The Pentax-AWS® rigid indirect video laryngoscope: clinical assessment of performance in 320 cases*

Authors


  • *

    This work was partially presented at Annual Meeting of American Society of Anesthesiologists 2007 in San Francisco as an abstract on 14th October, 2007

Akihiro Suzuki
E-mail: masuikasuzuki@yahoo.co.jp

Summary

The Pentax-AWS® airway scope system is a rigid indirect video laryngoscope with integrated tube guidance. Laryngoscopy and intubation are visualised using a built in LCD monitor which displays the view obtained by a CCD camera mounted in the tip of the laryngoscope. We describe its clinical performance in 320 patients. The Pentax-AWS significantly improved the laryngeal view compared to the Macintosh laryngoscope. Forty-six patients (14%) who were classified as Cormack Lehane glottic view grade 3 or 4 using the Macintosh laryngoscope were classified as grade 1 (45 cases) or 2a (1 case) using the Pentax-AWS airway scope. Laryngeal views measured by percentage of glottic opening score were improved significantly using the Pentax-AWS. Intubation using the Pentax-AWS was successful in all cases, 96% at the first and 4% at the second attempt. The mean (SD) time required to place the tracheal tube was 20 (10) s. The Cormack Lehane grade obtained with the Macintosh blade did not affect the total time to correctly position the tube using the Pentax-AWS. Intubation difficulty scale (score = 0 in 305 patients, score = 1 in 14 and score = 2 in one patient) indicates that tracheal intubation was performed easily in most cases. The Pentax-AWS not only improves the laryngeal view, but its tube guide also facilitates rapid, easy and reliable tracheal intubation under vision. It can be useful in routine anesthesia care and may be advantageous in the situation of unanticipated difficult intubation.

Introduction

The Pentax-AWS® system (AWS; Pentax Corporation, Tokyo, Japan) is a new rigid indirect laryngoscope which has an integrated tube passage function. The AWS consists of: a camera, monitor, and single-use blade (Pblade®) incorporating a channel for passage of the tracheal tube (Fig. 1). The Pblade is anatomically designed to pass behind the tongue. The channel on the right side of the Pblade guides the tube. The images from the charge-coupled device (CCD) camera are displayed on a built-in 2.4-in colour liquid crystal device (LCD) monitor. Visualisation of the vocal cords is achieved without the need to displace the tongue laterally to obtain a direct line of sight, as required with direct laryngoscopy [1]. A target mark on the monitor indicates the tracheal tube direction of advance, and is intended to facilitate intubation.

Figure 1.

 Pentax-AWS® (Airway Scope®) rigid video laryngoscope system. Left: The Pentax-AWS system (AWS). The Pentax-AWS is a rigid indirect video laryngoscope. The view from the CCD camera is displayed on the built-in colour LCD monitor. The monitor screen has a wide viewing angle so that other staff can verify progress of the tracheal intubation. On the monitor, a cross hair line (target mark) indicates the tracheal tube direction from the tube channel. The images can also be displayed on slave monitor by using the external video output. The Pblade® is a single use laryngoscope blade which is anatomically designed to pass behind the tongue. The Pblade is supplied sterile and fits over camera cable of the AWS to eliminate cross-infection risk. The channel on the right side guides the tracheal tube. Middle: Assembled AWS with the tube in the channel. Right: AWS inserted in the pharynx of the manikin. Its J-shape avoids the requirement for a line of sight from above the maxillary teeth.

The objective of this study was to evaluate the efficacy of the AWS for tracheal intubation. We compared the laryngeal views obtained using the AWS and Macintosh laryngoscope (MacL) in 320 patients undergoing elective surgery. In addition, the number of attempts required for intubation, the time taken to pass the tracheal tube, any difficulties encountered during the intubation process and any postoperative complaints were documented.

Material and Methods

The protocol was approved by the clinical research committee of each institution, and written informed consent was obtained from 320 unpremedicated ASA I – III patients aged > 18 years scheduled for elective surgery, requiring general anaesthesia with tracheal intubation. The research period was from 1 November 2006 to 30th September 2007. Patients who had unstable teeth or has mouth opening of < 18 mm were excluded from the study as the AWS blade is 18 mm thick. All intubations were performed by anaesthetists who had > 2 years anaesthetic experience, and were skilled in both the AWS and the MacL in order to eliminate a learning process bias. Mallampati classification without phonation and thyromental distance were evaluated prior to surgery.

