Anesth Analg 2008; 107:1248-1252
© 2008 International Anesthesia Research Society
doi: 10.1213/ane.0b013e318185cecb
TECHNOLOGY, COMPUTING, AND SIMULATION
A New Curved Laryngoscope Blade for Routine and Difficult Tracheal Intubation
Koji Nishikawa, MD, PhD*,
Koki Yamada, MD, PhD*, and
Atsuhiro Sakamoto, MD, PhD
From the *Department of Anesthesiology, Nippon Medical School Tama Nagayama Hospital, Tokyo, Japan; and Department of Anesthesiology, Nippon Medical School Hospital, Tokyo, Japan.
Address correspondence and reprint requests to Dr. Koji Nishikawa, Department of Anesthesiology, Nippon Medical School Tama Nagayama Hospital, Nagayama 1-7-1, Tama-shi, Tokyo 206-8512, Japan. Address e-mail to nagayamamasui60{at}yahoo.co.jp.
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Abstract
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We have designed a new curved laryngoscope blade based on a new concept of reversing the peardrop phenomenon to facilitate a view of the larynx sufficient for intubation in a greater variety of patients than the current Macintosh blade affords. The new design has a bifid tip and S-shaped spatula to exert more effective pressure in the vallecula area, elevate the epiglottis and change directions of the forces on the tongue to prevent posteroinferior displacement of the compressed tongue in the submandibular space during laryngoscopy. A radiograph laryngoscopy technique was used to guide the new blade curvature design and compare the performance of the new blade with the Macintosh blade in patients with or without a difficult airway. Our results confirm that the new blade provides a laryngeal view sufficient to accomplish intubation by compressing the root of the tongue in an anterocephalad direction in the submandibular space and elevating the epiglottis effectively in patients with or without unanticipated difficult airway. The new curved blade can also effectively move the U-shaped epiglottis out of the laryngeal view to facilitate intubation in pediatric patients aged 2 mo–13 yr.
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Introduction
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Unanticipated difficult laryngoscopy and/or difficult intubation is an important problem as management may be complicated by the lack of preparation of alternate intubation techniques.
In an editorial, Hung and Morris emphasized that a clear understanding of the dynamic anatomy of the upper airway during laryngoscopy is required to develop new strategies to manage the unanticipated difficult airway.1 The peardrop phenomenon, noted by Horton et al. with radiograph laryngoscopy,2 is caused by posteroinferior displacement of the compressed tongue in the submandibular space, which impairs a direct view of the airway due to the inability to elevate the epiglottis during laryngoscopy with the Macintosh blade.
The submandibular space has a flexible wall and is the main space for the displacement of the tongue during laryngoscopy. We hypothesize that reduced compliance of the submandibular space may limit its potential anterior space and compress the root of the tongue backward into a peardrop shape. The peardrop-shaped tongue can then press the less curved distal portion of the Macintosh blade3 to fold the epiglottis down toward or against the posterior pharyngeal wall, which may obscure the view of the vocal cords and be associated with difficult intubation.
Although the reasons for encountering the unanticipated difficult airway are not fully understood, we speculate that the mechanism of the unanticipated difficult airway may be principally related to the peardrop phenomenon in patients with reduced submandibular compliance. Current methods for preoperative airway assessment would not predict reduced submandibular compliance. A possible way to reverse the peardrop phenomenon is to modify the laryngoscope blade curvature with the goal of modifying the forces on the tongue and preventing posteroinferior displacement of the compressed tongue into the submandibular space (Fig. 1).
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Figure 1. The peardrop phenomenon. The compressed tongue displaces in a posteroinferior direction in the submandibular space and becomes molded into a peardrop shape during laryngoscopy. The peardrop-shaped tongue can then press the less curved distal portion of the Macintosh blade to fold the epiglottis down toward the posterior pharyngeal wall, which may cause difficult laryngoscopy and/or difficult intubation. The dashed line XY is the expected curve of the laryngoscope under normal circumstances. P1: the anterocephalad force on the root of the tongue in the submandibular space; P2: the downward force within the tongue in the mandibular space. It is possible to change the blade curvature for reversing the peardrop effect by increasing P1 and decreasing P2.
