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Forces Applied to the Maxillary Incisors During Video-Assisted Intubation

Lee, Ruben A. BE(hons)*; van Zundert, André A. J. PhD, MD, FRCA†‡; Maassen, Ralph L. J. G. MD†; Willems, Remi J. MD†; Beeke, Leon P. BSc*; Schaaper, Jan N. BSc*; van Dobbelsteen, Johan PhD*; Wieringa, Peter A. PhD*

doi: 10.1213/ane.0b013e31818d1904
Technology, Computing, and Simulation: Research Reports

BACKGROUND: Modern, video laryngoscopes provide an easier view of the glottis, possibly facilitating easier intubations. We describe an objective method for evaluating the benefits of video-assisted laryngoscopy, compared with standard techniques using force measurements.

METHOD: Macintosh and video laryngoscopes (both Karl Storz, Tuttlingen, Germany) were used on the patients until the anesthesiologist was convinced he or she had the best possible view of the glottis. Actual intubation was only performed with the second of the laryngoscopes. Sensors measured the forces directly applied to the patients’ maxillary incisors. Additionally, common subjective pre- (e.g., Mallampati) and intraintubation (e.g., Cormack-Lehane [C&L]) metrics of intubation difficulty were evaluated by the anesthesiologists.

RESULTS: All patients (24 female, [50 ± 16 yr], 20 male [56 ± 13 yr]) included in the study were successfully intubated with both the classic and video laryngoscopes. The forces recorded for the classic Macintosh blade ranged from 0 to 87.4 N with a median of 15.3 N, whereas the video laryngoscope forces ranged from 0 to 45.2 N, with a median of 2.1 N. The only factor determined to be significantly influential on the associated forces applied to the maxillary incisors was the laryngoscope type (P < 0.01). Video-assisted laryngoscopes reduced the applied forces over standard blades. Mallampati and C&L grade were not predictive of the forces applied.

CONCLUSIONS: Video-assisted laryngoscopes seem beneficial when considering forces applied to the maxillary incisors as an objective metric of intubation difficulty. In this study, we could not support that Mallampati and C&L grades predict the forces that are applied to the maxillary incisors.

IMPLICATIONS: Video laryngoscopy helps reduce the forces applied to the maxillary incisors of patients during intubation, regardless of common metrics of intubation difficulty (e.g., Mallampati grade).

From the *Department of Biomechanical Engineering, Delft University of Technology, Delft, The Netherlands; †Department of Anesthesiology, Intensive Care, and Pain Therapy, Catharina Hospital–Brabant Medical School, Eindhoven, The Netherlands; and ‡Department of Anesthesiology, University Ghent Hospital, Gent, Belgium.

Accepted for publication June 23, 2008.

Presented, in part, at the XIV World Congress of Anesthesiology in Cape Town, South Africa, 2008.

No financial support was obtained for this study. No financial arrangement between authors, no personal relationship with other people or organizations exist that could bias our submitted work.

I hereby state that the authors had full access to all data and that I take final responsibility for decision to submit this manuscript.

The large number of authors is attributable to the collaborative effort of the Delft University of Technology and Catharina Hospital.

Address correspondence to André A. J. van Zundert, PhD, MD, FRCA, Department of Anesthesiology, Intensive Care, and Pain Therapy, Catharina Hospital—Brabant Medical School, Michelangelolaan 2, 5623 NL, Eindhoven, The Netherlands. Address e-mail to

Reprints will not be available from the author.

The recent introduction of intubation blades incorporating optics in the tip for video imaging has been proven advantageous for improved viewing of the glottis, with subsequently assumed fewer traumas to the patient and faster intubation times, especially in typically problematic cases.1,2

All too common sequelae of difficult intubation are damage to soft tissues of the mouth and throat and, more seriously, damage to the patient’s maxillary incisors.3,4 Even more critically, difficulties during intubation in emergency situations may jeopardize the patient. Poor vision of the glottis makes intubation more difficult. There are guidelines, however, for predicting both pre- and intraprocedural intubation difficulty,5–7 yet these guidelines may have less relevance to modern video-assisted laryngoscope blades. Previous studies have indicated that video-equipped laryngoscopes improve scoring with such metrics, especially for more complicated intubations.1,2,8 However, it is of clinical relevance to know if this similarly implies quicker, less traumatic intubation. To this end, the measurement of forces applied to the teeth by the laryngoscope may be a more objective assessment of the intubation difficulty.9,10

During intubation the anesthesiologist uses the laryngoscope blade to distract the tongue and other soft tissues of the mouth to achieve the best view of the glottic opening. When the forces are correctly applied in an upwards direction (away from the maxilla), forces are distributed over the soft tissues and mandibular incisors (Fig. 1A). It is extremely unlikely that any dental damage will be caused in such a situation. In contrast, in difficult situations in which the anesthesiologist uses the maxillary incisors as a fulcrum to lever the soft tissues upwards, there are much higher forces applied to the upper teeth (Fig. 1B). In an effort to quantify the difference between conventional and video laryngoscopes, we have chosen to analyze the forces applied to the maxillary incisors during routine intubation.

