SEARCH

SEARCH BY CITATION

Keywords:

  • Equipment, laryngoscopes; Macintosh, McCoy;
  • Anatomy; vertebral column

Abstract

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

We studied cervical spine movement in 10 patients scheduled for elective surgery under general anaesthesia. Each patient was fitted with a rigid cervical collar before undergoing direct laryngoscopy for orotracheal intubation. Laryngoscopy was performed using the McCoy laryngoscope in the activated position and the standard Macintosh blade. Displacement of the cervical spine at laryngeal exposure was measured using lateral cervical spine X-rays. Flexion and extension movements of the cervical spine during the use of the two laryngoscope blades were compared. For each blade, the greatest degree of extension occurred at the joint between the first and second cervical vertebrae. There was no significant difference in cervical spine movement when the two blades were compared.

Patients with confirmed or suspected cervical spine instability may require tracheal intubation for a number of reasons, sometimes in pressing circumstances. Laryngoscopy in these patients creates a conflict between minimising cervical spine movement and allowing sufficient laryngeal exposure to allow tracheal intubation. In routine anaesthetic practice, direct laryngoscopy involves extension of the head at the occipito-atlanto-axial complex and flexion of the lower cervical vertebrae in order to align the oral, pharyngeal and laryngeal axes and therefore allow intubation under direct vision. It is believed that patients with unstable cervical spines may be at risk during these manoeuvres and case reports exist that describe neurological deterioration following intubation [1, 2], although the actual contribution of direct laryngoscopy to these injuries remains debatable [3] and the overall risk has been estimated to be low [4]. However, there is evidence that the severity of neurological sequelae may be reduced when cervical injury is recognised early [5] and, presumably, more care is taken to minimise movement of the cervical spine. Minimising cervical movement during laryngoscopy should therefore be a natural aim in these patients.

The McCoy laryngoscope blade [6] has a hinged tip, allowing elevation of the epiglottis from a fulcrum within the pharynx. The role of the McCoy blade in anaesthetic practice is still being investigated but it has certain characteristics that might prove advantageous for intubation in patients with a suspected cervical spine injury. These include easier laryngoscopy and tracheal intubation in patients with poor laryngeal exposure [7], a reduction in the associated haemodynamic responses to laryngoscopy [8] (an important factor in combined head and neck injuries) and a reduction in the anterior–posterior forces across the cervical region during tracheal intubation [9]. In patients with cervical spine instability, this reduction in force may allow sufficient laryngeal exposure with minimal movement of the cervical spine and therefore a reduction in the risk of neurological damage. Using radiological measurements, we investigated movement of the cervical spine during laryngoscopy with the McCoy blade in patients with simulated cervical injuries.

Methods

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

Following Local Ethics Committee approval, 10 patients were recruited, all of whom gave informed verbal consent before the study. All patients were under 55 years of age, were ASA grade I or II and required orotracheal intubation as part of their planned anaesthetic technique. Specific exclusions were proven or potential difficulties with the airway or neck, including known or predicted difficult intubation, risk of gastric aspiration, cervical spine abnormality, restricted neck mobility, surgery to the airway and those patients for whom X-rays might pose a risk, i.e. those who may have been pregnant. Standard precautions for radiation protection were employed. The two laryngoscope blades compared were a Macintosh Size 3 and a McCoy III (Penlon, Abingdon, UK), with each patient acting as his or her own control.

After institution of full monitoring and pre-oxygenation for 3 min with a face mask, anaesthesia was induced with fentanyl 100 μg and a sleep dose of propofol (2–2.5 mg.kg−1). Neuromuscular blockade was provided by vecuronium 0.08 mg.kg−1. Anaesthesia was maintained with 40% oxygen in nitrous oxide and isoflurane 1–2% using a facemask and Bain coaxial breathing system. Once satisfactory manual ventilation of the patient's lungs was established and neuromuscular blockade was confirmed using a peripheral nerve stimulator, the patient's pillow was removed and an appropriately sized two-piece rigid cervical collar (Laerdal StifneckTM, Orpington, UK) was applied. A first baseline lateral cervical spine radiograph was taken in this position using a mobile image intensifier (Siemens Siremobil 4K). Laryngoscopy was then performed with one of the two blades (picked at random), employing only the minimum force required to obtain a view of the glottis that would allow a gum elastic bougie to be passed between the vocal cords. At this point, a second radiograph was taken, after which manual ventilation was continued before repeating the procedure with the other laryngoscope blade. The McCoy laryngoscope blade was inserted in the neutral position and then used in the activated position, i.e. with the hinge tip elevated. Once the third radiograph had been taken, the patient's trachea was intubated with an appropriately sized cuffed tracheal tube using a gum elastic bougie. The patient's normal grade of laryngoscopy was later confirmed when the rigid collar had been removed. All laryngoscopies were performed by the same anaesthetist who was experienced in tracheal intubation and the use of the McCoy laryngoscope. A second anaesthetist was present at all times to direct the positioning of the radiograph, so that the first four cervical vertebrae, the dorsal part of the hard palate and the caudal portion of the occiput were viewed.

