Comparing Video and Direct Laryngoscopy for Nasotracheal Intubation

This institutional review board approved (IRB #10272) randomized clinical trial included as eligible subjects all patients who presented to University Medical Center in New Orleans, LA for oral and maxillofacial surgery under general anesthesia with nasotracheal intubation between December 15, 2018, and May 30, 2019, under the care of the senior author. Inclusion criteria for the study were as follows: a planned general anesthetic with nasotracheal intubation, ASA (American Society of Anesthesiologists) I, and II, and at least 18 years of age. Prisoners and patients with anticipated difficult airways were excluded from this study. Anticipated difficult airways, as determined by the anesthesiologist, included a previous history of difficult intubation or 3 or more of the following factors: body mass index > 35 kg/m², neck range of motion < 30 degrees, Mallampati score IV, neck circumference > 42 cm, and thyromental distance < 6 cm.

The study was a single center, parallel group, unblinded randomized controlled trial with an allocation ratio of 1:1. An investigator explained the risks and benefits of study participation in detail to each patient and enrolled patients after obtaining signed informed consent. A coinvestigator, not involved with enrollment or the anesthetic management, used a computerized research randomization program to perform block randomization using varying block sizes of 4, 6, and 8.¹⁰  Group allocation was concealed using opaque envelopes sealed after the group assignment paper was placed inside. When a patient was enrolled in the study, the next envelope in the sequence was labeled with the patient's study ID and opened by the principal investigator who directed the anesthetic team to perform the assigned laryngoscopy technique. The DL group utilized a standard laryngoscope with a properly sized Macintosh or Miller blade during direct visualization of the larynx at the discretion of the intubating anesthetist. The VL group utilized a C-MAC S Video Laryngoscope with a size 3 blade (Karl Storz, Tuttlingen, Germany) during indirect visualization of the larynx. Under the direct supervision of a board-certified anesthesiologist, all intubations were performed by certified registered nurse anesthetists who had been in clinical practice for > 5 years and had extensive experience with nasotracheal intubations using the C-MAC device, DL, and Magill forceps.

The primary outcome variable was the total time required to successfully complete nasotracheal intubation. Time for intubation, as recorded for each case by an independent observer using a stopwatch, started upon removal of the bag mask and ended after the confirming bilateral breath sounds, end-tidal CO₂ waveform, and lack of a cuff leak. These confirmation steps were generally accomplished in 10 to 20 seconds, and the authors felt it important to include these steps in the total intubation time to avoid stopping and starting the stopwatch for intubations with torn cuffs or esophageal placement.

The secondary outcome variables were the success of first intubation attempts, the number of attempts, the number of endotracheal tube exchanges after placement, and the use of Magill forceps. Successful intubation was defined as endotracheal tube placement confirmed by equal bilateral breath sounds, end-tidal CO₂ waveform, and lack of cuff leak. A failed intubation attempt was defined as removal of the laryngoscope prior to insertion of the endotracheal tube and restarting bag mask ventilation or if any of the above mentioned intubation criteria were not achieved. Tube exchanges were recorded if they occurred. Magill forceps usage during intubation was recorded by an independent observer. The Magill forceps were used only if the nasotracheal tube was unable to be passed through the vocal cords with manipulation of head position, cricoid pressure, and gentle advancement of the tube.

The predictor variable was the laryngoscopy technique employed: VL (the experimental group) or DL (the control group). Subject demographic variables recorded were age, gender, ASA score, Mallampati score, maximum incisal opening, assisted maximum incisal opening (measured after induction), body mass index, thyromental distance, neck circumference, and procedure performed.

An a priori power analysis was used to establish the required sample size of 48 patients (24 in each arm) with an 80% power, an alpha level of 0.05, and an effect size of a difference of 7 seconds in total intubation time, which was based on a previous study in a similar patient population, using a standard method described elsewhere.^3,11 

Data were compiled and imported into IBM SPSS Statistics (IBM Corp, Version 26.0, Armonk, NY, 1989–2019). The relationship of the primary outcome variable (total intubation time) to the primary predictor (VL vs DL) was analyzed using a Mann-Whitney U test. The relationship between the secondary outcome variable (intubation success) and subject demographic variables was examined using simple logistic regression and Fisher's exact test, and, as appropriate for data type. The relationship of the other secondary outcome variables to the primary predictor (VL vs DL) was analyzed using Fisher's exact tests. A p-value of less than .05 was considered significant. A preliminary multiple variable linear regression model was created with all variables associated with the primary outcome variable to a significance level of .05. A final multivariable linear regression model for intubation time was created using a backwards elimination technique until all variables had p-values of <.1.

One hundred seven patients were screened for enrollment in the study, 50 of which met the inclusion criteria. Eligibility, exclusions, enrollment, and allocation data are detailed below (Figure). Demographic variables for the 25 patients randomly assigned to the VL group and the 25 patients to the DL group were compared (Table 1). Despite random assignment, there were more men enrolled in the DL group (21) than the VL group (14), which was statistically significant (p = .031), but no other subject variables were statistically significant between the 2 groups.

Statistical analysis of the demographic variables relationship to the primary outcome variable (total intubation time) and the secondary outcome variable (first intubation success rate) are presented below (Tables 2 and 3). Only a decreased assisted maximum incisal opening and a decreased thyromental distance were associated with statistically significant increased intubation times (p = .003 and p = .032). Increased assisted maximum incisal opening was associated with statistically significant increased first intubation success rates (p = .037).

Analysis of the relationship of the primary predictor (VL vs DL) to the primary outcome variable (total intubation time) is presented below (Table 4). The mean total intubation time was 142 ± 90 seconds in the DL group and 94 ± 54 seconds in the VL group (p = .011).