Anaesthetic technique

Patients were placed in the supine position, the neck was in a neutral position with a pillow under the head. The patients breathed 100% oxygen for 3 min. Induction of anaesthesia was performed using fentanyl (1 μg.kg−1) and propofol (1.5–2.0 mg.kg−1) intravenously. Following loss of consciousness, confirmed by loss of eyelid reflex, positive pressure mask ventilation was initiated. Anaesthesia was then maintained using sevoflurane (3–5%) in oxygen. Once adequate mask ventilation was confirmed, vecuronium (0.1–0.12 mg.kg−1) was administered.

Airway management

Following verification of complete muscular paralysis using a peripheral nerve stimulator, laryngoscopy was performed with the MacL. The operator was allowed to change the patient’s head and neck position, if necessary, in order to achieve the best laryngeal view. This was evaluated with both Cormack and Lehane classification (CL) modified by Cook [2, 3] and the Percentage of Glottic Opening (POGO) [4]. External laryngeal manipulation [5] was not applied during the laryngeal evaluation with MacL. This laryngoscope was then removed, and further mask ventilation was given. Then the AWS, with the tracheal tube (standard, curved tube, 7.0–7.5 mm ID for females and 7.5–8.0 mm for males) pre-inserted in the tube channel, was rotated over the dorsum of the tongue. Anatomical structures were visualised on the monitor as the scope was advanced. The tip of the AWS was passed posterior to the epiglottis, and a gentle lifting force was then used to elevate the epiglottis. The position of the AWS was adjusted to optimise the glottis view. The CL grade and POGO score were evaluated (Fig. 2). The tube was then advanced into the trachea under vision until the depth marker was at the level of the vocal cords. If the tube impinged on the arytenoids cartilages or other structures, the AWS position was further adjusted to complete the tube passage. External laryngeal manipulation was not applied during laryngoscopy and intubation. The AWS was connected to a hard disk drive recorder in order to store a visual record throughout the procedure.

Figure 2.

 Modified Cormack and Lehane classification (CLAWS) for the Pentax-AWS. The epiglottis should be lifted with the tip of the AWS to achieve the best laryngeal view, a technique which is similar to the straight and different from the Macintosh laryngoscope. The AWS laryngoscopy grade was based on Cook’s modified Cormack and Lehane classification [3].

When any of the following situations occurred, the AWS was removed and mask ventilation employed: i) If the SpO2 fell below 95% during the procedure. ii) The view was inadequate due to fogging or secretions. iii) The tracheal tube came out of the channel or any other unexpected technical problem occurred during the procedure. In these situations, the AWS and the tube were restored to their initial setting. If the tracheal tube could not be placed at an ideal depth within three attempts, the operator was allowed to use a different device for intubation. If ventilation proved impossible, placement of LMA Fastrach® (LMA North America, Inc., San Diego, CA, USA) was recommended.

The total time for intubation was defined as the time taken from the blade first passing the incisors until passage of the tracheal tube was completed. The number of intubation attempts and the incidence of the tube tip impingement on glottis tissue were recorded. The Intubation Difficulty Scale (IDS) score [6] was calculated after each intubation.

The AWS was inspected for blood staining in order to evaluate oropharyngeal trauma during the procedure. Following surgery, all patients were asked to report if they had a sore throat or laryngeal hoarseness.

Statistics

Paired Student’s t-test was used for comparison of the POGO score between the MacL and AWS. Mann–Whitney U-test was used for comparison of CL grade between MacL and AWS. Kruskal–Wallis analysis of variance (ANOVA) was employed to compare the time taken for ETT placement and the number of intubation attempts for each CL grades obtained with MacL. Data are expressed as mean (SD) unless otherwise noted. A value of p < 0.05 was considered to indicate statistical significance.

Results

The patient details studied are shown in Table 1.

Table 1.   Patient profiles. Data are presented as mean (SD) (min–max), or the actual number.
 n = 320
  1. ASA PS, American Society of Anesthesiologists Physical Status.

