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Anesth Analg 2007; 105:1118-1126 © 2007 International Anesthesia Research Society doi: 10.1213/01.ane.0000278734.34434.60
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Abstract |
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 Top Abstract IntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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BACKGROUND: The view obtained during direct laryngoscopy is only seen by a single anesthesiologist. The inability of instructors to observe the view poses problems for teaching the technique. The anatomic interactions affecting laryngoscopy are largely internal, hampering efforts to understand why some patients are unexpectedly difficult to intubate. In response, we have constructed a full scale, adjustable, two-dimensional model showing the head and neck in the sagittal plane. In this article, we validate the mannequin and test how various conditions or changes in equipment affect the laryngoscopic view.
METHODS: Model parameters were compared with literature values. Glottic exposure was evaluated over a range of jaw lengths and interincisor gaps for Macintosh 3, Miller 2, and Macintosh 4 blades.
RESULTS: Thirty segmental airway distances and 10 angles were within 1 standard deviation from published values. Spine and jaw mobilities approximated normal range of motion. Glottic exposure decreased steeply for mouth openings below a threshold. A larger mouth opening was required to obtain a view when the mandible was short. None of the blades exposed the glottis when mouth opening was narrow, 2.4 cm. The Macintosh 4 blade was closest to success, within 7 mm of viewing the posterior cords.
CONCLUSIONS: The model reflects an average 16-yr-old male patient in size, proportion, and mobility. It can be used to explicate how anatomic relationships affect laryngoscopy. An objective assessment is necessary to determine the model’s utility for teaching and as a tool for researching the mechanisms responsible for laryngoscopic difficulty.
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Introduction |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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Experienced anesthesiologists meet with difficulty visualizing the glottis by direct laryngoscopy in 1.5%–13% of patients (1). The source of the difficulty often rests with unfavorable anatomic or functional characteristics of the patient’s oropharynx, jaw, neck, cervical spine, or larynx (2). The risk of limited visualization increases in patients with more than one unfavorable anatomic or functional characteristic. Thus, some investigators have described multivariate models for predicting laryngoscopy difficulty from combinations of factors (1,3,4).
The importance of anatomic structures and their interactions for laryngoscopy are not easy to appreciate, because laryngoscopy takes place inside a patient’s mouth and pharynx. Teaching is complicated because trainees and instructors cannot observe the other’s movement or view. Training mannequins help in developing technique. However, most three-dimensional laryngoscopy models simulate a single patient’s anatomy and provide no experience in how variations in shape and size affect difficulty or information on how to cope with such variation. The inability to vary the model anatomy is a significant limitation in teaching laryngoscopy, because trainees develop their technique for the mannequin on which they train, and have difficulty shifting to a new setting (5). Some laryngoscopy trainers are capable of limiting airway compliance or neck range of motion, but the dimensions and proportions of head and neck structures are static.
The capability of adjusting anatomy could be added to a standard airway mannequin, but from a design perspective, it is easier to build a laryngoscopy training tool with variable anatomical features in two dimensions. Such a mannequin could allow motion by incorporating different layers. A two-dimensional model makes sense because the movements of the head, neck, and laryngoscope lie primarily in the sagittal plane. Therefore, we have designed and constructed a full-scale two-dimensional mannequin that can model the primary aspects of laryngoscopy and allow variation in the key features that affect difficulty. The model allows direct inspection of the internal mechanics of laryngoscopy to aid in teaching and understanding laryngoscopy. In addition, it can be used to address how different anatomic factors, singly or in combination, affect the ability to obtain a line of sight from the mouth to the vocal cords with a laryngoscope. This article describes the model, compares its dimensions, proportions, and motion with reported values for patients and illustrates how it can be used to evaluate the effects of anatomic factors on laryngoscopy.
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METHODS |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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Model Concepts
Model components representing cervical spine, thyroid cartilage, epiglottis, hyoid bone, mandible, incisors, tongue, mandible, cranium, and laryngoscope were organized on a plastic backboard. The C7 vertebra was fixed in position with the rest of the spine, the occiput, and the mandible connected in series to C7 and mobile on the backboard. Models of the thyroid cartilage, epiglottis, and hyoid bone were attached, but their position could be adjusted.
The model was intended to simulate a static picture of laryngoscopy at the point of attempting to expose the vocal cords. The mouth opening, jaw displacement, head extension spine flexion and laryngeal configuration were arranged in positions that would occur with optimal effort during laryngoscopy. Then the determination was made whether the vocal cords would be visible. Endotracheal intubation was not modeled.
