Cervical spine movement during laryngoscopy using the Airway Scope compared with the Macintosh laryngoscope

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1 doi: /j x APPARATUS Cervical spine movement during laryngoscopy using the Airway Scope compared with the Macintosh laryngoscope Y. Hirabayashi, 1 A. Fujita, 2 N. Seo 1 and H. Sugimoto 2 1 Department of Anaesthesiology and Critical Care Medicine, 2 Department of Radiology, Jichi Medical University, Yakushiji, Shimotsuke-shi, Tochigi , Japan Summary The Airway Scope is a new rigid laryngoscope. This intubation device provides a non-line-of sight view of the glottis. A non-line-of sight view is expected to cause less movement of the cervical spine during laryngeal visualisation. We compared the degree of cervical spine movement during laryngoscopy with the Airway Scope and conventional direct laryngoscope. Twenty patients requiring general anaesthesia and tracheal intubation were studied. Movements of the cervical spine were measured using radiography in the same patient during laryngoscopy with the Airway Scope and a Macintosh laryngoscope. Cervical spine movement during laryngoscopy with the Airway Scope was 37%, 37% and 68% less than that with the Macintosh laryngoscope at the C0 C1, C1 C2 and C3 C4 motion segments, respectively (p < 0.05). The movement of the atlanto-occipital distance using the Airway Scope was 42% less than that during laryngoscopy using the Macintosh laryngoscope (p < 0.05). Laryngoscopy using the Airway Scope involves less movement of the cervical spine compared to conventional laryngoscopy using the Macintosh laryngoscope.... Correspondence to: Yoshihiro Hirabayashi, MD yhira@jichi.ac.jp Accepted: 19 May 2007 Direct laryngoscopy using the Macintosh laryngoscope requires movement of the cervical spine to align the oral, pharyngeal and laryngeal axes. Elevation of the laryngoscope blade to achieve laryngeal visualisation causes superior rotation of the atlanto-occipital and atlanto-axial joints [1]. In contrast, a non-line-of sight view of the glottis using rigid fibreoptic laryngoscopes requires less cervical spine extension [2, 3]. A non-line-of sight view of the glottis is preferable in patients who require immobilisation of the cervical spine following trauma, to reduce movement of the cervical spine. The Airway Scope (Pentax, Tokyo, Japan, Fig. 1) is a new rigid laryngoscope for tracheal intubation that provides a non-line-of sight view of the glottis [4, 5]. The purpose of this study was to compare the extent of cervical spine movement during laryngoscopy using the Airway Scope and conventional direct laryngoscopy. Methods The study was approved by the ethics committee of Jichi Medical University, and written informed consent was obtained from 20 patients classified as American Society of Anesthesiologists physical status 1 2 who were undergoing general anaesthesia using tracheal intubation. Exclusion criteria included history of cervical spine injury, a difficult airway, gastro-oesophageal reflux disease, or body mass index > 30 kg.m )2. The patient rested in the supine position on the operating table without a pillow. General anaesthesia was induced using fentanyl 2 lg.kg )1 and propofol mg.kg )1. Muscle relaxation was produced using vecuronium 0.15 mg.kg )1 and complete muscle relaxation was confirmed using a nerve stimulator (TOF- Watch Ò SX, Organon, Dublin, Ireland). Anaesthesia was maintained using sevoflurane (expiratory tidal 1050 Journal compilation Ó 2007 The Association of Anaesthetists of Great Britain and Ireland

