CLINICAL situations arise in which it is necessary to visualize the subglottic region, either to monitor the extent of disease or to confirm intratracheal placement of an airway device. In patients with abnormal laryngeal anatomy and a narrowed glottic opening, it may be difficult to pass a fiberoptic scope, 1and a minitracheotomy is necessary to visualize the larynx from below. 2
In patients who require an elective intratracheal surgical access procedure, 2it is possible that the finder needle position tip is outside the trachea and that forcible insertion of the airway device into the neck could result in traumatic complications. An added measure of safety would be added to these procedures if intratracheal entry were to be visually demonstrated before insertion of the airway device.
We present the results of a pilot feasibility imaging study involving the use of a thin miniendoscope in 3 human cadavers and 10 dogs for the transtracheal examination of the glottis and the trachea. In the animal study, the device was also used to visually confirm intratracheal access before performance of a surgical airway procedure, either cricothyrotomy 3,4or percutaneous tracheostomy. 2
Materials and Methods
With approval of the University of Southern California Institutional Review Board (Los Angeles, California), three adult human cadavers fixed with formalin were studied.
With approval of the University of Southern California Animal Care Committee and with veterinarian supervision, 10 mixed hounds (mean age, 1.3 yr; range, 1–3 yr; mean weight, 22 kg; range, 18–31 kg) were induced intravenously with acepromazine, sodium pentothal, atropine, and pancuronium. Standard monitors (electrocardiograph, noninvasive blood pressure, and pulse oximetry) were applied. Animals were intubated with an 8- to 9-mm-ID endotracheal tube (ETT), provided with 100% oxygen–isoflurane general anesthesia, and mechanically ventilated with a Narkomed Drager anesthesia machine (North American Drager, Telford, PA). At the end of the experiment, animals were euthanized with pentobarbital.
Recent developments in endoscopic technology have allowed the creation of extremely thin miniendoscopes. Figure 1shows a Karl Storz miniendoscope (Karl Storz Endoscopy, Tuttlingen, Germany) with a distal needle-shaped semirigid fiberoptic telescope (spanned by the two fingers), which is 1.2 mm in OD and 10 cm in length. The telescope has a 0° direction (straight-forward looking) and a 70° angular sector of visualization. The telescope is connected to a 30-cm-long fiberoptic cable that terminates in a remote eyepiece. A Xenon/Nova (Karl Storz Endoscopy) light source connects at the perpendicular adapter. For insertion through tissue, the distal telescope can be contained and advanced within a sharp-pointed sheath with an infusion port. In this study, the endoscopic image was viewed on a video monitor screen. Although the needle endoscope is thin enough to pass through an 18-gauge catheter, we chose to use a 14-gauge catheter for all animal procedures.
In three adult human cadavers, the endoscope was inserted through the cricothyroid membrane and was used to view the glottis from below (cephalad view) and to view the trachea from above (caudad view).
In 10 anesthetized intubated dogs, the 14-gauge intravenous needle-in-catheter was inserted either through the cricothyroid space (in 8 cases) or through a lower tracheal site (in 2 cases). Initial confirmation of intratracheal placement was with air aspiration into a saline-filled syringe. The needle was withdrawn, and the catheter was left in place. The thin miniendoscope was advanced through the catheter and was used to confirm intratracheal placement of the catheter tip. Intratracheal confirmation was verified by recognition of familiar structures, such as the vocal folds and anterior tracheal rings.
In eight dogs, a cricothyrotomy kit (Melker Emergency Cricothyrotomy Catheter Set; Cook Critical Care, Bloomington, IN) was used. In this technique, a wire guide is passed through an intratracheal catheter, the catheter is removed, and an enlarging scalpel incision is made. A tapered 12-French dilator is placed in a curved 4-mm-ID cannula and is advanced over the guide wire. The dilator and guide wire are removed, and the curved cannula is left in place.
In the two other dogs, a Blue Rhino percutaneous tracheostomy kit (Cook Critical Care) was used. The Blue Rhino is a curved, tapered dilator that creates, in a single pass, a tracheostomy opening. First, a guide wire is passed through an intratracheal catheter. After use of a small dilator, the Blue Rhino is passed over the guide wire to dilate the tracheal orifice. The Blue Rhino is removed, and a tracheostomy tube with an inner cannula is inserted. In this study, the two Blue Rhino procedures resulted in a low-lying tracheostomy several centimeters below the cricothyroid membrane.
Adult Human Cadavers
In each of the three human cadavers, the miniendoscope was inserted through the cricothyroid membrane. The scope tip was steadily advanced through soft tissue until it entered the tracheal lumen. Confirmation of intratracheal placement was made through the identification of anterior tracheal rings. With the scope pointed downward (caudad), the tracheal rings were readily seen. When the scope was pointed upward (cephalad), the glottic opening could be seen, and it was readily possible to pass the needle-sized endoscope through the triangular-shaped vocal cords. In figure 2, the three pairs of cadaver images are shown.