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Furthermore, the bifid tip of the laryngoscope blade, designed by Bowen and Jackson was confirmed to be more effective than the Macintosh blade for elevating the epiglottis by increasing the pressure in the vallecula.4 We used cervical magnetic resonance imaging or computed tomography scans of normal individuals to guide a redesign of the bifid tip to improve the performance of the new blade tip.
Based on the new concept of reversing the peardrop phenomenon, we have developed an alternate design to the Macintosh blade with a streamlined bifid tip and S-shaped spatula. This blade design is intended to provide an adequate view of the larynx for intubation in more patients than the current Macintosh blade and to reduce the risk of unanticipated difficult airway. The performance of the new blade was objectively evaluated with the radiograph laryngoscopy technique in patients with or without a difficult airway.
Description of the Modification
The spatula consists of three continuous curves in an S shape: the proximal, intermediate, and distal curves. The distal curve is an inverted shape. The intermediate curve is markedly anterior compared with the Macintosh blade (Fig. 2a). LN is a horizontal line drawn from the handle-mounting block. T is the blade tip. TN is perpendicular to LN. LN and the under surface of the blade mid-portion cross at point M. The ratio of MN to LM is approximately 0.62. The angle TMN varies with different blade sizes in the range 35 to 39 degrees (Fig. 2b).
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Figure 2. (a) View of the new blade curvature. (b) Right side view of new blade (Size 3). Reference lines and points for analysis of the new blade shape: LN is a horizontal line drawn from the handle-mounting block. T is the blade tip. TN is perpendicular to LN. LN and the under surface of the blade mid portion cross at point M. MN/LM: 0.62. The angle TMN: 38 degrees. (c) View of the new blade tip and cervical computed tomography scans of the normal individual show the shape of the vallecula. The distance from S1 to S2 is 3–5 mm.
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The new blade has a streamlined bifid tip, which was designed based upon the dimensions of the vallecula on cervical magnetic resonance imaging or computed tomography scans of 10 normal individuals, to provide a larger contact area in the vallecula than that of the current Macintosh blade. The distance from image S1 to image S2 is approximately 3–5 mm (Fig. 2c).
There are four sizes of the new blades commercially available. The material used in the manufacture of the blade is chromium-plated brass. To avoid contact of the handle with the patient’s chest, a short folding handle is used.
Design Process of the New Blade Using Radiograph Laryngoscopy
To determine the best ratio of MN to LM, 2 versions of prototype blades were empirically made in a ratio of MN to LM 0.74 and 0.42. We confirmed the laryngeal view and performance of each version of the prototype blade during laryngoscopy with radiograph laryngoscopy and noted that it was slightly difficult to conduct laryngoscopy with the version in the ratio 0.74; the laryngeal view could be worsened by the intermediate curve of the blade in the ratio 0.42 compared to the Macintosh blade in some cases. Finally, a golden ratio (MN/LM: 0.62) was introduced to optimize the curvature of the blade and reduce the interference of the blade mid-portion with the laryngeal view. The angle TMN was evaluated in the same fashion. The new version of the prototype blade shown in this study was designed and tested. (Fig. 3).
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Figure 3. Design process using radiograph laryngoscopy. Lateral cervical radiograph shows laryngoscopy with the prototype blade in the ratio MN to LM 0.42 (Size 3) on a female patient with a Cormack and Lehane grade 2 view in whom the laryngeal view was grade 1 with the Macintosh blade. Reference lines and points for analysis of the radiograph are the same as those mentioned in Figure 2b. A golden ratio (MN/LM: 0.62) was introduced to revise the blade curvature to improve the laryngeal view. S line, the expected blade curvature; Eyeline 1, the actual eyeline; Eyeline 2, the expected eyeline; F line, the expected flange edge.