2 Y. Hirabayashi et al. Æ Cervical spine movement using the Airway Scope Figure 1 Photograph of the Airway Scope. a) Lateral view. b) Front view. concentration %) in oxygen via a bag-mask. Each patient underwent laryngoscopy using the Airway Scope and direct laryngoscopy using a Macintosh size #3 blade; tracheal intubation was completed as part of the second laryngoscopy. The order in which the laryngoscopes were used was randomly assigned at the start of the study, based on the number drawn from a random number table. Between laryngoscopies, the lungs were ventilated using a bag and mask to avoid hypoxaemia. All laryngoscopies were performed by one anaesthetist. A mobile X-ray machine (Sirius Ò 125 MP, Hitachi Medical, Tokyo, Japan) was positioned for a lateral view of the patient s cervical spine. The equipment was kept in that position throughout the examination period. A baseline radiograph was taken with the head and neck in a neutral position. During each laryngoscopy, a radiograph was taken when the best view of the larynx was obtained. A metallic bar was included in the three images as a common line, which served as a global reference. A radio-opaque scale was imposed at the mid-sagittal plane of the patient. All images were exported into the picture archiving and communication system (Synapse, Fujifilm Medical, Tokyo, Japan) for examination. Two radiologists with subspeciality training in musculoskeletal imaging measured vertebral body angles. The radiologists knew that the purpose was to compare laryngoscopes but were not familiar with either of the instruments. They had no knowledge of head extension during each laryngoscopy and the order in which the laryngoscopies were performed. The reference for the occiput (C0, McGregor line) was defined by a line drawn between the posterior margin of the hard palate and the opisthion. The C1 reference was a line between the anterior and posterior arches of the atlas. The C2-C4 reference was a tangent through the anterior and posterior Figure 2 Representative radiograph showing the reference lines for skull base (C0) and each vertebral level (C1-C4). In this figure, the lines are intentionally emphasised for identification. The actual lines used in the examination were drawn at hairline thickness for accurate measurements of the angles (not shown here). AOD ¼ atlanto-occipital distance. basal plate of the respective vertebral bodies (Fig. 2). The reference lines were determined with the consensus of the two radiologists. The angles between adjacent levels were calculated based on differences between the angles; for example, C1-C2 angle ¼ (C1 to common line angle) (C2 to common line angle). Positive angles denote extension, and negative angles flexion. The distance between the occiput and C1 (atlanto-occipital distance, AOD) was measured in mm as the length of a vertical line drawn from C1 to the occiput (Fig. 2). Data are expressed as mean (standard deviation, SD). Statistical analysis was performed using analysis of variance, followed by posthoc test using the Student-Newman- Keuls test. Significance was accepted at the p < 0.05 level. Results The patients included one male and 19 females, with a mean age of 45 (12) years, weight 57 (7) kg and height 158 (8) cm. The use of the Airway Scope allowed full visibility of the vocal cords in all cases, whereas the latter could not be viewed in two of the 20 patients when the Macintosh laryngoscope was used. However, the posterior part of the vocal cords was visible in the remaining two patients, which was sufficient to introduce a tracheal tube into the trachea. Journal compilation Ó 2007 The Association of Anaesthetists of Great Britain and Ireland 1051

3 Y. Hirabayashi et al. Æ Cervical spine movement using the Airway Scope Anaesthesia, 2007, 62, pages Table 1 Angle between occiput and C1 (C0-C1) and between adjacent vertebral bodies (C1-C2, C2-C3, C3-C4) and between occiput and C4 (C0-C4), atlanto-occipital distance (AOD) in neutral position of cervical spine (Baseline), during laryngoscopy with the Airway Scope and Macintosh laryngoscope. Values are mean (SD). n ¼ 20. Baseline Airway Scope Macintosh C0-C1; )11.0 (6.4) )0.5 (4.6)*, 5.9 (4.3)* C1-C2; 22.4 (6.2) 30.5 (5.7)*, 34.3 (4.7)* C2-C3; )0.5 (3.5) 2.1 (3.5)* 3.5 (3.4)* C3-C4; )0.8 (2.6) 1.5 (2.1)*, 5.1 (2.7)* C0-C4; 10.2 (6.0) 33.6 (6.5)*, 48.8 (5.8)* AOD; mm 16.3 (3.5) 12.7 (3.1)*, 10.5 (2.8)* *p < 0.05, relative to baseline. p < 0.05, relative to Macintosh laryngoscopy. The measured angles in the neutral cervical spine position during laryngoscopy using the Airway Scope and the Macintosh laryngoscope are shown