Anaesthetists may encounter the situation in which it is required to secure the airway in the lateral position such as accidental loss of airway patency during surgery, inadequate regional anaesthesia requiring conversion to general anaesthesia and intubation in the presence of oropharyngeal bleeding to reduce the risk of aspiration.1,2 Also, some failed intubation drills recommend tracheal intubation in the left lateral position as a rescue technique for difficult airway.3,4 However, McCaul et al.5 have pointed out that direct laryngoscopic intubation in this position is more difficult than that in the supine position. Moreover, many experienced anaesthetists may be unfamiliar with tracheal intubation in the lateral position.6 Thus, a reliable method for securing the airway in this position is necessary.
The Airway Scope (AWS, AWS-S100; HOYA-PENTAX, Tokyo, Japan), which has been available in Japan since June 2006, provides clear views of the glottis without difficulty in the majority of patients and its excellent tracheal tube passage system facilitates fast and reliable tracheal intubation under vision.7,8 Tracheal intubation using this device has been shown to be useful in many clinical situations, for example, difficult airway and potential cervical spinal cord injury.9,10 Moreover, there is evidence of rapid skill acquisition for intubation with the AWS and this method requires less operator skill.11 Thus, we hypothesised that the AWS could be used as a reliable airway device to secure the airway in the lateral position. However, there are no studies regarding use of the AWS in this position. Therefore, we evaluated the efficacy of the AWS on tracheal intubation in patients in the lateral position in comparison with the Macintosh laryngoscope.
Participants and methods
The study was approved by the Institutional Ethical Committee (Nippon Steel Yawata Memorial Hospital, Kitakyushu, Japan), and written informed consent to participate in the study was obtained from all patients. Seventy patients classified as American Society of Anesthesiologists physical status 1–3 who were scheduled for elective non-obstetric surgery in the lateral position requiring general anaesthesia with tracheal intubation were studied. Patients with BMI of more than 30 kg m−2, cervical spine abnormality, pharyngolaryngeal disorder, anticipated difficult airway and an increased risk of aspiration were excluded. The patients were randomly assigned into two groups using a sealed envelope technique: intubation with the Macintosh laryngoscope and that with the AWS.
All patients were premedicated with 50 mg ranitidine. On arrival in the operating room, each patient lay supine on the operating table, and standard monitors were applied. Following adequate pre-oxygenation, general anaesthesia was induced with 1–2 mg kg−1 propofol and 0.5–1.5 μg kg−1 fentanyl, and muscle relaxation was produced with 0.6 mg kg−1 rocuronium. The patient's lungs were ventilated with 1–4% sevoflurane in oxygen via a face mask until the train-of-four response evoked by ulnar nerve stimulation was abolished. The patient was turned to either the right or left lateral position that was determined by the site of the surgical procedure, and the patient's head was placed in the sniffing position. After confirming ease of mask ventilation in this position, an experienced anaesthetist who stood at the head of the operating table performed laryngoscopy using the AWS or the Macintosh blade in the standard fashion. External laryngeal manipulation and adjustment of the patient's head and neck position were performed as necessary. After the laryngoscopic view was assessed using the classification described by Cormack and Lehane,12 the trachea was intubated with 7.5 and 7.0 mm internal diameter standard polyvinyl chloride tracheal tubes (Smiths Medical Ltd., Hythe, Kent, UK) for men and women, respectively. The tracheal tube to which a stylet was set prior to laryngoscopy was used for intubation with the Macintosh laryngoscope in all patients. Two anaesthetists (KA and TI), who were not informed about the laryngoscope being used until the patient arrived in the operating room, performed all laryngoscopies and intubations. They experienced more than 5000 intubations with the Macintosh laryngoscope and more than 300 intubations with the AWS in the supine position. However, as they had few experiences in the lateral position, they practised tracheal intubation in this position with a mannequin (Airway Management Trainer, Laerdal Medical, Tokyo, Japan) before the study. The mannequin attached to a baseboard was placed in the right or left lateral position by holding the baseboard at 90° to the horizontal plane. The anaesthetists practised until the trachea of this mannequin could be successfully intubated with both laryngoscopes in both lateral positions within 60 s.
An assistant (IT) measured time to intubation that was defined as the time taken from insertion of the Macintosh or AWS blade between the teeth until the tracheal tube cuff was passed through the vocal cords. When the laryngoscopist did not see passage of the tube through the vocal cords, the intubation attempt was not considered complete until successful intubation was confirmed by detecting end-tidal carbon dioxide.13 To maximise patient safety, only one intubation attempt was performed in the lateral position. If intubation attempt took longer than 60 s or the tracheal tube was inserted into the oesophagus, it was deemed a failure, which was calculated as 61 s.5,13,14 Also, the patient in whom drop of SpO2 to 95% occurred during intubation attempt or mask ventilation was difficult in either the supine or lateral position was excluded from this study. In case of a failed intubation, difficult mask ventilation or occurrence of desaturation, the patient was immediately returned to the supine position. We collected data on patient demographics, Cormack and Lehane laryngoscopic grade,12 intubation time, intubation difficulty scale (IDS) score proposed by Adnet et al.15 for objective assessment of intubation difficulty and success or failure of intubation.
