Cervical Spine Motion During Tracheal Intubation with Manual In-Line Stabilization: Direct Laryngoscopy versus GlideScope® Videolaryngoscopy

Abstract

BACKGROUND: 

The optimal tracheal intubation technique for patients with potential cervical (C) spine injury remains controversial. Using continuous cinefluoroscopy, we conducted a prospective study comparing C-spine movement during intubation using direct laryngoscopy (DL) or GlideScope® videolaryngoscopy (GVL), with uninterrupted manual in-line stabilization of the head by an assistant.

METHODS: 

Twenty patients without C-spine pathology were studied. After induction of general anesthesia with neuromuscular blockade, both DL and GVL were performed on every patient in random order. Cinefluoroscopic images of C-spine movement during GVL and DL were acquired and divided into four stages: a baseline image before airway manipulation, glottic visualization, insertion of the endotracheal tube into the glottis, and tracheal intubation. Peak segmental motion from the occiput to C5 was measured offline for each patient and each stage, averages were calculated, and movements induced by each instrument were compared using a two-way ANOVA. Also studied were the proportion of patients with occiput-C1 rotation exceeding 10, 15, or 20 degrees, and the quality of glottic visualization.

RESULTS: 

No significant difference was found between DL and GVL regarding average segmental spine movement at any level (P values between 0.22 and 0.70). During both techniques, motion was mainly an extension concentrated in the rostral C-spine and occurred predominantly during glottic visualization. The proportion of patients with occiput-C1 extension of more than 10, 15, or 20 degrees was not significantly different. Glottic visualization was significantly better with GVL compared with DL.

CONCLUSION: 

During intubation under general anesthesia with neuromuscular blockade and manual in-line stabilization, the use of GVL produced better glottic visualization, but did not significantly decrease movement of the nonpathologic C-spine when compared with DL.

In Brief

IMPLICATIONS: During intubation with manual in-line stabilization by an assistant, videolaryngoscopy produced better glottic visualization than direct laryngoscopy, but did not significantly decrease movement of the nonpathologic cervical spine.

When tracheal intubation is required in patients with an injured cervical spine (C-spine), securing the airway while minimizing C-spine motion to prevent neurological damage can prove challenging. In the nonemergent situation, awake intubation over a flexible bronchoscope is often preferred¹ as it minimizes C-spine motion in cadaver studies.² In the emergent situation, however, direct laryngoscopy (DL) with manual in-line stabilization (MILS) by an assistant in the anesthetized patient is most commonly used,³ as it is quicker, less affected by blood, secretions, and vomitus present in the airway and does not require patient collaboration.^4,5 DL nevertheless causes more C-spine movement,² particularly in the often-injured rostral portion^6,7 and has been associated with catastrophic neurological deterioration.^8,9

Numerous alternatives to DL and fiberoptic bronchoscopy have been studied,¹⁰ including different blade designs, laryngeal mask airways, and indirect rigid fiberoptic laryngoscopes. None of these methods, however, combines the convenience of DL and the C-spine immobility afforded by intubation over a fiberoptic bronchoscope.

Videolaryngoscopy, which has recently developed extensively and become more widely available, has the potential of combining the advantages of both DL and intubation over a fiberoptic bronchoscope. Indeed, it provides an indirect view of the glottis, which could diminish C-spine movement, but its handling shares many similarities with DL, which could make it more convenient than the flexible bronchoscope. The GlideScope® videolaryngoscopy (GVL) system is gaining acceptance for both the difficult airway^11,12 and the patient with an injured C-spine.¹³ In the only study examining C-spine movement during DL and GVL, Turkstra et al.¹⁴ found no difference in movement at the rostral level but showed significantly less movement of the inferior C-spine with GVL. In this study, C-spine stabilization was provided by taping the patient's head to a Mayfield horseshoe. Since MILS is a standard component of airway management of the patient with an unstable C-Spine,¹⁵ we conducted a prospective cinefluoroscopic study comparing C-spine motion during DL and GVL in patients with an intact C-spine stabilized by MILS. We postulated that GVL would induce less movement than DL.

METHODS

After approval from our institutional Research Ethics Board, written informed consent was obtained from 20 patients scheduled to undergo an elective interventional neuroradiological procedure under general anesthesia between October 2004 and September 2005. Patients incapable of informed consent, having clinical or radiological evidence of C-spine abnormalities, and those requiring rapid-sequence induction or an induction without a neuromuscular blocking drug, were excluded.