Anatomically, the dog larynx differs from the human one in that the dog has a barking structure, referred to as a vestibular fold, which sits inside the vocal folds. The triangular shape of the vocal folds, containing the inner vestibular folds, were clearly distinguishable (fig. 3, first row ).
With the scope positioned facing the glottic opening, one could visualize the vocal folds and then advance the ETT forward through them, in a controlled reenactment of an actual intubation, (fig. 3, second row ) or conversely and then back through the folds, as in an extubation. By keeping the scope fixed in position, it was possible to guide the scope directly into the lumen of the ETT, in effect achieving the equivalent of a subglottic retrograde intubation. The scope tip could also be maneuvered alongside the ETT in the immediate proximity of the vocal cords.
With cephalad positioning of the miniendoscope, the tracheal lumen and its anterior rings could be identified consistently (fig. 3, third row, left ). In one dog, with the miniendoscope directed caudad, a Univent tube (Fuji Systems, Tokyo, Japan) was advanced under direct visualization into the left mainstem bronchus for one-lung ventilation. Finally, with conventional laryngoscopic visualization, one could see the lighted end of the fiberoptic scope protruding through the vocal cords (fig. 3, third row, right ).
The current pilot study indicates that subglottic visualization with a thin miniendoscope is readily possible. The major potential applications for such a miniendoscope include (1) diagnostic purposes (e.g. , examination of the vocal cords and the trachea in patients with laryngeal cancer, burns, or prolonged ETT placement) and (2) performance of elective airway procedures (e.g. , retrograde intubation, 5,6elective percutaneous tracheostomy 2).
Difficulties in using the instrument were related primarily to the fiberoptic scope being a semirigid structure. To achieve a better view of the glottis, the scope often had to be flattened, i.e. , it had to be placed almost parallel to the neck. This often required repositioning of the catheter, followed by the fiberoptic scope. A common error was to insert the catheter too far so that its tip was close to the opposite tracheal wall, which in turn limited the length of scope that could be introduced into the trachea. Depending on the position of the scope, a helpful maneuver at times was the manual lifting of the trachea. In three dogs, a small amount of heme was noted, which initially obscured the view. However, syringe flushing with a few milliliters of saline readily cleared the field.
The current needle-shaped design for the miniendoscope is not optimal for subglottic visualization. Ideally, the shaft of the endoscope should be curved so that, on entering the tracheal interior, it is less likely to encounter the opposite tracheal wall and can be better directed either cephalad (even beyond the cords) or caudad. If the miniscope were extendable in length, it could be used as a lighted stylet for a retrograde intubation. 5,6With a longer and better designed scope, which either serves as the guiding stylet itself or which encompasses a guide wire, visually guided retrograde intubation would be quite easy. Alternatively, the scope could function as a distal guiding light to direct the laryngoscopist in an attempted conventional intubation.
If time permitted and no major bleeding was present, an added measure of safety might be added to emergency airway procedures—transtracheal jet ventilation, 7,8cricothyrotomy, 3,4and retrograde intubation 5,6—if the needle endoscope was visually demonstrated to have its tip inside the trachea (1) before the jet ventilator was activated, (2) before the cricothyrotomy catheter was forcibly inserted over a stylet through tissue, and (3) before the retrograde wire was advanced through the needle in the cricothyroid space.
In the absence of visual confirmation, it is possible that, with blind advancement, the tracking needle tip may not end up inside the trachea, but rather outside the trachea within the neck. By using the needle endoscope to visually confirm intratracheal placement before insertion of the larger and thicker airway access device, it seems that the risk of device misplacement would be reduced. This, in turn, would lead to a reduced incidence of traumatic complications (subcutaneous or mediastinal emphysema of the neck with a jet ventilator, hematoma from vessel injury, esophageal perforation, thyroid damage, and pulmonary barotrauma). Even if the needle-shaped endoscope were to be advanced incorrectly into a key neck structure, the extent of the damage would be far less than that associated with the larger airway devices.
Because the clinical occasion for the use of these airway devices is rare, many practitioners, especially those unfamiliar with emergency airway care, are ill at ease with the use of these devices, and they are often hindered by the knowledge that device misplacement in the neck could lead to significant complications. 9It seems that these individuals would be more inclined to use such a device if it were able to provide visual evidence of proper insertion.
In summary, a thin miniendoscope can be used to visually confirm intratracheal access. We believe that further studies will reveal that this type of miniendoscope can be used for diagnostic, therapeutic, and procedural purposes in the subglottic airway.
The authors thank Diane McIntee (Special Projects Coordinator, University of Southern California, Los Angeles, California), Abraham Raphael (Graphics Consultant, Reelix, Inc, Valley Village, California), and Mike Garcia (Anesthesia Tech, LAC + USC Medical Center, Los Angeles, California).