonance imaging studies showed that head elevation to bring the patient’s sternum onto the horizontal plane of the external auditory meatus (EAM) reduces the angle of the pharyngeal and laryngeal axes, thus demonstrating how the back-up position improved the laryngeal view and ease of intubation during DL [ 18 , 19 ]. We hypothesized that improved alignment of the pharyngeal and laryngeal axes using the back-up position (to align the sternal notch and EAM) would help optimize the laryngeal view and facilitate tracheal intubation during Mac-VL-guided intubation Therefore, this study investigated the effect of the back-up position on laryngeal visualization (primary outcome) and ease of tracheal intubation (secondary outcome) using McGrath MAC VL with a Macintosh-geometry blade in simulated difficult airway with a cervical collar to restrict neck movement and mouth opening. Methods This was a prospective, crossover, randomized controlled trial of a simulated difficult airway with concurrent limited mouth opening and neck movement. Ethical approval for this study was approved by the institutional review board of Hallym University Kangnam Sacred Heart Hospital (approval number: 2020-09-009). The trial was registered prior to patient enrolment at www.clinicaltrials.gov (NCT04716218) on 20/01/2021, and conducted and reported according to the Consolidating Standards of Reporting Trials (CONSORT) 2010 statement [ 20 ]. This study was conducted from January 2021 to March 2021. We recruited 64 adult patients undergoing general anesthesia with tracheal intubation for elective surgery. All patients provided written informed consent and had ASA physical status of I–III. The exclusion criteria were aspiration risk, bleeding tendency, uncontrolled hypertension, obstructive sleep apnea, and a known or anticipated difficult airway (e.g. Mallampati class III–IV, body mass index > 35 kg/m 2 , inter-incisor distance < 3.5 cm, thyromental distance < 6 cm). Using a computer-generated randomization table ( www.randomizer.org ), the enrolled patients were randomly assigned to one of the two sequences. In sequence 1, the back-up position was used first, followed by the neutral position. In sequence 2, the neutral position was used first, followed by the back-up position. The sequence allocation was concealed in an opaque envelope, which was opened by the investigator responsible for randomization immediately before the initiation of anesthesia. Study protocol A McGrath™ MAC VL (Aircraft Medical, Ltd., Edinburgh, UK) with a curved blade (resembling a Macintosh blade) and a camera was set up according to the standard practice at our institution. Tracheal intubation was performed using a size 3 McGrath™ MAC VL blade, and a TT with an internal diameter of 6.5 mm for females and 7.5 mm for males (Mallinckrodt, St. Louis, MO, USA). A malleable stylet was not initially used to facilitate tracheal intubation, because the McGrath MAC VL had a Macintosh-geometry blade [ 7 , 21 ]. All patients were premedicated with glycopyrrolate (0.2 mg) at least 30 min before initiation of anesthesia. After entering the operating room, each patient was placed in the supine position on the operating table, with their head on a soft pillow. Standard monitoring (continuous pulse oximetry, electrocardiography, capnography, and non-invasive blood pressure) was applied. After removing the pillow, an observer blinded to the sequence assignment assessed the airway based on the Mallampati class, thyromental distance, and inter-incisor distance at maximal mouth opening. Then, a semi-rigid cervical collar (Philadelphia Cervical Collar Co., Thorofare, NJ, USA) of appropriate size was placed around the patient’s neck, and the inter-incisor distance at maximal mouth opening was re-measured. The back-up position was created by flexing the operating table at the trunk–thigh hinge and then raising the back section. For each patient, a digital inclinometer and tripod water level were used to determine the table ramp angle required to align the sternal notch and EAM in the same horizontal plane. Anesthesia was induced with propofol (1.5 mg/kg) and remifentanil (0.5–1 µg/kg), followed by rocuronium (0.6 mg/kg). After verifying neuromuscular blockade using a nerve stimulator, tracheal intubation was performed using the McGrath MAC VL according to the assigned sequence. In sequence 1, the initial assessment of laryngeal view was performed in the back-up position (end of period 1). Subsequently, the patient was placed in the neutral position and the second assessment of laryngeal view was performed (start of period 2). The trachea was then intubated in the second position. For the sequence 2, the procedure was reversed. Thus, the laryngeal view was assessed in both the neutral and back-up positions, but the trachea was intubated only once (in the second position). When assessing laryngeal visualization, external laryngeal manipulations (ELMs) were not permitted. To eliminate interobserver variation, a single experienced anesthesiologist (> 100 previous tracheal intubations using the McGrath MAC VL) performed all VL procedures. Outcome measurement The following data were collected by two investigators (Eun Hee Chun and Joo Hyun Jun) not involved in the VL procedures. The primary outcome was the laryngeal view in both the neutral and back-up positions (periods 1 and 2, respectively), which was assessed according to the percentage of glottic opening (POGO) score and modified Cormack–Lehane (MCL) grade. The POGO score reflects the proportion of the glottic area that is visible: a score of 100% denotes visualization of the whole glottis, from the inter-arytenoid notch to the anterior commissure, whereas a score of 0% denotes visualization of none of the glottis [ 22 ]. Based on the MCL grade, the laryngeal view was classified as easy (laryngeal inlet visible; MCL grade 1 or 2a), restricted (posterior glottic structures or epiglottis visible, where the latter could be lifted; MCL grade 2b or 3a), and difficult (epiglottis could not be lifted or no laryngeal structures visible; MCL grade 3b or 4) [ 23 ]. The data collection form included illustrations of the MCL grades and POGO scores to promote standardization. The secondary outcome was the ease of intubation, which was measured in the second position (period 2). To evaluate this outcome, we recorded the optimization maneuvers used for successful tracheal intubation, such as withdrawing and reinserting the blade, increasing the lifting force, applying ELMs, bending the TT into a steeper curve, adding a stylet, or rotating the TT during passage into the trachea to avoid impacting the anterior wall of the subglottic space [ 24 , 25 ]. The time to intubation, Intubation Difficulty Scale (IDS) score [ 26 ], reasons for failed intubation, and adverse effects were also recorded. Time to intubation represented the time between insertion into and removal of the VL blade from the mouth. Statistical analysis Statistical analyses were performed using SAS (version 9.4; SAS Institute, Cary, NC, USA), SPSS (version 27.0; IBM Corp., Armonk, NY, USA), and R software (version 4.1.0; http://www.R-project.org ). The distribution of continuous data was evaluated using the Shapiro–Wilk test. Normally distributed continuous variables are provided as the mean ± standard deviation and were analyzed using the paired and independent t-tests . Non-normally distributed continuous variables are provided as the median (interquartile range) and were analyzed using Wilcoxon’s signed rank and Mann–Whitney U tests. Categorical data are expressed as n (%) for proportions and were compared using the McNemar and χ 2 tests, with bootstrapping applied as appropriate. To evaluate possible carryover effects, the sum of the POGO scores in the first position (period 1) and second position (period 2) was calculated for each subject and compared across the two sequences using the unpaired t test. To evaluate possible period effects, the difference in POGO scores between the two periods was calculated for each subject and compared across the two sequences using the unpaired t-test [ 27 ]. In all analyses, P <0.05 was considered statistically significant. We calculated the sample size based on the POGO score using PASS software (version 15.0; NCSS, LLC, Kaysville, UT, USA). Assuming that a 20% difference in POGO score was clinically important (standard deviation of 35%) [ 28 ], we determined that 28 pairs were required in each sequence for two-sided testing in this 2 × 2 crossover study, with 95% power and an alpha level of 5%. Therefore, we enrolled 32 participants in each sequence group, assuming a dropout rate of 10%. Results In total, 68 potential participants were screened between January 2021 and March 2021. Three patients were excluded because of mobile teeth ( n = 2) or uncontrolled hypertension ( n = 1), and one declined to participate in the study. The remaining 64 patients were randomized (32 per group) and included in the analyses (Fig. 1). Table 1 shows the characteristics of the patients (including the airway). After applying the cervical collar, neck motion was severely restricted, and median inter-incisor distance at maximal mouth opening decreased significantly from 40 (40 to 50) to 32 (30 to 35) mm ( P <0.0001). The mean table ramp angle for aligning the EAM and sternal notch in the back-up position was 17.5 ± 4.1°. Primary outcome Compared with the neutral position, the back-up position provided significantly better or similar laryngeal visualization. The mean POGO score improved significantly in the back-up position (59.4 ± 23.8%) compared with the neutral position (37.5 ± 24%) (difference in means, 21.9%; 95% confidence interval [CI] 17.1% to 26.7%; P < 0.0001). An easy laryngeal view (MCL grade 1 or 2a) was achieved in 56 patients (85.9%) in the back-up position and only 28 (43.7%) in the neutral position (difference in proportions, 42.2%; 95% CI 30% to 54.4%; P < 0.0001) (Fig. 2, Table 2). In 38 of 64 patients (59.4%), the back-up position improved the glottic view by one or two MCL grades, while the MCL grade was similar between positions in the remaining 26 patients (40.6%). The back-up position did not worsen the MCL grade in any patient (Fig. 3, additional file 1). Secondary outcome The results of intubation in the second position (period 2) are summarized in Table 3. Intubation was successful on the first attempt in all patients. The proportion of patients requiring optimization maneuver(s) for tracheal intubation was higher in the neutral position ( n = 17; 53%) than in the back-up position ( n = 7; 22%) ( P =0.01). There were no significant differences between positions in median time to intubation ( P = 0.11) or median IDS score ( P = 0.13). Slight mucosal bleeding attributed to laryngoscopy or tracheal intubation was noted in one patient. No major complications (e.g. dental damage and, palatal perforation) were