Demographic and morphometric data for each patient were recorded preoperatively. In the neuroradiology suite, patients were positioned with the C-spine in the neutral position, their occiput and shoulders resting directly on the firm horizontal surface of the fluoroscopy table with their head in the anatomical position. Each patient was equipped with standard monitoring, including a neurostimulator. A trained assistant, positioned at the patient's head, maintained MILS of the C-spine throughout airway maneuvers by grasping the mastoid processes bilaterally with the fingertips while cupping the occiput in the palms of the hands¹⁵; no traction was applied. For each patient, two cuffed endotracheal tubes (ID 7.0 mm for females and 8.0 mm for males) were prepared, each fitted with a malleable stylet: one in the classic “hockey stick” position (i.e., straight with the distal tip angulated anteriorly approximately 30 degrees), and the other with its distal end angulated upward by about 60 degrees to match the angle of the GVL blade (as per the manufacturer's instructions). Adequacy of patient positioning relative to the fluoroscopy camera was assessed and, once it was deemed appropriate, neither the table nor the fluoroscopy camera was allowed to move.

Administration of 100% O₂ by mask was performed. The choice of the anesthesia induction technique was left to the attending anesthesiologist, but always included neuromuscular blockade. Mask ventilation was provided after loss of consciousness and between airway techniques if deemed necessary. C-spine movement was minimized during mask ventilation, and oral airways could be used if needed. Neuromuscular block was confirmed by neurostimulation before initiating laryngoscopy. Both DL (using a Macintosh size 3 or 4 blade) and GVL (using a “large,” 18-mm thick blade) were performed on every patient in an order determined by a randomization table, with randomization occurring after patient positioning but before induction of general anesthesia. To avoid any bias resulting from the order in which the techniques were performed in any given patient, the randomization table was constructed, so that DL and GVL were each performed first in half of the patients. During the first of the two techniques, the glottis was visualized and an appropriately shaped endotracheal tube was then advanced until it reached the glottic aperture. Afterwards, it was withdrawn without actually entering the trachea. The second technique was then performed, with the tube being advanced this time into the trachea. Correct placement was confirmed by capnography and bilateral auscultation of the lungs. Each subject was therefore only intubated once.

Glottic view, graded according to Cormack and Lehane,¹⁶ was recorded for DL and GVL. With both techniques, only exposure sufficient to pass the endotracheal tube was sought. If necessary, external laryngeal manipulations were used to obtain sufficient glottic visualization. All intubations were performed by two senior anesthesiology residents (A.R. and M.H.T.) having performed both laryngoscopy techniques at least 30 times at the beginning of the study.

During airway maneuvers, continuous lateral fluoroscopy (Axiom Artis biplane digital system, Siemens, Erlangen, Germany) was used to record movements of the skull base, craniovertebral junction, and cervical vertebrae down to C5. Throughout the maneuvers, the person performing the techniques could not see the fluoroscopic images.

Video of the procedure was reviewed offline on the Leonardo Workstation (Siemens, Erlangen, Germany) and, for purposes of analysis, each intubation sequence was divided into four distinct stages: the “baseline” stage was defined as the last image before the introduction of the laryngoscopy instrument inside the oropharynx; the “visualization” stage corresponded to the positioning of the instrument for glottic visualization and included laryngeal manipulations performed if the view obtained initially was insufficient to successfully intubate the trachea; the “tube” stage corresponded to the insertion of the endotracheal tube up to the glottic aperture; the final stage, “intubation,” was defined as the penetration of the tube inside the trachea and the removal of the stylet and laryngoscopy instrument. Film for the last three stages was systematically reviewed frame by frame to determine peak C-spine displacement for each stage compared to the baseline image. The three frames thus selected, plus the baseline image, were then analyzed to follow the movement of all six osseous elements (occiput to C5) throughout the intubation process.

All measurements were made on eFilm Workstation software version 1.9 (Merge Healthcare, Milwaukee, WI) by a senior radiology resident under the supervision of an attending neuroradiologist. C-spine movement was estimated from a reference outside the patient, which was defined as a horizontal line parallel to the angiosuite table. Localization of the cervical vertebrae in two dimensions was defined as a straight line intersecting two reference points: the first landmark was the corner formed by the junction of the anterior cortex of the vertebral body and the inferior vertebral end-plate, and the second was defined as the inferior limit of the spinolamellar line, a constant radiological landmark that defines the posterior limits of the bony spinal canal. Position of the occiput was followed by drawing a straight line parallel to the axis of the lower part of the clivus (Fig. 1). If anatomical variations or limitations in fluoroscopic visualization prevented the use of standard reference points, alternative reference points were determined case by case by the radiologist. The same anatomical markers were used to measure movement in all fluoroscopic sequences for a given subject. If the two-point vertebral landmarks were not visible for a specific vertebra for each intubation step, the vertebra was rejected and excluded from the analysis.

Figure 1.: Representative digitized cinefluroscopic image showing a reference line for each osseous element from the skull base down to C5 vertebrae. The occiput-C1 angle is drawn as an example of a measured segmented angle.

