Introduction
Controlled ventilation in patients can be performed with different airway devices [1] [2] [3] [4,5] [6–8] [9]
Over the last few years, video-assisted endoscopic techniques have successfully been introduced into various surgical disciplines. In contrast, anaesthetists have been reluctant to take up the advantages of the video technique for their purposes. First attempts were undertaken with improvised instruments combining laryngoscopes and flexible fibrescopes [10] [11–14]
This prospective randomized trial investigated whether a Macintosh-shaped video laryngoscope [direct-coupled interface (DCI) video laryngoscope] and a specifically designed video laryngoscope (GlideScope) can improve glottic view and intubation conditions in patients with a potentially difficult airway compared with a conventional direct laryngoscope.
Patients and methods
Study design and patients
One hundred and twenty patients scheduled for elective minor surgery requiring endotracheal intubation according to our routine anaesthesiological management were enrolled in this study. Institutional Review Board (IRB) approval was secured from our local ethics committee. After examination during the routine premedication visit, written informed consent was obtained from all patients. Enrolment criteria included patients of at least 18 years of age, ASA classification ≤3, presenting with at least one positive predictor of a difficult airway: Mallampati score at least 2, reduced mobility of the atlantooccipital joint (≤15°), mouth opening (≤38 mm) and thyromental distance (≤65 mm). Exclusion criteria were refusal of participation, indication for rapid sequence induction and known difficult face-mask ventilation. The investigation was carried out by two board-certified anaesthetists (G.S., V.D.). Both were familiar with all the laryngoscopes investigated (≥50 intubations each).
Laryngoscopes
Conventional direct laryngoscopy was performed with a battery handle laryngoscope equipped with Macintosh blades (Heine Optotechnik, Herrsching, Germany) as routinely used in our department (Fig. 1 ). A Macintosh blade size 3 was routinely used for female and male patients; a size 4 blade was used only for tall individuals.
The DCI video laryngoscope (Karl Storz, Tuttlingen, Germany) was designed as a further refinement of previous Macintosh video laryngoscopes by Berci (Figs 1 and 2 ) [15]
The GlideScope video laryngoscope (Saturn Biomedical, Burnaby, Canada, now Verathon Medical, Bothwell, USA, Figs 1 and 2 ) was developed in 2003 [16] Figs 1 and 2 ). Insertion of the GlideScope into the oral cavity resembles conventional laryngoscopy, whereas further advancement to the plica glossoepiglottica has to be performed under indirect visual control via video monitor. No preparation routine is required to use the GlideScope.
Study protocol
After establishing an intravenous catheter and standard cardiovascular monitoring, including ECG, noninvasive blood pressure, peripheral oxygen saturation and capnography (all S/5 Datex-Ohmeda, Helsinki, Finland), patients were preoxygenated with 100% oxygen via a face mask for 3 min. General anaesthesia was induced with remifentanil at a rate of 0.3 μg kg−1 min−1 and 2 mg kg−1 propofol and maintained with 1 minimum alveolar concentration (MAC) desflurane and remifentanil at a rate of 0.3–0.5 μg kg−1 min−1 . After checking face-mask ventilation, patients were paralysed with 0.6 mg kg−1 rocuronium. Complete neuromuscular blockade was verified by train-of-four stimulation of the ulnaric nerve.
Repeated laryngoscopy was performed in a randomized sequence (allocation of patients by opening of a sealed envelope) to avoid systematic bias. For each laryngoscopy, the time was measured starting from touching the laryngoscope until achievement of the best glottic view. In between the laryngoscopies, patients were ventilated by face mask. Subsequent to the third laryngoscopy, the patients were intubated. Successful ventilation was assured with capnometry and bilateral lung auscultation. Owing to this study design, three intubation subgroups of patients (n = 40) were established for each laryngoscope. We used standard cuffed endotracheal tubes with an inner diameter of 7.5 mm for female and 8.5 mm for male patients. All endotracheal tubes were equipped with stylets preformed depending on the laryngoscope type scheduled for intubation. The use of the GlideScope requires a distinctively curved stylet resembling the blade form (Fig. 2 d). For direct laryngoscopy and DCI video laryngoscopy, stylets were bent moderately. The number of intubation attempts was documented. It should be noted that taking the laryngoscope completely out was considered as another intubation attempt. After two failed intubation attempts, the study protocol was stopped in order to ensure patients' safety. The airway was maintained using routine clinical practice (e.g. Bonfils intubation endoscope or LMA-Fastrach; LMA Germany, Bonn, Germany). If complications such as oxygen desaturation or severe cardiocircular depression occurred, the protocol was stopped immediately.
