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Patient Safety: Research Reports

A Comparison of Three Videolaryngoscopes: The Macintosh Laryngoscope Blade Reduces, but Does Not Replace, Routine Stylet Use for Intubation in Morbidly Obese Patients

Maassen, Ralph MD*†; Lee, Ruben BE; Hermans, Boukje MD*†; Marcus, Marco MD, PhD; van Zundert, André MD, PhD, FRCA

Author Information
doi: 10.1213/ANE.0b013e3181b7303a

Difficult tracheal intubation using direct laryngoscopy is more common in morbidly obese patients (body mass index [BMI] ≥35 kg/m2).1,2 It has been shown that videolaryngoscopy improves intubation conditions in this challenging (and growing) patient population.3 The routine use of a styletted endotracheal tube (ETT) with videolaryngoscopy is advocated by some manufacturers4–6 but has been questioned recently7 because styletted ETTs have rare but potentially important complications when videolaryngoscopes (VLSs) are used.6,8–16 We therefore hypothesize that it is important to reserve stylets for difficult intubations.

In this study, we compared 3 VLSs in morbidly obese patients undergoing intubation for elective surgery: the GlideScope® Ranger™, the Storz® V-MAC™, and the McGrath® VLS and tested whether it is feasible to intubate the tracheas of obese patients with a VLS without using a stylet.

METHODS

After approval from the hospital medical ethics committee (Catharina Hospital, Eindhoven, The Netherlands) and obtaining informed consent, 150 consecutive adult patients scheduled for surgeries requiring endotracheal intubation, with a BMI more than 35 kg/m2, were randomly selected to undergo tracheal intubation using one of 3 VLSs: GlideScope (Ranger, Verathon, Bothell, WA); V-MAC Storz Berci DCI (Karl Storz, Tuttlingen, Germany); and McGrath (McGrath Series 5, Aircraft Medical, Edinburgh, UK). Exclusion criteria were ASA physical status III–V, patients younger than 18 yr, patients with a BMI <35 kg/m2, and patients who had known airway pathology or cervical spine injury. Randomization was done prospectively with a sealed envelope technique.

The preanesthetic visit (performed by an anesthesiologist not involved in this study) ascertained history of difficult intubation, measurement of common predictive indices for difficult intubation (BMI, thyromental distance, Mallampati grade, interincisal [or intergingival] distances), and evaluation of status of dentition and neck movement.

When the patients arrived at the operating room, they were connected to standard monitoring devices and administered oxygen for at least 3 min. Anesthesia induction consisted of IV fentanyl 1.5 μg/kg and propofol 3 mg/kg based on estimated lean body weight. Manual mask ventilation and inflation of the lungs was attempted via a face mask using sevoflurane in oxygen before rocuronium 0.7 mg/kg was administered once the ability to mask ventilate the patient was verified.

After approximately 3 min, all patients underwent an initial direct laryngoscopy, by an independent anesthesiologist, using a classic metal Macintosh (Heine Optotechnik GmbH & Co. KG, Herrsching, Germany) blade to score the Cormack-Lehane (C&L) grade, although no intubation took place. After subsequent mask positive pressure ventilation, the trachea was intubated using the patient's designated VLS. The anesthesiologist performing the intubation was blinded to the laryngoscopy (C&L) score as assessed by the first anesthesiologist. All tracheal intubations were performed by one of 5 different anesthesiologists, all of whom were experienced in anesthesia and the use of the VLS studied (introductory videolaryngoscopy course in airway skills laboratory and a minimum of 50 uses of each VLS). During intubation, intraprocedural metrics of intubation difficulty were measured, as well as the dependent variables of intubation time, number of attempts, and use of rigid stylet to facilitate intubation. In a pilot study, using a stylet was favored to a gum-elastic bougie. Therefore, the choice was to use a specific rigid stylet17,18 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, B.C., Canada), and was the first option if intubation was not feasible after 2 intubation attempts in all 3 VLSs. A gum-elastic bougie was always at hand, as was the difficult intubation cart.

The number of intubation attempts was counted as each approach of the ETT to the glottic 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 tube until the vocal cords were between the 2 black line markings on the distal end of the ETT. Interim bag and mask time, if needed, was not included in the intubation time. More than 5 attempts or 120 s was regarded as failure of intubation. If failure to secure the airway occurred with the VLS, then conventional difficult intubation protocols approved by the hospital were to be implemented. Attention was paid to insert and remove the VLS smoothly in order 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, bleeding, dental damage, or other possible complications.