It is likely that the addition of video has little additional value for easier intubations. Anesthesiologists are well versed in using the classic Macintosh type laryngoscopes, and the additional modality may be redundant. In contrast, most difficult intubations are often effectively blind (requiring additional instruments, e.g., gum elastic bougy, stylet, trachlight, fiberoptic etc.), and the addition of video imaging may drastically improve intubation time, applied force, and reduce the necessity of extra instrumentation. This study was designed to compare the forces applied to the maxillary incisors by both video-assisted and Macintosh laryngoscope blades during intubations in a population of elective surgical patients.

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Patients were randomly selected and consented before their designated elective surgery to participate in this study (Table 1). We obtained ethical consent for the study from the Medical Ethical Hospital Committee (Catharina Hospital, Eindhoven, The Netherlands) and registered with the national auditing commission for human trials. Exclusion criteria were patients younger than 18 yr, patients requiring other than blade size III of laryngoscope, patients of ASA IV or higher, and patients requiring surgery of the face or throat.

After 3 min of oxygen administration, IV induction of anesthesia with 1 μg/kg fentanyl, 3 mg/kg propofol, and rocuronium 0.7 mg/kg was performed. The lungs were manually inflated via a facemask using sevoflurane in oxygen. The laryngoscope was inserted at least 1.5 min after completion of induction, and only when the anesthesiologist considered the level of anesthesia adequate for intubation.

The patients had both the classic Macintosh and video laryngoscope (both Karl Storz, Tuttlingen, Germany) placed in their mouths (in randomized order), and the anesthesiologist continued until he or she had ascertained the best possible view of the glottis and a tube was brought into position in front of the glottis. This sequence was repeated by two anesthesiologists. Actual intubation was only performed with the second of the laryngoscopes, by the second anesthesiologist. The time for a single study was limited to 2 min for patient safety, which placed little time pressure on the anesthesiologist. However, this did preclude (fully) performing the sequence by both anesthesiologists on some patients. In total, there were 78 measurements for the classic Macintosh laryngoscope and 71 for the video laryngoscope. Geometrically, the two blades were almost exactly similar. Ten anesthesiologists (4 specialists, 6 residents), all familiar with the video laryngoscope (minimum 30 uses) and classical intubation practice, participated in the study. The sensors were manually cleaned with a cetrimide/ chlorhexidine solution, followed by alcohol 70% solution; autoclaving was impossible given the plastic laminate of the piezoresistive sensors.

Patient characteristics (i.e., weight, age, gender, height), specific metrics of intubation difficulty (i.e., Mallampati grade, thyromental distance, neck movement, mouth opening, dentition, Cormack-Lehane (C&L) grade, subjective difficulty, number of attempts, and any complications) were noted. Forces applied to the maxillary teeth (dependent parameter) were measured for the duration of insertion of each laryngoscope.

The measurement of the forces was performed with Flexiforce® sensors (A201-25, Tekscan, MA) fixed to the blade of the laryngoscope at the area of contact with the teeth (Fig. 2). Three sensors were mounted along the length of the blade to sufficiently cover the contact surface with the teeth, given that the contact point (teeth-laryngoscope) varies for each patient. The sensors are rated to 110 N. Calibration was performed by applying a known mass (from 1 to 12 kg, in steps of 1 kg) using a flat-headed screw driver (as geometrical approximation of the contact with the teeth) to the sensors mounted upon the blade. The sensors were invariant to the contact point of the applied load. Each of the sensors was individually calibrated in situ. Data acquisition was achieved with a National Instruments® DAQ6009 (National Instruments Corporation, TX) card at 500 Hz, using Labview® 7.0 (National Instruments Corporation, TX) on a laptop computer (Hewlett Packard Company, CA). Peak forces were subsequently noted for each of the patients.

Each type of laryngoscope, Mallampati grade, anesthesiologists (specialist versus resident), subjective difficulty, dentition, C&L grade, and number of attempts were analyzed against the applied force to the maxillary incisors to ascertain significant relationships using the Kruskal-Wallis one-way analysis of variance (nonparametric, to forego assumptions of normality of the force distribution) using MATLAB® 7.2 (R2006a) (Mathworks, MA). Corrections were made for the multiple hypotheses tested using Bonferroni (i.e., significance = α/n). A Pearson product-moment correlation coefficient was calculated to determine if patient-specific characteristics were significantly and linearly correlated to the applied forces, again using MATLAB 7.2. The relationship of C&L grade (most common metric of intraprocedural intubation difficulty) with blade type was analyzed, again using Kruskal-Wallis one-way analysis of variance. P < 0.05 was taken to be statistically significant.

