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Airway Ultrasound
Introduction
Airway management is one of the most important and critical skills in the practice of Anesthesiology. Errors in this critical step of patient care is a major contributor to poor patient outcomes including brain hypoxia and death.
Not surprising, ultrasound has a role in this critical aspect of care that can assist in clinical decision making. Ultrasound can be used to evaluate vocal cord dysfunction and pathology before induction of anesthesia. It can be used to differentiate between tracheal and esophageal intubation as well as endobronchial intubation. It can be used to determine airway size and predict the approximate size of endotracheal tubes including double lumen tubes. It can also identify the cricothyroid membrane for emergency airway access plus distinguish tracheal rings for ultrasound guided tracheostomy.
Equipment and Technique:
The evaluation of the upper airway involves an evaluation of the floor of the mouth and the neck and its associated structures. The floor of the mouth can be examined with a curvilinear probe (1-8MHz). The neck and its associated structures are better visualized with a linear high frequency probe (13-6Mhz).
It is important to recognize the scanning planes of the different structures that will be introduced with the use of the following images.
Scanning planes
Transverse (or short axis) and long axis planes are defined according to the spatial relationship between the ultrasound probe and the structure of interest. In the following images we see the probe footprint position and its corresponding name.
Imaging planes used for airway ultrasound. On gray, long axis or longitudinal view of the floor of the mouth; red, short axis view of the floor of the mouth; green, long axis or longitudinal view of the neck and in blue, transverse or short axis view of the neck.
Anatomy of the Airway through a 3D model
We will be exploring ultrasound images with the use of this airway model to help us understand anatomic relations and orientation.
Anatomic model created and modified from Z-anatomy
Sonographic anatomy of the Airway, an Overview
Lets have a closer look at the corresponding images that we get with those scanning planes and later explore the clinical implications they have later in this chapter.
Anatomy of the larynx for reference purposes. Image By Olek Remesz
Floor of the mouth
Ultrasound images of the floor of the mouth. In these images, the floor of the mouth is explored in long and short axis. On the long axis view we appreciate the acoustic shadow caused by the mentum (M) and hyoid bones (H) with the probe moved down the mouth and neck.
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H
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Floor of the mouth in long (label 1) and short axis (label 2) planes. Images obtained with a curvilinear probe and less than 5 cm of sector depth. The hyperechoic line deep in the image corresponds to the air-tissue interface of the tongue.
US of the neck
Here the floor of the mouth is explored in long and short axis. Bony structures create acoustic shadowing and thus limits the penetration of ultrasound waves into deeper tissues the hyperechoic line seen on these images corresponds to the air-tissue interface. We can use the following views to locate the cricothyroid membrane and the location of tracheal rings. If the esophagus lies directly posterior the trachea, is will not be visible on US.
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Thy
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US of the neck in both the long axis (label 1) and short axis planes (label 2). The probe is moving down from the head to the base of the neck. T, thyroid cartilage; C, cricoid; S, tracheal rings creating the 'string of pearls' configuration', Thy, thyroid gland appears deeper in the neck as the probe is moved down. Images here obtained using a linear probe with less than 4cm of sector depth.
Sonographic anatomy of the Airway on Short Axis
Lets have a closer look at what the different anatomical that you would see according to the interrogation planes along the short axis of the larynx. A diagram of the larynx and trachea is seen with slices taken at different levels as planes. Their corresponding ultrasound images are seen.
1. Vocal cords
5.Trachea
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Anatomy of the larynx . Image By Olek Remesz
2. Thyroid cartilage
3. Cricothyroid membrane
4.Cricoid cartilage
Sonographic anatomy of the Airway on Sagittal Plane
Lets have a closer look at what the different anatomical that you would see according to the interrogation planes along the long axis of the larynx. A diagram of the larynx and trachea is seen with a single slice corresponding to the movement of the ultrasound moving from head to the base of neck.
Larynx viewed on long axis. The blue plane corresponds to the cut generated by the ultrasound beam. On the right we see the corresponding B mode clips while the ultrasound probe is moved in direction of the arrow above. Image By Olek Remesz
Anatomical structures and clinical implications.
Ultrasound of airway has various clinical implications that improve airway management which is what we will explore in this segment.
Cricothyroid Membrane
We often struggle to identify the cricothyroid membrane by visualization or external palpation which leads to a low success rate of cricothyrotomy so it is imperative that identification of this membrane happens before induction of anesthesia if possible. Ultrasound can help in this regard as it greatly improves its identification and the depth necessary to reach the airway. Lets identify this membrane on the planes described above.
String of Pearls Technique
Let's first look back at the anatomic model to visualize what bony structures we might be seeing. The clips below show a probe and sector moving up the neck in a sagittal plane towards the thyroid cartilage as the neck is extended.
This is the most well published and proven technique compared to palpation alone. This technique can also be used for optimal tracheostomy tube placement since it can accurately identify trachea rings.
Start by placing the probe at the base of the neck in a transverse orientation to get the short axis of the trachea in view which is visualized as a horseshoe shaped dark structure. Make sure that the midline of the probe corresponds to the midline of the trachea. Next, rotate the probe 90 degrees so that the indicator is pointing towards the head. A number of dark hypoechoic structures will appear corresponding to the anterior part of the tracheal rings that correspond to the 'pearls' of the String of Pearls technique. From here translate the probe upwards to the head. The cricoid is initially seen as a larger elongated hypoechoic 'pearl'. As the probe moves up the inferior portion of the thyroid cartilage is seen. The cricothyroid membrane can thus be located and can be marked.
String of Pearls technique. Probe movements and corresponding ultrasound images. A 3D model with a linear probe showing its projecting beam on the left. The probe is first placed transverse to identify the center of the trachea. The probe is then rotated 90 degrees and moved up for the String of Pearl technique. On the right, the corresponding US display. The string of pearls is seen before we can observe the cricoid and then the thyroid cartilage. The hyperechoic line that joins all the structures corresponds to the air-muscosa interface. See text above for more information. The label string on the clip corresponds to the last pearl of the string of pearls seen before the cricoid is seen. The 3D model was modified from Z-anatomy as are the rest of the models on this page.
Thyroid-Airline-Cricoid-Airline or TACA technique
Let's again first take a look at the 3D model to gain insight into the relevant structures and the orientation necessary to get this view. Here the probe and sector are seen moving down and then up in a transverse plane with an extended neck.
This technique is recommended when the SOP technique is not feasible due to limited neck mobility or very short neck. This technique can also be used in conjunction to the SOP technique for proper localization of the cricothyroid membrane.
Start at the level where you identified the thyroid cartilage and place the ultrasound probe over this structure. This cartilage appears a hyperechoic triangular structure. The transducer is then moved caudally with the cricothyroid membrane appearing as a hyperechoic white line. As the transducer is moved down further a C shaped structure corresponds to the cricoid. The transducer can then be moved back up the neck for marking of the cricothyroid membrane.
