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PATIENT SAFETY

Poor Visualization During Direct Laryngoscopy and High Upper Lip Bite Test Score Are Predictors of Difficult Intubation with the GlideScope® Videolaryngoscope

From the Department of Anesthesiology, CHUM, Hôpital Notre-Dame, University of Montréal, Montréal, Canada.

Address correspondence and reprint requests to Stephan Williams, MD, PhD, Department of Anesthesiology, CHUM, Hôpital Notre-Dame, 1560 Sherbrooke East, Montréal, Canada, H2L 4M1. Address e-mail to stephan.williams{at}umontreal.ca.

Abstract

BACKGROUND: The GlideScope® videolaryngoscope allows equal or superior glottic visualization compared with direct laryngoscopy, but predictive features for difficult GlideScope intubation have not been identified. We undertook this prospective study to identify patient characteristics associated with difficult GlideScope intubation.

METHODS: Demographic and morphometric factors were recorded preoperatively for 400 patients undergoing anesthesia with endotracheal intubation. After induction, direct laryngoscopy was performed in all patients to assess the Cormack and Lehane grade of glottic visualization followed by GlideScope intubation. The number of attempts and time needed for intubation were recorded. Univariate and multivariate analyses were performed to identify the characteristics associated with difficult GlideScope intubation.

RESULTS: Intubation required 1, 2, and 3 attempts in 342, 48, and 9 participants, respectively, with one failure. Mean time for intubation was 21 ± 14 s. After univariate analysis, the following characteristics were significantly correlated (P < 0.05) with longer time to intubate and/or multiple attempts: older age, male sex, history of snoring, high Mallampati class, small mouth opening, short sternothyroid and manubriomental distances, large neck circumference, high upper lip bite test score, and high Cormack and Lehane grade during direct laryngoscopy. However, after introducing these variables in nominal logistic and proportional hazard multiple regression models, only high Cormack and Lehane grade during direct laryngoscopy, high upper lip bite test score, and short sternothyroid distance were significantly associated with multiple attempts or lengthier intubations.

CONCLUSION: Despite a high success rate, intubation with the GlideScope is likely to be more challenging in patients with high Cormack and Lehane grade during direct laryngoscopy, high upper lip bite test score, or short sternothyroid distance.

Endotracheal intubation is usually accomplished using direct laryngoscopy. However, in some situations, direct laryngoscopy may prove difficult or impossible. Physical findings, such as the Mallampati classification or measurements of the thyromental distance, mouth opening, and neck extension, have been validated to help anticipate such situations.1 An algorithm for managing difficult airways2 recommends early conversion to an alternative technique when direct laryngoscopy is difficult to reduce the risk of airway compromise and associated morbidity.3–5 Part of the anesthesiologist's task is to select the alternative approach best suited to each patient's specific features. The GlideScope® is a new videolaryngoscopic device that allows glottic visualization without a direct line-of-sight view. Despite its increasing use for both elective and unexpectedly difficult intubation, predictive criteria for successful airway management with the GlideScope have not been proposed.

We therefore undertook a prospective study to determine the patient characteristics, if any, that predict difficult intubation when using the GlideScope for perioperative orotracheal intubation in an elective surgical population.

METHODS

After IRB approval and written informed consent, 400 adult patients, scheduled to undergo elective neurosurgical, othorhinolarynoglogic, cardiac, general, urologic, gynecologic, orthopedic, and plastic surgery under general anesthesia with orotracheal intubation were prospectively enrolled. Patients were excluded if they required a rapid sequence induction or if an induction was planned without using a neuromuscular blocking drug. Two senior residents in anesthesiology (M.H.T. and A.R.) who had each used the GlideScope (Saturn Biomedical Systems, Burnaby, British Colombia, Canada, first-generation model with a maximum blade width of 18 mm) for more than 30 intubations performed all intubations. Before anesthesia, we recorded each patient's age, sex, weight, height, ASA classification, history of snoring, Mallampati class,6 mandibular protrusion assessed with the upper lip bite test,7 mouth opening (interincisor distance), thyromental and sternothyroid distances with the patient in sitting position and neck extended, manubriomental distances in flexion and extension, and neck circumference. The upper lip bite test7 was performed according to the following criteria: class 1 = lower incisors can bite the upper lip above the vermilion line; class 2 = lower incisors can bite the upper lip below the vermillion line; class 3 = lower incisors cannot bite the upper lip.