Baseline position of the occiput and of each cervical vertebra was measured in every patient. The degree of rotation was assessed by measuring the angle formed by the intercept of the vertebral line and the reference. For each vertebra and each step of the intubation, rotation in the vertical plane was measured to calculate variation from the baseline value.

Movement of the C-spine was described in a manner similar to Sawin et al.,⁶ that is, by following segments defined by adjacent osseous units (occiput-C1, shown as an example in Fig. 1, down to C4–5): an increase in the angle between two units compared with baseline value (defined as zero), corresponding to an extension, was given a positive value; a decrease in the angle, corresponding to a flexion, was given a negative value. This algebraic convention is consistent with previous studies of the same nature.^6,17,18

Mean values of angular change for each segment, during each stage, and with each of the two techniques were calculated using the absolute value of the angles. To compare the frequency of large C-spine movements, the proportion of stages in each group where segmental movement exceeded 10, 15, and 20 degrees was calculated.

To compare motion of every segment across the four stages during both techniques, two-way analysis of variance (ANOVA) was used. To compare extremes of movement between both techniques, a Fisher's exact test was used. To compare grades of glottic visualization between both techniques, a Mann– Whitney test was performed. Unless stated otherwise, results are given as a mean ± sd. Two studies already published were used for sample size calculation.^2,19 They suggested that 16 patients would be necessary to find a reduction in spinal vertebrae movement amplitude of 5 degrees between the two techniques (α error 0.05, β error 0.2, estimated standard deviation 5 degrees).

RESULTS

Characteristics of our study group of 20 patients are summarized in Table 1. Mean segmental movement of adjacent osseous units in the sagittal plane is depicted in Figure 2. Data could be obtained in all patients for all segments and for all stages, with the exception of segments occiput-C1 and C4–5 in which data were missing in a minority of patients, as indicated in Figure 2, because these segments could not always be visualized on the images obtained. No significant difference was found between techniques at any level (P values between 0.22 and 0.70). During both GVL and DL, C-spine motion was predominantly an extension concentrated in the rostral C-spine (the atlanto-occipital, and to a lesser extent the atlantoaxial segment). The more caudal segments (C2–3 to C4–5) underwent minimal movement. C-spine displacement occurred mainly during glottic visualization, whereas maximal amplitude of movement (i.e., the largest deviation from the baseline) was during the passage of the endotracheal tube for both GVL and DL.

Figure 2.: Segmental cervical (C)-spine motion with GlideScope® videolaryngoscopy (GVL, dark grey) and direct laryngoscopy (DL, light grey) during the three stages of airway manipulation, compared with baseline. Given as mean ± sd. Numbers at the base of each column correspond to the number of patients with data available to calculate each mean.

Data for motion of occiput-C1 (the segment with the greatest movement during both techniques) in every individual subject are presented in Figure 3, as well as mean movement and SD. Occiput-C1 movement exceeding 10 degrees occurred in 11 of 47 segments (23%) with DL and in 11 of 50 segments (22%) during GVL (P = 0.54); movement exceeding 15 degrees occurred in five segments (11%) with DL and in two segments (4%) during GVL (P = 0.21); movement exceeding 20 degrees occurred in three segments (6%) with DL and in none during GVL (P = 0.11).

Figure 3.: Motion of the occiput-C1 segment with videolaryngoscopy and direct laryngoscopy during the four stages of airway manipulation. Individual patient data points (algebraic values) shown in grey. Mean ± sd (of absolute values) shown in black.

Endotracheal intubation was successful in all 20 patients. With GVL, half the subjects¹⁰ showed a Cormack and Lehane glottic visualization grade¹⁶ of 1 on the video screen, whereas the other half showed a grade 2. With DL, 13 patients (65%) showed a grade 2 view, with the seven remaining patients (35%) presenting a grade 3 laryngeal view initially, but with all but 1 (5%) being reduced to grade 2 views with laryngeal manipulations (P = 0.0002 for comparison between techniques).

DISCUSSION

This study shows that GVL with the large GlideScope blade and DL induce similar movements of the intact C-spine stabilized with MILS. Furthermore, both greatest C-spine motion during any given stage and maximal amplitude of movement are reached during the same stages of airway management in both techniques, respectively, during glottic visualization and during intubation of the trachea.

Our findings are consistent with those of Turkstra et al.¹⁴ who found no difference in rostral C-spine motion during DL and GVL. Also consistent was our finding that most movement for both techniques was concentrated at the atlanto-occipital and atlantoaxial articulations. Our findings differed concerning the caudal C-spine. We found no significant difference in motion at this level, whereas Turkstra et al. found a small but statistically significant reduction with GVL compared to DL (3 vs 6 degrees, P < 0.04). The smaller C-spine motions we observed for both techniques and for every C-spine segment in this study, which could be attributed to C-spine stabilization with MILS, may explain this difference in results. Overall, since the only significant difference in C-spine movement found by Turkstra et al. was small and possibly not clinically significant, the findings of our study seem to confirm the previous study's conclusion of similar C-spine movement with both techniques.