The model could emulate patients in whom anatomy, proportions, and range of motion were normal and abnormal. For example, the laryngeal components could be fastened at any position in a grid of holes in the backboard spanning 100 mm x 70 mm in the cranial-caudal direction and anteroposterior directions, respectively. Spine and jaw mobility could be altered and dimensions of other features could change as well. An expanded description of these features appears below. A supplemental document that discusses model design and construction in detail can be downloaded at either http://anes-som.ucsd.edu/Hastings/Laryngoscopy_Model_Supplement.pdf or http://www.delsonlabs.ucsd.edu/laryn/public/publications/Shea_Online.pdf.
Model Design
Head design was based on the Ann Arbor atlas of cephalometric data from the University of Michigan School of Dentistry, a compilation of measurements made in 47 Caucasian male and Caucasian 36 female patients between 6 and 16 yr of age (6). The atlas presents age- and sex-specific tables of close to 200 segmental lengths and angles relating anatomic points and landmarks. We drew the pattern for the head from 16 reference points that, in our opinion, influence access to the airway (Table 1). The Ann Arbor atlas summarizes distance and angle relationships for groups of two and three points, but it does not map the location of the entire set of points. Our contribution was to manually arrange the 16 points in a two-dimensional plane in a way that maintained 30 segmental distances and 10 different angles as closely as possible to the mean values for the 16-yr-old male patients in the Ann Arbor data (n = 23). We used soft tissue profiles in the atlas to draw an outline of the head around the points (Fig. 1A).
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The head was constructed in two parts, the upper skull and the mandible. In turn, these components were each divided into anterior and posterior pieces that could be separated or approximated with adjustable connectors (seen in right panel in Fig. 2). These adjustments allowed alterations in the length of the face and the mandible. Profiles of the upper and lower incisors were incorporated into the maxilla and mandible. The jaw opened in a superoinferior direction around a point hinge connecting the mandible and skull. A sliding mechanism allowed the jaw to extend anterior to the maxilla along the axis of the mandible, a prognathic direction (Figs. 2, and please see Supplementary Fig. S1 available at www.anesthesia-analgesia.org). The mandibular condyle rested on the maxilla, shifted away from the surface on which the maxilla rested. The two components were separated to avoid interfering with each other at the hinge. Raising the mandible also created room for the model laryngoscope to pass from the mouth to the vallecula. On the right side of Figure 2, one can see the laryngoscope entering the mouth and passing underneath the mandible toward the larynx.
A plastic block was inserted under the anterior portion of the mandible, occupying roughly one-third of the mandibular space. This block represented the volume of tongue that would be compressed by the laryngoscope blade during laryngoscopy (7). It limited how far the model laryngoscope could be advanced in the mandibular space (please see Supplementary Fig. S5 available at www.anesthesia-analgesia.org), just as the tongue would during laryngoscopy in a patient.
The model cervical spinal column included the seven vertebrae and the disks. The shapes of the individual bodies became progressively broader and less angled going in the cephalad to caudal direction, producing normal lordosis (Fig. 1B). The C7 vertebral body was fixed to a backboard, while C1 was attached to the occiput. Joints between the occiput, C1 and each successive vertebra, could flex and extend in the sagittal plane. The atlantooccipital joint was modeled as a single point hinge, whereas the other cervical joints consisted of two-point hinges between the anterior portion of the body below and the mid-portion of the body above. Each level included a foam rubber intervertebral disk. The hinges were designed and the thickness and compliance of the disks chosen to give a range of motion close to the physiologic range in patients.
The model larynx consisted of hyoid bone, epiglottis, and thyroid cartilage (Fig. 1C). The hyoid and epiglottis were joined in one unit, representing the connection through the hyoepiglottic ligament. The thyroid cartilage was attached to the backboard anterior to the spine and below the jaw, and the hyoid-epiglottis complex was placed a few mm superior to it, secured at a pivot point (Fig. 2). The design allowed the hyoid and epiglottis to be rotated in an anterior and inferior course, modeling the movement the structures take when a curved laryngoscope blade applies pressure in the vallecula of a patient (7). A 15-mm line was scored on the cranial edge of the thyroid cartilage to identify the approximate position of the glottic opening in the sagittal plane (11). A smaller glottic opening could be modeled as a shorter length. As a standard position, the thyroid cartilage was placed 1.5 cm anterior to the lower border of C4, the hyoid bone was aligned with the C3–4 interspace and the epiglottis was oriented with C3 (12,13).