The choice of the anesthesia induction technique was left to the attending anesthesiologist, but had to include a neuromuscular blocking drug. All attempts at intubation were performed after confirmation of adequate paralysis, defined as no response to the train-of-four or single twitch stimulation of the ulnar nerve. The patient's head was placed in the sniffing position and the following sequence was followed for each participant. First, a regular direct laryngoscopy using a Macintosh size 3 or 4 blade was performed for the sole purpose of assessing the Cormack and Lehane grade of glottic visualization.8 Second, the GlideScope was inserted down the midline of the mouth as recommended by the manufacturer.9 The view of the glottis displayed on the monitor screen of the GlideScope was noted, again using the Cormack and Lehane scale. Finally, an endotracheal tube mounted on a malleable stylet with its distal end angulated upward by about 60 degrees to match the curve of the GlideScope's blade was inserted toward the glottis. The stylet was removed when the tip of the endotracheal tube was seen entering the trachea. The accurate positioning of the endotracheal tube was confirmed by capnography and lung auscultation. Cuffed endotracheal tubes of 6.0 mm internal diameter were used for patients undergoing microlaryngeal surgery, whereas 7.0 and 8.0 mm internal diameter tubes were used for all other female and male patients respectively.

Ease of intubation with the GlideScope was measured using two outcomes: the time to intubate and the number of attempts needed for successful intubation. Time to intubate was measured from the time the GlideScope entered the patient's mouth until the endotracheal tube was seen going through the vocal cords. An unsuccessful attempt occurred when the GlideScope or the endotracheal tube needed to be withdrawn from the mouth. At this moment, the distal end of the endotracheal tube was angulated more upward and/or the GlideScope's blade was repositioned. A maximum of three attempts was allowed.

Univariate analyses were performed to evaluate the association between patient characteristics and time or number of attempts needed for intubation. Pearson's and Spearman's coefficients were used respectively for continuous and ordinal data. Variables that exhibited a significant correlation with time required for intubation were entered into a proportional hazard model to identify those variables that remained significantly and independently correlated with time to intubate. Nominal logistic regression was used to analyze the interaction between patient characteristics that exhibited a significant correlation during univariate analysis and the number of attempts divided in two categories (one attempt and more than one attempt) to create a two-category nominal variable. One-way ANOVA followed with Tukey's post hoc test was used to assess the relationship of the Cormack and Lehane grades during direct laryngoscopy with intubation time. In addition, receiver-operator-characteristic (ROC)10,11 curves were constructed to assess the discrimination ability of each factor identified as predictor of multiple attempts during uni- and multivariate analyses except for snoring history (a dichotomous variable). No patient was excluded from the analysis. Unless stated otherwise, data are presented as mean ± sd. A P value <0.05 was considered significant.

RESULTS

Patient characteristics, including Cormack and Lehane grade at direct laryngoscopy, are shown in Table 1. Intubation required 1 attempt in 342 patients, 2 attempts in 48 patients, and 3 attempts in 9 patients. One patient (0.25%) could not be intubated with the GlideScope after three attempts; this patient was a grade 1 glottic visualization with a conventional laryngoscopy. The problem was difficulty coordinating endotracheal tube advancement with stylet removal. He was intubated with direct laryngoscopy on the first attempt. In 54 of the 69 supplemental attempts, it was deemed necessary to reshape the endotracheal tube by increasing its upward angulation because the tip of the endotracheal tube could not be positioned anteriorly enough to enter the glottic opening. In the remaining cases, the GlideScope's blade was repositioned to give more space for the passage of the endotracheal tube or to develop a better view of the glottis. Mean intubation time for all intubations with the GlideScope was 21 ± 14 s. The mean intubation time for patients with Cormack and Lehane grades 1, 2, and 3-4 were 18 ± 9 s, 25 ± 17 s, and 37 ± 21 s respectively. Time to intubate Cormack and Lehane grade 2 patients was significantly longer than Cormack and Lehane grade 1 patients (P <0.001). The same was true for Cormack and Lehane 1 vs 3-4 (P < 0.001) and 2 vs 3-4 (P < 0.001) (Fig. 1). In all patients, including the 27 patients with Cormack and Lehane grades 3 or 4 at direct laryngoscopy, a Cormack and Lehane grade 1 (n = 357, 89.5%) or 2 (n = 43, 10.5%) laryngeal view was obtained with the GlideScope.