The radiographs were analysed by an experienced radiologist, blinded to the purpose of the study. Reference lines were constructed for the occiput (C0) and the first four cervical vertebrae (C1–C4). These were drawn through the basal plates of the vertebral bodies for C2–C4 and as a tangent between the anterior and posterior arches of C1 (Fig. 1). The McGregor line, which joins the most dorsal and caudal part of the occiput to the posterior part of the hard palate, was used as the reference line for C0. Using the upper edge of the radiograph as a common reference, the angle between each vertebral level was measured using a goniometer. Angles between adjacent levels were calculated by difference, e.g. C1–C2 angle = (C1 to common line angle) − (C2 to common line angle). The differences in angular movement at each level produced by the two laryngoscope blades were compared using Wilcoxon's signed rank-sum test, as it could not be assumed that the data were normally distributed. Statistical significance was accepted at the p < 0.05 level.

image

Figure 1. Lateral cervical spine X-ray showing reference lines for the measurement of the angles between adjacent cervical vertebrae.

Download figure to PowerPoint

Results

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

Ten patients were recruited to the study. Six were male and four were female, with a mean (range) age of 39.3 (25–54) years. Laryngoscopy grade (Cormack and Lehane [10]) without the cervical collar was grade 1 in all 10 patients. The grade at laryngoscopy with the cervical collar applied was not recorded, as the end point of the investigation was laryngeal exposure just sufficient to achieve tracheal intubation. In four patients, we were unable to measure movements at the C0–C1 and C3–C4 joints because of technical difficulties encountered in positioning the imaging equipment.

Excursions and median values of all the investigated joints are shown in Tables 1 and 2. The greatest extension of the cervical spine was recorded at the atlanto-axial joint (C1–C2) and movement at this joint was extension in nine of the ten patients using the Macintosh blade and in all ten patients using the McCoy blade. The range of extension at this joint was 3–25°. In the other joints, flexion and extension were recorded but calculation of the median values for cervical movement revealed extension at C0–C1 and C2–C3. Movement in the cervical spine below this level was generally flexion. Movement covering the C0–C3 segment is shown in Table 3. Overall median segmental movement from C0 to C3 was 7.5° of extension using the Macintosh blade and 6.5° with the McCoy blade. There were no significant differences between the Macintosh and McCoy blades in the excursions of the cervical vertebrae during intubation.

Table 1.  Cervical spine extension and flexion during laryngoscopy using the Macintosh and McCoy laryngoscope blades in patients fitted with a rigid cervical collar. A negative value denotes flexion, a positive value denotes extension.Thumbnail image of
Table 2.  Median change in angles at segmental levels of the cervical spine during laryngoscopy using the Macintosh and McCoy laryngoscope blades. A negative value denotes flexion, a positive value denotes extension.Thumbnail image of
Table 3.  Movement of the C0–C3 segment during laryngoscopy. A negative value denotes flexion, a positive value denotes extension.Thumbnail image of

In no patient was movement of the cervical spine prevented despite the application of a rigid cervical collar.

Discussion

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

In this study, we could find no evidence for a reduction in cervical spine movement when the McCoy blade was used, although it is acknowledged that the measurements made represented only a ‘snapshot’ of the intubation sequence and that the study was performed on a small number of patients with intact cervical spines. There were also certain limitations to our protocol, namely that the end point was subjective and that the operator could not be blinded to the type of laryngoscope blade in use. However, these problems are inherent in studies of this nature.

A number of studies have investigated cervical spine movement during laryngoscopy with various designs of blade. The Miller blade [11] and the Belscope [12] have been compared with the Macintosh blade but neither showed a reduction in cervical spine movement sufficient to warrant a move away from the design familiar to most anaesthetists. The Bullard laryngoscope has been shown to produce significantly less cervical spine movement when compared with the Macintosh blade [11], although the results achieved with it were no better than those in Sawin's study [13], which used a Macintosh blade to produce laryngeal exposure at which it was just possible to intubate the trachea. However, as the two studies used different end points of laryngeal exposure, they are not strictly comparable.

Sawin et al.'s study [13], which used continuous lateral fluoroscopy, showed that the greatest movement of the cervical spine during direct laryngoscopy occurred when lifting the epiglottis forward to expose the glottis. Hence, in the present study we chose this as a logical end point at which to assess any advantage of the McCoy laryngoscope blade. Sawin et al.'s [13] study also showed that the most significant movement occurred above the level of C4 and thus displacement of the cervical vertebrae below this level was not recorded. Continuous lateral fluoroscopy was not available to us but the technique of using lateral cervical spine X-rays to measure vertebral movement has been validated in previous studies [11, 12, 14].