Male/female163/157
Height; cm159 (12) [135–187]
Weight; kg 59 (13) [28–123]
Thyromental distance; cm  7.4 (1.0) [4.0–10.5]
Age; years 55 (19) [18–90]
ASA PS (I/II/III)133/146/41
Mallampati class (I/II/III/IV)122/143/49/6

Tracheal intubation using the AWS was successful in all patients.

The laryngeal views in the 320 patients in whom both MacL and AWS laryngoscopy were performed are compared in Table 2. The CL grades obtained with the MacL were distributed from grade 1–4. However, the CL obtained with the AWS were grade 1 in 317 cases (99.1%) and grade 2a in the remaining three cases. All forty-six patients (14.4%) who were CL grade 3 or 4 using the MacL were grade 1 (45) or grade 2a (1) when the AWS was used. No grades of 2b or greater were seen. The AWS significantly improved the CL grade compared to the MacL. Risk factors for difficulty with MacL such as increased body mass index, Mallampati classification three or four, and the thyromental distance were not associated with failure to visualise the larynx or an increase in IDS score (Table 3). The POGO score was significantly improved with the AWS (Fig. 3), in particular when the MacL CL was 2a or greater (90 (13%) vs 13 (18%), p < 0.0001, n = 145). The influence of CL grade with MacL on duration and number of attempts at intubation with the AWS is shown in Table 4 (Table 4). The total time required to pass the tracheal tube was 20 (10) s and there were no significant differences between the MacL CL groups. The tube was successfully placed at the first attempt in 308 cases (96.3%) and the remaining 12 patients required two attempts. The cases which required two attempts were caused by an inadequate view due to fogging or secretions (11 cases), or the tube becoming detached from the channel (one case). The SpO2 did not fall below 95% in any case. The number of intubation attempts was 1.04 (0.2), and only the MacL CL grade 4 had a significant effect.

Table 2.   Comparisons of laryngeal view obtained by Macintosh blade (MacL) and Pentax-AWS® (AWS) in 320 patients. Data are presented as actual number (percentage).
 View by MacL
CL 1 54.7%CL 2a 21.3%CL 2b 9.7%CL 3a 7.8%CL 3b 5.3%CL 4 1.3%
  1. There are no CLAWS greater than grade 2a with the AWS despite the widely distributed CL values with the MacL. View was significantly improved with AWS (*p < 0.001).

  2. CL, Cormack and Lehane classification; CLAWS, modified CL classification for AWS.

View by AWS*
 CLAWS 1 (99.1%)173683125173
 CLAWS 2a (0.9%)200001
 CLAWS 2b000000
 CLAWS 3a000000
 CLAWS 3b000000
 CLAWS 4000000
Table 3.   ‘Risk’ characteristics vs laryngeal view and IDS with the Pentax-AWS.
CategoryNumberCLAWS1/2a/≥ 2bIDS 0/1–5/≥ 6
  1. Obesity (BMI > 30), high Mallampati class, and reduced TMD did not appear to be associated with difficult visualisation or intubation using the Pentax-AWS.

  2. IDS, Intubation difficulty scale; CLAWS, modified Cormack Lehane classification for AWS; BMI, Body mass index; TMD, Thyromental distance.

BMI > 304242/0/041/1/0
Mallampati
class 3–45554/1/054/1/0
TMD < 7 cm6361/2/061/2/0
Figure 3.

 Comparison of the Percentage of glottic opening (POGO) score for Macintosh (MacL) and Pentax-AWS (AWS). Left box: comparison of all cases of Cormack and Lehane grade with MacL (n = 320). Right box: comparison of cases of MacL CL grade 3 to 4 (n = 46). The bold horizontal bar represents the median value of the POGO score for each laryngoscope (box = interquartile range; whisker = range) *p < 0.0001 compare to MacL.

Table 4.   Intubation data with Pentax-AWS®. Data are presented as mean (SD) (range), or the actual number.
Laryngeal view obtained with MacLGrade 1 n = 175Grade 2a n = 68Grade 2b n = 31Grade 3a n = 25Grade 3b n = 17Grade 4 n = 4Overall n = 320
  1. The time to intubation is not affected by the Macintosh grade. The number of intubation attempts was not different except for MacL grade 4.