View larger version (12K): [in this window] [in a new window]   Figure 1. Mean time to intubate with the GlideScope® videolaryngoscope related to Cormack and Lehane grades at direct laryngoscopy. The mean time to intubate increased with increasing Cormack and Lehane grade. Time to intubate patients with grade Cormack and Lehane 2 was significantly longer than for patients with grade Cormack and Lehane 1 (P < 0.001), as well as grade Cormack and Lehane 1 vs 3-4 (P < 0.001) and 2 vs 3-4 (P < 0.001) (One-way ANOVA followed with Tukey's posttests). For each Cormack and Lehane grade at direct laryngoscopy, the number of patients requiring 1/2/3/>3 (= failed intubation) attempts with the GlideScope is specified in brackets.

The univariate analyses demonstrated many patient characteristics that were correlated with either time to intubate, number of attempts, or both (Tables 2 and 3). These variables included Cormack and Lehane grade at direct laryngoscopy, results of the upper lip bite test, sternothyroid distance, Mallampati class, age, male sex, neck circumference, mouth opening, history of snoring, and manubriomental distance in extension. However, when these variables were analyzed using multiple regression methods, only high Cormack and Lehane grade at direct laryngoscopy, high upper lip bite test score, and short sternothyroid distance were found to be significantly correlated with both time to intubate and the number of intubation attempts (Tables 2 and 3). The area under the ROC curves for those factors that were associated with multiple intubation attempts was 0.68 for Cormack and Lehane grade during direct laryngoscopy, 0.65 for upper lip bite test score, 0.59 for Mallampati class, 0.62 for sternothyroid distance, and 0.60 for manubriomental distance in extension.

DISCUSSION

The current study demonstrates that poor glottic visualization during direct laryngoscopy and high upper lip bite test score are the best predictive factors for challenging intubation with the GlideScope videolaryngoscope. A clinician confronted with these factors can expect to face a longer but generally successful videolaryngoscopic endotracheal intubation, and more than one attempt may be required. A short sternothyroid distance is also significantly associated with more difficult GlideScope intubation, but the strength of this association is weaker.

Other authors have reported their experience with the GlideScope. Cooper et al. found no relation between morbid obesity, reduced neck extension, or small oral aperture (30 mm) and intubation success with the GlideScope.12 Their study was mostly descriptive and they only compared Cormack and Lehane grades obtained during direct laryngoscopy and GlideScope in a subset of their population. Conversely, Rai et al. reported increased intubation difficulty with the GlideScope in 4 of 50 patients.13 These four patients had "high" body mass index (28-32 kg/m²), and 2 of the 4 were Cormack and Lehane grade 3 on direct laryngoscopy. This sample was too small to determine which factors were most likely to predict a more challenging intubation. Conversely, Sun et al. reported that intubation times were not affected by Cormack and Lehane grades at direct laryngoscopy.14 However, the surprisingly frequent incidence of patients with Cormack and Lehane grade 3 in this study (16.5%), the exclusion of 4 of the 9 patients requiring more than 1 attempt at intubation from the analysis, and the fact that the method of measuring time to intubate was modified during the study make these results difficult to interpret. Interestingly, none of these studies evaluated the predictive ability of the upper lip bite test. Limited mandibular protrusion has recently been associated with both difficult intubation using direct laryngoscopy and difficult mask ventilation.7,15,16

Among the 400 patients recruited in this study, many had nonreassuring airway features: limited mouth opening, short thyromental distance, Mallampati class 3 or 4, limited neck extension, large neck circumference, and high body mass index. None of these features was found to be independently associated with the GlideScope intubation performance when the Cormack and Lehane grade and the result of the upper lip bite test were included in the multivariate analysis. Still, many of these factors are likely to contribute to more challenging GlideScope intubation, as shown during univariate analysis (Tables 2 and 3). The fact that these criteria were not deemed significant in the multivariate analysis only shows that a high Cormack and Lehane grade during direct laryngoscopy and limited mandibular protrusion are the most comprehensive predictors of difficult videolaryngoscopic intubation. In clinical practice, many patients suspected of airway management difficulties do not have a recent Cormack and Lehane grade evaluation in their chart. In these cases, the upper lip bite test, the sternothyroid distance and, to a lesser degree, the other seven criteria identified during univariate analyses (high Mallampati class, older age, male gender, large neck circumference, small mouth opening, history of snoring, and reduced manubriomental distance in extension) can also help clinicians anticipate more challenging GlideScope intubation before proceeding with airway instrumentation.