observed. Carryover and period effect There was no apparent carryover effect for the primary outcome (POGO score; P =0.32). However, a period effect was observed for the POGO score ( P = 0.03) (Table 4). Although the difference in POGO scores between the neutral and back-up positions was lower in period 2 than period 1, the POGO scores remained better in the back-up position than in the neutral position during period 2 ( P =0.008) (additional file 2). Discussion Our study evaluated laryngeal visualization and ease of tracheal intubation in the back-up position using an individualized table-ramp angle (17.5 ± 4.1°) degrees) to align the sternal notch and EAM. Use of this individualized approach led to significantly improved laryngeal visualization in the back-up position, compared to the neutral position, when using the Mac-VL in patients with a simulated difficult airway. Specifically, the mean POGO score improved from 37.5 ± 24 to 59.4 ± 23.8%, and the proportion of patients with an easy (MCL grade 1 or 2a) view increased from 43.7–85.9%. This position was also associated with reduced use of optimization maneuvers during Mac-VL-guided tracheal intubation. In the current study, these maneuvers were used in only 21.9% of patients in the back-up position, in contrast to 53.1% of patients in the neutral position. However, the time to intubation and IDS score did not differ significantly between positions. Laryngeal exposure in the neutral position in this study was inferior to that in Klein-Brueggeney et al., who used the same difficult airway scenario (application of a cervical collar to limit mouth opening and neck movement), where an easy laryngeal view (MCL grade 1 or 2a) was achieved in 110/180 patients (91.6%) [ 29 ]. In the neutral position, the nasopharynx and upper part of the laryngeal vestibule are below the glottis [ 18 ], which can lead to a more anterior laryngeal position. In this situation, optimization maneuvers, such as ELM or upward and forward lifting force of the blade, are necessary to raise the epiglottis and visualize the laryngeal opening during laryngoscopy [ 30 , 31 ]. However, we reported a “first achieved view” that did not allow these maneuvers during scoring of laryngeal visualization, to independently evaluate the effects of changes in airway anatomy associated with positional changes on the laryngeal view, whereas Klein-Brueggeney et al.[ 29 ] reported the “best achievable view”. This difference in laryngeal exposure between our study and that of Klein-Brueggeney et al. may reflect the fact that Mac-VL requires optimization maneuvers to improve airway axis alignment, to obtain an optimal laryngeal view in cases with a genuine difficult airway. Likewise, in a recent study using a Mac-VL as the primary intubation tool, the optimal view of the larynx to facilitate successful tracheal intubation was not achieved in 12–30% of cases, thereby requiring switching to a HA-VL [ 32 ]. However, previous studies comparing HA-VL and Mac-VL in simulated or “predicted” airways imply that the enhanced laryngeal view does not always lead to easier or quicker tracheal intubation [ 10 , 12 ]. A similar mismatch was also observed in a previous study that used an HA-VL (McGrath Series 5) in a simulated difficult airway with in-line stabilization of the neck, which provided a better laryngeal view but had a longer time to intubation (36 s) than in the present study (22–26 s) [ 33 ]. A HA-VL requires that a styleted TT be bent at an acute angle to match the blade’s curvature, which can contribute to an increased time to intubation [ 10 , 12 ]. It also results in partly blind oropharyngeal passage of the styleted TT; this may increase the risk of soft-tissue injury, such as oropharyngeal perforation [ 34 ]. By contrast, the lower angulation of the Mac-VL provides more room to manipulate the TT, allowing easier intubation and reducing the risk of trauma [ 35 ]. Therefore, when a Mac-VL is used for routine intubation, the back-up position may improve safety by obviating the need for conversion to alternative techniques (i.e. use of a hyper-angulated blade). Indirect laryngoscopy with a VL does not require a direct line of sight to the laryngeal opening, which is helpful for difficult tracheal intubation but complicates TT insertion [ 7 ]. This is especially problematic for HA-VL, but can also be an issue for the Mac-VL [ 7 ]. This may also prevent VLs from becoming the standard of care for all tracheal intubations. To compensate for their geometric limitations, some authors have suggested deliberate worsening of the view (by withdrawing the VL blade) to reduce the TT introduction angle, thereby facilitating intubation [ 21 ]. However, this strategy negates the perceived advantage of VL, i.e. a good laryngeal view. In contrast, the back-up position used in the present study allowed relatively straight passage of the TT and facilitated successful tracheal intubation, as determined based on the need for optimization maneuvers without compromising laryngeal visualization. This study has some limitations. First, we enrolled patients with a simulated difficult airway. Although the use of a cervical collar is common when simulating difficult airways for research purposes [ 29 , 36 , 37 ], caution is required when extrapolating the results to genuine difficult airways. However, we did not enroll patients with genuine difficult airways because this is a rare and possibly life-threatening condition [ 38 ]. Second, as our study was performed in the specific scenario of limited neck extension and mouth opening, our results may not be applicable to other types of difficult airway. Third, all laryngoscopy/intubation procedures were performed by a single anesthesiologist experienced in simulated difficult airways. Caution is required when extrapolating our results to less experienced physicians and/or patients with naturally difficult airways. Finally, as with any crossover study, a carryover effect from the first to the second period was possible. Stress-induced relaxation of the tongue and pharyngeal tissues occurs with laryngoscopy; we randomized the position order to reduce the influence of this phenomenon. Furthermore, our analysis implied that there were no significant carryover effects. Conclusions Compared with a neutral position, use of the back-up position to align the EAM and sternum in the same horizontal plane not only improved glottic visualization but also facilitated successful tracheal intubation, as indicated by the reduced proportion of patients for whom optimization maneuvers were performed during Mac-VL-guided tracheal intubation. Our results imply that the back-up position may be useful for problematic Mac-VL-guided intubation in patients with an unexpected difficult airway. Abbreviations CI: confidence interval; DL: direct laryngoscopy; EAM: external auditory meatus; ELMs: external laryngeal manipulations; HA-VL: hyper-angulated videolaryngoscope; IDS: Intubation Difficulty Scale; Mac-VL: Macintosh-geometry videolaryngoscope; MCL: modified Cormack–Lehane; POGO: percentage of glottic opening; TT: tracheal tube; VL: videolaryngoscope Declarations Ethics approval and consent to participate This study was performed according to the Declaration of Helsinki after the approval of Institutional Review Board of Hallym University Kangnam Sacred Heart Hospital (2020-09-009). Written informed consent was obtained from the enrolled patients before surgery. Consent for publication Not applicable. Availability of data and materials The datasets generated and/or analyzed during the current study are not publicly available due to the regulation of Institutional Review Board, but are available from the corresponding author after getting permission from IRB for sharing the dataset on reasonable request. Competing interests The authors declare that they have no competing interests. Funding This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. H202002950001). The funding body had no influence on study design, data collection and analysis, decision to publish, or preparation of the manuscript Authors’ contributions EHC: concept/design, data collection, drafting article, approval of article.. MHC: data collection, critical revision of article, approval of article. JEK: data collection, critical revision of article, approval of article. KMK: data analysis/interpretation, critical revision of article, approval of article. HSL: data analysis/interpretation, critical revision of article, approval of article. JMS: data collection, critical revision of article, approval of article. JHP: data collection, critical revision of article, approval of article. JHJ: concept/design, critical revision of article, approval of article. All authors read and approved the final manuscript. Acknowledgements Not applicable Author details 1 Department of Anesthesiology and Pain Medicine, Kangnam Sacred Heart Hospital, Hallym University, College of Medicine, Seoul, Korea 2 Department of Anaesthesiology and pain medicine, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea 3 Department of Biostatistics, Yonsei University College of Medicine, Seoul, Korea References Paolini JB, Donati F, Drolet P: Review article: video-laryngoscopy: another tool for difficult intubation or a new paradigm in airway management? 