Airway & Respiratory

Ultrasound as an Adjunct for Prehospital Airway Management

Issue 8 and Volume 41.

You’re dispatched to a 45-year-old male involved in a high-speed motorcycle accident. On scene you find the un-helmeted rider unresponsive with a Glasgow coma scale of 5 and significant facial trauma. The patient is breathing, but is in significant respiratory distress. You determine he’s unable to protect his own airway and begin to think about how to manage the airway effectively. You anticipate endotracheal (ET) intubation and bag-valve mask (BVM) ventilation will be difficult given his facial injuries, and mentally prepare for the possibility of needing to perform a cricothyroidotomy. Your extraglottic airway device and end-tidal carbon dioxide (EtCO2) detector are at the ready, but what else might help you successfully manage this likely difficult airway?

Critical Airway

As this case illustrates, there are many challenges faced by prehospital providers when managing the airway of a critically ill or injured patient. It may have been months or even years since the last time you intubated a patient in the field, and many providers can go their entire careers without performing a cricothyroidotomy. These low-frequency, high-risk procedures can be anxiety-provoking even for experienced EMS providers. There’s no substitute for the clinical experience of performing these procedures and regularly engaging in high-fidelity simulation to maintain skill competency. However, point-of-care ultrasound is a valuable tool that can increase providers’ confidence when patients require advanced airway procedures.

We rely on visual identification of upper airway and glottic landmarks when performing direct or video laryngoscopy, and both visual and tactile recognition of the laryngoskeleton when performing cricothyroidotomy. Point-of-care ultrasound can be used to confirm ET tube placement within the trachea, to recognize inadvertent esophageal intubation before ventilations are administered, and to assist with identification of the cricothyroid membrane and other important structures when performing a cricothyroidotomy.

Sonographic assessment of the airway can be performed prior to performing any airway intervention, during attempts at ET intubation, and for confirmation after securing the airway with an ET tube.

Relevant Anatomy

The tissues that are visualized on ultrasound in the upper airway include the tracheal rings, the cricoid cartilage, the thyroid cartilage, the thyroid gland, common carotid arteries and the esophagus.

The ultrasound waves emitted by the ultrasound transducer pass quite easily through cartilage and the soft tissues of the airway. For this reason, cartilage appears hypoechoic (i.e., dark) on the ultrasound screen, as very few sound waves are echoing back to the transducer. Air is a poor medium for the passage of ultrasound waves, however, and the junction between the soft tissues of the airway and the air within the trachea—an air-mucosal interface—generates a hyperechoic (i.e., bright) echo signal back to the transducer.

ET Tube Placement Confirmation

Ultrasound for ET tube placement confirmation requires two providers: one to perform the intubation attempt, and one to perform the focused ultrasound.

Ultrasound gel is applied liberally to the patient’s neck. This gel improves the quality of the image by eliminating any trapped air at the interface of the skin surface and the transducer, and creates a relatively frictionless surface on which the ultrasound transducer can move across the patient’s body. The long, flat, linear array, high-frequency transducer is applied to the trachea just above the top of the sternum in a transverse orientation with the probe marker pointing to the patient’s right side. (See Figure 1, below.) The esophagus is identified posterior and to the patient’s left of the trachea, and its concentric muscular layers are easily seen on ultrasound.

Figure 1: Proper positioning of the linear array transducer for
ultrasound guidance of an endotracheal intubation attempt.

EMS use ultrasound for prehospital airway management

Because the ultrasound waves don’t pass easily through the air within the trachea, there’s significant shadowing and artifact, making it difficult to visualize the posterior aspect of the airway. This can make visualizing the ET tube itself difficult, although distension of the trachea as the tube is advanced is sometimes observed. The color flow Doppler feature on the ultrasound machine can be used to identify placement of the ET tube within the trachea. When the trachea distends and is moved by the entry of the ET tube, a Doppler signal is generated, and color flow is seen within the lumen of the trachea.

Because the esophagus is usually collapsed and doesn’t contain as much air as the trachea, passage of an ET tube into the esophagus creates a second air-mucosal interface posterior and lateral to the air-mucosal interface of the trachea. This “double tract sign” is much more readily identified than if the tube is placed within the trachea itself. (See Figure 2, below.) If this is observed by the provider performing the ultrasound, the intubating provider should immediately withdraw the ET tube.

Figure 2: The “double tract sign” seen when an esophageal intubation has occurred
EMS use ultrasound for prehospital airway management
Note that there are two areas shadowing seen as the result of air-mucosal
interfaces: one in the trachea and one in the esophagus. Image courtesy Laleh Gharahbaghian

Doppler color flow imaging can also be used to detect esophageal intubation in the same manner as tracheal intubation. If the ET tube passes into the esophagus, the deformation of the esophageal walls by the presence of the tube will generate a color flow signal that can be detected by ultrasound Doppler. Using these techniques, confirmation of ET tube placement is performed by excluding esophageal intubation.

