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NEUROSURGICAL ANESTHESIOLOGY AND NEUROSCIENCE

Airway Scope and StyletScope for Tracheal Intubation in a Simulated Difficult Airway

Ryu Komatsu, MD^*, Kotoe Kamata, MD^*, Keiko Hamada, MD^*, Daniel I. Sessler, MD, and Makoto Ozaki, MD

From the *Department of Anesthesia, Kosei Hospital, Japan; Department of Outcomes Research, The Cleveland Clinic, Cleveland, Ohio; and Department of Anesthesiology, Tokyo Women’s Medical University, Tokyo, Japan.

Address correspondence and reprint requests to Daniel I. Sessler, MD, Department of Outcomes Research, Cleveland Clinic, 9500 Euclid Ave.MP77, Cleveland, OH 44195. Address e-mail to ds{at}or.org or www.or.org.

Abstract

BACKGROUND: Direct laryngoscopy is difficult when the cervical spine is immobilized. The Airway Scope® and StyletScope® are new laryngoscopes designed to facilitate intubation under these circumstances. Thus, in patients wearing a rigid cervical collar to simulate a difficult airway, we tested the hypothesis that the intubation success rates of the Airway Scope and StyletScope are similar, but that intubation with Airway Scope is faster.

METHODS: Adult patients requiring tracheal intubation as part of anesthesia were enrolled. After anesthesia induction and muscle relaxation, patients’ necks were stabilized with a rigid Philadelphia collar and patients were randomly assigned to tracheal intubation with Airway Scope (n = 50) or StyletScope (n = 50). Overall intubation success rate, time required for intubation, the number of attempts required for successful intubation, and airway complications related to intubation were recorded.

RESULTS: Overall intubation success rates were 98% with Airway Scope and 96% with StyletScope. Intubation was 19 s faster with Airway Scope (32[8] s; mean) versus StyletScope (51[29] s). The number of required intubation attempts was similar with each device: 26/18/5 (first/second/third attempt) for Airway Scope versus 26/17/5 for StyletScope. The incidence of mucosal trauma and lip injury was similar, except esophageal intubation occurred only with StyletScope (n = 6); neither dental injury nor hypoxia occurred.

CONCLUSIONS: Both the Airway Scope and StyletScope offer high success rates in a simulated difficult airway achieved by a rigid collar. However, the Airway Scope is faster and less likely to cause esophageal intubation.

According to the Advanced Trauma Life Support protocol, possible cervical spine injury requires the use of a rigid cervical collar to immobilize the neck in neutral position.1 Although removal of a cervical collar and application of manual in-line stabilization (MILS) during airway management of these patients is recommended,2 this maneuver requires experienced personnel who may be unavailable in a suboptimal prehospital setting (e.g., a roadside resuscitation). In such circumstances, a cervical collar may be left in place because it reduces cervical spine movement, at least, to some degree.3 However, direct laryngoscopy in these patients is difficult.4–6 Consequently, a number of devices have been developed to facilitate intubation in patients immobilized by a rigid cervical collar. These include Airway Scope® (Pentax, Tokyo, Japan) and StyletScope® (Nihon Kohden Corporation, Tokyo, Japan).

The Airway Scope is a new video laryngoscope for tracheal intubation (Fig. 1). It has a built-in 2.4 in. liquid crystal display powered by standard AA batteries, which displays an image from a charge-coupled device camera attached to its tip. A single-use blade (Intlock) protects the camera unit from oral contamination and accepts tracheal tubes with outside diameters between 8.5 and 11.0 mm (Fig. 2). The Intlock is designed to be positioned posterior to the epiglottis and to slightly elevate it. In this position, the camera is in front of the glottis opening. Once the glottis opening has been visualized on the display screen, a target signal, which is shown on the display, is aligned with the glottis opening, and the tracheal tube is then passed through the vocal cords. The Airway Scope allows intubation without alignment of the oral, pharyngeal, and tracheal axis. In contrast, StyletScope is a fiberoptic stylet. The StyletScope uses a plastic fiberoptic imaging system incorporated into an endotracheal tube stylet. It can be used with a tracheal tube with an internal diameter of 7.0 mm or larger. The proximal handle of the stylet connects to a 15 mm tracheal tube adaptor. An eyepiece is mounted on the handle of the scope. By depressing the lever attached to the proximal handle, the distal tip of the stylet, together with the tracheal tube, can be flexed anteriorly to provide direct access to anterior airways. (Figs. 3 and 4).