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De Jong A, Pouzeratte Y, Laplace A, Normanno M, Rolle A, Verzilli D, Perrigault PF, Colson P, Capdevila X, Molinari N et al : Macintosh Videolaryngoscope for Intubation in the Operating Room: A Comparative Quality Improvement Project . Anesth Analg 2021, 132 (2):524-535. Taylor AM, Peck M, Launcelott S, Hung OR, Law JA, MacQuarrie K, McKeen D, George RB, Ngan J: The McGrath(R) Series 5 videolaryngoscope vs the Macintosh laryngoscope: a randomised, controlled trial in patients with a simulated difficult airway . Anaesthesia 2013, 68 (2):142-147. Van Zundert A, Wyssusek K: In reference to Review of videolaryngoscopy pharyngeal wall injuries . Laryngoscope 2018, 128 (2):E83. van Zundert A, Pieters B, van Zundert T, Gatt S: Avoiding palatopharyngeal trauma during videolaryngoscopy: do not forget the 'blind spots' . Acta Anaesthesiol Scand 2012, 56 (4):532-534. Ali QE, Das B, Amir SH, Siddiqui OA, Jamil S: Comparison of the Airtraq and McCoy laryngoscopes using a rigid neck collar in patients with simulated difficult laryngoscopy . J Clin Anesth 2014, 26 (3):199-203. Jeon WJ, Shim JH, Cho SY, Baek SJ: Stylet- or forceps-guided tube exchanger to facilitate GlideScope intubation in simulated difficult intubations--a randomised controlled trial . Anaesthesia 2013, 68 (6):585-590. Cook TM, Woodall N, Frerk C, Fourth National Audit P: Major complications of airway management in the UK: results of the Fourth National Audit Project of the Royal College of Anaesthetists and the Difficult Airway Society. Part 1: anaesthesia . Br J Anaesth 2011, 106 (5):617-631. Tables Table 1 Baseline patient and airway characteristics Overall ( n = 64) Sequence 1* ( n = 32) Sequence 2‡ ( n = 32) Age, years 52.7 ± 13.9 53.4 ± 14.9 51.9 ± 13.0 Male/female 38/26 20/12 18/14 BMI, kg/m 2 24.2 ± 2.9 23.6 ± 2.8 24.8 ± 2.8 ASA I/II/III 1/52/11 1/27/4 0/25/7 Mallampati class I/II 26/38 13/19 13/19 TMD, cm 8 (7 to 9) 8 (7 to 9) 8 (7 to 9) ID without collar, mm 40 (40 to 50) 43 (38 to 50) 40 (40 to 48) ID with collar, mm 32 (30 to 40) 30 (30 to 40) 35 (30 to 39) Table-ramp angle, ° 17.5 ± 4.1 17.1 ± 4.2 17.8 ± 4.1 Data are presented as mean ± standard deviation, n , or median (interquartile range). * Sequence 1: back-up position, followed by neutral position. ‡Sequence 2: neutral position, followed by back-up position. BMI, body mass index; TMD, Thyromental distance; ID, inter-incisor distance at maximal mouth opening. Table 2 POGO scores and MCL grades in the neutral and back-up positions Neutral position ( n = 64) Back-up position ( n =64) Difference (95% CI) P -value POGO score, % 37.5 ± 24 59.4 ± 23.8 21.9 (17.1 to 26.7) < 0.0001 MCL grade Easy: Grade 1/2a 11/17 (43.7) 27/28 (85.9) 42.2 (30 to 54.4) < 0.0001 Restricted: Grade 2b/3a 28/8 (56.3) 7/2 (14.1) - Difficult: Grade 3b/4 0/0 (0) 0/0 (0) - Data are presented as mean ± standard deviation or n (%). CI, confidence interval, POGO, percentage of glottic opening; MCL grade, Modified Cormack–Lehane grade. Table 3 Ease of intubation in the second position (during period 2) Neutral position ( n = 32) Back-up position ( n = 32) P -value Patients requiring optimization maneuvers 17 (53.1) 7 (21.9) 0.01 Blade withdrawal 2 (6.2) 2 (6.2) External laryngeal manipulation 11 (34.4) 7 (21.9) Increased lifting force 7 (21.9) 1 (3.1) Tracheal tube bending 2 (6.2) 1 (3.1) Stylet added 4 (12.5) 2 (6.2) Tracheal tube rotation 1 (3.1) 0 (0) Time to intubation, s 26.6 (22.1 to 32.7) 22 (19.4 to 30.1) 0.11 Intubation Difficulty Scale score 1 (1 to 3) 1 (0.5 to 1) 0.13 Data are presented as n (%) or median (interquartile range). Table 4 Carryover and period effects Sequence 1* ( n = 32) Sequence 2‡ ( n = 32) Difference (95% CI) P -value POGO score, % Sequence effect‡ 102.3 ± 52.1 91.4 ± 33.4 10.9 (−11.0 to 32.9) 0.32 Period effect§ 16.4 ± 16.3 27.3 ± 20.4 −10.9 (−20.2 to −1.7) 0.02 Data are presented as mean ± standard deviation. *Sequence 1: back-up position, followed by neutral position. †Sequence 2: neutral position, followed by back-up position. ‡Sequence effect: sum of POGO scores in the first position (period 1) and second position (period 2). §Period effect: difference between POGO scores in the back-up and neutral positions (back-up minus neutral). CI, confidence interval; POGO, percentage of glottic opening. Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-1448707","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":92895995,"identity":"1ebce401-3923-47c5-9f0f-60b5610b83ad","order_by":0,"name":"Joo Hyun Jun","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAuklEQVRIiWNgGAWjYBACNnYGxgNAWo4ELcwMDEAtBsYkWAPVkthAtA4+Zt4DBz62/Unfzn7G8HEFg52cLiHNbMx8CQdnthnk7uzJMTY8w5BsbHaAoBYeg8O8QC0bDqSlSTYwHEjcRpSWv20G6Qbnn6X/JF4LY5tBgsGN5GOMRGoB+qXnnLHhhhuPD0s2GBDhF/n23oMPfpTJyRucT2z82FBhJ0dQCwMDDzLHgKByDC2jYBSMglEwCrAAAG4ePssqUmydAAAAAElFTkSuQmCC","orcid":"","institution":"Department of Anesthesiology and Pain Medicine, Kangnam Sacred Heart Hospital, Hallym University College of Medicine, Seoul","correspondingAuthor":true,"prefix":"","firstName":"Joo","middleName":"Hyun","lastName":"Jun","suffix":""},{"id":92895988,"identity":"654bcb92-3a01-4654-8b86-e7f170b94d08","order_by":1,"name":"Eun Hee Chun","email":"","orcid":"","institution":"Department of Anesthesiology and Pain Medicine, Kangnam Sacred Heart Hospital, Hallym University College of Medicine, Seoul","correspondingAuthor":false,"prefix":"","firstName":"Eun","middleName":"Hee","lastName":"Chun","suffix":""},{"id":92895989,"identity":"c781a4eb-6e80-4c76-84a1-ea5d6184caab","order_by":2,"name":"Mi Hwa Chung","email":"","orcid":"","institution":"Department of Anesthesiology and Pain Medicine, Kangnam Sacred Heart Hospital, Hallym University College of Medicine, Seoul","correspondingAuthor":false,"prefix":"","firstName":"Mi","middleName":"Hwa","lastName":"Chung","suffix":""},{"id":92895990,"identity":"d4d0817f-7672-41d1-bb61-89898e20cc0a","order_by":3,"name":"Jung Eun Kim","email":"","orcid":"","institution":"Department of Anesthesiology and Pain Medicine, Kangnam Sacred Heart Hospital, Hallym University College of Medicine, Seoul","correspondingAuthor":false,"prefix":"","firstName":"Jung","middleName":"Eun","lastName":"Kim","suffix":""},{"id":92895991,"identity":"b11e6ab1-6de9-4936-beae-b38dab976ec3","order_by":4,"name":"Kyung Mi Kim","email":"","orcid":"","institution":"Department of Anesthesiology and pain medicine, Asan Medical Center, University of Ulsan College of Medicine, Seoul","correspondingAuthor":false,"prefix":"","firstName":"Kyung","middleName":"Mi","lastName":"Kim","suffix":""},{"id":92895992,"identity":"95e71357-7f18-4153-97de-ad7b100689bb","order_by":5,"name":"Hye Sun Lee","email":"","orcid":"","institution":"Department of Biostatistics, Yonsei University College of Medicine, Seoul","correspondingAuthor":false,"prefix":"","firstName":"Hye","middleName":"Sun","lastName":"Lee","suffix":""},{"id":92895993,"identity":"92ee2d1a-8045-4ae9-a169-4c6a96fa9e74","order_by":6,"name":"Jung Mo Son","email":"","orcid":"","institution":"Department of Anesthesiology and Pain Medicine, Kangnam Sacred Heart Hospital, Hallym University College of Medicine, Seoul","correspondingAuthor":false,"prefix":"","firstName":"Jung","middleName":"Mo","lastName":"Son","suffix":""},{"id":92895994,"identity":"9d1bf709-936c-46c0-bbc3-cb40b7d9bae3","order_by":7,"name":"Jiho Park","email":"","orcid":"","institution":"Department of Anesthesiology and Pain Medicine, Kangnam Sacred Heart Hospital, Hallym University College of Medicine, Seoul","correspondingAuthor":false,"prefix":"","firstName":"Jiho","middleName":"","lastName":"Park","suffix":""}],"badges":[],"createdAt":"2022-03-14 06:29:16","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-1448707/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-1448707/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":19583284,"identity":"30ae0057-8ecd-41b8-9f2b-32651b7381f1","added_by":"auto","created_at":"2022-03-24 19:05:22","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":18449,"visible":true,"origin":"","legend":"\u003cp\u003eStudy flow chart.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-1448707/v1/bae91e3bc1c82a29fdb8ad93.png"},{"id":19583282,"identity":"3fa95d1c-1d78-4aba-a4c7-4667d3dcccac","added_by":"auto","created_at":"2022-03-24 19:05:20","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":10884,"visible":true,"origin":"","legend":"\u003cp\u003eLaryngeal view based on modified Cormack–Lehane (MCL) grades in the neutral and back-up positions. E = easy laryngeal view (MCL grade of 1 or 2a); R = restricted laryngeal view (MCL grade of 2b or 3a); D = difficult laryngeal view (MCL grade of 3b or 4); Blue lines = improved laryngeal view in the back-up position; green lines = no difference in laryngeal view between positions\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-1448707/v1/28c53c64bba7c17683490edd.png"},{"id":19583278,"identity":"1adc815a-b875-4dec-acff-4bb64a83c175","added_by":"auto","created_at":"2022-03-24 19:05:17","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":21988,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in the modified Cormack–Lehane (MCL) grade in the back-up position compared to the neutral position. Blue lines = improved MCL grade in the back-up position; green lines = no difference in MCL grade between positions. MCL grade did not worsen in the back-up position in any patient.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-1448707/v1/938cdbb5052ef119fd08d954.png"},{"id":19703041,"identity":"3dfa3024-3878-4187-a05e-4d06058c3c77","added_by":"auto","created_at":"2022-03-28 20:06:14","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1344949,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-1448707/v1/4ac3152b-4892-4f2b-b9b3-33cc146bbdea.pdf"},{"id":19583204,"identity":"87bce475-7303-4c96-b286-3caa2e9e6dea","added_by":"auto","created_at":"2022-03-24 19:04:22","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":16556,"visible":true,"origin":"","legend":"","description":"","filename":"aditioanlfile1.docx","url":"https://assets-eu.researchsquare.com/files/rs-1448707/v1/203912373a2b59c624e9dabd.docx"},{"id":19583267,"identity":"4d326f45-345e-4ded-b3da-8f4440087e47","added_by":"auto","created_at":"2022-03-24 19:04:50","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":16160,"visible":true,"origin":"","legend":"","description":"","filename":"aditioanlfile2.docx","url":"https://assets-eu.researchsquare.com/files/rs-1448707/v1/2fab4e3f68471fcf842dc61f.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effects of back-up position on tracheal intubation using a Macintosh-geometry videolaryngoscope in patients with a simulated difficult airway: A prospective randomized crossover study","fulltext":[{"header":"Background","content":"\u003cp\u003eOwing to the many advances that have resulted in improved laryngeal views, reduced intubation difficulty, and decreased incidence of failed intubation, videolaryngoscopes (VLs) are increasingly being used as first-line tools not only for anticipated difficult tracheal intubation but also as a substitute for direct laryngoscopy (DL), even in routine airway care [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. These changes are being rapidly implemented due to the coronavirus disease 2019 (COVID-19) pandemic, because of the perceived risk of infection due to possible aerosolization during the intubation process [\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThere are several types of VLs, with different device designs and blade geometries. Each VL has unique features that may be advantageous under some conditions but disadvantageous under others [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. VLs have two main blade types: hyper-angulated (HA-VL) and Macintosh-geometry (Mac-VL). The HA-VL, originally designed for use in difficult airways, provides an improved view of the larynx with minimal head and neck manipulation. However, the HA-VL also prevents direct visualization of the larynx and has the potential to make it difficult to guide the tracheal tube (TT) towards the glottis despite obtaining a good laryngeal view [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. In comparison, Mac-VL can be used to perform both indirect and direct laryngoscopy. The TT is inserted in much the same way as in DL, with or without a stylet. Combining the benefits of DL and VL in one device, the Mac-VL can serve as a standard intubation device for both routine airway management and educational