There is clear evidence that indirect videolaryngoscopy offers improved viewing of the glottic entrance over direct classic laryngoscopy.1,2 A better view is assumed to facilitate easier intubations, but this is not entirely confirmed,3,4 as a good laryngeal view does not guarantee easy or successful endotracheal tube (ETT) insertion. Several manufacturers are producing videolaryngoscopes (VLSs) with differing specifications, user interface, and geometry. However, the relative performance of the different models is unknown because no comparative studies have been published. Most tracheal intubations using direct laryngoscopy are performed without instruments other than the ETT itself. However, the 60° angle of some VLS flanges limits the advancement of the ETT through the vocal cords into the trachea.5–7 Some manufacturers of VLSs advocate the use of stylets with the ETT to facilitate an easier insertion into the trachea.8–10 However, recent reports have shown rare but potentially serious complications associated with styletted ETT and VLSs.3,11–16
In this prospective, randomized comparative study, we evaluated, in clinical circumstances, the effectiveness of three commonly used VLSs in laryngoscopy and tracheal intubation of patients with normal airways and tested whether it is feasible to intubate the trachea with indirect videolaryngoscopy without using a stylet. Our first objective was to assess the necessity of stylet use with the VLS. It is of clinical interest to determine which VLS has the most suitable geometric and ergonomic designs for tracheal placement of a standard ETT without the use of a stylet. We hypothesized that there are significant differences in the ease of insertion of the ETT given the substantially different laryngoscope blade designs of the VLSs studied. Second, we considered intubation conditions between the blades with use of a styletted ETT. Because the stylet essentially compensates for the geometrical mismatch of the VLS with the laryngeal anatomy of the patient, we expected no discernable differences among the three VLSs. Finally, we performed a comparative assessment of the glottic view among the respective VLSs and classic direct Macintosh laryngoscopy. We hypothesized that all indirect VLSs are equal or superior to classic direct laryngoscopy in terms of the glottic view and that there are no significant differences in visualization quality among the respective VLSs.
After approval of the hospital medical ethics committee (Catharina Hospital, Eindhoven, the Netherlands) and obtaining informed consent, 450 consecutive adult patients, undergoing intubation for elective surgery, were randomly selected to receive general anesthesia and tracheal intubation using one of three VLSs: GlideScope® (Ranger, Verathon Bothell, WA), V-Mac™ Storz® Berci DCI® (Storz, Karl Storz, Tuttlingen, Germany), and McGrath® (McGrath Series 5, Aircraft Medical, Edinburgh, UK) (Fig. 1). Randomization was done through sealed envelope. Exclusion criteria were physical status ASA Class III–V; age <18 yr; body mass index (BMI) >35 kg/m2; and patients with known airway pathology or cervical spine injury.
The preanesthetic visit of the patient (performed by an anesthesiologist not involved in this study) determined history of difficult intubation, measurement of common predictive indices for difficult intubation (BMI, thyromental distance, Mallampati grade, interdental [or intergum] distances), and evaluation of status of dentition and neck movement.
When the patients arrived at the operating room, they were placed in the “sniffing position” with their head placed on a pillow, connected to standard monitoring devices and breathed 100% oxygen for at least 3 min. Anesthesia induction consisted of IV fentanyl 1.5 μg/kg, propofol 3 mg/kg, and rocuronium 0.7 mg/kg, and the lungs were manually inflated through a face mask using sevoflurane in oxygen.
After approximately 3 min, guided by objective confirmation of adequate degree of neuromuscular blockade (train-of-four monitoring), the same independent anesthesiologist not involved in this study performed an initial direct laryngoscopy using a classic metal Macintosh (Heine Optotechnik GmbH & Co. KG, Herrsching, Germany) blade; laryngoscopic view was scored according to the Cormack-Lehane (C&L) grading system, although no intubation took place. After subsequent positive-pressure ventilation using a face mask and an oxygen-sevoflurane mixture for 1 min, the trachea was intubated using one of the three VLSs available in our hospital. The anesthesiologist performing the intubation was blinded to the C&L laryngoscopy score given by the first anesthesiologist. All tracheal intubations were performed by one of five different anesthesiologists, all of whom were experienced in anesthesia, and the use of the VLS was studied (introduction of VLS course in airway skills laboratory and minimal of 30 uses with each VLS). During intubation, intraprocedural metrics of intubation difficulty (C&L grade) and our dependent variables of intubation time, number of attempts, and use of extra tools to facilitate tracheal intubation were measured. In a pilot study, the use of a stylet was favored over the gum-elastic bougie. Therefore, the choice was made to use a specific rigid stylet, formed in the shape of a hockey stick with a 90° bend, optimized for use with the VLS (GlideScope Rigid Stylet, Saturn Biomedical Systems, Burnaby, BC, Canada), as the first option if intubation was not feasible after two intubation attempts.17,18
The number of intubation attempts was counted as each approach of the ETT to the glottis entrance. Intubation time was measured (by an assistant) as the time between picking up the ETT (Hi-contour™, Mallincrodt Medical, Athlone, Ireland) and the visual passage of the ETT until the vocal cords were between the two black line markings on the distal end of the ETT. Interim bag and mask ventilation time, if needed, was not included in the total intubation time. More than five attempts or 120 s were regarded as failure of intubation. If failure to secure the airway occurs with the VLS, then conventional difficult intubation protocols from the hospital were to be implemented.