View larger version (138K): [in this window] [in a new window]   Figure 1. Airway Scope with attached Intlock blade and tracheal tube.

View larger version (166K): [in this window] [in a new window]   Figure 2. Intlock blade (disposable part).

View larger version (161K): [in this window] [in a new window]   Figure 3. StyletScope.

View larger version (173K): [in this window] [in a new window]   Figure 4. StyletScope with flexed fiberoptic tip.

METHODS

With approval of the Human Research Committee at Kosei Hospital and written informed consent, 100 patients scheduled for various surgical procedures requiring tracheal intubation as part of anesthesia were enrolled. Patients were aged 22 to 84 yr and ASA physical status I, II, or III. Exclusion criteria were increased risk of pulmonary aspiration, cervical spine pathology, anticipated airway difficulties (i.e., Mallampati grade IV or thyromental distance <5 cm), and ASA physical status >III.

Protocol
Before anesthesia, patients’ heads were elevated 7 cm with a pillow. Anesthesia was induced with fentanyl 2 µg/kg and propofol 2 mg/kg, muscle relaxation was achieved with vecuronium 0.1 mg/kg, and anesthesia was maintained with sevoflurane 2% in oxygen during the study period.

After full muscle relaxation was confirmed with a nerve stimulator, the pillow was removed and an appropriately sized, rigid Philadelphia collar (Tracheostomy Philadelphia Collar, Philadelphia Cervical Collar CO, Thorofare, NJ) was positioned around the neck. Patients were randomly assigned to intubation with an Airway Scope or StyletScope. Randomization was based on computer-generated codes maintained in sequentially numbered opaque envelopes.

For patients assigned to the Airway Scope group, an Airway Scope with a preloaded, curved tracheal tube (7 mm internal diameter for women and 8 mm for men) was inserted into the mouth and positioned at the glottic opening, which was shown at the center of the cross mark on the scope’s monitor. The endotracheal tube was then advanced into the trachea, and the scope was detached from the tube and removed from the mouth. Finally, the respiratory circuit was connected and ventilation confirmed by capnography.

For patients assigned to the StyletScope group, a StyletScope with a primed tracheal tube (7 mm internal diameter for women and 8 mm for men) was inserted into the mouth. Using a #3 Macintosh laryngoscope, the best possible view of the glottis was obtained. If the glottic view was Cormack-Lehane grade 1 or 2a,12 the tube was advanced into the trachea under direct vision. As the tube was advanced, the angle of the tip was adjusted as necessary by manually depressing the lever on the handle. However, if the glottic view was Cormack-Lehane grade 2b or more, the tracheal tube was advanced by direct vision (Cormack-Lehane grade 2b, 3a, 3b) or fiberoptically (Cormack-Lehane grade 4) to the epiglottis, which was identified by manipulation of the tip under fiberoptic vision, and then further advanced into the trachea. Finally, the scope was detached and removed, and the respiratory circuit was connected and ventilation was confirmed.

In each group, tracheal intubation was considered a failure if it could not be accomplished within 3 min or 3 attempts. Any single forward movement of either device was considered an intubation attempt. If intubation failed, the Philadelphia collar was removed, and the trachea was subsequently intubated under direct vision using a #3 Macintosh blade. All intubations were performed by a single anesthesiologist (RK) whose previous experience included 15 intubations with Airway Scope and more than 50 intubations with StyletScope.

Measurements
Morphometric data, Mallampati score, mouth opening (inter-incisor distance), thyromental distance, and sternomental distance (with head extended in upright position) were measured by an observer blinded to group assignment. With the patient in supine position and cervical collar in place, mouth opening was measured before intubation by an observer blinded to group assignment.

The following outcomes were recorded by an unblinded observer: 1) overall intubation success rate; 2) number of intubation attempts; 3) intubation time (defined as the time from picking up the Airway Scope or Macintosh laryngoscope to confirmation of intubation by capnography); 4) frequency of esophageal intubation; 5) mucosal trauma (i.e., blood detected on the devices or on the Macintosh laryngoscope for the StyletScope group); 6) lip or dental injury; and 7) hypoxia (Spo₂ <95%). Further, Cormack-Lehane laryngoscopy grade was reported with a #3 Macintosh blade in the StyletScope group. Finally, reasons for any failed intubation attempt or overall unsuccessful intubation were also recorded.