purposes [\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. However, the Mac-VL may not provide as dramatic an improvement in the laryngeal view as the HA-VL because it lacks the capacity to view structures around corners, so glottic exposure using the Mac-VL may be incomplete in unexpected difficult airways [\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOptimal patient positioning maximizes the likelihood of successful laryngoscopy and tracheal intubation [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. However, the ideal head and neck position for VL has not yet been evaluated. The back-up position during DL improves the laryngeal view, lessening the need for assisted maneuvers and shortening the intubation time [\u003cspan additionalcitationids=\"CR15\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Moreover, placing the patient in a back-up position is a simple maneuver to improve safety during tracheal intubation [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Magnetic resonance imaging studies showed that head elevation to bring the patient\u0026rsquo;s sternum onto the horizontal plane of the external auditory meatus (EAM) reduces the angle of the pharyngeal and laryngeal axes, thus demonstrating how the back-up position improved the laryngeal view and ease of intubation during DL [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWe hypothesized that improved alignment of the pharyngeal and laryngeal axes using the back-up position (to align the sternal notch and EAM) would help optimize the laryngeal view and facilitate tracheal intubation during Mac-VL-guided intubation Therefore, this study investigated the effect of the back-up position on laryngeal visualization (primary outcome) and ease of tracheal intubation (secondary outcome) using McGrath MAC VL with a Macintosh-geometry blade in simulated difficult airway with a cervical collar to restrict neck movement and mouth opening.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eThis was a prospective, crossover, randomized controlled trial of a simulated difficult airway with concurrent limited mouth opening and neck movement. Ethical approval for this study was approved by the institutional review board of Hallym University Kangnam Sacred Heart Hospital (approval number: 2020-09-009). The trial was registered prior to patient enrolment at \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e\u003ca href=\"http://www.clinicaltrials.gov\" target=\"_blank\"\u003ewww.clinicaltrials.gov\u003c/a\u003e\u003c/span\u003e\u003cspan address=\"http://www.clinicaltrials.gov\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (NCT04716218) on 20/01/2021, and conducted and reported according to the Consolidating Standards of Reporting Trials (CONSORT) 2010 statement [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. This study was conducted from January 2021 to March 2021. We recruited 64 adult patients undergoing general anesthesia with tracheal intubation for elective surgery. All patients provided written informed consent and had ASA physical status of I\u0026ndash;III. The exclusion criteria were aspiration risk, bleeding tendency, uncontrolled hypertension, obstructive sleep apnea, and a known or anticipated difficult airway (e.g. Mallampati class III\u0026ndash;IV, body mass index\u0026thinsp;\u0026gt;\u0026thinsp;35 kg/m\u003csup\u003e2\u003c/sup\u003e, inter-incisor distance\u0026thinsp;\u0026lt;\u0026thinsp;3.5 cm, thyromental distance\u0026thinsp;\u0026lt;\u0026thinsp;6 cm).\u003c/p\u003e \u003cp\u003eUsing a computer-generated randomization table (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e\u003ca href=\"http://www.clinicaltrials.gov\" target=\"_blank\"\u003ewww.randomizer.org\u003c/a\u003e\u003c/span\u003e\u003cspan address=\"http://www.randomizer.org\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e), the enrolled patients were randomly assigned to one of the two sequences. In sequence 1, the back-up position was used first, followed by the neutral position. In sequence 2, the neutral position was used first, followed by the back-up position. The sequence allocation was concealed in an opaque envelope, which was opened by the investigator responsible for randomization immediately before the initiation of anesthesia.\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eStudy protocol\u003c/h2\u003e \u003cp\u003eA McGrath\u0026trade; MAC VL (Aircraft Medical, Ltd., Edinburgh, UK) with a curved blade (resembling a Macintosh blade) and a camera was set up according to the standard practice at our institution. Tracheal intubation was performed using a size 3 McGrath\u0026trade; MAC VL blade, and a TT with an internal diameter of 6.5 mm for females and 7.5 mm for males (Mallinckrodt, St. Louis, MO, USA). A malleable stylet was not initially used to facilitate tracheal intubation, because the McGrath MAC VL had a Macintosh-geometry blade [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAll patients were premedicated with glycopyrrolate (0.2 mg) at least 30 min before initiation of anesthesia. After entering the operating room, each patient was placed in the supine position on the operating table, with their head on a soft pillow. Standard monitoring (continuous pulse oximetry, electrocardiography, capnography, and non-invasive blood pressure) was applied.\u003c/p\u003e \u003cp\u003eAfter removing the pillow, an observer blinded to the sequence assignment assessed the airway based on the Mallampati class, thyromental distance, and inter-incisor distance at maximal mouth opening. Then, a semi-rigid cervical collar (Philadelphia Cervical Collar Co., Thorofare, NJ, USA) of appropriate size was placed around the patient\u0026rsquo;s neck, and the inter-incisor distance at maximal mouth opening was re-measured. The back-up position was created by flexing the operating table at the trunk\u0026ndash;thigh hinge and then raising the back section. For each patient, a digital inclinometer and tripod water level were used to determine the table ramp angle required to align the sternal notch and EAM in the same horizontal plane.\u003c/p\u003e \u003cp\u003eAnesthesia was induced with propofol (1.5 mg/kg) and remifentanil (0.5\u0026ndash;1 \u0026micro;g/kg), followed by rocuronium (0.6 mg/kg). After verifying neuromuscular blockade using a nerve stimulator, tracheal intubation was performed using the McGrath MAC VL according to the assigned sequence. In sequence 1, the initial assessment of laryngeal view was performed in the back-up position (end of period 1). Subsequently, the patient was placed in the neutral position and the second assessment of laryngeal view was performed (start of period 2). The trachea was then intubated in the second position. For the sequence 2, the procedure was reversed. Thus, the laryngeal view was assessed in both the neutral and back-up positions, but the trachea was intubated only once (in the second position). When assessing laryngeal visualization, external laryngeal manipulations (ELMs) were not permitted. To eliminate interobserver variation, a single experienced anesthesiologist (\u0026gt;\u0026thinsp;100 previous tracheal intubations using the McGrath MAC VL) performed all VL procedures.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eOutcome measurement\u003c/h2\u003e \u003cp\u003eThe following data were collected by two investigators (Eun Hee Chun and Joo Hyun Jun) not involved in the VL procedures.\u003c/p\u003e \u003cp\u003eThe primary outcome was the laryngeal view in both the neutral and back-up positions (periods 1 and 2, respectively), which was assessed according to the percentage of glottic opening (POGO) score and modified Cormack\u0026ndash;Lehane (MCL) grade. The POGO score reflects the proportion of the glottic area that is visible: a score of 100% denotes visualization of the whole glottis, from the inter-arytenoid notch to the anterior commissure, whereas a score of 0% denotes visualization of none of the glottis [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Based on the MCL grade, the laryngeal view was classified as easy (laryngeal inlet visible; MCL grade 1 or 2a), restricted (posterior glottic structures or epiglottis visible, where the latter could be lifted; MCL grade 2b or 3a), and difficult (epiglottis could not be lifted or no laryngeal structures visible; MCL grade 3b or 4) [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. The data collection form included illustrations of the MCL grades and POGO scores to promote standardization.\u003c/p\u003e \u003cp\u003eThe secondary outcome was the ease of intubation, which was measured in the second position (period 2). To evaluate this outcome, we recorded the optimization maneuvers used for successful tracheal intubation, such as withdrawing and reinserting the blade, increasing the lifting force, applying ELMs, bending the TT into a steeper curve, adding a stylet, or rotating the TT during passage into the trachea to avoid impacting the anterior wall of the subglottic space [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. The time to intubation, Intubation Difficulty Scale (IDS) score [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e], reasons for failed intubation, and adverse effects were also recorded. Time to intubation represented the time between insertion into and removal of the VL blade from the mouth.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eStatistical analysis\u003c/h2\u003e \u003cp\u003eStatistical analyses were performed using SAS (version 9.4; SAS Institute, Cary, NC, USA), SPSS (version 27.0; IBM Corp., Armonk, NY, USA), and R software (version 4.1.0; \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://www.R-project.org\u003c/span\u003e\u003cspan address=\"http://www.R-project.org\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e). The distribution of continuous data was evaluated using the Shapiro\u0026ndash;Wilk test. Normally distributed continuous variables are provided as the mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation and were analyzed using the paired and independent \u003cem\u003et-tests\u003c/em\u003e. Non-normally distributed continuous variables are provided as the median (interquartile range) and were analyzed using Wilcoxon\u0026rsquo;s signed rank and Mann\u0026ndash;Whitney U tests. Categorical data are expressed as \u003cem\u003en\u003c/em\u003e (%) for proportions and were compared using the McNemar and χ\u003csup\u003e2\u003c/sup\u003e tests, with bootstrapping applied as appropriate. To evaluate possible carryover effects, the sum of the POGO scores in the first position (period 1) and second position (period 2) was calculated for each subject and compared across the two sequences using the unpaired \u003cem\u003et\u003c/em\u003e test. To evaluate possible period effects, the difference in POGO scores between the two periods was calculated for each subject and compared across the two sequences using the unpaired \u003cem\u003et-test\u003c/em\u003e [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. In all analyses, \u003cem\u003eP\u003c/em\u003e \u0026lt;0.05 was considered statistically significant.