in Table 1. Laryngoscopy with the Macintosh laryngoscope required extension of the cervical spine. All levels of the cervical spine examined had undergone significant movement compared to baseline (p < 0.05, Table 1). Compared with the Macintosh laryngoscope, laryngoscopy using the Airway Scope produced less extension of the cervical spine and this differed significantly from the baseline at each level (p < 0.05, Table 1). Extension of the cervical spine during laryngoscopy using the Airway Scope was 37%, 37% and 68% less than that produced during Macintosh laryngoscopy at the C0 C1, C1 C2 and C3 C4 motion segments, respectively (p < 0.05, Fig. 3). The cervical movement between the occiput and C4 Figure 4 Cervical spinal extension (angle C0-C4) during laryngoscopy with the Macintosh blade and the Airway Scope. Data are mean (SD). *p < 0.05, relative to baseline. p < 0.05, relative to Macintosh laryngoscopy. (C0 C4) during the Airway Scope was 39% less than that during the Macintosh laryngoscopy (p < 0.05, Fig. 4). Figure 5 shows representative radiographs in one patient demonstrating the condition of the cervical spine at baseline and during laryngoscopy with the two laryngoscopes. The Airway Scope required less extension at the cranial end of the cervical spine. Deviation of the axis of the vertebral bodies from the baseline was smaller using the Airway scope than during Macintosh laryngoscopy (Fig. 6). The average atlanto-occipital distance of 16.3 mm measured in the neutral position was significantly reduced with both laryngoscopy techniques (p < 0.05, Table 1), but the movement while using the Airway Scope was 42% less than while using the Macintosh laryngoscopy (p < 0.05). Discussion Figure 3 Extension of the cervical spine (angle C0 C1 to C3 C4) during laryngoscopy with Airway Scope and the Macintosh laryngoscope. Data are mean (SD). *p < 0.05, relative to baseline. p < 0.05, relative to Macintosh laryngoscopy. Our study has demonstrated that laryngoscopy using both the Macintosh and Airway Scope involved movement of the cervical spine. However, the extension of the cervical spine between the occiput and C4 was 39% less during laryngoscopy with the Airway Scope. Movement of the atlanto-occipital region was also 42% less when the Airway Scope was used. Direct laryngoscopy using the Macintosh laryngoscope requires extension of the cervical spine to align the oral, pharyngeal, and laryngeal axes. In the present study, Macintosh laryngoscopy involved significant extension of the cervical spine and the greatest movement of the atlanto-occipital and C1-C2 segments. This movement 1052 Journal compilation Ó 2007 The Association of Anaesthetists of Great Britain and Ireland

4 Y. Hirabayashi et al. Æ Cervical spine movement using the Airway Scope Figure 5 Lateral radiographs taken in one patient showing the condition of the cervical spine at baseline with the head and neck in a neutral position without pillows (a) and during laryngoscopy with Airway Scope (b) and the Macintosh laryngoscope (c). Laryngoscopy with the Macintosh laryngoscope involves a significant elevation of the glottis and cervical spinal extension. For example, the upper cervical spine is more curved at (c) than at (a) or (b). The angle between the occiput and C4 (C0 C4) was 16, 35 and 56, in (a), (b) and (c), respectively. The atlanto-occipital distance (AOD) was 16 mm, 10.6 mm and 7.7 mm, in (a), (b) and (c), respectively. Figure 6 Deviation of the axis of the cervical vertebral bodies from the baseline during two laryngoscopic techniques. The X-ray figures during the Airway Scope (a) and Macintosh laryngoscopy (b) were superimposed over the same baseline figure. Solid circles: centres of the vertebral bodies (C2, C3 and C4) at baseline, open circles: centres of the vertebral bodies (C2, C3 and C4) during laryngoscopy. Deviation of the axis of the cervical spine seems to be significantly less in the Airway Scope than in the Macintosh laryngoscope. of the cervical spine during Macintosh laryngoscopy was consistent with previously reported results [1, 6]. In patients