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METHODS
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Initial Clinical Experience and Evaluation Using Radiograph Laryngoscopy
This study was approved by the hospital ethics committee of our institution and informed consent was obtained from each patient. We have compared the direct laryngeal views obtained with both the Macintosh and new curved blades, and then successfully accomplished tracheal intubation with a Cormack and Lehane grade 1–2 view using the new blade in 150 adult (including 4 patients with an unanticipated difficult airway) and 20 pediatric patients. In the present study, we defined the unanticipated difficult airway as a grade 3–4 view during laryngoscopy at the first attempt with the Macintosh blade despite the application of optimal external laryngeal pressure in patients without obvious anatomical factors indicating a difficult airway.
The radiograph laryngoscopy technique was used to compare the performance of the new blade with the current Macintosh blade in some typical cases with or without a difficult airway. Synapse Workstation Software (FUJIFILM Medical CO) was used to analyze the eyeline, the blade tip position, the distance from the mid-point on the anterior surface of C3 to the mid-point on the under surface of the blade mid-portion, the distance from the arytenoid cartilage to the posterior pharyngeal wall and cross-section area of the root of the tongue in the submandibular space.
According to our initial clinical experience, the advantages of the new blade were found to be: (a) it was as easy to obtain the same laryngeal view as when using the Macintosh blade in patients without a difficult airway (Figs. 4a and b), (b) it was effective for converting a Cormack and Lehane grade 3 view with the Macintosh blade into a grade 2 or 1 view and elevating the downfolded epiglottis by withdrawing the blade slightly to displace the root of the tongue in the submandibular space anterocephalad into the mandibular space and mouth in patients with a difficult airway (Figs. 5a and b), (c) there was a widely opened glottis without deterioration of the view caused by the U-shaped epiglottis during laryngoscopy with the new blade (Size 1 or 2) in all pediatric patients aged 2 mo–13 yr in this study, and (d) it provided a better anatomical condition for facilitating external laryngeal pressure by increasing the space of the hypopharynx (CD) and freeing the thyrohyoid membrane (Figs. 4 and 5).
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Figure 4. Reference lines and points for analysis of radiographs. AB, distance from the mid-point on the anterior surface of C3 to the mid-point on the under surface of the blade mid-portion between the lower border of the mandible and the upper margin of the hyoid; CD, distance from the arytenoid cartilage to the posterior pharyngeal wall; Tarea, cross-section area of the root of the tongue in the submandibular space. An eyeline is shown from the upper incisors along the under surface of the blade. Radiological variables mentioned above were measured by Synapse Workstation Software (FUJIFILM Medical CO). Lateral cervical radiographs show (a) laryngoscopy with the Macintosh blade (Size 3.5) on a male patient without a difficult airway in the sniffing position (a Cormack and Lehane grade 1 view); AB = 48 mm, CD = 15 mm and Tarea = 982 mm2. (b) laryngoscopy with the new blade (Size 3.5) on the same patient (a Cormack and Lehane grade 1 view); AB = 56 mm, CD = 18 mm and Tarea = 843 mm2. Note that the new blade creates a larger pharyngeal space by compressing the root of the tongue effectively. The main bulk of the tongue is pulled cephalad into the mouth (arrow).
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Figure 5. Reference lines and points for analysis of radiographs are the same as those mentioned in Figure 4. Lateral cervical radiographs show (a) the peardrop phenomenon during laryngoscopy with the Macintosh blade (Size 3.5) in a male patient with a difficult airway (a Cormack and Lehane grade 3 view). Note that the blade tip is below the thyrohyoid membrane and folds the epiglottis down toward the posterior pharyngeal wall to obstruct the airway; AB = 39 mm, CD = 10 mm and Tarea = 771 mm2. (b) The peardrop phenomenon is successfully reversed for ease of intubation with the new blade (Size 3.5) in the same patient (a Cormack and Lehane grade 2 view); AB = 48 mm, CD = 19 mm and Tarea = 703 mm2. Note that the blade tip is below the hyoid body and the downfolded epiglottis in Figure (a) is sufficiently elevated by the new blade. The main bulk of the tongue is pulled cephalad into the mouth (arrow).