\u003c/p\u003e \u003cp\u003eWe calculated the sample size based on the POGO score using PASS software (version 15.0; NCSS, LLC, Kaysville, UT, USA). Assuming that a 20% difference in POGO score was clinically important (standard deviation of 35%) [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e], we determined that 28 pairs were required in each sequence for two-sided testing in this 2 \u0026times; 2 crossover study, with 95% power and an alpha level of 5%. Therefore, we enrolled 32 participants in each sequence group, assuming a dropout rate of 10%.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cp\u003eIn total, 68 potential participants were screened between January 2021 and March 2021. Three patients were excluded because of mobile teeth (\u003cem\u003en\u0026nbsp;\u003c/em\u003e= 2) or uncontrolled hypertension (\u003cem\u003en\u0026nbsp;\u003c/em\u003e= 1), and one declined to participate in the study. The remaining 64 patients were randomized (32 per group) and included in the analyses (Fig. 1). Table 1 shows the characteristics of the patients (including the airway). After applying the cervical collar, neck motion was severely restricted, and median inter-incisor distance at maximal mouth opening decreased significantly from 40 (40 to 50) to 32 (30 to 35) mm (\u003cem\u003eP\u003c/em\u003e \u0026lt;0.0001). The mean table ramp angle for aligning the EAM and sternal notch in the back-up position was 17.5 \u0026plusmn; 4.1\u0026deg;.\u0026nbsp;\u003c/p\u003e\n\u003ch2\u003e\u003cstrong\u003ePrimary outcome\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eCompared\u0026nbsp;with\u0026nbsp;the neutral position, the back-up position provided significantly better or similar laryngeal\u0026nbsp;visualization.\u0026nbsp;The mean POGO score improved significantly\u0026nbsp;in the back-up position (59.4 \u0026plusmn; 23.8%) compared with\u0026nbsp;the neutral position\u0026nbsp;(37.5 \u0026plusmn; 24%)\u0026nbsp;(difference\u0026nbsp;in means,\u0026nbsp;21.9%; 95% confidence interval [CI] 17.1% to 26.7%; P\u0026nbsp;\u0026lt;\u0026nbsp;0.0001). An easy laryngeal view (MCL grade 1 or 2a) was achieved in 56 patients\u0026nbsp;(85.9%)\u0026nbsp;in the back-up position and only 28 (43.7%) in the neutral position\u0026nbsp;(difference\u0026nbsp;in proportions,\u0026nbsp;42.2%; 95% CI 30% to 54.4%;\u0026nbsp;P\u0026nbsp;\u0026lt;\u0026nbsp;0.0001) (Fig. 2, Table 2). In 38 of 64 patients (59.4%), the back-up position improved the glottic view by one or two MCL grades, while the MCL grade was similar between positions in the remaining 26 patients (40.6%). The back-up position did not worsen the MCL grade in any patient (Fig. 3, additional file 1).\u0026nbsp;\u003c/p\u003e\n\u003ch2\u003e\u003cstrong\u003eSecondary outcome\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eThe results of intubation in the second position (period 2) are summarized in Table 3. Intubation was successful on the first attempt in all patients. The proportion of patients requiring optimization maneuver(s) for tracheal intubation was higher in the neutral position (\u003cem\u003en\u0026nbsp;\u003c/em\u003e= 17; 53%) than in the back-up position (\u003cem\u003en\u003c/em\u003e = 7; 22%) (\u003cem\u003eP\u003c/em\u003e =0.01). There were no significant differences between positions in median time to intubation (\u003cem\u003eP\u003c/em\u003e = 0.11) or median IDS score (\u003cem\u003eP\u003c/em\u003e = 0.13). Slight mucosal bleeding attributed to laryngoscopy or tracheal intubation was noted in one patient. No major complications (e.g. dental damage and, palatal perforation) were observed.\u003c/p\u003e\n\u003ch2\u003e\u003cstrong\u003eCarryover and period effect\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eThere was no apparent carryover effect for the primary outcome (POGO score; \u003cem\u003eP\u003c/em\u003e=0.32). However, a period effect was observed for the POGO score (\u003cem\u003eP\u003c/em\u003e =\u0026nbsp;0.03) (Table 4). Although the difference in POGO scores between the neutral and back-up positions was lower in period 2 than period 1, the POGO scores remained better in the back-up position than in the neutral position during period 2 (\u003cem\u003eP\u003c/em\u003e=0.008) (additional file 2).\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eOur study evaluated laryngeal visualization and ease of tracheal intubation in the back-up position using an individualized table-ramp angle (17.5\u0026thinsp;\u0026plusmn;\u0026thinsp;4.1\u0026deg;) degrees) to align the sternal notch and EAM. Use of this individualized approach led to significantly improved laryngeal visualization in the back-up position, compared to the neutral position, when using the Mac-VL in patients with a simulated difficult airway. Specifically, the mean POGO score improved from 37.5\u0026thinsp;\u0026plusmn;\u0026thinsp;24 to 59.4\u0026thinsp;\u0026plusmn;\u0026thinsp;23.8%, and the proportion of patients with an easy (MCL grade 1 or 2a) view increased from 43.7\u0026ndash;85.9%. This position was also associated with reduced use of optimization maneuvers during Mac-VL-guided tracheal intubation. In the current study, these maneuvers were used in only 21.9% of patients in the back-up position, in contrast to 53.1% of patients in the neutral position. However, the time to intubation and IDS score did not differ significantly between positions.\u003c/p\u003e \u003cp\u003eLaryngeal exposure in the neutral position in this study was inferior to that in Klein-Brueggeney et al., who used the same difficult airway scenario (application of a cervical collar to limit mouth opening and neck movement), where an easy laryngeal view (MCL grade 1 or 2a) was achieved in 110/180 patients (91.6%) [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. In the neutral position, the nasopharynx and upper part of the laryngeal vestibule are below the glottis [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], which can lead to a more anterior laryngeal position. In this situation, optimization maneuvers, such as ELM or upward and forward lifting force of the blade, are necessary to raise the epiglottis and visualize the laryngeal opening during laryngoscopy [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. However, we reported a \u0026ldquo;first achieved view\u0026rdquo; that did not allow these maneuvers during scoring of laryngeal visualization, to independently evaluate the effects of changes in airway anatomy associated with positional changes on the laryngeal view, whereas Klein-Brueggeney et al.[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e] reported the \u0026ldquo;best achievable view\u0026rdquo;. This difference in laryngeal exposure between our study and that of Klein-Brueggeney et al. may reflect the fact that Mac-VL requires optimization maneuvers to improve airway axis alignment, to obtain an optimal laryngeal view in cases with a genuine difficult airway.\u003c/p\u003e \u003cp\u003eLikewise, in a recent study using a Mac-VL as the primary intubation tool, the optimal view of the larynx to facilitate successful tracheal intubation was not achieved in 12\u0026ndash;30% of cases, thereby requiring switching to a HA-VL [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. However, previous studies comparing HA-VL and Mac-VL in simulated or \u0026ldquo;predicted\u0026rdquo; airways imply that the enhanced laryngeal view does not always lead to easier or quicker tracheal intubation [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. A similar mismatch was also observed in a previous study that used an HA-VL (McGrath Series 5) in a simulated difficult airway with in-line stabilization of the neck, which provided a better laryngeal view but had a longer time to intubation (36 s) than in the present study (22\u0026ndash;26 s) [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. A HA-VL requires that a styleted TT be bent at an acute angle to match the blade\u0026rsquo;s curvature, which can contribute to an increased time to intubation [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. It also results in partly blind oropharyngeal passage of the styleted TT; this may increase the risk of soft-tissue injury, such as oropharyngeal perforation [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. By contrast, the lower angulation of the Mac-VL provides more room to manipulate the TT, allowing easier intubation and reducing the risk of trauma [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. Therefore, when a Mac-VL is used for routine intubation, the back-up position may improve safety by obviating the need for conversion to alternative techniques (i.e. use of a hyper-angulated blade).\u003c/p\u003e \u003cp\u003eIndirect laryngoscopy with a VL does not require a direct line of sight to the laryngeal opening, which is helpful for difficult tracheal intubation but complicates TT insertion [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. This is especially problematic for HA-VL, but can also be an issue for the Mac-VL [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. This may also prevent VLs from becoming the standard of care for all tracheal intubations. To compensate for their geometric limitations, some authors have suggested deliberate worsening of the view (by withdrawing the VL blade) to reduce the TT introduction angle, thereby facilitating intubation [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. However, this strategy negates the perceived advantage of VL, i.e. a good laryngeal view. In contrast, the back-up position used in the present study allowed relatively straight passage of the TT and facilitated successful tracheal intubation, as determined based on the need for optimization maneuvers without compromising laryngeal visualization.\u003c/p\u003e \u003cp\u003eThis study has some limitations. First, we enrolled patients with a simulated difficult airway. Although the use of a cervical collar is common when simulating difficult airways for research purposes [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e], caution is required when extrapolating the results to genuine difficult airways. However, we did not enroll patients with genuine difficult airways because this is a rare and possibly life-threatening condition [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Second, as our study was performed in the specific scenario of limited neck extension and mouth opening, our results may not be applicable to other types of difficult airway. Third, all laryngoscopy/intubation procedures were performed by a single anesthesiologist experienced in simulated difficult airways. Caution is required when extrapolating our results to less experienced physicians and/or patients with naturally difficult airways. Finally, as with any crossover study, a carryover effect from the first to the second period was possible. Stress-induced relaxation of the tongue and pharyngeal tissues occurs with laryngoscopy; we randomized the position order to reduce the influence of this phenomenon. Furthermore, our analysis implied that there were no significant carryover effects.