Data Analysis
Assuming the overall intubation success rate in the StyletScope group would be above 98% from the study of Kihara et al.10 in which the neck was immobilized by MILS, we decided that a 14% difference in overall intubation success rate between the groups would be clinically important. Therefore, 50 patients in each group would be necessary to detect such a difference ( = 0.05, β = 0.2, 1-tailed).

Nonparametric data were compared with the Mann-Whitney U-test. The incidence of intubation complications, overall intubation success rate, and sex distribution between the groups was tested by Fisher’s exact or ² tests as appropriate. Parametric data were compared using unpaired Student’s t-test. To evaluate learning during the study period, we compared the time required for the first 10 and last 10 intubations for each device.

Statistical analysis was performed using StatView version 5.0 (SAS Institute, Cary, NC) and Sample Power 2.0 (SPSS, Chicago, IL). Values are expressed as means unless otherwise specified; P < 0.05 was considered statistically significant.

RESULTS

Morphometric and airway assessment data of patients assigned to either Airway Scope or StyletScope were similar (Table 1).

Table 2 compares the intubation data of the two laryngoscopes. First, second, and third attempts at intubation were 26,18, and 5 for the Airway Scope and 26, 17, and 5 for the StyletScope (P = 0.93). Overall intubation success rates were 98% for Airway Scope and 96% for StyletScope (P > 0.99). Time to intubation was significantly faster with the Airway Scope (32[8] s) than with StyletScope (51[29] s; P < 0.001).

Intubation times for the first and last 10 intubations with the Airway Scope were 32(9) and 31(6) s, respectively (P = 0.70); times for the first and last 10 intubations for the StyletScope were 61(29) and 56(41) s, respectively (P = 0.80).

In the StyletScope group, the laryngoscopic view with the collar in place was graded according to the modified Cormack-Lehane scoring system (Table 2). None of the patients had Cormack-Lehane 1, 2a, or 3b views; 9 patients with a Cormack-Lehane grade 2b laryngeal view had a mean intubation time of 52(32) s; 37 patients with a grade 3a view had an intubation time of 51(29) s; and 4 patients with a grade 4 view had an intubation time of 41(15) s.

Intubation complications are listed in Table 3. Notably, 6 esophageal intubations occurred with the StyletScope compared to none with the Airway Scope (P = 0.013). Furthermore, mucosal trauma occurred in seven patients with the StyletScope versus three patients with the Airway Scope, and the incidence of lip injury was one patient with the StyletScope versus four patients with the Airway Scope. Other complications, such as dental injury or hypoxia (Spo₂ <95%), were not observed.

Reasons for failed intubation (at the first, second, or third attempt) using the StyletScope were tactile resistance (n = 12), fiberoptic view obstructed by persistent secretion or bleeding (n = 9), failure to locate the glottis (n = 6), and esophageal intubation (n = 6). Intubations failed with the Airway Scope due to the inability to position the Intlock tip posterior to the epiglottis, consequently advancing the blade into the vallecula (n = 28), and premature detachment of the tracheal tube from the Intlock (n = 3).

After three unsuccessful attempts, intubation was considered a failure. This occurred in two patients using StyletScope and in one patient using Airway Scope. In the StyletScope group, esophageal intubation occurred once and the laryngoscopic view was obstructed twice by mucosal secretions in one patient with Cormack-Lehane grade 2b view. In a second patient with Cormack-Lehane grade 3a view, intubation failed because of repeated tactile resistance (three times). In the Airway Scope group, intubation repeatedly failed in one patient because of premature detachment of the tracheal tube from the Intlock channel (three times). In each case after the third failed attempt, the cervical collar was removed and intubation was attempted with a #3 Macintosh laryngoscope. Without the collar, the laryngeal view of each patient was Cormack-Lehane grade 1 with no unusual upper airway anatomical characteristics, and thus each was easily intubated.