An overall satisfaction score of the intubation conditions was rated on a scale from 0 to 4: 0 = failure, intubation not possible; 1 = poor (had to use a tool other than the VLS); 2 = fair (need for an extra tool plus intubation time >90 s); 3 = moderate (need for extra tool to intubate the trachea, but intubation time <90 s); and 4 = good (intubation successful on first or second attempt, within 90 s, and no need for extra tools to secure the airway). Attention was paid to insert and remove the VLS smoothly not to damage the oral cavity, the tongue, or the patient’s dentition. After removal of the VLS, the oral cavity was inspected for any bruises, lacerations, bleedings, dental damage, or other possible complications.
A priori sample size testing was conducted assuming an analysis of variance (ANOVA) model for the time measurements. Using three treatments (blade), an effect size of 5 s from clinical experience, a high-desired statistical power level of 0.95 and α level of 0.05, we calculated a sample size of 50 patients. We expanded the patient group to account for the three hypotheses tested and necessary correction of the sample size (i.e., Bonferroni). Data were reported as mean (±sd) and incidences (both absolute and percentage). ANOVA was used to assess any differences among the groups regarding the patient-specific characteristics (i.e., age, BMI, thyromental distance, and interdental distance). Nonparametric patient metrics (i.e., gender, ASA PS, and dentition) were evaluated for differences among the groups using Kruskal–Wallis one-way ANOVA. The differences in the dependent parameters of intubation time, attempts, use of additional tools, and overall satisfaction for the respective VLS groups was evaluated using Kruskal–Wallis nonparametric one-way ANOVA (to forgo assumptions of normality) and Bonferroni correction for the multiple hypothesis testing. Finally, the C&L grades were compared for each of the tested VLSs and the classical Macintosh blade again using Kruskal–Wallis one-way ANOVA. All statistical analysis was performed using MATLAB® 7.2 (R2006a) (Mathworks, MA). P < 0.05 was considered statistically significant.
Patient characteristics and preprocedural intubation conditions did not differ among patient groups (Table 1). Patients underwent a large variety of general surgery, orthopedic surgery, urology, gynecology, and plastic surgery. Peripheral oxygen saturation was maintained above 95% in all patients throughout the laryngoscopy and intubation period. All operations were completed uneventfully. We did not detect any injury of the palatoglossal arch or dental injury in any patient. Minor lip lacerations were seen in four patients.
The first hypothesis concerned the differences between the VLSs studied in their effectiveness for intubation without a stylet. Intubation was successful on the first attempt in 46 patients (53%) with the GlideScope, 118 patients (84%) with the Storz, and 32 patients (52%) with the McGrath VLS, all without using a stylet (Table 2). A stylet had to be used to successfully intubate almost half the patients in the GlideScope group (n = 64, 43%) and the McGrath group (n = 88, 59%); however, it was less for the Storz group (n = 10, 7%), (P < 0.01) (Fig. 2). The Storz group required fewer attempts to secure the airway (Kruskal–Wallis, χ2 = 126, P < 0.01), (Table 2) with and without stylet. The average intubation times were 34 ± 20 s for the GlideScope, 18 ± 12 s for the Storz, and 38 ± 23 s for the McGrath VLS (Table 2), again considering the total intubation attempts, both with and without stylet. Intubation with the Storz VLS was faster than the other two VLSs tested (Kruskal–Wallis, χ2 = 116, P < 0.01) (Fig. 3).
Concerning the second hypothesis, there were no differences using a stylet among the three VLSs with regard to the number of required intubation attempts (P > 0.05). A successful intubation was achieved on the first pass with the stylet in 49 patients (76%) who used the GlideScope, eight patients (80%) who used the Storz, and 65 patients (74%) who used the McGrath. However, because we did not differentiate the time before and after using a stylet, we cannot draw any conclusions regarding differences in intubation time. Subjectively, there did not seem to be any differences concerning the intubation time among the three VLSs when using a stylet. All VLSs performed well in terms of effective intubation time necessary for securing the airway because it was not necessary to revert to standard difficult airway protocols.