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eCompared with a neutral position, use of the back-up position to align the EAM and sternum in the same horizontal plane not only improved glottic visualization but also facilitated successful tracheal intubation, as indicated by the reduced proportion of patients for whom optimization maneuvers were performed during Mac-VL-guided tracheal intubation. Our results imply that the back-up position may be useful for problematic Mac-VL-guided intubation in patients with an unexpected difficult airway.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eCI: confidence interval; DL: direct laryngoscopy; EAM: external auditory meatus; ELMs: external laryngeal manipulations; HA-VL: hyper-angulated videolaryngoscope; IDS: Intubation Difficulty Scale; Mac-VL: Macintosh-geometry videolaryngoscope; MCL: modified Cormack\u0026ndash;Lehane; POGO: percentage of glottic opening; TT: tracheal tube; VL: videolaryngoscope\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was performed according to the Declaration of Helsinki after the approval of Institutional Review Board of Hallym University Kangnam Sacred Heart Hospital (2020-09-009). Written informed consent was obtained from the enrolled patients before surgery.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets generated and/or analyzed during the current study are not publicly available due to the regulation of Institutional Review Board, but are available from the corresponding author after getting permission from IRB for sharing the dataset on reasonable request.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. H202002950001). The funding body had no influence on study design, data collection and analysis, decision to publish, or preparation of the manuscript\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026rsquo; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eEHC: concept/design, data collection, drafting article, approval of article.. MHC: data collection, critical revision of article, approval of article. JEK: data collection, critical revision of article, approval of article. KMK: data analysis/interpretation, critical revision of article, approval of article. HSL: data analysis/interpretation, critical revision of article, approval of article. JMS: data collection, critical revision of article, approval of article. JHP: data collection, critical revision of article, approval of article. JHJ: concept/design, critical revision of article, approval of article. All authors read and approved the final manuscript.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor details\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003csup\u003e1\u003c/sup\u003e Department of Anesthesiology and Pain Medicine, Kangnam Sacred Heart Hospital, Hallym University, College of Medicine, Seoul, Korea\u003c/p\u003e\n\u003cp\u003e\u003csup\u003e2\u003c/sup\u003e Department of Anaesthesiology and pain medicine, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea\u003c/p\u003e\n\u003cp\u003e\u003csup\u003e3\u0026nbsp;\u003c/sup\u003eDepartment of Biostatistics, Yonsei University College of Medicine, Seoul, Korea\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003e Paolini JB, Donati F, Drolet P: 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Siemonsma M, Buise M, Wiepking M: \u003cstrong\u003eA Macintosh laryngoscope blade for videolaryngoscopy reduces stylet use in patients with normal airways\u003c/strong\u003e. \u003cem\u003eAnesth Analg\u003c/em\u003e2009, \u003cstrong\u003e109\u003c/strong\u003e(3):825-831.\u003c/li\u003e\n \u003cli\u003e Ochroch EA, Hollander JE, Kush S, Shofer FS, Levitan RM: \u003cstrong\u003eAssessment of laryngeal view: percentage of glottic opening score vs Cormack and Lehane grading\u003c/strong\u003e. \u003cem\u003eCan J Anaesth\u003c/em\u003e1999, \u003cstrong\u003e46\u003c/strong\u003e(10):987-990.\u003c/li\u003e\n \u003cli\u003e Cook TM: \u003cstrong\u003eA new practical classification of laryngeal view\u003c/strong\u003e. \u003cem\u003eAnaesthesia\u003c/em\u003e2000, \u003cstrong\u003e55\u003c/strong\u003e(3):274-279.\u003c/li\u003e\n \u003cli\u003e Kramer DC, Osborn IP: \u003cstrong\u003eMore maneuvers to facilitate tracheal intubation with the GlideScope\u003c/strong\u003e. \u003cem\u003eCan J Anaesth\u003c/em\u003e2006, \u003cstrong\u003e53\u003c/strong\u003e(7):737.\u003c/li\u003e\n \u003cli\u003e Kwak HJ, Lee SY, Lee SY, Kim YB, Kim JY: \u003cstrong\u003eIntubation without use of stylet for McGrath videolaryngoscopy in patients with expected normal airway: A randomized noninferiority trial\u003c/strong\u003e. \u003cem\u003eMedicine\u003c/em\u003e2016, \u003cstrong\u003e95\u003c/strong\u003e(48):e5498.\u003c/li\u003e\n \u003cli\u003e Adnet F, Borron SW, Racine SX, Clemessy JL, Fournier JL, Plaisance P, Lapandry C: \u003cstrong\u003eThe intubation difficulty scale (IDS): proposal and evaluation of a new score characterizing the complexity of endotracheal intubation\u003c/strong\u003e. \u003cem\u003eAnesthesiology\u003c/em\u003e1997, \u003cstrong\u003e87\u003c/strong\u003e(6):1290-1297.\u003c/li\u003e\n \u003cli\u003e Wellek S, Blettner M: \u003cstrong\u003eOn the proper use of the crossover design in clinical trials: part 18 of a series on evaluation of scientific publications\u003c/strong\u003e. \u003cem\u003eDtsch Arztebl Int\u003c/em\u003e2012, \u003cstrong\u003e109\u003c/strong\u003e(15):276-281.\u003c/li\u003e\n \u003cli\u003e Kim EH, Lee JH, Song IK, Kim JT, Kim BR, Kim HS: \u003cstrong\u003eEffect of head position on laryngeal visualisation with the McGrath MAC videolaryngoscope in paediatric patients: A randomised controlled trial\u003c/strong\u003e. \u003cem\u003eEur J Anaesthesiol\u003c/em\u003e2016, \u003cstrong\u003e33\u003c/strong\u003e(7):528-534.\u003c/li\u003e\n \u003cli\u003e Kleine-Brueggeney M, Greif R, Schoettker P, Savoldelli GL, Nabecker S, Theiler LG: \u003cstrong\u003eEvaluation of six videolaryngoscopes in 720 patients with a simulated difficult airway: a multicentre randomized controlled trial\u003c/strong\u003e. \u003cem\u003eBr J Anaesth\u003c/em\u003e2016, \u003cstrong\u003e116\u003c/strong\u003e(5):670-679.\u003c/li\u003e\n \u003cli\u003e Nam K, Lee Y, Park HP, Chung J, Yoon HK, Kim TK: \u003cstrong\u003eCervical Spine Motion During Tracheal Intubation Using an Optiscope Versus the McGrath Videolaryngoscope in Patients With Simulated Cervical Immobilization: A Prospective Randomized Crossover Study\u003c/strong\u003e. \u003cem\u003eAnesth Analg\u003c/em\u003e2019, \u003cstrong\u003e129\u003c/strong\u003e(6):1666-1672.\u003c/li\u003e\n \u003cli\u003e Kaplan MB, Hagberg CA, Ward DS, Brambrink A, Chhibber AK, Heidegger T, Lozada L, Ovassapian A, Parsons D, Ramsay J\u003cem\u003eet al\u003c/em\u003e: \u003cstrong\u003eComparison of direct and video-assisted views of the larynx during routine intubation\u003c/strong\u003e. \u003cem\u003eJ Clin Anesth\u003c/em\u003e2006, \u003cstrong\u003e18\u003c/strong\u003e(5):357-362.\u003c/li\u003e\n \u003cli\u003e De Jong A, Pouzeratte Y, Laplace A, Normanno M, Rolle A, Verzilli D, Perrigault PF, Colson P, Capdevila X, Molinari N\u003cem\u003eet al\u003c/em\u003e: \u003cstrong\u003eMacintosh Videolaryngoscope for Intubation in the Operating Room: A Comparative Quality Improvement Project\u003c/strong\u003e. \u003cem\u003eAnesth Analg\u003c/em\u003e2021, \u003cstrong\u003e132\u003c/strong\u003e(2):524-535.\u003c/li\u003e\n \u003cli\u003e Taylor AM, Peck M, Launcelott S, Hung OR, Law JA, MacQuarrie K, McKeen D, George RB, Ngan J: \u003cstrong\u003eThe McGrath(R) Series 5 videolaryngoscope vs the Macintosh laryngoscope: a randomised, controlled trial in patients with a simulated difficult airway\u003c/strong\u003e. \u003cem\u003eAnaesthesia\u003c/em\u003e2013, \u003cstrong\u003e68\u003c/strong\u003e(2):142-147.\u003c/li\u003e\n \u003cli\u003e Van Zundert A, Wyssusek K: \u003cstrong\u003eIn reference to Review of videolaryngoscopy pharyngeal wall injuries\u003c/strong\u003e. \u003cem\u003eLaryngoscope\u003c/em\u003e2018, \u003cstrong\u003e128\u003c/strong\u003e(2):E83.\u003c/li\u003e\n \u003cli\u003e van Zundert A, Pieters B, van Zundert T, Gatt S: \u003cstrong\u003eAvoiding palatopharyngeal trauma during videolaryngoscopy: do not forget the \u0026apos;blind spots\u0026apos;\u003c/strong\u003e. \u003cem\u003eActa Anaesthesiol Scand\u003c/em\u003e2012, \u003cstrong\u003e56\u003c/strong\u003e(4):532-534.\u003c/li\u003e\n \u003cli\u003e Ali QE, Das B, Amir SH, Siddiqui OA, Jamil S: \u003cstrong\u003eComparison of the Airtraq and McCoy laryngoscopes using a rigid neck collar in patients with simulated difficult laryngoscopy\u003c/strong\u003e. \u003cem\u003eJ Clin Anesth\u003c/em\u003e2014, \u003cstrong\u003e26\u003c/strong\u003e(3):199-203.\u003c/li\u003e\n \u003cli\u003e Jeon WJ, Shim JH, Cho SY, Baek SJ: \u003cstrong\u003eStylet- or forceps-guided tube exchanger to facilitate GlideScope intubation in simulated difficult intubations--a randomised controlled trial\u003c/strong\u003e. \u003cem\u003eAnaesthesia\u003c/em\u003e2013, \u003cstrong\u003e68\u003c/strong\u003e(6):585-590.\u003c/li\u003e\n \u003cli\u003e Cook TM, Woodall N, Frerk C, Fourth National Audit P: \u003cstrong\u003eMajor complications of airway management in the UK: results of the Fourth National Audit Project of the Royal College of Anaesthetists and the Difficult Airway Society. Part 1: anaesthesia\u003c/strong\u003e. \u003cem\u003eBr J Anaesth\u003c/em\u003e2011, \u003cstrong\u003e106\u003c/strong\u003e(5):617-631.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cdiv align=\"center\"\u003e\n \u003ctable border=\"1\" cellpadding=\"0\" cellspacing=\"0\" width=\"99%\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"4\" valign=\"top\" width=\"98.01980198019803%\"\u003e\n \u003cp style=\"text-align: center;\"\u003e\u003cstrong\u003eTable 1\u003c/strong\u003e\u003c/p\u003e\n \u003cp style=\"text-align: center;\"\u003eBaseline patient and airway characteristics\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"1.9801980198019802%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"31.95876288659794%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"22.68041237113402%\"\u003e\n \u003cp\u003eOverall\u003c/p\u003e\n \u003cp\u003e(\u003cem\u003en\u0026nbsp;\u003c/em\u003e=\u0026nbsp;64)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"22.68041237113402%\"\u003e\n \u003cp\u003eSequence 1*\u003c/p\u003e\n \u003cp\u003e(\u003cem\u003en\u0026nbsp;\u003c/em\u003e=\u0026nbsp;32)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\" width=\"22.68041237113402%\"\u003e\n \u003cp\u003eSequence 2\u0026Dagger;\u003c/p\u003e\n \u003cp\u003e(\u003cem\u003en\u0026nbsp;\u003c/em\u003e=\u0026nbsp;32)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"31.95876288659794%\"\u003e\n \u003cp\u003eAge, years\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e52.7 \u0026plusmn;\u0026nbsp;13.