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All patients (24 female [50 ± 16 yr], 20 male [56 ± 13 yr]) were successfully intubated in the study with both the classic and video laryngoscope. Furthermore, no injuries or dental damage were recorded during the study, except for minor laceration of the lip of one patient. Peripheral oxygen saturation was more than 95% for all patients during the intubation sequence. The forces recorded for the classic Macintosh blade ranged from 0 to 87.4 N with a median of 15.3 N, whereas forces recorded for the video laryngoscope ranged from 0 to 45.2 N, with a median of 2.1 N.

The only factor determined to be of direct significance on the associated forces applied to the maxillary incisors was the laryngoscope type (Kruskal-Wallis, χ2 = 40.51, P < 0.01). Video-assisted laryngoscopes reduced the applied forces over classic blades. Figure 3 presents box plots of the forces exerted on the maxillary incisors for the classic Macintosh and video laryngoscope, respectively. Mallampati grade, subjective difficulty, dentition, C&L grade and number of attempts were not statistically significant influences on the applied forces. Table 2 presents the forces associated with the respective blades for the Mallampati grades, and Table 3 shows the same for C&L grade.

One of the most evident results of the study, as observed by each of the participating anesthesiologists, was the almost optimal C&L grades for every patient when using the video laryngoscope (Table 4). There was a very strong relationship between the type of blade used, and the C&L grade (Kruskal-Wallis, χ2 = 25.2, P < 0.01).

There was no significant difference in the forces applied by the anesthesiologists, across all various experience levels (Fig. 4). Additionally, there was no significant correlation between thyromental distance, Body Mass Index (Figs. 5 and 6), mouth opening, and age with the measured force.

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When using the video laryngoscope, the forces exerted by the anesthesiologist on the patient’s maxillary incisors are reduced when compared with the classic Macintosh laryngoscope. Although exertion of any force is incorrect following textbook protocols, it is unavoidable in many patients. Indeed, the incidence of accidental dental damage points to this phenomenon, which undoubtedly goes hand-in-hand with difficulty of the intubation.4

It is not uncommon with classic laryngoscope designs to effectively perform the intubation “blind,” and the temptation to use some leverage to achieve greater separation of soft-tissues and mouth opening is great. Video imaging of the glottis, at the end of the blade, reduces this temptation with near perfect viewing of the vocal cords for every patient. It would be a likely hypothesis to suppose that the additional modality of the video laryngoscope would be redundant for “easier” patients. Anesthesiologists are well versed in use of intubation equipment, and it seems unlikely that the video modality would help for simpler cases. However, the video laryngoscope was superior to the classic design for patients with better Mallampati grades.

An interesting observation of this study was the correlation between C&L score and the type of laryngoscope. There were only four incidents of C&L score other than perfect (I) in this study for the video laryngoscope. This brings into question the value of C&L grades when using video modality. Indeed, in further clinical practice outside of this study, there have been near zero incidences of difficult view of the glottis, for even the most difficult patients intubated in the participating hospital with video laryngoscopes. A caveat of the current study is that good view of the glottis does not guarantee an easy intubation. This implicitly illustrates the inadequacy of the C&L scoring system for predicting a successful intubation. Additionally, there was no significant correlation of Mallampati grade with applied forces.

It was noted during the experimentation that anesthesiologists who participated in the study had a small learning curve in adjusting to the video laryngoscope, especially the slightly different eye-hand coordination. This was, however, not evident in the force profiles. We intend to investigate learning curves with various models of video laryngoscopes.

There are many new video laryngoscopes entering the market. This study restricts itself to the comparison of two geometrically similar blades, (one with, one without video functionality) from the same manufacturer. Initial impressions with different models from the various suppliers indicate that the video imaging provides an equally good view of the glottis, yet the ease of intubation differs markedly. Although outside the scope of this study, it is an interesting point of research for investigating the subjective and objective ease of intubation with the different models. Our initial supposition is that the geometry of the blades, specifically in relation to the ease of the insertion of the endotracheal tube, is the bottleneck given that viewing of the glottis is sufficient with the video modality. Furthermore, in an ongoing study we will investigate the differences in intubation difficulty for especially problematic cases. We found lower forces were applied to the maxillary incisors when we were using indirect video laryngoscopy, and higher forces when using direct classic laryngoscopy.

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Video-assisted laryngoscopy results in significantly lower forces exerted on the maxillary incisors relative to classical direct laryngoscopy. This conclusion is regardless of anesthesiologist experience, patient characteristics, or common metrics of intubation difficulty. The clinical implications include that video-assisted laryngoscopes are potentially superior even for easier patients. Furthermore, in this study the contemporary metrics for determination of difficult airways (e.g., Mallampati, C&L etc.) do not predict the forces applied to the patient’s teeth, and may be unsuitable for predicting intubation success when using a video laryngoscope. Given these findings, the incidence of dental damage during laryngoscopy may be reduced by adopting video laryngoscopes for intubation. Further study is needed to determine the impact of using video laryngoscopes on the actual incidence of dental injury during intubation.

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The authors thank John Dukker of the Precision Mechanical Workshop (3ME), Technische Universiteit Delft, for his effort in the arduous task of precisely placing the sensors, and Kees Slinkman for the construction of the data acquisition system.

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