DISCUSSION

Neither the Airway Scope nor StyletScope has been formally tested in patients with their cervical spines immobilized by a rigid collar. Thus, we compared tracheal intubation with these two devices in patients wearing a rigid Philadelphia collar. Overall intubation success rates were 98% with Airway Scope and 96% with StyletScope, with intubation time being 19 s shorter with the Airway Scope. The number of intubation attempts and complications were similar in the two groups, except esophageal intubation occurred only with the StyletScope. In the Airway Scope group, intubation took 23 to 36 s among patients with lip or mucosal injuries. This range was similar to the average intubation time of this group. In the StyletScope group, it took 55 to 114 s among patients with lip or mucosal injuries, which was longer than the average intubation time of the group. Most injuries related to the Airway Scope occurred when the Intlock was initially introduced into the mouth probably due to the bulkiness of the Intlock in comparison to limited mouth opening of the patients wearing a collar. However, a further scooping movement of the Intlock was required in some cases to position the scope posterior to the epiglottis; this movement is not apparently related to an increased incidence of injuries. However, in the StyletScope group, a back and forth movement of this device in the mouth seemed to cause increased injuries; longer intubation times and multiple intubation attempts were also probably associated with more frequent injuries. Nonetheless, the results of the current study confirm the hypothesis that intubation success rates for Airway Scope and StyletScope were similar, but intubation with Airway Scope was faster.

StyletScope has been tested in other similar difficult airway management scenarios, such as with MILS, which simulated cervical spine immobilization.10 Kihara et al.,10 for example, showed that intubation with StyletScope was successful within 3 attempts in 64 of 65 patients exhibiting Cormack-Lehane grade 3/4 laryngeal views; the mean intubation time was 41 s and the first time success rate was 80%. However, in the current study, the mean intubation time was 51 s and the first time success rate was 52%, even when 9 Cormack-Lehane grade 2b views were included. A slightly longer mean intubation time and a lower first-time success rate were probably due to the restricted mouth opening (approximately 20 mm), resulting from use of the cervical collar. In contrast, there was presumably little, if any, restriction of the mouth opening when MILS was used.

In the current study, similar intubation times (approximately 50 s) were observed in Cormack-Lehane 2b and 3a patients. This result is predictable because, once the epiglottis is elevated from the posterior wall of the pharynx and visible with direct laryngoscopy, the tip of the StyletScope can be slipped easily to the posterior of the epiglottis. Thus, a good fiberoptic view of the glottis resulted after tip flexion.

We were concerned that the StyletScope would prove ineffective when grade 3b or 4 views were encountered because of the limited space available to slip the tip of the StyletScope beyond the epiglottis. However, we were unable to test this theory because no grade 3b views were encountered, and only four grade 4 airways were observed. Nonetheless, each of the four grade-4 patients were successfully intubated. Intubation was possible because once the tip of the device was inserted blindly into the pharynx, the epiglottis could be seen somewhat elevated from the posterior pharyngeal wall. However, it seems unlikely that the tip of a #3 laryngoscope blade could be inserted sufficiently deeply in patients wearing a cervical collar since mouth opening would be restricted and maneuverability of the blade limited, although it remains possible that the blade would sufficiently displace the base of the tongue to simultaneously lift the epiglottis.

Intubation with the StyletScope failed on 12 occasions due to tactile resistance and the esophagus was intubated 6 times. Tactile resistance was the cause of all esophageal intubations. No intubation was attempted if the glottis was not visible. However, if the view of the glottis was partial, then an intubation attempt was made by advancing the tracheal tube, which was mounted on the StyletScope. Under these conditions, the axis of the StyletScope and the trachea were frequently oblique and smooth passage of the tracheal tube through the vocal cords was often impeded. Thus, in some cases, the tip of the StyletScope advanced into the mucosal tissue surrounding the vocal cords (tactile resistance) with temporary loss of sight of the vocal cords. Consequently, the tip sometimes slipped into the esophagus, resulting in an esophageal intubation. Although the tip of the StyletScope is maneuverable and can be flexed, flexion is limited only to the vertical plane. Consequently, it is difficult to guide the device into the trachea when the glottis is approached obliquely. Presumably, this would be less difficult in patients whose mouths are wide-open and with sufficient space to align the axis of tip flexion with the sagittal axis of the patient. Other intubation attempts failed because of the inability to locate the glottis or because the fiberoptic view was obstructed by mucosal secretions. Unfortunately, the StyletScope offered a narrow view, which makes recognition of the tip’s whereabouts difficult when easily identifiable landmarks such as the epiglottis are obscured.