No dropouts were encountered, as the tracheas of all patients could be intubated using the VLSs. All the VLSs tested offered an equal or better view of the glottis as assessed by the mean C&L grade compared with traditional direct intubation techniques (P < 0.01) (Fig. 4). Only very few patients had C&L grades other than Grade I with the VLS, and none of the patients included in this study had a C&L grade poorer than II. The VLSs tested were rated favorably for their larger viewing angle of the glottic entrance compared with classic laryngoscopy techniques. This proved useful for guidance of the ETT and nasogastric tube into position and avoidance of contact with soft tissues of the mouth and pharynx. In general, visualization of the glottis entrance with the VLS was not a problem, although intubation was not always straightforward. In the context of this study, overall satisfaction was greater using the Storz VLS compared with the other two VLSs (Kruskal–Wallis, χ2 = 95, P < 0.01).
This study compares three VLS devices in a clinical setting during endotracheal intubation and confirms that a stylet is not necessary with some VLSs (Storz) but highly recommended when other VLSs (e.g., GlideScope and McGrath) are used. When using a stylet, the VLSs studied did not differ in intubation time or number of intubation attempts. This study also confirms the excellent unobstructed view of the glottic opening obtained indirectly with a VLS as opposed to the direct classic Macintosh laryngoscope.
No conversion to direct laryngoscopy was necessary in any patients studied. An equal or better C&L grade could be obtained in all cases, which is in agreement with other studies.19,20 There were no significant differences in the visualization quality of the glottis among the three VLSs except for the dimension of the device monitor. Good visualization of the glottic entrance is paramount for successful tracheal intubation. However, providing a good view of the glottis does not always correlate with successful intubation. Indeed, this study shows that devices that offer the same unhindered view of the glottis are not alike in their ease of use.
This study demonstrates that stylet use is not always required with certain VLSs. The Storz VLS group was associated with a shorter intubation time, required significantly fewer attempts to secure the airway compared with the other two groups, and a stylet was required only in a minority (7%) of the patients, whereas a styletted ETT had to be used in almost half of the patients in the GlideScope and McGrath VLS groups. Presumably, this is due to the fact that the Storz VLS uses the same Macintosh laryngoscope blade as with direct laryngoscopy, providing a better view and better access, which decreases the need for stylet use. During direct laryngoscopy, a stylet is rarely used routinely at first attempt in our clinical practice. Given the fact that styletted ETT have rare but potentially significant complications, we believe it is important to reserve their use for difficult intubations.3,11–16
Using a styletted ETT with the GlideScope VLS, the first pass success rates were higher in the studies performed by Sun et al.21 (94%) and Xue et al.22 (97%) compared with our study. However, the other studies used a different definition of a “single pass” in which a successive attempt was only recorded on retracting the ETT completely out of the mouth; in our study, each approach to the vocal cords was counted as an attempt. Similarly, Shippey et al.10 also found a first pass success rate of 93% when using the McGrath VLS and a styletted ETT. In our study, successful intubation (without using a stylet) in the Storz group was 93%, supporting our contention that a stylet is not needed at all times and, thus, preventing potential complications. Successful intubation with the Storz VLS (without stylet) is as good as with the reported success rate in the literature using the GlideScope/ McGrath VLS (with stylet).10,20–22 The direct laryngoscope and the indirect Storz VLS are generally inserted on the right side of the tongue, which is compressed and deflected laterally, whereas the indirect GlideScope and McGrath VLS are inserted in the midline and advanced over the tongue because there is no need to sweep the tongue laterally. It might be that the greater success rate using the Storz VLS without a stylet depends on the angle of its blade, which is similar to the conventional direct Macintosh laryngoscope and the fact that the tongue is displaced laterally. Presumably, use of a styletted ETT from the outset would mitigate the differences among the VLSs that we found; however, this may also increase the risk of injury. The addition of a stylet essentially compensates for the geometrical shortcomings of some of the VLS designs.
Since its commercial introduction in 2002, numerous studies have reported the efficacy and safety of the GlideScope VLS for tracheal intubation in patients (and simulators) with easy and difficult airways.1,5,20,23,24 However, some authors have noted that the GlideScope VLS is difficult to insert into the patient’s mouth, does not reach deep enough in some cases, insertion of ETT is not easy, the advancement of the ETT after removal of the stylet is difficult,22 and complications because of the use of a styletted ETT and a VLS may occur.3,11–16
The arytenoid cartilages, the interarytenoid soft tissues, anterior commissure of the glottis, or the anterior wall of the cricoid cartilage sometimes interfere with advancement of the ETT into the trachea. Manipulation of the ETT orientation is often not sufficient because the curvature of the distal end of the ETT is insufficient; in such cases, an extra tool is necessary. Additionally, patient characteristics, such as dentition and mouth opening, may greatly influence the ease of insertion of the ETT. The VLS essentially positions the operator’s eye proximal to the larynx. Therefore, care should be taken to do the initial introduction (passing the teeth and first part of mouth) of the ETT directly, until the distal end comes into view of the VLS. Indeed, the VLS is essentially a standard laryngoscope in form and function until the critical insertion of the ETT through the vocal cords is performed.
