Tube Mechanics - Tip Interaction with Trachea Rings

The mechanical impediment to tube advancement results from the interaction of the tube tip with the anterior tracheal rings, and the fact that the trachea itself is a relatively small pipe through which the tube is placed. 

High-resolution endoscopic image (at left) shows first tracheal ring between vocal cords marked with white arrows. With an optical stylet inside the trachea, the rings appear as a row of horseshoes (middle and center). Note that the rings are present anteriorly but are not complete circles; the flat posterior or membranous trachea (at bottom of two right images) has no rings and lies against the esophagus.

If there is resistance to tube insertion, a clockwise (rightward) turn will lower the leading edge of the tube (disengaging it from the rings).  By turning the tube 90 degrees clockwise the standard left-facing bevel of the tracheal tube moves from facing leftward to facing upward.  The leading edge of the tube rotates downward, and it disengages from the tracheal rings. Additionally, withdrawing the stylet will allow the tube to have greater flexibility. 

Unlike standard left-facing beveled tracheal tubes Parker tubes have a ski-tip design to their distal tip.  This symmetric, soft-tipped tube does not have the same mechanical issues with the tracheal rings as left-facing beveled tubes noted above. 

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Mechanical problems associated with tube advancement (and tip interaction with the trachea) are also a function of stylet shaping.  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.  A better term is “straight-to-cuff” stylet shaping, using a bend 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. This phenomenon explains why it is possible to have a correctly sized tube between the vocal cords and be unable to pass the tube, even though the trachea itself is large enough to accept it. 

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.