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e53.4 \u0026plusmn;\u0026nbsp;14.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" width=\"22.68041237113402%\"\u003e\n \u003cp\u003e51.9 \u0026plusmn;\u0026nbsp;13.0\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"31.95876288659794%\"\u003e\n \u003cp\u003eMale/female\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e38/26\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e20/12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" width=\"22.68041237113402%\"\u003e\n \u003cp\u003e18/14\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"31.95876288659794%\"\u003e\n \u003cp\u003eBMI, kg/m\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e24.2 \u0026plusmn;\u0026nbsp;2.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e23.6 \u0026plusmn;\u0026nbsp;2.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" width=\"22.68041237113402%\"\u003e\n \u003cp\u003e24.8 \u0026plusmn;\u0026nbsp;2.8\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"31.95876288659794%\"\u003e\n \u003cp\u003eASA I/II/III\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e1/52/11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e1/27/4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" width=\"22.68041237113402%\"\u003e\n \u003cp\u003e0/25/7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"31.95876288659794%\"\u003e\n \u003cp\u003eMallampati class I/II\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e26/38\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e13/19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" width=\"22.68041237113402%\"\u003e\n \u003cp\u003e13/19\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"31.95876288659794%\"\u003e\n \u003cp\u003eTMD, cm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e8 (7 to 9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e8 (7 to 9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" width=\"22.68041237113402%\"\u003e\n \u003cp\u003e8 (7 to 9)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"31.95876288659794%\"\u003e\n \u003cp\u003eID without collar,\u0026nbsp;mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e40 (40 to 50)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e43 (38 to 50)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" width=\"22.68041237113402%\"\u003e\n \u003cp\u003e40 (40 to 48)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"31.95876288659794%\"\u003e\n \u003cp\u003eID with collar,\u0026nbsp;mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e32 (30 to 40)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e30 (30 to 40)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" width=\"22.68041237113402%\"\u003e\n \u003cp\u003e35 (30 to 39)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"31.95876288659794%\"\u003e\n \u003cp\u003eTable-ramp angle, \u0026deg;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e17.5 \u0026plusmn;\u0026nbsp;4.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.68041237113402%\"\u003e\n \u003cp\u003e17.1 \u0026plusmn;\u0026nbsp;4.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" width=\"22.68041237113402%\"\u003e\n \u003cp\u003e17.8 \u0026plusmn;\u0026nbsp;4.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"4\" valign=\"top\" width=\"98.01980198019803%\"\u003e\n \u003cp\u003eData are presented as mean \u0026plusmn; standard deviation, \u003cem\u003en\u003c/em\u003e, or median (interquartile range). \u003csup\u003e*\u003c/sup\u003eSequence 1: back-up position, followed by neutral position. \u0026Dagger;Sequence 2: neutral position, followed by back-up position. BMI, body mass index; TMD, Thyromental distance; ID, inter-incisor distance at maximal mouth opening.\u003c/p\u003e\n \u003cp\u003e\u003csup\u003e\u0026nbsp;\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"1.9801980198019802%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cdiv align=\"center\"\u003e\n \u003ctable border=\"1\" cellpadding=\"0\" cellspacing=\"0\" width=\"100%\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"5\" valign=\"top\" width=\"100%\"\u003e\n \u003cp style=\"text-align: center;\"\u003e\u003cstrong\u003eTable 2\u003c/strong\u003e\u003c/p\u003e\n \u003cp style=\"text-align: center;\"\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003ePOGO scores and MCL grades in the neutral and back-up positions\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"0%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"23.232323232323232%\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.22222222222222%\"\u003e\n \u003cp\u003eNeutral position (\u003cem\u003en\u003c/em\u003e = 64)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.22222222222222%\"\u003e\n \u003cp\u003eBack-up position (\u003cem\u003en\u003c/em\u003e=64)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.19191919191919%\"\u003e\n \u003cp\u003eDifference (95% CI)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" width=\"13.131313131313131%\"\u003e\n \u003cp\u003e\u003cem\u003eP\u003c/em\u003e-value\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"23.232323232323232%\"\u003e\n \u003cp\u003ePOGO score, %\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.22222222222222%\"\u003e\n \u003cp\u003e37.5 \u0026plusmn; 24\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.22222222222222%\"\u003e\n \u003cp\u003e59.4 \u0026plusmn; 23.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"19.19191919191919%\"\u003e\n \u003cp\u003e21.9 (17.1 to 26.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" width=\"13.131313131313131%\"\u003e\n \u003cp\u003e\u0026lt; 0.0001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"23.232323232323232%\"\u003e\n \u003cp\u003eMCL grade\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.22222222222222%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.22222222222222%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"19.19191919191919%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" width=\"13.131313131313131%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"23.232323232323232%\"\u003e\n \u003cp\u003eEasy: Grade 1/2a\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.22222222222222%\"\u003e\n \u003cp\u003e11/17 (43.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.22222222222222%\"\u003e\n \u003cp\u003e27/28 (85.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"19.19191919191919%\"\u003e\n \u003cp\u003e42.2 (30 to 54.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\" width=\"13.131313131313131%\"\u003e\n \u003cp\u003e\u0026lt; 0.0001\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"23.232323232323232%\"\u003e\n \u003cp\u003eRestricted: Grade 2b/3a\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.22222222222222%\"\u003e\n \u003cp\u003e28/8 (56.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.22222222222222%\"\u003e\n \u003cp\u003e7/2 (14.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"19.19191919191919%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\" width=\"13.131313131313131%\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"23.232323232323232%\"\u003e\n \u003cp\u003eDifficult: Grade 3b/4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.22222222222222%\"\u003e\n \u003cp\u003e0/0 (0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"22.22222222222222%\"\u003e\n \u003cp\u003e0/0 (0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"19.19191919191919%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\" width=\"13.131313131313131%\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"23.1311706629055%\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd width=\"22.143864598025388%\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd width=\"22.143864598025388%\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd width=\"19.464033850493653%\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd width=\"12.552891396332862%\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003ctd width=\"0.5641748942172073%\"\u003e\u003cbr\u003e\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eData are presented as mean \u0026plusmn; standard deviation or \u003cem\u003en\u003c/em\u003e (%). CI, confidence interval, POGO, percentage of glottic opening; MCL grade, Modified Cormack\u0026ndash;Lehane grade.\u003cstrong\u003e\u003cbr\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp style=\"text-align: center;\"\u003e\u003cstrong\u003eTable 3\u003c/strong\u003e\u003c/p\u003e\n\u003cp style=\"text-align: center;\"\u003eEase of intubation in the second position (during period 2)\u003c/p\u003e\n\u003cdiv align=\"center\"\u003e\n \u003ctable border=\"1\" cellpadding=\"0\" cellspacing=\"0\" width=\"100%\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"43.43434343434343%\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"24.242424242424242%\"\u003e\n \u003cp\u003eNeutral position\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e(\u003cem\u003en\u003c/em\u003e = 32)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"24.242424242424242%\"\u003e\n \u003cp\u003eBack-up position\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e(\u003cem\u003en\u003c/em\u003e = 32)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"8.080808080808081%\"\u003e\n \u003cp\u003e\u003cem\u003eP\u003c/em\u003e-value\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"43.43434343434343%\"\u003e\n \u003cp\u003ePatients requiring optimization maneuvers\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e17 (53.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e7 (21.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.080808080808081%\"\u003e\n \u003cp\u003e0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"43.43434343434343%\"\u003e\n \u003cp\u003eBlade withdrawal\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e2 (6.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e2 (6.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.080808080808081%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"43.43434343434343%\"\u003e\n \u003cp\u003eExternal laryngeal manipulation\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e11 (34.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e7 (21.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.080808080808081%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"43.43434343434343%\"\u003e\n \u003cp\u003eIncreased lifting force\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e7 (21.