undergoing general anaesthesia, elevation of the laryngoscope blade for laryngeal visualisation causes the greatest extension at the atlanto-occipital and C1-C2 joints, whilst the subaxial cervical segments (C2-C5) are displaced only minimally [1]. In healthy volunteers using topical anaesthesia, extension at the craniocervical junction is maximal during direct laryngoscopy and there is progressively increasing extension from C4 to the base of the skull [6]. These cervical spinal movements induced by direct laryngoscopy have been reported to be little influenced by the type of the laryngoscope blade, including Macintosh, Miller and McCoy blades [2, 7]. In comparison with direct laryngoscopy, a non-line-of sight view of the airway is likely to require less extension of the cervical spine. The Airway Scope resulted in less cervical extension compared with the Macintosh laryngoscope. The extension of the cervical spine between the occiput and C4 was 39% less during laryngoscopy with the Airway Scope than with the Macintosh laryngoscope. Movement in the atlanto-occipital region was also 42% Journal compilation Ó 2007 The Association of Anaesthetists of Great Britain and Ireland 1053

5 Y. Hirabayashi et al. Æ Cervical spine movement using the Airway Scope Anaesthesia, 2007, 62, pages less with the Airway Scope. The Bullard laryngoscope is the most widely used rigid laryngoscope for tracheal intubation in patients with difficult airway and or cervical spine injury. Watts et al. compared cervical spine extension and time to intubation using the Bullard and Macintosh laryngoscopes during a simulated emergency with cervical spine precautions taken [3]. In their hands, the use of the Bullard laryngoscope resulted in reduced extension of the cervical spine. Extension of the cervical spine (occiput-c5) was reduced from 25.9 (2.8) using the Macintosh laryngoscope to 12.6 (1.8) using the Bullard laryngoscope. Hastings et al. also reported that the median atlanto-occipital extension angle was 12 and 6 for the Macintosh and Bullard laryngoscopes, respectively [2]. Bullard laryngoscopy requires a significantly smaller degree of extension compared with the direct laryngoscopy approach. From the point of view of reduced movement of the cervical spine during laryngoscopy, a non-line-of sight view of the airway using anatomically shaped rigid laryngoscopes would seem to be more suitable than the Macintosh laryngoscope. Our measurements were obtained in a neutral head and neck position without the use of pillows, and hence the results cannot be applied to patients with their heads held in the sniffing position, which is commonly recommended for direct laryngoscopy. The presence of a pillow for sniffing position provides greater occipito-atlanto-axial angulation than simple head extension in healthy volunteers [8]. As the presence of a pillow significantly affects the configuration of the cervical spine, we opted to exclude it from our protocol. The aim of this study was to identify which intubation method is preferable from the point of view of reducing the movement of the cervical spine in trauma patients who require immobilisation of the cervical spine. The anatomically shaped blade of the Airway Scope is likely to offer advantages when the patient s head and neck are held in-line. In the present study, the exposed glottis with the Airway Scope was always in full view, whereas this was achieved in only 18 of the 20 patients using the Macintosh laryngoscope. In comparison with the Macintosh laryngoscope, the Airway Scope provided the airway operator with improved laryngeal exposure, without excessive extension of the cervical spine. Manual in-line immobilisation is one of the methods recommended for reducing cervical spine extension. It reduces total spinal movement during laryngoscopy [2, 9], but often leads to a reduced laryngoscopic view [10]. Although manual in-line immobilisation is expected to be an effective stabilising technique, it may make direct laryngoscopy more difficult in some patients than when no immobilising forces are applied [7]. In this