A preliminary multivariable linear regression model was created for total intubation time with the predictor variables that reached statistical significance in the bivariate analysis (gender, thyromental distance, assisted maximum incisal opening, and VL or DL group). The final model was then created as described above, and gender was removed as a nonsignificant variable, leaving thyromental distance, assisted maximum incisal opening, and laryngoscopy group as predictors approaching statistical significance, for intubation time (Table 5). Collinearity analysis using variance inflation factor was performed and no multicollinearity was detected.

Statistical analysis of the laryngoscopy group (VL vs DL) to the secondary outcome variables is presented below (Table 6). Only the use of Magill forceps was found to be significant, with 56% use in the DL group compared with 16% in the VL group (p = .007). There were 6 patients, 4 in the DL group and 2 in the VL group, in whom the first intubation attempt was unsuccessful. In all 6, intubation was successful on the second attempt. VL was used on the second attempt for 2 patients allocated to the DL group due to an unexpected difficult airway.

The purpose of the study was to compare the total intubation times and success rates of VL and DL in routine nasotracheal intubation for oral and maxillofacial procedures. The use of VL was associated with a statistically significant 48-second decrease in total intubation time (p = .011) and a nonsignificant higher rate (92 vs 84%) of first intubation attempt success (p = .384) compared with DL. There was also a much greater variance in total intubation time with the DL group, the standard deviation of the mean was 90 seconds versus 54 seconds in the VL group. Cuff tears after intubation requiring tube exchange occurred equally between the 2 groups (4 vs 4%).

Only 1 of the secondary outcome variables was found to be significantly associated with the use of VL, decreased Magill forceps use. The finding of maximum incisal opening and thyromental distance being significant predictors of increased total intubation times as noted in the final multivariable analysis, along with laryngoscopy type, is unsurprising. Such clinical findings have long been associated with more challenging airway management and intubations, as identified in numerous studies on the topic.^12–15 

The present study has several key differences as it relates to other studies. The first difference is the method for measuring intubation time; however, no 2 previous studies measured intubation time in the same manner. Because the investigators measured total intubation time as the time of laryngoscope insertion until confirmation of proper placement, the measured intubation times are longer than has been reported in similar studies (1:58 in this study vs approximately 36–40 seconds in other studies).^3,8  However, these studies measured a much narrower timeframe of events involved. One study starting timing after the nasotracheal tube was already inserted into the nose and stopped the intubation time when the tube was passed through the vocal cords,³  and another study stopped timing only after detection of end-tidal CO₂.⁸  The methods used in these other studies would not include the time normally utilized to detect malpositioning of the tube and cuff leaks. Although the method used in the present study captured all of the differences in the intubation duration between the 2 techniques, it could be influenced by factors other than the laryngoscopy type.

Tabrizi et al³  also showed a much lower first attempt intubation success rate in all groups, ∼70%. Their study included only patients with bilateral mandibular fractures, which they postulated were more technically challenging. Our study also included a number of patients with bilateral mandibular fractures, but we did not find an association between the procedure and intubation success or duration.

In the study by Kwak et al,⁸  Magill forceps were also used much more frequently in the DL group. However, Kwak et al⁸  did not measure cuff tears. VL was associated with a drastic reduction in Magill use in both our study and the study from Kwak et al.⁸  However, the association of VL to fewer cuff tears was not significant in this study.

Although there are many studies demonstrating the benefits of VL, the studies are not uniformly positive. Particularly in studies in the intensive care unit, emergency room, and prehospital settings, VL does not appear to confer a benefit.^6,16,17  This could be due to the learning curve associated with using VL. For those practitioners who have less experience with intubation, such as emergency medical technicians and emergency room physicians compared with trained anesthetists and anesthesiologists, the utilization of indirect vision could be more difficult. It is also possible that the prehospital, emergency room, and ICU settings have other factors present that make VL difficult, such as increased secretions, airway swelling, and/or blood in the oropharynx. However, studies utilizing VL for oral intubation in the operating room have consistently demonstrated similar benefits to the findings presented in this study. Pieters et al⁴  performed a systematic review and meta-analysis showing a clear benefit for VL in patients with difficult airways: higher first attempt success, better glottis views, and less mucosal trauma. These benefits were repeated in obese patients by Yumul et al.⁷  A systematic review by Lewis et al⁵  demonstrated the benefits of VL for routine oral endotracheal intubation. Overall, it appears that the same benefits demonstrated in this study were evident in other studies on oral intubations performed with VL.

There are several limitations of the study. The first is the limited sample. As the study size was based on a power analysis using a difference in intubation time of 7 seconds based on a previous study, it is possibly underpowered to detect differences in secondary outcomes such as cuff tears and intubation success. If a future study examining the intubation success rates using these techniques were to use our data to calculate a sample size, an estimated 256 patients would be required in each group. Another weakness of this study is that it did not examine some of the outcomes pertaining to the 2 techniques as reported in previous studies (dental trauma, mucosal tears, intubation difficulty score). A final limitation is the lack of blinding, which could create a performance bias and an observer bias as both the anesthetist and the time keeper knew the type of laryngoscope in use. Although it would be impossible to blind the anesthetist, the time keeper could have been blinded though we chose not to do this for practical considerations. Blinding the time keeper would require setting up vision barriers around crucial equipment, which would be cumbersome to the anesthesia team. It would also require objective reporting of the events for which the only cue is visual, such as insertion of the laryngoscope, from another source that could introduce similar biases.

In conclusion, this study demonstrates that VL for nasotracheal intubation results in faster total intubation times and fewer uses of Magill forceps compared with conventional DL. These results were obtained with experienced anesthetists, and future studies could be directed at the measurement of these outcomes with inexperienced operators who are more prone to have difficulty with intubation.