The IDS score is defined below.15
The IDS score is the sum of the following seven variables (0–15):
N1: number of intubation attempts greater than 1
N2: number of operators greater than 1
N3: number of alternative intubation techniques used
N4: glottic exposure (Cormack and Lehane grade minus 1)
N5: lifting force required during laryngoscopy (0 = normal; 1 = increased)
N6: necessity for external laryngeal pressure (0 = not applied; 1 = applied)
N7: position of the vocal cords at intubation (0 = abduction/not visualised; 1 = adduction).
The primary endpoint was time to intubation. A minimal sample size was estimated on the basis of time to intubation. Based on the study by McCaul et al.,5 we projected that time to intubation with the Macintosh laryngoscope in the lateral position would be 39 s with a SD of 19 s. Previous studies demonstrated that intubating laryngeal mask airway and illuminating stylet were useful airway devices in the lateral position and the mean time to intubation with these devices in the lateral position was similar to that in the supine position.16,17 Because time required for intubation with the AWS in the supine position was reported to be 16.3–23.8 s,8,10,13,18 we considered that an important change in intubation time would be a 35% absolute change. Thus, an estimated minimal sample size of 32 patients per group was required to detect this change with an α equal to 0.05 and β equal to 0.2 in a two-sided test. Statistical analysis was carried out using StatView 5.0 (SAS Institute Inc., Cary, North Carolina, USA). Demographic data were compared using a Student's t-test. Time to intubation, laryngoscopic view and IDS score were analysed using the Mann–Whitney U-test. Success rate was compared using the χ2 test with Yates correction or the Fisher's exact probability test. A P value of less than 0.05 was considered to be significantly different.
A total of 69 patients (30–88 years) were entered into the study (Table 1). Seventy patients consented to participate, but one patient was not entered into the study because of cancellation of surgery. There were no patients in whom mask ventilation was difficult. None of the patients desaturated during anaesthesia induction or intubation attempts.
In all 35 patients in whom laryngoscopy with the AWS was performed, the laryngoscopists obtained a Cormack and Lehane grade 1 glottic view, could see passage of the tracheal tube through the vocal cords during intubation attempt and successfully intubated at the first attempt (Table 1). In contrast, the Cormack and Lehane grades obtained with the Macintosh laryngoscope were distributed from grade 1 to 3 (Table 1), and there were six patients in whom the tube passage through the vocal cords was invisible. The experienced anaesthetists could not intubate within 60 s in five of 34 patients (14.7%). The trachea in four of the five patients with a failed intubation was intubated with the Macintosh laryngoscope after returning to the supine position. In one remaining patient, the trachea was intubated with the AWS in the supine position because of difficulty in intubation with the Macintosh laryngoscope. The median times to intubation with the AWS and with the Macintosh laryngoscope were 14 (interquartile range, 9–19) s and 29 (20–31) s, respectively, and a significant difference was found between them (P < 0.01, Table 1). Also, use of the AWS significantly reduced the IDS score compared with that of the Macintosh laryngoscope (P < 0.01, Table 1).
There were no significant differences in demographic data, a Cormack and Lehane grade, intubation time, an IDS score and success rate between the right and left positions in the Macintosh laryngoscope and in the AWS (Table 2). No significant difference in time to intubation between the two anaesthetists was shown.
Use of the AWS for tracheal intubation in the lateral position resulted in shorter intubation time, lower IDS score and higher success rate in comparison with that of the Macintosh laryngoscope. The AWS has potential as a useful airway device to secure the airway in the lateral position.
Although direct laryngoscopic intubation in the lateral position is an important concern for anaesthetists, there are few studies.5,6 In the current study, tracheal intubation with the Macintosh laryngoscope in the lateral position was achieved in 29 s and was successful in 85.3% of patients, which was comparable with the results of McCaul et al.5 When using the same definition of intubation time as ours, time required for direct laryngoscopic intubation in the supine position was reported to be 12–20 s.13,18,19 The lateral position markedly prolonged intubation time comparing to the supine position, although the experienced anaesthetists performed all intubations. Also, 85.3% intubation success rate was considered to be low, although there have been no studies that addressed success rate in the supine position using our definition. Direct laryngoscopic intubation in the lateral position was difficult, which was consistent with findings of McCaul et al.5
Anatomic distortion of the tongue and other tissues of the upper airway in the lateral position is different from that in the supine position because of the effect of gravity, which can make direct laryngoscopy and subsequent intubation difficult. McCaul et al.5 showed that turning from the supine position to the left lateral position resulted in deterioration of direct laryngoscopic view in about one third of patients and improvement in none. However, in this study, a grade 3 laryngoscopic view was observed in two of the 34 patients (5.9%), which was not frequent in comparison with results in the supine position in previous studies.20 A possible reason for prolonged time to intubation and low success rate using the Macintosh laryngoscope in the lateral position could be technical difficulty in an unfamiliar posture. It took longer to obtain a good glottic view. Furthermore, even when a grade 1 or 2 laryngoscopic view was attained, it took longer to confirm successful intubation because passage of the tracheal tube through the vocal cords was not seen in some patients. Although the laryngoscopist had thoroughly practised intubation in the lateral position in the mannequin prior to the study, the mannequin could not reproduce the effect of the lateral position on the upper airway tissues in humans.6 Thus, practice with the mannequin might not be sufficient for acquiring the skill of intubation in this position.