Current guidelines recommend removal of the front of the cervical collar and application of manual in-line stabilisation before laryngoscopy, although adequately skilled assistance to perform this may not always be available. We chose to use a fully applied collar in order to create a situation where the advantages of the McCoy blade could be maximised if the difficulty of laryngoscopy were increased using a rigid cervical collar. Therefore, for a fixed end point of laryngeal exposure, any reduction in cervical spine movement using the McCoy blade might be expected to become apparent.

The rigid cervical collar has been shown not to prevent movement of the cervical spine [15[16][17]–18]. In awake volunteers asked to move their heads, Johnson et al. [16] found that a rigid collar only reduced cervical spine movements by 70, 67 and 34% for flexion/extension, rotation and lateral bending, respectively. This was compared with a reduction of > 95% for all movements using a halo and plastic body vest. Majernick et al. [18] looked at cervical spine subluxation during intubation and, although only four patients were studied, found no difference in cervical movement between maintaining the head in a neutral position and applying a rigid cervical collar. With in-line manual immobilisation, subluxation was reduced by ≈60%. Table 4 combines Johnson et al.'s [16] results with studies investigating cervical spine extension during intubation. It is clear that direct laryngoscopy in the unrestrained neck to produce full glottic exposure can produce maximal or supramaximal extension of the intact upper cervical spine. These movements can be approximately halved if the head is maintained in a neutral position and laryngeal exposure is kept to a minimum.

Table 4.  Summary of studies investigating cervical spine extension in the awake and anaesthetised patient. Mean values are quoted. GA, general anaesthetic.Thumbnail image of

The application of a rigid cervical collar provides no additional benefit but, as it does not prevent movement of the cervical spine, any difference between the performance of the blades should have been detected in this study. Conversely, as the rigid cervical collar makes direct laryngoscopy more difficult due to the limitation of mouth opening produced, this could actually negate the advantages of the McCoy blade [19]. It might be useful to repeat the study that measured the forces across the cervical region during laryngoscopy with the McCoy laryngoscope [9] but with the addition of manual in-line stabilisation.

Table 5 summarises the studies investigating the relationship between method of cervical spine stabilisation, ease of laryngoscopy and type of blade. One in five patients with in-line manual immobilisation are recorded as Cormack & Lehane Grade 3 or worse and, although Gabbott [7] found no difference, this can increase to two out of three with the application of a rigid cervical collar [19]. What Gabbott [7] did report was that if the McCoy was used in its activated position, the incidence of grade 3 views returns to 2%. In that study, of the 26 patients recorded as difficult intubations, the activated McCoy blade produced a grade 1 or 2 glottic view in 24. In the ‘Macintosh’ (nonactivated) configuration, the difficulty of laryngoscopy may actually be increased [20].

Table 5.  Summary of laryngoscopy grades (Cormack & Lehane [10]) using different techniques of cervical spine stabilisation. Thumbnail image of

Awake fibreoptic intubation is probably the technique of choice in situations where minimal movement of the cervical spine is desirable. However, its use may be less appropriate in the emergency setting, as it requires a co-operative patient, the availability of specialised equipment and a sufficient level of operator expertise [21]. Reductions in cervical spine movement have also been shown with blind nasal [15] and oral intubation using a device such as the Augustine guide [22], although they are limited by the uncertainty of success. Blind intubation through a laryngeal mask airway, or the recently introduced intubating laryngeal mask [23], may prove more successful, though this latter device has yet to be evaluated fully, particularly in the management of the trauma patient.

In many emergency situations, direct laryngoscopy with adequate control of the cervical spine probably remains the best compromise to allow prompt tracheal intubation in patients with a potentially unstable cervical spine [24]. There is good evidence that a rigid neck collar is no better than maintaining the head in a neutral position and, if fitted, the front portion of the collar should be removed to allow mouth opening and intubation should be accompanied by in-line manual stabilisation. Sandbags and tape across the forehead can be used to reduce lateral and rotational movement [19] but no axial traction should be applied to the cervical spine [25]. Laryngoscopy should not be to full glottic exposure but to that just allowing intubation with or without the use of a bougie [26]. The McCoy blade in the activated position provides a superior view of the glottis in approximately one out of five patients with in-line manual immobilisation and it should therefore be readily available in all Accident and Emergency departments. Doctors involved in intubating the tracheas of patients with suspected cervical injury should also be skilled in its use.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

We thank the members of the Radiology Department of the Whittington Hospital for their assistance with this study and the orthopaedic surgeons for their patience. We would also like to thank the Whittington Hospital Anaesthetic Department for supplying the funding that enabled the completion of the study.

References

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References