In the current study, similar numbers of patients required multiple intubation attempts with each device. All failed attempts in Airway Scope patients (except the single patient in whom intubation completely failed due to premature release of the tracheal tube) resulted from the tip of the Intlock failing to reach the epiglottis and advance into the vallecula. However, this was easily corrected by partially withdrawing the device and, with a subsequent scooping movement of the Intlock, lifting the epiglottis which allowed insertion of the tracheal tube. This maneuver required only a few seconds and is the likely reason the Airway Scope usually secured the airway more quickly than the StyletScope. This is reflected in the range of intubation times, which were narrower with the Airway Scope than the StyletScope (18–56 s and 21–158 s, respectively). The finding that both devices required multiple attempts of intubation and that the view of the StyletScope was obscured by blood and secretion suggests that the utility of these devices may be limited in actual trauma situations, where blood and secretions in the upper airway are frequent and oxygenation tends to be compromised.

No patients in the Airway Scope group experienced esophageal intubation. The design and functionality of the Airway Scope provides a nearly complete view of the larynx and allows the clinician to observe advancement of the tube into the trachea from outside the larynx. This continuous view presumably allows the intubator to detect advancement of tube in an inappropriate direction, which can be corrected before esophageal intubation occurs. Part of the Intlock which covers the camera unit is located posterior to the device’s tip. Hence, this part covering the camera unit is free from direct contact with mucosal tissue, which may be the reason this view was not obstructed by blood or secretion. Furthermore, the Airway Scope could be used in patients with a mouth opening as small as 20 mm despite the relative bulk of the Intlock.

The current study had a number of limitations. First, correlation between Airway Scope efficacy and Cormack-Lehane laryngeal view scores was not obtained, because direct laryngoscopy is not required with the Airway Scope. However, assuming that laryngeal scores were similar between randomized groups and given the similar performance of the two devices, it is reasonable to speculate that the Airway Scope was as effective as StyletScope for grade 3a laryngeal views. Second, no grade 3b laryngeal views were observed, prohibiting the evaluation of these devices when that view was encountered. Third, the intubating investigator had more experience with the StyletScope than the Airway Scope. Yet, this seemed to have little influence in the study, since intubation time was nonetheless significantly faster with the Airway Scope. Furthermore, there was no evidence of learning as the first and last 10 intubations required similar amounts of time. This lack of learning effect is consistent with the Airway Scope reportedly being easy to use even for novice intubators.13,14 Fourth, patients in the StyletScope group were on average 4 kg heavier than those in the Airway Scope group; they were also 2 cm taller than Airway Scope patients. However, Body Mass Index (Table 1) differed by only 1 kg/m² between the two groups. Hence, given the similar airway assessment data of both groups, it is unlikely that the difference in body weight between the groups is clinically relevant as it relates to intubation. Fifth, the current study did not include a third study arm comparing the Macintosh laryngoscope, the most commonly used intubating modality, to the Airway Scope and StyletScope. However, application of a cervical collar was expected to produce frequent Cormack-Lehane 3 or 4 views,4,6 and Nolan and Wilson reported that the success rate of intubation with direct laryngoscopy using a Macintosh blade, when Cormack-Lehane grade 3 views were generated by application of MILS and cricoid pressure, was only 62%.15 Thus, a study limb with the Macintosh laryngoscope was not included in the current study considering the expected low success rate of this modality. Furthermore, in previous studies we observed that intubation with the intubating laryngeal mask airway in patients wearing a cervical collar resulted in a 96% success rate with an average intubation time of 60 s.16 This is relatively slow and the reason the intubating laryngeal mask airway was not included in the present study. Other investigators showed that the use of the Bullard laryngoscope in patients wearing a cervical collar offered <90% success rate of intubation,6 which is relatively low and was the reason that the Bullard laryngoscope was not included in the present study. Sixth, an unblinded observer collected intubation data, and thus the incidence of intubation complications may be a source of bias.

In conclusion, both the Airway Scope and StyletScope offer high success rates in a simulated difficult airway achieved by a rigid collar. However, the Airway Scope is faster and will likely be preferred in emergency settings where accidental esophageal intubation is best avoided.

Footnotes

Accepted for publication June 7, 2008.

No corporate funds supported this project. Instruments were loaned to the investigators by the manufacturers. Dr. Sessler’s time was supported by NIH Grant GM 061655 (Bethesda, MD) and the Joseph Drown Foundation (Los Angeles, CA). None of the authors has a personal financial interest in the outcome of this research.

Received from the Department of Anesthesia, Kosei Hospital; the Department of Outcomes Research, The Cleveland Clinic; and the Department of Anesthesiology, Tokyo Women’s Medical University.

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