The time needed from touching the endotracheal tube until cuff inflation of the inserted tube was documented. This was added to the corresponding laryngoscopy time, thus defining intubation time. The laryngoscopic view for each laryngoscope type was documented using the classification of Cormack and Lehane, as modified by Yentis and Lee [17] χ 2 test. A P value less than 0.05 was considered significant.
Results
In total, 120 patients were enrolled in this study; none had to be excluded for data analysis.
There were no significant differences between groups with regard to patients' characteristics and predictors of a difficult airway (Table 1 ). There was a significant difference in laryngoscopic views according to the Cormack and Lehane classification as modified by Yentis and Lee (C&L) between laryngoscopes. Overall, both video laryngoscopes enabled significantly better visualization of the glottic opening than the direct laryngoscope (direct laryngoscopy, P < 0.001; Table 2 ). The GlideScope was also superior to the DCI video laryngoscope (P < 0.05). Thirty per cent of the patients showed insufficient laryngoscopic view (C&L ≥ III) when performing direct laryngoscopy. In contrast, C&/L ≥ III was obtained in only 10.8% when using the DCI laryngoscope (P < 0.001) and in 1.6% when using the GlideScope (P < 0.001; Table 2 ).
Table 1: Patients' characteristics and predictors for a potentially difficult airway
Table 2: Laryngoscopic view
An improvement up to C&L ≤ IIb in the specific 36 individuals with insufficient direct laryngoscopic view (C&L ≥ III) could be achieved significantly (P < 0.01) more often with the GlideScope (94.4%) than with the DCI laryngoscope (63.8%; Table 3 ). The DCI laryngoscope most frequently improved glottic view by one grade (54.2%) compared with the direct laryngoscope (Table 3 ). Improvement of two or three grades (10%) was seen less frequently and the DCI did not alter glottic view in 34.2%. The GlideScope enabled an improvement of two or three grades (26.7%) more often than the DCI (P < 0.001; Table 3 ). Deteriorated glottic view compared with direct laryngoscopy occurred in two cases (1.7%), one with the DCI laryngoscope and one with the GlideScope.
Table 3: Individual differences in Cormack and Lehane classification grading compared with direct laryngoscopy
The median time needed to achieve the best laryngeal view did not differ between the three laryngoscopes (Table 4 ). In contrast, tracheal intubation performed with the direct laryngoscope [22.5 (12–49) s] was significantly faster than that with the DCI laryngoscope [27 (17–94) s; P < 0.05] and with the GlideScope [33 (18–68) s, P < 0.001) (Table 4 )]. Compared with the DCI (2.5%) and GlideScope (2.5%), the intubation failure rate was higher using the direct laryngoscope (10%) (Table 4 ).
Table 4: Times for laryngoscopy and intubation and numbers of intubation attempts and intubation success rates
Complications such as oxygen desaturation or severe cardiovascular depression did not occur in any patient.
Discussion
Difficult laryngoscopy has been defined as the inability to visualize the vocal cords, thus potentially leading to difficult intubation, with an incidence rate varying from 3 to 13% [18,19] [4,5] [6–8] [9]
The recently introduced video laryngoscopy may be a suitable alternative. With this technique, indirect laryngoscopy enables a ‘look-around-the-corner’, whereas the video technique allows a magnified display on the monitor. This study investigates the capability of two different video laryngoscopes to improve laryngoscopic view in patients with potentially difficult airways.