The intubating anesthesiologist was asked to provide an overall satisfaction score of the intubation conditions scoring from 0 to 4: 0 = failure (intubation not possible); 1 = poor (had to use an adjunct other than the VLS); 2 = fair (need for an extra adjunct plus intubation time >90 s); 3 = moderate (need for extra adjunct to intubate the trachea but intubation time <90 s); 4 = good (intubation successful on first or second attempt, within 90 s, and no need for extra adjunct to secure the airway).

A priori sample size testing was conducted assuming an analysis of variance (ANOVA) model for the time measurements. Using 3 treatments (blades), an effect size of 5 s from clinical experience, a high desired statistical power of 0.95, and α value of 0.05, we calculated a sample size of 50 patients. Data are reported as mean (±sd) and incidences (both absolute and percentage). ANOVA analysis was used to assess any differences among the groups regarding the patient-specific characteristics (BMI, thyromental distance, and interincisal distance). Nonparametric patient metrics (gender, ASA status, and dentition of the patient) were evaluated for differences among the groups using Kruskal-Wallis one-way ANOVA. The differences in the dependent variables of intubation time, number of attempts, use of additional adjuncts, and overall satisfaction for the respective VLS groups were calculated using Kruskal-Wallis nonparametric one-way ANOVA (to forgo assumptions of normality). We applied a conservative correction using Bonferroni. The C&L grades were compared for each of the tested VLSs and the classical Macintosh blade using Kruskal-Wallis one-way ANOVA. All statistical analyses were performed using SPSS 16.0 (SPSS, Chicago, IL). P < 0.05 was considered statistically significant.

RESULTS

Patient characteristics and preprocedural intubation conditions did not differ among patient groups (Table 1). Peripheral oxygen saturation was maintained above 95% in all patients throughout the laryngoscopy and intubation period. In this study, no dropouts were encountered, because all patients were intubated successfully using the VLS. No patients required conversion to classic Macintosh laryngoscopy, and all operations proceeded uneventfully. We did not detect any injury of the palatoglossal arch or dental injury in any of the patients.

Table 1
Table 1:
Patient Characteristics and Preprocedural Intubation Conditions

All of the VLSs tested offered an equal or better view of the glottis (Grades I and II) as assessed by the C&L grade, compared with traditional direct laryngoscopy intubation techniques (Grades I–IV) (P < 0.01). In general, visualization of the glottic entrance with the VLS was never a problem, although intubation was not always straightforward. The number of attempts necessary to intubate the trachea differed significantly among the VLS blades (average 2.6 ± 1.0 attempts for the GlideScope, 1.4 ± 0.7 for the Storz, and 2.9 ± 0.9 for the McGrath VLS). The Storz group required statistically fewer attempts to secure the airway (Kruskal-Wallis, χ2 = 65, P < 0.01) (Fig. 1) when considering the total number of intubation attempts (i.e., with and without stylet). Intubation was successful on the first attempt in 8 patients with the GlideScope, 34 with the Storz, and 4 with the McGrath, all without using a stylet (Table 2). A stylet had to be used in the majority of the patients in the GlideScope group (n = 30, 60%) and the McGrath group (n = 38, 76%). However, this was not the case for the Storz group, which showed less need for a stylet (n = 5, 10%) (P < 0.01).

Figure 1
Figure 1:
Figure 1.
Table 2
Table 2:
Intubation Metrics

The C&L grade as assessed with classic VLS influenced the number of attempts necessary (P < 0.01) to intubate the trachea (Fig. 2). All VLSs performed well in terms of effective intubation time necessary for securing the airway. The average intubation times were 33 ± 18 s for the GlideScope, 17 ± 9 s for the Storz, and 41 ± 25 s for the McGrath, again considering the total intubation time, both with and without stylet. Intubation with the Storz VLS was significantly faster than with the other 2 VLSs tested (Kruskal-Wallis, χ2 = 60, P < 0.01) (Fig. 3). In the context of this study, overall satisfaction was greater using the Storz VLS (Kruskal-Wallis, χ2 = 56, P < 0.01).

Figure 2
Figure 2:
Figure 2.
Figure 3
Figure 3:
Figure 3.

DISCUSSION

This study confirms that a styletted ETT is highly desirable to intubate the trachea in morbidly obese patients when certain VLSs are used (e.g., GlideScope and McGrath), whereas with the Storz VLS, the use of a stylet is not essential. This finding is in agreement with our previous study in a group of 450 nonobese adult patients (ASA physical status I–II).7 The importance of the VLS in improving intubation conditions in this challenging (and growing) patient population has been shown.3 The various VLSs available on the market differ significantly in their ease of use (in terms of intubation time, number of attempts, and the need for additional adjuncts), even when the visual quality is essentially identical.