regard, the Airway Scope may provide better laryngoscopic view than does the direct laryngoscope using manual in-line immobilisation. To minimise interoperator variability, all laryngoscopies were performed by a single investigator in the operating room under ideal conditions. The results of the present study may underestimate the degree of cervical spine extension compared with true emergency situations. The laryngoscopist, however, endeavoured to obtain the best view of the larynx during each laryngoscopy and the results were still valid for direct comparison of two devices. The airway operator could not be blinded to the laryngoscopes used. The airway operator may have potentially introduced bias that could have resulted in cervical spine extension. However, the degree of cervical extension with direct laryngoscope in our study was similar to that reported in a previous study [11]. Radiographic biases were also possible, because the laryngoscopes appeared on the radiographs. However, the radiologists who examined the radiographs were unfamiliar with the laryngoscopes and completely blinded to the position of the cervical spine during laryngoscopy. In conclusion, laryngoscopy using the Airway Scope involved less extension of the cervical spine compared with conventional laryngoscopy using the Macintosh laryngoscope. From the point of view of reduced movement of the cervical spine during laryngoscopy, intubation with the aid of the Airway Scope would seem to be a more suitable method than with the Macintosh laryngoscope. Acknowledgements The authors thank radiology technologists, Sho Kawai and Yasuhiro Koshiji, for their excellent assistance. This study was performed at Jichi Medical University Hospital. Support was provided solely from institutional and or departmental sources. Conflict of interest The authors have no affiliation with any manufacturer of any devices described in the manuscript. The authors declare that we have no conflict of interest. References 1 Sawin PD, Todd MM, Traynelis VC, et al. Cervical spine motion with direct laryngoscopy and orotracheal intubation. An in vivo cinefluoroscopic study of subjects without cervical abnormality. Anesthesiology 1996; 85: Hastings RH, Vigil AC, Hanna R, Yang B-Y, Sartoris DJ. Cervical spine movement during laryngoscopy with the Bullard, Macintosh, and Miller laryngoscopes. Anesthesiology 1995; 82: Journal compilation Ó 2007 The Association of Anaesthetists of Great Britain and Ireland

6 Y. Hirabayashi et al. Æ Cervical spine movement using the Airway Scope 3 Watts AD, Gelb AW, Bach DB, Pelz DM. Comparison of the Bullard and Macintosh laryngoscopes for endotracheal intubation of patients with a potential cervical spine injury. Anesthesiology 1997; 87: Koyama J, Aoyama T, Kusano Y, et al. Description and first clinical application of AirWay Scope for tracheal intubation. Journal of Neurosurgical Anesthesiology 2006; 18: Asai T, Enomoto Y, Okuda Y. Airway Scope for difficult intubation. Anaesthesia 2007; 62: Horton WA, Fahy L, Charters P. Disposition of the cervical vertebrae, atlanto-axial joint, hyoid and mandible during x-ray laryngoscopy. British Journal of Anaesthesia 1989; 63: Crosby ET. Airway management in adults after cervical spine trauma. Anesthesiology 2006; 104: Takenaka I, Aoyama K, Iwagaki T, Ishimira H, Kadoya T. The sniffing position provides greater occipito-atlanto-axial angulation than simple head extension: a radiological study. Canadian Journal of Anesthesia 2007; 54: Majernick TG, Bieniek R, Houston JB, Hughes HG. Cervical spine movement during orotracheal intubation. Annals of Emergency Medicine 1986; 15: Nolan JP, Wilson ME. Orotracheal intubation in patients with potential cervical spine injuries. An indication for the gum elastic bougie. Anaesthesia 1993; 48: Rudolph C, Schneider JP, Wallenborn J, Schaffranietz L. Movement of the upper cervical spine during laryngoscopy: a comparison of the Bonfils intubation fiberscope and the Macintosh laryngoscope. Anaesthesia 2005; 60: Journal compilation Ó 2007 The Association of Anaesthetists of Great Britain and Ireland 1055