The lack of difference in average C-spine motion between DL and GVL suggests that abolishing the need to establish a direct line of sight between the operator's eye and the glottis may not reduce C-spine motion during laryngoscopy. Although the findings of this study cannot necessarily be generalized to patients who are difficult to intubate, they do indicate that, despite providing an indirect means of glottic visualization, GVL is no different than DL concerning C-spine motion during glottic visualization.

A benefit of GVL in this study was better glottic visualization compared with DL for the same amount of C-spine movement. Improved visualization grade with the GlideScope compared with DL is already established in the literature,^20–22 and the MILS performed on all patients in this study appears to accentuate this difference. Impeding C-spine movement renders the establishment of a direct line of sight with DL more difficult, as is illustrated by our finding that a large proportion (35%) of our subjects presented a grade 3 view on DL, despite presenting no criteria predicting difficult DL on physical examination. In these patients, laryngeal manipulations, which might be relatively contraindicated in the presence of actual C-spine instability,²³ were necessary to intubate the patient. This result is consistent with the literature on the effect of MILS on the quality of glottic visualization.²⁴ Whether this improved glottic visualization with GVL actually translates into faster, easier intubation remains to be established, since our study was too small (10 patients intubated with each technique) to draw any firm conclusions. However, this study suggests that in a patient predicted to be a difficult intubation with DL and MILS who cannot be intubated awake, one should consider GVL since it might offer better glottic visualization than DL without resorting to laryngeal manipulations.

This study has several limitations, the foremost being the use of an intact spine model since the presence of intact bony and ligamentous structures limits cervical movement. Although cadaver studies with unstable C-spines generally tend to show amounts of flexion and extension comparable with those found in stable C-spine models,^2,25,26 differences have been shown.²⁷ One must therefore use caution when extrapolating our results to patients with actual cervical instability. Further complicating generalization is the fact that C-spine instability is a heterogeneous pathology,⁷ and the influence of different patterns of injury on C-spine mobility, as well as neurological morbidity during intubation, is largely unknown.

Another limitation to the interpretation of our findings is the lack of a clear threshold establishing dangerous C-spine movement.²⁸ With continuous cinefluoroscopy, it is now possible to precisely track cervical motion, but separating statistically significant differences from clinically significant ones remains difficult. In addition, a clinically significant movement in one patient might not cause prejudice in another, since many other factors such as hemodynamic instability and tissue edema are thought to compound the mechanical effect of C-spine motion on damaged neurological tissue.²⁹

A potential source of bias in this study could be the lack of blinding, since both the operators performing the laryngoscopies and the image assessors knew which technique was being executed, blinding being impossible to perform in the former and extremely difficult to achieve in the latter. Care was taken however to avoid any bias by trying to minimize C-spine motion during all laryngoscopies and by using objective criteria for image analysis. Another potential source of bias could be the order in which both techniques were performed, although this should have been minimized by the randomization process. Post hoc ANOVA on the data including the order in which the techniques were performed as a factor did not reveal any significant effect.

Although in the literature average movements have generally been used to compare two techniques,^2,14,30–33 extremes of C-spine movement could be as pertinent to examine, since it is plausible that the patients most likely to develop neurologic damage during airway management are those experiencing the greatest amount of movement. In the absence of a recognized threshold, we used cutoff values of 10, 15, and 20 degrees and did not find a statistically significant difference between techniques. Whether a larger study would have shown a significant difference is an open question.

Since the end of our recruitment phase, the introduction of a smaller, 14 mm thick, GVL blade raises the question of whether this improved GVL might decrease C-spine motion compared to the initial 18 mm thick GVL blade.

Finally, certain limitations relate to our imaging method. First, lateral cinefluoroscopy permits only the study of movement in the sagittal plane. However, both DL and GVL predominantly involve forces exerted in that plane. Second, as in previous studies,^6,26,27 we were only able to consistently image the C-spine down to C5, more caudal vertebrae being obscured by the shoulders; the study of caudal C-spine instability will require a different methodology. Finally, only the four stages where C-spine movement had been shown to be significant by Sawin et al.⁶ were examined in this study.

In conclusion, this study shows that, in patients with intact C-spines undergoing orotracheal intubation with MILS, neither average nor extreme C-spine motion was significantly different during DL or GVL. Both techniques induce movements of similar nature and amplitude, which also follow the same temporal pattern. For the same amount of C-spine motion, laryngeal visualization was superior using GVL and intubation was successful in all patients with both techniques. Further studies are necessary to specify how these findings translate to patients with actual C-spine instability.