Outlines of a laryngoscope handle and Macintosh 3, Miller 2 and Macintosh 4 laryngoscope blades were produced with specifications from Penlon® (www.penlon.com) and Welch Allyn (http://www.welchallyn.com/medical/products/catalog).
Model Construction
Patterns were drawn with Smartsketch version 4.0 (Intergraph Corporation, Huntsville, AL) and modified with AutoCad 2000 (Autodesk, Inc., San Rafael, CA). A LaserCAMM machine cut the parts from acrylic plastic, controlled by LaserCAMM user software version 11.0. The laser beam is between 0.01 and 0.02 inches in diameter. Holes were tapped by hand. Intervertebral disks were cut manually from flat 1/4-inch foam rubber using templates (for patterns to scale, please see Supplementary Fig. S6 available at www.anesthesia-analgesia.org).
Model Validation
To evaluate whether our model was a reasonable representation of an average male head and neck, we compared segmental distances and angles involving our arrangement of the 16 reference features with values in the Ann Arbor data (6). In addition, we matched the model against cephalometric measurements of airway structures in normal subjects found in the sleep apnea literature (14,15). The reference data were taken from control groups of people without sleep apnea. Finally, we compared range of motion of the cervical spine during flexion and extension, mouth opening, and extent to which the jaw could extend into a prognathic position to literature values for normal adults (8–10,16,17). These studies covered a range of ages from late adolescence to old age.
Effects of Anatomy on Laryngoscopy
We tested whether the model could be used to analyze how changes in mouth opening and thyromental distance affected laryngoscopy difficulty. The model was held in a standard intubating position. The C3–C6 vertebral bodies were flexed 23°, the angle of the spine when the neck is flexed 35° for the sniff position (7). The face plane (perpendicular to the line connecting the opisthion and basion, Op and Ba in Fig. 1A, respectively) was extended 15° relative to horizontal as shown in Figure 2. Interincisor gap was varied from 4 to 2 cm and thyromental distances could be set to 6.1, 5.5, or 5.3 cm. The 5.5 and 5.3 cm lengths modeled patients with short jaws. The 6.1-cm thyromental distance is shorter than the average for adult males, probably because our model is based on measurements from 16-yr-old male patients. However, thyromental distances <7 cm occur in roughly 25% of the normal population, 54 of 207 adult males in one report (18).
To evaluate view in a given position, the laryngoscope was advanced through the mouth and pharynx to contact the hyoid bone with the tip of the blade. The blade was then shifted upward and forward, rotating the hyoepiglottic complex with the blade tip in contact with the hyoid. The movement ended when the blade was constrained by the incisors or the tongue block. Contact between the hyoid bone and the laryngoscope blade was based on radiographic observations during laryngoscopy reported by Charters (7). The line of sight was determined by passing a 1/16-inch rod past the upper incisors along a tangent with the blade. The length of the glottic opening under the line of sight and the distance between the vertebral column and the hyoid were measured with a ruler. The angle of the laryngoscope handle with respect to the horizontal in the sagittal plane was measured with a protractor.
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RESULTS |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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Model Proportions and Mobility
Our array of cranial landmarks in a two-dimensional pattern provided a close approximation to the cephalometric reference data (Table 2). For example, all 10 angles and 30 segmental distances used to position the landmarks lay within one standard deviation (SD) of the mean values listed in the Ann Arbor data. The model also approximated four angles and four segmental distances that were not considered when designing the model. Finally, the model also closely matched cephalometric distances between cranial and airway structures in normal subjects (lower portion of Table 2). In comparison with literature values for 47 segmental distances and 14 angles, our model parameters differed by 0.2 ± 0.1 cm (mean ± sd) for the distance measurements and 2° ± 2° for the angles.
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The full range of motion in extension of our cervical spine model at C0–1 (atlantooccipital joint) was 26°, close to values reported in the literature for normal adults of all ages [range 2°–30° with n = 70 (16) mean ± sd 27.4° ± 7.1° with n = 16 (17)]. From C2 to C7, the model possessed 29° flexion and 17° extension (Fig. 3) compared to the full range of 40° (range 23°–61°) reported in the literature. Combining the mobility from the occiput to C7, the model head displayed 69° flexion and 103° extension from the neutral position, compared to normal adult patients with mean flexion of 54° ± 8° (range of 36°–70°), and mean extension of 93° ± 12° (range of 70°–120°) (8–10).