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DISCUSSION
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The performance of the new blade curvature strongly supports the potential mechanism for contributing to the unanticipated difficult airway caused by the peardrop phenomenon during laryngoscopy. The inverted distal curve of the new blade is able to exert stronger forces to compress the root of the tongue in the submandibular space in an anterocephalad direction to decrease the cross-section area of the root of the tongue in that space. The enhanced intermediate curve can facilitate the anterocephalad displacement of the tongue by reducing the downward force within the tongue in the mandibular space and also enable the blade tip to be more easily placed in the optimal position, below or slightly cephalad to the hyoid bone.5 Consequently, the pharyngeal space is widened for ease of intubation.
The streamlined bifid tip of the new blade can facilitate insertion of the blade and provide a larger contact area in the vallecula than the current Macintosh blade tip for exerting more effective vertical and horizontal pressure in the vallecula area, and as a result the epiglottis can be more effectively elevated for ease of intubation.
Based on the analysis of radiograph laryngoscopy in patients with an unanticipated difficult airway, we found that there were two basic factors for determining the blade curvature to attain an optimal balance between the dynamic performance of the blade and visualization of the larynx. One factor is the angle TMN, the most effective range of the angle TMN is 35–39 degrees, similar to that advocated by Gabuya and Orkin.6 Another factor is the ratio of MN to LM, a golden ratio 0.62 was the best ratio to balance the design for the new blade. It is very interesting that a similar ratio can also be shown for the prototype of the Macintosh blade.7 For a certain length of the spatula (for example, a 15-cm spatula) with the end-points L and T fixed, a shift of point M toward point N (a decrease in the ratio of MN to LM) will cause a decrease in the visualization of the larynx because of increasing eyeline deviation.8
Curved blades are traditionally not used for pediatric intubations because the pediatric glottis is higher up in the neck and the size of the tongue is relatively larger compared to the oral cavity, which may cause difficult laryngoscopy with curved blades in pediatric patients. The new curved blade may effectively compress the root of the tongue and exert more effective vertical and horizontal pressure in the vallecula area to facilitate glottic exposure in pediatric patients.
Because we only presented our initial clinical experience with the new curved blade in this study, a further clinical investigation in patients with both normal and difficult airways is needed to confirm the usefulness and limitation of the new blade in anesthesia practice. We believe that the new blade may reduce the risk of unanticipated difficult airways.
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ACKNOWLEDGMENTS
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We wish to thank Mr. Y. Kobayashi at DAIWA Manufacture Co. for his ingenious design of the LED lamp and making the prototype of the new blade and the radiological technicians at Nippon Medical School Tama Nagayama Hospital for their invaluable help. The new blade is commercially available through Acoma Medical Industry Co.
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Footnotes
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Accepted for publication June 19, 2008.
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REFERENCES
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- Hung OR, Morris I. Dynamic anatomy of upper airway: an essential paradigm. Can J Anaesth 2000;47:295–8[Medline]
- Horton WA, Fahy L, Charters P. Factor analysis in difficult tracheal intubation: laryngoscopy-induced airway obstruction. Br J Anaesth 1990;65:801–5[Abstract/Free Full Text]
- Jephcott A. The Macintosh laryngoscope. A historical note on its clinical and commercial development. Anaesthesia 1984;39:474–9[Web of Science][Medline]
- Bowen RA, Jackson I. New inventions: a new laryngoscope. Anaesthesia 1952;7:254–6[Medline]
- Horton WA, Fahy L, Charters P. Disposition of cervical vertebrae, atlanto-axial, hyoid and mandible during X-ray laryngoscopy. Br J Anaesth 1989;63:435–8[Abstract/Free Full Text]
- Gabuya R, Orkin LR. Design and utility of a new curved laryngoscope blade. Anesth Analg 1959;38:364–9[Medline]
- Macintosh RR. A new laryngoscope. Lancet 1943;1:205
- Marks RR, Hancock R, Charters P. An analysis of laryngoscope blade shape and design: new criteria for laryngoscope evaluation. Can J Anaesth 1993;40:262–70[Web of Science][Medline]
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Abstract
Introduction
