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The limited opening of the mouth, the teeth, the manner in which the tongue falls around the laryngoscope blade, and the epiglottis, all restrict the visual field in direct laryngoscopy. Endotracheal intubation involves passing another instrument into this restricted space. When there is good mouth opening and laryngeal exposure intubation is rarely problematic. Conversely, when the larynx is poorly exposed, or mouth opening restricted, delivering a tube can be very challenging, especially if the operator does not appreciate the restricted optics and does not have a pre-planned strategy for how to accomplish intubation in such instances.
All instruments designed to be passed into narrow body cavities have a narrow long-axis dimension and an upward distal turn. Instruments that adhere to this shape are alligator forceps, laryngeal mirrors, and trachea tube introducers, a.k.a., bougies. The optical benefit of this shape is that the long straight section allows good maneuverability toward a target, while the upturned distal tip makes the end of the device visible.
The large curvature of standard PVC tracheal tubes (or those prepackaged with an matching arcuate-shaped stylet) is difficult to maneuver in the mouth and hypopharynx. They have a wide side-to-side dimension when viewed down the long axis, causing the tube to contact the sides of the mouth, tongue and teeth. As efforts are made to direct the tip upward, for example, the mid-section of the tube will contact the teeth when used with a stylet, causing bending of the mid-section of the tube and stylet. The other problem with an arcuate-shaped tube (and stylet) is that minor rotational change will cause the distal tip to move a large amount. When this occurs at the last moment of insertion toward the target, the mid-section of the tube may visually block the target itself, leading to inadvertent and unseen passage of the tube into the esophagus.
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 A straight-to-cuff shaped tube and stylet has a narrow long axis and offers significant visual advantages that a tube alone, or a tube with an arcuate-shaped stylet. When viewed down the long axis it has a very narrow dimension. It can be passed into the mouth and easily maneuvered without obscuring the target as it advanced toward the target. The ideal method of inserting a tracheal tube is to always pass the tube from beneath the line of sight, and bring the distal tip up from below, passing over the interarytenoid notch under direct vision without obscuring the target itself. The author prefers using a stylet for all airways because of this maneuverability and visualization advantage. When the tube is first inserted in the extreme right corner of the mouth, the tube is placed visually behind the maxilla, and the distal tip is not even seen. By rocking the proximal tube backward, the distal tip moves posteriorly form behind the maxilla, up into the line of sight, and anteriorly, until it is placed above the posterior landmarks of the larynx and into the trachea.
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Lateral neck radiograph shows how an arcuate tube and stylet block the line of sight (shaded green) down the laryngoscope lumen, obscuring visualization of the tube tip. The mid-section of the tube and stylet contacts the upper dentition, limiting up down mobility..
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1) With arcuate stylet shape, attempts to move up the tube tip causes the mid-section of the tube and stylet to bend over the upper dentition. Large curvature cannot be easily maneuvered.
2) An arcuate shaped tube can cause esophageal intubation through two mechanisms. First, the mid-section and distal third of the tube and stylet block the line of sight and obscure the tip as it approaches the target. Second, minor rotational change while holding an arcuate tube causes the distal tip to move significantly. This can easily cause the tip to rotate past the right aryepiglottic fold, missing the larynx entirely. Since the distal tip is difficult to visualize, the effect of rotation is often unappreciated.
3) The angle of the straight-to-cuff shape should not exceed 35 degrees. Beyond this angle, there is minimal optical benefit. Too acute an angle causes the tip to catch on the tracheal rings even after passing through the cords. |
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After insertion, the tube tip is directed below the line of sight, into the hypopharynx. Pivoting the tube (right corner of the mouth) makes the tip visible, without blocking the target. |
The narrow long-axis dimension of the straight-to-cuff stylet shape permits insertion behind maxilla without blocking the line of sight to the larynx. |
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Simultaneous visualization of the target and tube tip. An additional benefit of the straight-to-cuff shape is that the tip is being directed upward toward the target. This permits it to be exactly placed above the interarytenoid notch into the larynx, even when the glottic opening is poorly seen. The notch is the most posterior aspect of the laryngeal inlet.
The historic approach to stylet shaping is to use a “hockey stick,” but this does not define the optimal bend point, nor the proper angle. Dr. Levitan coined the term “straight-to-cuff” stylet shaping, and recommends an angle not exceeding 35 degrees. This narrow long-axis shape is ideal for tube delivery toward the target, without blocking the line of sight. It still provides enough of a bend upward allowing the distal tip to be easily seen. The bend point should be at the proximal cuff of the tracheal tube and the stylet should stop at the distal cuff. Stylets can be dangerous if they extend beyond the tube tip, and even if they extend just to the tip of the tube they can cause the tip to be too stiff. By stopping the stylet tip at the distal cuff they provide an effective bend without stiffening the tip of the tube.
Angles beyond 35 degrees confer no visual advantage and worsen maneuverability within the mouth and hypopharynx. After the tip has passed into the trachea bend angles above 35 degrees cause the tip of the tube to impact on the anterior tracheal rings.
In a cadaver study assessing the optimal bend angle for a straight-to-cuff stylet, there was a marked increase in mechanical insertion problems above 35 degrees; at 60 degrees more than half of the styletted tracheal tubes could not be advanced into the trachea:
Levitan RM, Pisaturo JT, Kinkle WC, Butler K, Everett WW.Stylet bend angles and tracheal tube passage using a straight-to-cuff shape. Acad Emerg Med. 2006; 13: 1255-8.
More about tracheal tube mechanics and bend angles |
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DISCLAIMER: The procedures discussed at this website, to be performed properly, rely on complex medical skills and should only be undertaken after formal instruction and hands-on clinical training with appropriate supervision, and after receiving professional certification and authorization to do so. The content presented herein is meant to serve as just one of many resources that trainees and health care professionals should consult in the course of the their skill acquisition and ongoing practice. Airway Cam Technologies, Inc., its authors, officers, agents and assigns, disclaim any and all responsibility for any adverse outcomes and or harmful acts committed by others persons who may have consulted this website or relied on information contained herein. |
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