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Our model mouth could open approximately 6 cm, measured from inferior tip of the upper incisors to the superior tip of the lower incisors, and the mandible could project 7 mm into a prognathic position. For comparison, maximum mouth opening is as much as 6 cm in some individuals with a mean of 3.9 ± 0.7 cm (19) and the ability of the jaw to extend in front of the face in adults averages of 5 ± 4 mm (20).
Glottic Exposure as a Function of Mouth Opening and Mandibular Length
In our defined standard intubating position, the full length of the glottic opening was visible with a normal mouth opening of 4 cm and with openings reduced to as little as 2.8 cm (Fig. 4, closed circles). Narrowing the oral opening to <2.8 cm reduced the length of visible glottic opening, and none of the glottic line was exposed at 2.4 cm. In a patient, this degree of exposure would constitute a grade II or III view with no more than corniculate cartilages of the posterior arytenoids or epiglottis visible. When the jaw was shortened, glottic view became limited at wider mouth openings. With a 5.2-cm thyromental distance, for example, a grade I view of the cords was lost at interincisor gaps below 3.1 cm and no glottis could be exposed if mouth opening were set at 2.8 cm (Fig. 4, open circles). Interestingly, when both mandible and maxilla were shortened by 0.6 cm, the view of the cords was not lost until mouth opening was reduced below 2.4 cm (Fig. 4, squares).
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When the mandible is shorter than the maxilla, the upper incisors, lower incisors, and compressed tongue constrain the laryngoscope blade to a more vertical angle, sufficient for entering the mouth, but inadequate to rotate around the base of the tongue toward the vallecula (Fig. 5, photograph on right). Because of the blade angle, forward and upward lift is impaired, displacement of the epiglottis is less and the epiglottis is more likely to impinge on the view of the glottis. With a normal jaw length, the blade can rotate farther into the pharynx and the handle, which is perpendicular to the blade, can take a more vertical angle. For example, the angle between the handle and horizontal is 31.5° when the jaw length is normal (left side of Fig. 5) compared to 23° when the jaw is short (right). The vertical handle angle appeared to equate with greater lift and better view. Over a large number of trials under different conditions, a better view of the glottic opening was generally achieved when the angle of the handle was greater. (Please see Supplementary Fig. S7 available at www.anesthesia-analgesis.org.)
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We investigated whether the problem of a narrow mouth opening might be mitigated with specific blades. Laryngoscopic views were assessed with Macintosh 3, Miller 2 and Macintosh 4 blades in a patient with a 2.4-cm mouth opening and otherwise standard position. None of the blades allowed visualization of the glottis under these conditions (Fig. 6). The Miller 2 blade lifts the epiglottis directly with greater displacement than the Macintosh 3 blade (Fig. 6B vs 6A). However, the straight blade did not provide an anterior angle on the line of sight that was available with the curved blade, and the view remained posterior to the cords. The longer Macintosh 4 blade could be inserted farther into the narrow mouth than the Macintosh 3. The result was greater rotation of the hyoid and greater lift of the epiglottis (Fig. 6C). The line of sight was still behind the glottic opening, but it was closer to the cords than for the other two blades, only 7 mm away (see inset on right). Conceivably, pressure on the thyroid cartilage directed superiorly and posteriorly might bring the cords into view.
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DISCUSSION |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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This project developed a physical model of the head, neck, and airway representing what could be considered an average adolescent, Caucasian male patient in terms of size, proportion, and mobility. In general, the distances among multiple pairs of anatomic landmarks and angles relating landmarks were close to the mean values reported in the cephalometric literature. The mobilities of the spine and jaw also approximated range of motion in normal patients. The model also met the goal of being adjustable across a range of normal and pathologic functional anatomies. It accommodates alterations in midline dimensions, such as length of the glottic opening, size of the tongue, length of the epiglottis or presence of midline tumors.
We modeled the anatomy of 16-yr-old Caucasian male patients because the greatest amount of cephalometric data for patients close to adult size has been collected for this group. The restricted patient population in the Ann Arbor data source may limit the degree to which our model can be applied to different groups of patients. In regard to age, minor increases in cephalometric parameters, notably 2–3 mm increments in nasion-mentum length and mandibular length, occur between ages 16 and 25 yr (21,22). These differences are small, and unlikely to significantly alter results obtained with this model. Female patients may have somewhat smaller dimensions than male patients and some proportions could be different, particularly around the jaw line and larynx. The model also does not necessarily reflect anatomic proportions that would be found in non-Caucasian populations. Sex-specific models could be constructed from the Ann Arbor data and race-specific models would be possible if the appropriate cephalometric information were available.