Use of the Storz VLS was associated with shorter intubation time, significantly fewer attempts to secure the airway compared with the other 2 VLS groups, and a need for a stylet in only a minority (10%) of the patients, whereas a styletted ETT had to be used in 60% and 76% of this patient population in the GlideScope and McGrath VLS groups, respectively. Presumably, this is due to the fact that the Storz VLS uses the same Macintosh laryngoscope blade as with direct laryngoscopy, affording additional room for ETT insertion, and limiting the need for stylet use. The study was not powered to show that any one blade was superior for all respective C&L grades, and this should be further investigated in subsequent clinical trials. Furthermore, the view provided by the VLS, especially in the case of the GlideScope and the McGrath VLS, does not necessarily correspond with the path followed by the ETT, making the hand-eye coordination exceptionally difficult. Indeed, the hockey stick profile of the stylet is necessary to match the sharp distal curve of the GlideScope and the McGrath VLS. Conceivably, the sharp angle of the GlideScope and McGrath VLSs might be advantageous in patients with difficult anatomy, such as micrognathia, neck immobility, or sublingual tonsillar hypertrophy.

We used a very stringent definition of an intubation attempt as the number of intubation attempts was counted as each approach of the ETT to the glottic entrance, even without complete withdrawal of the ETT out of the mouth. This strict criterion certainly influenced the observation that half of all patients required 3 or more intubation attempts. The ETT usually followed a path more dorsal to the view provided by the VLS. In other words, the view that is provided by the sharply curved VLS is not the same as the path of the less curved ETT (without stylet).

In direct laryngoscopy, a stylet is an adjunct rarely used at first attempt in our clinical practice. Given the fact that styletted ETTs have rare but potentially important complications with videolaryngoscopy,6,8–16 but also when direct laryngoscopy with a classic Macintosh blade is used,19–22 we believe it is important to reserve the use of a stylet for difficult intubations. Morbidly obese patients are rarely “routine” in their intubation requirements. However, morbid obesity does not automatically portend a difficult airway, and metrics that only consider macro patient characteristics are incomplete at best and less relevant to videolaryngoscopy.23 Besides often having more difficult airways, the time pressure is also greater, because desaturation develops more quickly in obese than in nonobese patients.24,25 Less routine use of a stylet for some VLSs does not imply that intubation adjuncts, such as a stylet or a difficult airway/intubation cart, should be not readily at hand for unanticipated difficult intubations.

This study has several limitations as follows: 1) The attending anesthesiologist was not blinded to the type of VLS used, which may have introduced slight bias, although the anesthesiologist was blinded to the preoperative metrics and initial C&L grade with the use of the classic Macintosh laryngoscope; 2) Failure to routinely use a stylet may have biased our study in favor of a device that most closely resembles a conventional Macintosh laryngoscope; 3) There are still other VLSs available on the market, so this review is not complete, but 3 of the most common manufacturers (as available in our hospital) were included; 4) Our study group consisted of morbidly obese patients, of which certainly a portion could be intubated easily by conventional direct laryngoscopy, because morbid obesity does not automatically define a population of difficult intubation patients; 5) Although we used the GlideScope stylet (90° angle) as our standard adjunct if intubation failed after 2 attempts, other shapes of stylets can be superior for one or more of the VLSs studied; and 6) Other intubation adjuncts were not studied and could have influenced the results in this study.

CONCLUSION

Videolaryngoscopy offers superior viewing of the glottis, which can be problematic in morbidly obese patients. However, the VLSs available on the market differ significantly in their ease of use, even when the visual quality is essentially identical. In this study, the Storz VLS performed better in overall satisfaction, intubation time, number of intubation attempts, and necessity of extra adjuncts, compared with the other 2 devices. Furthermore, successful intubation (without using a stylet) in the Storz group was 90%, supporting our hypothesis that a stylet may not be needed at all times when using a VLS, therefore preventing potential complications. A good view of the glottis is important but not alone sufficient. The interaction of the blade with the ETT is similarly critical and seems to be the remaining obstacle in developing an optimal laryngoscope design.

ACKNOWLEDGMENTS

The videolaryngoscopes were made available to the study center, on a temporary basis, at no cost by the respective manufacturers: Ranger™ GlideScope®, Verathon Inc., Bothell, WA; V-Mac™ Storz® Macintosh, Karl Storz, Tuttlingen, Germany; and McGrath® Series 5, Aircraft Medical, Edinburgh, United Kingdom.

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