  2. ETT, Tracheal tube; CL, Cormack and Lehane classification; AWS, Pentax-AWS®; IDS, Intubation difficulty scale. *p < 0.0001 to other grade.

Time to place the ETT with AWS; s 19.6 (8.2)  [7–45]21.8 (8.3) [9–51]24.1 (9.4) [10–52]22.4 (9.3) [11–37]19.1 (6.2) [11–30]40.0 (34.1) [12–87] 20.1 (9.6)  [7–87]
Number of intubation attempted with AWS  1.1 (0.2) [1–2] 1.0 (0) 1.0 (0) 1.0 (0.2) [1–2] 1.1 (0.2) [1–2] 1.5 (0.6*) [1–2]  1.0 (0.2) [1–2]
IDS with AWS (score 0/1–5/≥ 6)165/10/068/0/031/0/024/1/016/1/0 2/2/0305/15/0

The number of instances of tube impingement on the right arytenoid cartilage or other laryngeal structures occurred in 14 cases (4.4%), but these were easily resolved by adjusting the direction of the blade. The IDS score was 0 in 305 patients, and 1 in 14 patients, and 2 in one patient. The IDS scores of 1 were caused by two intubation attempts or CL grade > 1, and the IDS score of 2 was due to two intubation attempts by two operators.

Mild oropharyngeal trauma was confirmed with blood stain on the tip of the blade in 11 patients, postoperative sore throat was reported in 61 patients, and hoarseness in 27 patients. All symptoms disappeared within 48 h and there were no major adverse events recorded any of the 320 patients.

Discussion

Our results demonstrate that, in all patients, the AWS provides an improved view of the larynx compared with the Macintosh. Despite the full distribution of CL grades obtained with the Macintosh, the view obtained with the AWS was mostly grade 1, with a few grade 2a. This study includes patients with anticipated difficult airway, and therefore reflects typical clinical situations that anaesthetists may encounter. The POGO score was significantly improved using the AWS. In particular in cases where the glottic opening could not be seen with the Macintosh (POGO = 0%, CL grade 2b or higher, 77 cases), the average POGO score with the AWS was 88 (13%). This implies that, with the AWS, there will be an extremely low risk of encountering difficult laryngoscopy. This finding may be very important in clinical practice.

These results are probably a consequence of the structural and functional feature of the AWS. Firstly, the single use Pblade attached to the AWS is anatomically designed to conform to the shape of the mouth and pharynx and to be passed over the dorsum of the tongue so that the proximal end of the line of sight is posterior to the tongue. Minimal displacement of the tongue and other soft tissues is necessary. Secondly, direct elevation of the epiglottis, as with straight laryngoscopes, is more reliable than when using the indirect Macintosh technique [7].

The most important finding is that the AWS facilitates fast and reliable tracheal intubation under direct vision, particularly in patients who are CL grade 3 or 4 with the Macintosh laryngoscope. When using the AWS, all patients were intubated under vision within two attempts, there was no trauma associated with blind attempts at tube passage, and there were no failed intubations. Overall intubation success rate was 96% at the first attempt and reached 100% at the second attempt. Ninety-six per cent success ratio at the first attempt is higher than that achieved with other intubation devices such as the Wuscope (84%) [8] Lighted stylet (87%) [9], the Styletscope (94%) [10], and the Glidescope (94%) [11].

Many new rigid indirect video laryngoscopes are now available, but the improved laryngeal views are not always matched with intubation success rates, and include some failed intubations [11]. We believe that an integrated tracheal tube passage system, as in the AWS is likely to contribute to a high tracheal intubation success rate. The IDS score of 0 in most cases (95%) confirms the ease of intubation. Only 5% had an IDS score of 1 or 2, which represents slight difficulty. No cases had an IDS score > 5, which indicates moderate to major difficulty.

The time to place the tracheal tube was approximately 20 s, which means that the risk of developing hypoxaemia is low. In addition, the total time required to place the tube was independent of the CL grade obtained with the Macintosh. This is similar to the results obtained in our pilot study of 100 healthy patients without anticipated difficult airways (data not included in this study) [12]. This suggests the AWS can rapidly solve most of the difficult intubations associated with the Macintosh.