This model easily tests the effects of isolated changes in head, neck, and airway hard tissue configuration, but altering one part of the bony anatomy may have consequences on soft tissue elements in vivo. Atlantooccipital extension tends to displace the tongue posteriorly, for example (23). The model is disadvantaged in not featuring the tongue, ligaments, and other soft tissue components. Accordingly, our analysis of laryngoscopy with a receding jaw does not account for effects that could result from a change in the relative tongue size and mandible space. The size of the block in the submandibular space could be altered to model changes in tongue volume or compliance, but we have not pursued this point yet. Age-related changes in compliance and elasticity could change with age and affect ease of laryngoscopy. Our model does not include these parameters, but they could be modeled indirectly by reducing range of motion.
Lack of the third dimension limits what we can study with the mannequin. The model cannot account for the width of the pharynx, accommodate the lateral displacement of the tongue, or simulate the effects of airway tumors or pathology away from the midline. The model may allow useful analyses that do not depend on three-dimensional structure, but the limitations related to space and soft tissue must be kept in mind. Ultimately, any hypotheses generated from results with our model must be tested in human subjects.
We envision that the model will be used as a teaching aid for laryngoscopy. We have used the model to teach basic concepts of patient positioning, blade placement, laryngoscope handling, and view. We have also used it to demonstrate why factors measured in the standard airway examination, such as thyromental distance, mouth opening or spine mobility, effect laryngoscopy. Formal studies would be necessary to test whether the model provides advantages over other methods of teaching laryngoscopy.
We believe that this airway replica could also be exploited to analyze questions about laryngoscopy. For example, one might use the model to evaluate whether and why the sniff position is the best for laryngoscopy with a curved blade, adding to previous work on the topic (24). The mannequin could be used to test whether modifications in positioning or blade design would be beneficial in the face of specific anatomic limitations (Fig. 6). The model might help identify predictive factors for difficult laryngoscopy, such as the ratio of mandible to maxilla length (see below). In this study, we tested the effects of mouth opening and jaw length. Small mouth opening obstructed glottic visualization, as expected, and was more likely to be a problem with a receding jaw. Mouth opening became limiting over a relatively short range, 2–4 mm. A narrow window between easy and difficult laryngoscopy could explain, in part, why predicting difficult laryngoscopy is challenging.
The results for both mouth opening and jaw length indicate that unfavorable anatomic characteristics frequently interfere with positioning the laryngoscope, specifically rotating and lifting the blade into the necessary position. Other investigators have noted that laryngoscopic view is often better when the laryngoscope handle assumes a greater angle with the horizontal (12,25,26). The ability of a short face to compensate for a receding jaw suggests that a consideration of the maxillary length might be a valuable addition to the preanesthetic airway examination. In fact, Horton et al. (27) reported that laryngoscopy may be difficult if the ratio of a patient’s mandible length to maxilla length was <0.9. Patient studies might be indicated to determine whether combining some external physical examination measurement of mandible to maxilla length ratio with thyromental distance would improve discrimination between patients with easy and difficult laryngoscopy.
The model could provide guidance about which laryngoscope to use based on a particular patient’s anatomy. We compared the performance of Macintosh 3, Miller 2 and Macintosh 4 blades with a narrow mouth opening to test the principle. Our analysis in Fig. 6 suggested that the Macintosh 4 blade combined with thyroid pressure might provide the best chance for success of the three blades. Proof of the concept would require real-life trials. Relative blade performance could vary in different head and neck positions, but was not evaluated.
In summary, this article describes a two-dimensional, adjustable model of human upper airway anatomy relevant to laryngoscopy designed with cephalometric data. The model could be used as a teaching aid and may also generate ideas about how anatomic relationships affect the procedure. The model could provide new insights about interactions between different physical characteristics or new anatomic measurements that could improve prediction of difficult laryngoscopy. Further investigations are planned to determine the model’s utility in teaching and as a tool for researching the mechanisms responsible for laryngoscopic difficulty.
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Footnotes |
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This article has supplementary material on the Web site: www.anesthesia-analgesia.org.
Accepted for publication June 8, 2007.
Supported by the Department of Mechanical and Aerospace Engineering for model materials and LaserCAMM use.
None of the authors has a conflict of interest to report.
Reprints will not be available from the author.
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