When tracheal intubation with the AWS was performed in the lateral position, tracheal intubation was successful at the first attempt in all patients and the median intubation time was 14 s with an IDS score of 0. We recently showed that intubation with this device took 18 s in the supine position, which was similar to results in the lateral position.10 In this study, the laryngoscopist had no experience of tracheal intubation with the AWS in the lateral position in patients. In contrast to direct laryngoscopic intubation, practice with the mannequin appeared sufficient to acquire the skill for intubation with the AWS in the lateral position. Intubation with the AWS may not be influenced by the difference in anatomic distortion of the upper airway tissues between the supine and lateral positions because the AWS blade was inserted into the mouth in the midline with minimal displacement of these tissues. Hirabayashi and Seo11 clarified rapid skill acquisition for this method in the supine position in the study using novice residents. Relatively easy skill acquisition may also be applicable to the lateral position. In the lateral position, tracheal intubation with the AWS was easier than that with the Macintosh laryngoscope.
Tracheal intubation with the AWS was equally successful in the right and left lateral positions. This is probably because influence of anatomic distortion of the tongue and other tissues was minimised as mentioned above. In contrast, tracheal intubation with the Macintosh laryngoscope of which blade configuration is not symmetric is considered to be easy in the left lateral position. In this study, there was no significant difference in intubation time, IDS score and intubation success rate between the right and left lateral positions during intubation attempt using the Macintosh laryngoscope. We considered two reasons for this discrepancy. First, when performing direct laryngoscopy in patients in the left lateral position, the laryngoscopist tended to see the glottis from the right side of the patient. Because the tracheal tube was also inserted from the same side, the tube often obstructed a visual field even when the glottis was clearly seen before passing the tube. Second, in most of our patients, it was not necessary to displace the collapsed upper airway tissues to the opposite side for improving glottic visualisation, even in the right lateral position. In addition, Yamamoto et al.21 demonstrated that insertion of the Macintosh blade at a point above the left molar improved the glottic view in patients with difficult laryngoscopy. This approach is difficult to perform in the left lateral position. As our findings were drawn from a small number of examinations, large-scale studies regarding a difference in difficulty of direct laryngoscopic intubation between the right and left lateral positions are needed.
There are some potential limitations of our experimental design. First, all laryngoscopies and intubations were performed by experienced anaesthetists who had few experiences of tracheal intubation in the lateral position. Thus, results in this study may not be applicable to anaesthetists who are expert in tracheal intubation in this position. However, a large majority of anaesthetists may be unfamiliar with tracheal intubation in the lateral position.6 Thus, we believe that our observations apply to most anaesthetists. Second, it was impossible to blind the anaesthetist to the laryngoscope being used, which might cause the potential for bias in intubation time and laryngoscopic view. However, we believe that this bias was of minor importance because results in this study were similar to those described in other studies and our previous study.5,8,10,13,18 Third, we defined the endpoint of intubation attempt as either passage of the tracheal tube through the vocal cords or confirmation of successful intubation using end-tidal carbon dioxide if the tube passage was not seen.13 Because the ability to clearly see the tube passage throughout intubation attempt, particularly in the lateral position, is an important performance of the laryngoscope, we used two different endpoints in order to assess this performance. Finally, to maximise patient safety, we defined a failed intubation as an intubation attempt taking longer than 60 s according to the definition in the study by McCaul et al.,5 which was the only study regarding direct laryngoscopic intubation in the lateral position (to our knowledge). It should be cautioned that our definition differed from a standard one.22
There is evidence regarding the usefulness of intubating laryngeal mask airway, illuminating stylet and a combination of both devices for tracheal intubation in the lateral position.16,17,23,24 Although direct comparison between these devices and the AWS is difficult, these devices involve a blind technique. In contrast, the AWS provides a clear view of the glottis and has an excellent tube guide system that enables correct advancement of the tube into the trachea.7,8 Thus, the advantage of the AWS over these devices is the ability to visualise the passage of tube into the trachea.
In summary, in the lateral position, intubation with a Macintosh blade took longer and a satisfactory intubation success rate was not offered. In contrast, intubation with the AWS in the lateral position was as effective as that in the supine position. These findings indicate that, in the situation in which securing the airway in the lateral position is required, the AWS is more effective than the Macintosh laryngoscope.
The present work is attributable to the Department of Anaesthesia, Nippon Steel Yawata Memorial Hospital, Kitakyushu, Japan. Financial support was provided solely from institutional and/or departmental sources. There are neither declared conflicts of interest nor affiliations with any manufacturer of any medical devices described in the article.
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