The speed of tracheal tube passage is probably a consequence of the functions of tube channel of the Pblade and the target mark shown on the monitor. The distal end of the tube channel faces the vocal cords after the best laryngeal view is achieved. The centre of the target mark indicates the direction of tube travel from the tube channel. The optimal alignment of the integrated tube channel avoids the need to consider adjustment of the stylet angle [13], and this may represent a significant advantage over other video laryngoscopes such as the Glidescope® (Verathon Inc., Bothell, WA, USA) or the McGrath® laryngoscope (Aircraft Medical Limited, Edinburgh, UK). Tracheal tube impingement onto the arytenoid cartilages during intubation was observed in 4% but was easily managed by adjusting the blade direction. All these features contribute to fast and reliable intubation under vision.

We had a relatively high incidence of difficultly associated with laryngoscopy using the Macintosh laryngoscope (CL 3 and 4) and several reasons were considered for this. The only exclusion criteria were restricted mouth opening and unstable teeth, so that patients with other anticipated difficult airways were included and the two hospitals where the study was conducted have a high proportion of cases of ear-nose-throat and orthopaedic surgery. Most large prospective studies have excluded patients in whom difficult airway management was anticipated. In common with most other comparative studies, we did not apply external laryngeal manipulation during laryngoscopy with the Macintosh for CL grade comparison between direct and indirect laryngoscopes as previous studies [14, 15]. Omission of external laryngeal manipulation will have increased the incidence of CL grade 3 and 4 laryngeal views.

There were four cases of CL grade 4 with the Macintosh. It took 18 s to place the tube at the first attempt in a patient who was in Halo fixation following a cervical injury. The second patient had restricted mouth opening (1.9 cm) after radiation therapy for pharyngeal tumour. It took 87 s to place the tube at the second attempt but the main reason for the prolonged procedure was the time required to remove adherent clot and secretions, attached anterior to the vocal cords. In the third patient who had postburn contracture, it required 43 s to place the tube at the second attempt. The view was obscured by secretions and bleeding caused by the first attempt with the Macintosh. In the fourth case, the patient had restricted mouth opening (2.2 cm) due to arthritis of the temporomandibular joint and it took 12 s to place the tube. These cases of successful and rapid atraumatic intubation under vision with the AWS in difficult airways indicate the potential of the AWS.

Blood staining of the blade tip after intubation was noted on 11 occasions. Since at least two laryngoscopies (MacL and AWS) were performed, some of the bleeding might have been caused by the initial laryngoscopy with the Macintosh. The shape, structural features and lack of stylet may reduce the risk of pharyngeal trauma with the AWS. The airway is observed throughout the procedure with camera and monitor.

The Macintosh laryngoscope has been a ‘gold standard’ for routine tracheal intubation for over 60 years. However, failure to achieve a view of the glottis can result in difficult intubation, and recent studies show incidences of CL grade 3 or 4 view with the Macintosh of between 10–11%. [16, 17]. Attempted blind passage of the tracheal tube when the larynx is not visualised may require multiple attempts and cause serious soft tissue damage [18–21]. Therefore, alternative visual techniques have been recommended in the Difficult Airway Society guidelines [22]. These visual techniques require immediate availability of a sterile flexible fibreoptic laryngoscope. Part of the tube advancement is blind with some visual devices, such as Glidescope®, and palatopharyngeal arch perforation by the tracheal tube has been reported [23]. The ideal technique would involve visualisation throughout the procedure from blade entry into the oral cavity until correct tube placement into the trachea. The AWS functions in this way.

An additional advantage of the AWS is that, unlike indirect laryngoscopes which use an eye-piece, the view on colour monitor can be shared with assistants. This feature can be useful not only for training but also to facilitate management of difficult situations. The assistant can observe progress of laryngoscopy and intubation without the need for additional monitors. In addition, the sterile single-use Pblade meets modern standards for prevention of cross-infection.

In conclusion, the Pentax-AWS has potential advantages over standard Macintosh direct laryngoscopy for tracheal intubation. Tracheal intubation is achieved under vision in a much higher proportion of patients and all tracheal tube manipulations are observed. Some features of the design offer advantages over existing alternatives to the Macintosh laryngoscope. The Pentax-AWS has great potential to become a standard technique of laryngoscopy for tracheal intubation.

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