A number of techniques can be used during the intubation procedure to improve the success rate. Previously, Xue et al.22 noted that the use of a malleable stylet, preheating of the blade to body temperature, and avoiding the use of superfluous lubricant were important considerations for successful use of the GlideScope. Also, increased lifting force, withdrawal and reinsertion of the blade, and external laryngeal pressure have been proposed as helpful measures for successfully securing the airway.23 Several maneuvers may overcome the problem of inserting the ETT: relaxing the VLS; withdrawing the VLS 1–2 cm; use of a StyletScopeTM (Nihon Kohden Co., Tokyo, Japan) in which the operator can adjust the angle of the ETT tip between 30° and 90° by gripping the handle strongly;25 or the use of a stylet-ETT that can increase the angle between the axis of the ETT tip and the tracheal axis. We neither experience any improvement in the ease of intubation when using external laryngeal pressure nor withdrawal and reinserting the blade. In this study, it was noticed that the use of a stylet with a relatively pronounced curve (the best angle is reported to be 90°)17,18 at the distal end was most helpful in advancing the tip of the ETT to the glottic opening.
Further study is required to determine optimal geometrical forms for the stylet or gum-elastic bougies used for difficult intubations. More importantly, the integration of the ETT with the VLS blade is the major issue for redesign in future generations of VLS, especially considering that the classical problem of visualization now seems to be resolved (all patients in our study showed a C&L I or II with all three types of VLSs).
The McGrath VLS that uses a disposable blade and the recent introduction of the GlideScope Cobalt single-use disposable blades26 are promising developments, especially in busy settings in which there may not be sufficient time to sterilize the blades between uses. Portability of the VLS systems is also an issue, and there are clear advantages of the McGrath and GlideScope VLS over the Storz. The integration of an antifogging mechanism on the McGrath and the GlideScope VLS is advantageous over the Storz V-Mac VLS, which lacks this feature. Preheating the VLS with the former two is unnecessary because the light emitting diode heats a window over the video chip. If fogging does occur, it likely means that the VLS is defective. However, it is still possible to blur the view if the lubricated ETT makes contact with the imaging system.
This study has several limitations: 1) The attending anesthesiologist was not blinded to the type of VLS used, which this may have introduced bias, despite being blinded to the preoperative metrics and initial C&L grade with the use of the classic Macintosh laryngoscope; 2) There are more VLSs available on the market so this review is not complete, but the three most common models available in our hospital are included; 3) There was very low patient morbidity in this study, and it remains debatable how important the metrics of intubation time, attempts, and satisfaction are with regard to patient morbidity; 4) It is clear that if the study were performed using a stylet routinely in all cases, then the second or third intubation attempt would not have been necessary; 5) The selection of patients lacking features associated with a difficult airway may have reduced the potential superiority of VLS over direct laryngoscopy and minimized the differences among the three VLS models; 6) Failure to routinely use a stylet may bias our study in favor of a device which most closely resembles a conventional Macintosh laryngoscope; 7) A study of patients with difficult airway anatomy may be needed to determine the need for the routine use of a stylet; and 8) Finally, this study deals with a specific population of elective surgical patients with normal airways and no conclusions can be made for patients in whom difficult tracheal intubation is expected.
The use of a styletted ETT is not ideal during tracheal intubation because it can potentially contribute to complications. Our study confirms that a large proportion of patients with normal airway anatomy can be intubated successfully with certain VLS blades without using a stylet, although there is a large difference among the types of VLSs tested. Certainly, the problem of visualization of the glottic arch is resolved by a VLS, but this does not guarantee easy or successful endotracheal intubation. The stylet essentially compensates for the geometrical mismatch between the VLS blade and the laryngeal anatomy of the patient.
The Storz VLS performed better in overall satisfaction, intubation time, and number of attempts, including attempted intubations without a stylet, most probably due to the better view and access, which limited the need for stylet use. The GlideScope and McGrath VLS are equally successful in achieving good visualization and intubation in all patients. It seems that geometry and integration of ETT with the VLS is the next question that needs to be addressed in blade design for intubation.
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