Ultrasound in the Age of EtCO2 Sampling

Unquestionably, the availability of continuous EtCO2 monitoring has revolutionized airway management safety.1 It’s the position of the National Association of EMS Physicians (NAEMSP) that in a patient with a perfusing cardiac rhythm, end-tidal capnography may be superior to other methods of ET tube positon monitoring, yet NAEMSP acknowledges that no method of tube position confirmation is 100% reliable under all circumstances.2 Obtaining a continuous end-tidal capnogram requires a cardiac monitor with such capability, which may not always be available to the out-of-hospital provider, particularly in austere settings.1 For patients in low-flow states such as cardiac arrest or severe shock, end-tidal capnography may be less reliable, as the lungs are poorly perfused in these conditions and carbon dioxide isn’t as readily off-loaded.3

One advantage to using ultrasound for tube confirmation is that if the ET tube is placed in the esophagus, this can be seen immediately, before any ventilations are administered and the stomach is insufflated.4 This may potentially decrease the risk for aspiration of gastric contents. The sonographer can provide real-time feedback to the intubator if passage of the ET tube into the esophagus is evident sonographically.

Ultrasound may be particularly helpful for intubation during cardiac arrest when an intubation attempt is made while chest compressions are ongoing. The provider performing the ultrasound can be positioned on the opposite side of the patient as the person performing chest compressions, and can serve as another set of eyes that provide guidance for the intubator, who will be attempting intubation under the difficult conditions posed by ongoing CPR.3

Cricothyroidotomy Landmark Identification

If an airway is predicted to be difficult to secure, ultrasound can be used to identify and mark the cricothyroid membrane in anticipation of the patient requiring a cricothyroidotomy. This may expedite the cricothyroidotomy procedure, especially when anatomic landmarks are difficult to identify, such as in obese patients and those with short necks. Incidentally, these are also the patients who may be difficult to intubate as well.

Identification of cricothyroidotomy landmarks with ultrasound can be done at any point in the airway management algorithm for a patient whose clinical condition has prompted consideration for the possibility of cricothyroidotomy, but should occur prior to administering paralytic agents if rapid sequence intubation is to be performed. In this instance, if the patient can’t be intubated, the provider can proceed directly to cricothyroidotomy if the patient’s condition warrants.

Figure 3: Proper positioning of the linear array transducer for
ultrasound mapping of cricothyroidotomy landmarks

EMS use ultrasound for prehospital airway management

Similar to ultrasound for ET tube placement, a generous amount of gel is applied to the lower portion of the patient’s neck. The linear array high frequency transducer transducer is applied to the trachea just above the top of the sternum in the longitudinal orientation with the probe marker pointing towards the patient’s head. (See Figure 3, above.) It’s important for the transducer to be positioned in the midline. If a cervical spinal injury isn’t suspected, extending the patient’s neck will bring the laryngo-tracheal structures closer to the transducer and make them easier to visualize. The thyroid cartilage and cricoid rings are visible as discrete, hypoechoic structures, with the thyroid cartilage being larger than the cricoid rings and located the most cephalad (i.e., closest to the patient’s head). The air-mucosal interface is noted as a hyperechoic line just deep to the cartilages. The cricothyroid membrane can be identified as the space between the thyroid cartilage and the cricoid rings. (See Figure 4, below.) Once identified, the cricothyroid membrane is marked with a marker or pen so that it is easy to visualize once the transducer is removed from the neck.

Figure 4: Longitudinal view of the anterior neck on ultrasound
EMS use ultrasound for prehospital airway management
The thyroid cartilage (blue oval) is noted as the largest of the cartilaginous
structures. The cricoid rings (yellow ovals) are smaller and further
away from the patient’s head. The air-mucosal interface of the trachea
(pink line) is seen as a hyperechoic line just deep to the tracheal rings.
The cricothyroid membrane (yellow asterisks) is located between the
thyroid cartilage and the cricoid rings. Image courtesy Jenna M.B. White

Conclusion

Placing an ET tube between the vocal cords is only one facet of airway management. A successfully managed airway prevents or minimizes hypoxia, facilitates adequate gas exchange during attempts, secures the airway against the threat of aspiration and provides a reliable conduit for assisted ventilations.

Negotiating a difficult airway is one of the most stressful situations an EMS provider can encounter, yet it’s also an instance where a clear, stepwise approach can lead to improved patient outcomes. Using ultrasound to assist with identification of ET tube placement and cricothyroidotomy can inspire added confidence during these difficult clinical encounters.

References

1. Osman A, Chuan T, Rishya M. A feasibility study on bedside upper airway ultrasonography compared to waveform capnography for verifying endotracheal tube location after intubation. Crit Ultrasound J. 2013;5(7):1–11.

2. O’Connor R, Swor R. Verification of endotracheal tube placement following intubation. Prehosp Emerg Care. 1999;3(3):248–250.

3. Chou H, Chong K, Sim S, et al. Real-time tracheal ultrasonography for confirmation of endotracheal tube placement during cardiopulmonary resuscitation. Resuscitation. 2013;84(12):1708–1712.

4. Chou E, Dickman E, Tsou P, et al. Ultrasonography for confirmation of endotracheal tube placement: A systematic review and meta analysis. Resuscitation. 2015;90:97–103.

Resources

• Chou H, Tseng W, Wang C, et al. Tracheal rapid ultrasound exam (T.R.U.E.) for confirming endotracheal tube placement during emergency intubation. Resuscitation. 2011;82(10):1279–1284.

• White S, Slovis C. Inadvertent esophageal intubation in the field: Reliance on a fool’s “gold standard.” Acad Emerg Med. 1997;4(2):89–91.