Controlled ventilation in patients can be performed with different airway devices . Endotracheal intubation, considered to be the gold standard in securing the airway, is commonly performed using a direct laryngoscope. In addition to poor illumination, difficulties in performing conventional direct laryngoscopy commonly arise from the limited view angle of approximately 10–15°. Insufficient laryngoscopic view constitutes the main reason for difficult intubations . Without adequate visualization, intubation remains unsafe and associated with elevated risk for trauma, thus essentially contributing to anaesthesia-related morbidity and mortality . Therefore, different blade designs such as the McCoy leverage blade, Dörges universal blade and so on were developed to improve intubation success [4,5]. Owing to remaining intubation difficulties in some patients, instruments allowing indirect glottic view such as flexible and rigid fibrescopes, intubation endoscopes and optical stylets were introduced [6–8]. However, extensive costs and the need for special training essentially contributed to a limited spread of many of these devices . Therefore, anaesthetists are still searching for intubation devices combining excellent glottic visualization with simple and efficient use.
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 . Today, several elaborate video laryngoscopes are commercially available [11–14]. Whereas some devices feature a conventional Macintosh blade form, others show a distinct blade design. A pronounced curvature resembling oropharyngeal and hypopharyngeal anatomy enables a widened view.
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).
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) . The handle incorporates a small interchangeable camera-light unit connected via a DCI. The handle and blade form a fixed unit providing undisturbed phototelegraphy and preventing damage to attached cables and enabling hygienic processing. The light and image-transmitting fibres are guided through a metal tube ending 40 mm from the blade's distal end in a 0° optical lens. The visual angle is 60° (manufacturer's instructions); the visual field ends at the blade to the ventral side and at 30° to the dorsal side. The DCI-D1 camera-light unit is connected to a light source and camera control unit (MediPack; Karl Storz GmbH, Tuttlingen, Germany). The DCI video laryngoscope is inserted in the same manner as the conventional Macintosh laryngoscope. The blade's tip is positioned at the plica glossoepiglottica; the view obtained at this position is displayed on the 30 cm colour monitor. A Macintosh blade size 3 was routinely used for female and male patients and a size 4 blade for tall individuals. Preparation of the DCI video laryngoscope included white balancing, focus adjustment and application of antifog solution.
The GlideScope video laryngoscope (Saturn Biomedical, Burnaby, Canada, now Verathon Medical, Bothwell, USA, Figs 1 and 2) was developed in 2003 . The self-contained instrument incorporates a high-resolution complementary metal oxide semiconductor (CMOS) video chip and light-emitting diodes (LEDs) in a medical-grade plastic shell. The instrument is provided in different sizes; ‘standard adult’ (now called ‘large’) was used in this study. The laryngoscope is connected to a 17 cm LCD-colour monitor by a cable, supplying power and transmitting video signals. With an upward angulation of 60°, the blade design differs significantly from Macintosh blades. The camera pod is located at a marked inflection point from which the distal blade continues straight forward for another 58 mm (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.
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. 2d). 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 : grade I – full view of the glottis; grade IIa – partial view of the glottis; grade IIb – arytenoids or posterior portion of the cords visible; grade III – only the epiglottis visible; and grade IV – neither epiglottis nor glottis visible. Uniform data collection was secured by use of a standardized data sheet. Statistical analysis was performed using GraphPad Prism software (GraphPad Software Incorp., San Diego, California, USA). Significance was tested using nonparametric tests. The Friedman test with Dunn's posttest correction was used for dependent samples (laryngoscopic view and time) and the Kruskal–Wallis test with Dunn's posttest correction for independent samples (comparability of demographics and predictors between groups, intubation attempts and time). Proportions were compared using the χ2 test. A P value less than 0.05 was considered significant.
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).
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.
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).
Complications such as oxygen desaturation or severe cardiovascular depression did not occur in any patient.
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]. Many variations of laryngoscope blades have been developed to improve laryngoscopy [4,5], but as a mandatory straight line of sight on the glottic opening cannot be achieved in some patients with direct laryngoscopy, intubation devices enabling indirect laryngoscopy were designed [6–8]. As special training is mandatory for the successful use of these more complex instruments, the number of regular users still remains limited . Therefore, alternative intubation devices enabling easy and efficient use are required.
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]. The criteria used in our study have been demonstrated to offer acceptable sensitivity in 18 500 patients . Hence, the incidence of C&L ≥ III findings using direct laryngoscopy in our study was several times higher than that for unselected groups given in literature [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]. The best view was obtained with the GlideScope. Apparently, its angulated blade form offers a more effective ‘look-around-the corner’ than the DCI video laryngoscope.
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. . However, deterioration of glottic view also occurred in two patients without sufficient explanation. Fogging of the camera lens, as sometimes experienced using the DCI video laryngoscope, impedes a clear view but does not influence C&L grading.
In contrast, the GlideScope showed a pronounced ability to enhance glottic view by more than one grade, corresponding to recent findings [14,21]. Deterioration using the GlideScope also occurred in two patients with easy (C&L I) direct laryngoscopy.
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 . In contrast, the only failed intubation using the DCI video laryngoscope was associated with insufficient laryngoscopic view (C&L III). Compared with the incidence of unsuccessful intubation given in other studies [1,2,20], the intubation failure rate in our study was rather high. This may be explained by the selection criteria as well as by the study protocol limiting intubation attempts to a maximum of two.
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]. The DCI video laryngoscope also demonstrated improved laryngoscopic view in our study, which was comparable to recent findings [12,15]. Furthermore, the DCI video laryngoscope facilitates teaching of direct laryngoscopy and may thereby improve intubation skills, thus effectively helping to reduce the incidence of critical situations in airway management [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 .
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.
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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