As unpredicted difficult intubation may entail severe danger for the patient, numerable predictors for a difficult airway have been published [18,20] [20] [18,19]
Compared with the direct laryngoscope, our data clearly demonstrate superior laryngoscopic view for both video laryngoscopes, consistent with previous findings [12,14,15,21]
Our investigation also quantified the individual extent of glottic view improvement: the DCI video laryngoscope most commonly improved glottic view by one grade, which is in agreement with data given by Hofstetter et al . [12]
In contrast, the GlideScope showed a pronounced ability to enhance glottic view by more than one grade, corresponding to recent findings [14,21]
Improvement of a C&L ≥ III view up to ≤IIb is likely to enhance intubation success and safety. Whereas DCI video laryngoscope improved a direct C&L ≥ III view in nearly two-thirds of the patients, the GlideScope's capability for clinical improvement was even more remarkable. Therefore, video laryngoscopes, and the GlideScope in particular, may be helpful devices when difficult intubation is caused by insufficient laryngoscopic view.
Although introduction of both video laryngoscopes was often more delicate than with the conventional Macintosh laryngoscope, because of the attached cables and in case of the GlideScope because of the angulated blade form, the average laryngoscopy time did not differ between the three instruments. Obviously, these difficulties seemed to be compensated by rapid achievement of best glottic exposure.
In contrast, the intubation time using video laryngoscopy and especially the GlideScope was longer than that with direct laryngoscopy. Apparently, intubation under indirect visual control via a monitor requires a complex hand–eye coordination. As the investigators were familiar with both video laryngoscopes, there may be only minor potential for diminution of required time. When discussing these extended intubation times, an improvement in intubation success should be considered. Whereas four patients could not successfully be intubated using direct laryngoscopy, only one intubation failed in each video laryngoscope group.
On the other hand, our findings indicate that an improved glottic view does not guarantee intubation success, in particular with the GlideScope. The specific blade form of the GlideScope lifts up the larynx, thus tilting its axis, so that the tube may proceed in a steep angle through the glottic opening. If the tube and tracheal axis do not align, it may be helpful to rotate the tube and approach not in the midline, but more from the right side. In fact, one patient with failed intubation using the GlideScope showed a C&L grade IIa. We were unable to manipulate the endotracheal tube into the trachea due to a massive tongue and a narrow palatine. No subglottic disorder was responsible, as an 8.5 mm endotracheal tube could easily be introduced using the Bonfils intubation endoscope. Similar difficulties using the GlideScope have been reported by other authors, therefore suggesting use of a controllable stylet [14] [1,2,20]
Assessment of the role of video laryngoscopy in the management of the difficult airway has not yet been finalized. However, it may be concluded that video laryngoscopy proved to be very helpful in a variety of clinical conditions of expected difficult airway; for example, limited neck mobility, reduced thyromental distance, reduced intercessor distance or retrognathia. Our data indicate that the GlideScope enhances glottic visualization in patients with difficult conventional laryngoscopy and not only in unselected collectives [14,21] [12,15] [13,22]
Although routine use of video laryngoscopy may remain limited by extended costs in many institutions, it will certainly become more prevalent throughout university and teaching hospitals in the near future. Video laryngoscopy can certainly not replace awake fibreoptic intubation in many cases of a predictable difficult airway. However, awake intubation of patients with a difficult airway using the GlideScope has already been reported [23]
Until now, use of video laryngoscopy for managing the unexpected difficult airway has been hampered by restricted mobility and extended preparation time. Recent developments resulted in a variety of small and portable video laryngoscopes, most of them equipped with small integrated digital video cameras. Prehospital use may be one specific application area for these instruments, but innovations such as wireless signal transmission, compatibility with personal computers and integration of small hand-held LCD monitors may be beneficial for the use of video laryngoscopes in clinical routine as well.
Our study design was confronted with several limitations. First, the investigation was carried out by only two anaesthetists. However, neither of them was involved in the development of the laryngoscopes investigated. Furthermore, the study could not be blinded, thus exposing it to potential observer bias. As at least video laryngoscopic views could be witnessed by the nursing staff to control observer bias, the main findings of this study have to be considered reliable. To validate our findings, further investigation (e.g. multicentre studies) is warranted to approve increased intubation success by use of video laryngoscopes.
In conclusion, both video laryngoscopes and the GlideScope in particular enable significantly better visualization of the glottic opening compared with the direct laryngoscope. They may, therefore, be a useful alternative for the management of the difficult airway.
Acknowledgements
We are grateful to Karl Storz GmbH, Tuttlingen, Germany, and Saturn Biomedical, Burnaby, Canada, for supplying the video laryngoscopes and we are much obliged to the involved nursing staff of our department for their dedicated assistance.
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