Other authors have used ROC curves and multivariate analysis to establish the discriminating power of variables associated with airway management difficulties.16,17 In the present study, the areas under the ROC curves confirmed that Cormack and Lehane grade during direct laryngoscopy and upper lip bite test score were the most discriminating factors. It would be interesting in a subsequent study to validate a combined index that includes the factors available during the preoperative assessment to create a better predictive tool of difficult intubation with the GlideScope.

With direct laryngoscopy, poor visualization of the glottic aperture is correlated with difficult intubation. The situation is quite different with the GlideScope. The findings of the present study suggest that the intubations that prove difficult with the GlideScope occur despite a good view of the glottic aperture. In agreement with these results, most of the reported failures to intubate the trachea with the GlideScope can be attributed to the operator's lack of experience or inappropriate preshaping of the endotracheal tube with the stylet.12,13 Inability to perform tracheal intubation with the GlideScope occurred in only 1 patient (0.25%) in this study. This failure rate is comparable to what is observed with direct laryngoscopy,1,3 and low (0.25%) compared with the 3.7% failure rate reported when GlideScope intubation when attempted by operators with good to no previous experience.12 The incidence of three or more GlideScope attempts was 2.5% in this study, similar to the 2% incidence reported with direct laryngoscopy in other studies.1,18 Jones et al. have suggested that a 90 degrees endotracheal tube angulation, rather than the 60 degrees angulation recommended by the GlideScope Operator and Service Manual, should be used initially when intubation is attempted with the GlideScope.19 Endotracheal tube angulation was increased in 78% of supplemental attempts in this study, supporting the concept that a more pronounced initial endotracheal tube angulation could have reduced the time and number of attempts needed to intubate patients who exhibit a high Cormack and Lehane grade with direct laryngoscopy.

Recently, three cases of palatopharyngeal arch injury have been reported with the use of the GlideScope.20,21 In this study, a systematic examination of the oropharyngeal cavity was not performed after each intubation, but any unusual events were noted in the study database. Neither the personnel involved in the study nor the attending anesthesiologists reported any significant pharyngeal or soft tissue injury. When resistance in endotracheal tube advancement was encountered, the blade of the GlideScope was slightly withdrawn to see the tip of the endotracheal tube, which was then advanced under direct visual control. This may have reduced the possibility of soft tissue injury.

This study has some limitations. First, the lack of an independent observer to assess the Cormack and Lehane grade during direct laryngoscopy may have influenced the way the operator subsequently performed the intubation with the GlideScope. This experimental design mimicked the clinical setting in which unexpected difficult intubations are typically discovered while performing direct laryngoscopy, and was followed by attempts with alternative methods, usually by the same person. Second, only two participants exhibited a Cormack and Lehane grade of 4 during direct laryngoscopy. Although they were both successfully intubated, a much larger series will be necessary before definitive conclusions regarding the safety and efficacy of GlideScope in Cormack and Lehane grade 4 patients can be reached. Third, we performed this study in an adult population for which rapid sequence induction was not required.

In conclusion, the success rate associated with GlideScope videolaryngoscopy in this series of adult patients undergoing endotracheal intubation for elective surgery was 399 of 400. Predictive factors for longer intubation times or multiple attempts at endotracheal intubation with the GlideScope were restricted laryngeal visualization during direct laryngoscopy, limited mandibular protrusion, and short sternothyroid distance.

Footnotes

Accepted for publication December 28, 2007.

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Tremblay, M.-H. Articles by Drolet, P. Search for Related Content PubMed PubMed Citation Articles by Tremblay, M.-H. Articles by Drolet, P. Related Collections Airway Preoperative Evaluation Patient Safety