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e1 (3.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.080808080808081%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"43.43434343434343%\"\u003e\n \u003cp\u003eTracheal tube bending\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e2 (6.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e1 (3.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.080808080808081%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"43.43434343434343%\"\u003e\n \u003cp\u003eStylet added\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e4 (12.5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e2 (6.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.080808080808081%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"43.43434343434343%\"\u003e\n \u003cp\u003eTracheal tube rotation\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e1 (3.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e0 (0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.080808080808081%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"43.43434343434343%\"\u003e\n \u003cp\u003eTime to intubation, s\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e26.6 (22.1 to 32.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e22 (19.4 to 30.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.080808080808081%\"\u003e\n \u003cp\u003e0.11\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"43.43434343434343%\"\u003e\n \u003cp\u003eIntubation Difficulty Scale score\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e1 (1 to 3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"24.242424242424242%\"\u003e\n \u003cp\u003e1 (0.5 to 1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.080808080808081%\"\u003e\n \u003cp\u003e0.13\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eData are presented as \u003cem\u003en\u003c/em\u003e (%) or median (interquartile range).\u003cstrong\u003e\u003cbr\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cdiv align=\"center\"\u003e\n \u003ctable border=\"1\" cellpadding=\"0\" cellspacing=\"0\" width=\"100%\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"5\" width=\"100%\"\u003e\n \u003cp style=\"text-align: center;\"\u003e\u003cstrong\u003eTable 4\u003c/strong\u003e\u003c/p\u003e\n \u003cp style=\"text-align: center;\"\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eCarryover and period effects\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"25.510204081632654%\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.346938775510203%\"\u003e\n \u003cp\u003eSequence 1*\u003c/p\u003e\n \u003cp\u003e(\u003cem\u003en\u003c/em\u003e = 32)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.346938775510203%\"\u003e\n \u003cp\u003eSequence 2\u0026Dagger;\u003c/p\u003e\n \u003cp\u003e(\u003cem\u003en\u003c/em\u003e = 32)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"29.591836734693878%\"\u003e\n \u003cp\u003eDifference\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e(95% CI)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"10.204081632653061%\"\u003e\n \u003cp\u003e\u003cem\u003eP\u003c/em\u003e-value\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"25.510204081632654%\"\u003e\n \u003cp\u003ePOGO score, %\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.346938775510203%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.346938775510203%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"29.591836734693878%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"10.204081632653061%\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"25.510204081632654%\"\u003e\n \u003cp\u003e\u0026nbsp;Sequence effect\u0026Dagger;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.346938775510203%\"\u003e\n \u003cp\u003e102.3 \u0026plusmn; 52.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.346938775510203%\"\u003e\n \u003cp\u003e91.4 \u0026plusmn; 33.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"29.591836734693878%\"\u003e\n \u003cp\u003e10.9 (\u0026minus;11.0 to 32.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"10.204081632653061%\"\u003e\n \u003cp\u003e0.32\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" width=\"25.510204081632654%\"\u003e\n \u003cp\u003e\u0026nbsp;Period effect\u0026sect;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"17.346938775510203%\"\u003e\n \u003cp\u003e16.4 \u0026plusmn; 16.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"17.346938775510203%\"\u003e\n \u003cp\u003e27.3 \u0026plusmn; 20.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"29.591836734693878%\"\u003e\n \u003cp\u003e\u0026minus;10.9 (\u0026minus;20.2 to \u0026minus;1.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" width=\"10.204081632653061%\"\u003e\n \u003cp\u003e0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"5\" valign=\"top\" width=\"100%\"\u003e\n \u003cp\u003eData are presented as\u0026nbsp;mean \u0026plusmn; standard deviation. *Sequence 1: back-up position, followed by neutral position. \u0026dagger;Sequence 2: neutral position, followed by back-up position. \u0026Dagger;Sequence effect: sum of POGO scores in the first position (period 1) and second position (period 2). \u0026sect;Period effect: difference between POGO scores in the back-up and neutral positions (back-up minus neutral). CI, confidence interval; POGO, percentage of glottic opening.\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"bmc-anesthesiology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"bane","sideBox":"Learn more about [BMC Anesthesiology](http://bmcanesthesiol.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/bane","title":"BMC Anesthesiology","twitterHandle":"BMC_series","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Laryngoscopy, videolaryngoscope, Macintosh blade, tracheal intubation","lastPublishedDoi":"10.21203/rs.3.rs-1448707/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-1448707/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eBackground: \u003c/strong\u003eBoth Macintosh-geometry and hyper-angulated videolaryngoscopes provide a better laryngeal view than conventional direct laryngoscope. However, the Macintosh-geometry videolaryngoscopes may not provide as dramatic an improvement in the laryngeal view as the hyper-angulated videolaryngoscopes because they lack the capacity to view structures around corners, so glottic exposure using the Macintosh-geometry videolaryngoscopes may be incomplete in some difficult airway. Patient positioning has an important effect on laryngeal visualization and successful tracheal intubation. However, the ideal head and neck position for videolaryngoscope has not yet been evaluated. We evaluated the effects of the back-up position on laryngeal visualization and ease of intubation using a Macintosh-geometry videolaryngoscope in patients with a simulated difficult airway.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eMethods: \u003c/strong\u003eSixty-four patients were included in this prospective, crossover, randomized controlled trial. Limited neck movement and mouth opening were simulated using a cervical collar. The laryngeal view was assessed in both neutral and back-up positions, in a randomized order, using a Macintosh-geometry videolaryngoscope, based on the percentage of glottic opening (POGO) score and modified Cormack–Lehane (MCL) grade. When scoring laryngeal visualization, external laryngeal manipulation was not permitted. The trachea was intubated only once (in the second position). The ease of intubation was assessed based on the need for optimization maneuvers, Intubation Difficulty Scale (IDS) scores, and time to intubation.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eResults:\u003c/strong\u003e The mean POGO score improved significantly in the back-up position (59.4 ± 23.8%) compared with the neutral position (37.5 ± 24%) (\u003cem\u003eP\u003c/em\u003e \u0026lt;0.0001). MCL grade 1 or 2a was achieved in 56 (85.9%) and 28 (43.7%) of patients in the back-up and neutral positions, respectively (\u003cem\u003eP \u003c/em\u003e\u0026lt;0.0001). Optimization maneuvers for intubation were required in 7 (21.9%) and 17 (53.1%) patients in the back-up and neutral positions, respectively (\u003cem\u003eP \u003c/em\u003e\u0026lt;0.0001). The IDS and time to intubation did not differ significantly between positions. \u003c/p\u003e\u003cp\u003e\u003cstrong\u003eConclusion:\u003c/strong\u003e The back-up position significantly improved the laryngeal view during Macintosh-geometry videolaryngoscope-guided intubation. It also improved ease of intubation, reflected in a reduced need for optimization maneuvers.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eTrial registration:\u003c/strong\u003e This trial was registered with \u003ca href=\"http://www.clinicaltrials.gov/\" rel=\"noopener noreferrer\" target=\"_blank\"\u003ewww.clinicaltrials.gov\u003c/a\u003e, NCT04716218, on 20/01/2021.\u003c/p\u003e","manuscriptTitle":"Effects of back-up position on tracheal intubation using a Macintosh-geometry videolaryngoscope in patients with a simulated difficult airway: A prospective randomized crossover study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-03-24 15:50:32","doi":"10.21203/rs.3.rs-1448707/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revision","date":"2022-04-08T17:00:42+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2022-03-28T10:43:22+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"c84ad737-fd52-4675-a36c-74b4e9b65557","date":"2022-03-26T10:48:33+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"9b4cdf93-8aae-44c4-8343-33df4f5aa03b","date":"2022-03-23T11:27:52+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2022-03-23T10:51:13+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2022-03-23T09:08:10+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2022-03-23T06:12:34+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2022-03-23T06:09:48+00:00","index":"","fulltext":""},{"type":"submitted","content":"BMC Anesthesiology","date":"2022-03-14T06:28:06+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"bmc-anesthesiology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"bane","sideBox":"Learn more about [BMC Anesthesiology](http://bmcanesthesiol.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/bane","title":"BMC Anesthesiology","twitterHandle":"BMC_series","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"b9d5bd77-8914-4b29-9594-06a388afe2ca","owner":[],"postedDate":"March 24th, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2022-05-25T03:29:13+00:00","versionOfRecord":[],"versionCreatedAt":"2022-03-24 15:50:32","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-1448707","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-1448707","identity":"rs-1448707","version":["v1"]},"buildId":"_2-kVJe1T_tPrBINL-cwx","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}