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.
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.
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.
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.
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
Anatomy of the larynx . Image By Olek Remesz
2. Thyroid cartilage
3. Cricothyroid membrane
Sonographic anatomy of the Airway on Long Axis
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.
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
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. An outline of the bony structures is overlayered on the maneuver to better understand the corresponding US image. The maneuver consists on rotating the probe once the trachea is identified and moving the probe upwards until a string of pearls is seen. The probe is moved 90 degrees towards the head once the trachea's midlines is in line with that of the probe. 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.
Thyroid-Airline-Cricoid-Airline or TACA technique
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.
TACA technique. On the left the ultrasound probe movements with an overlay of the relevant anatomical structures. The movements on this maneuver have been exaggerated for demonstration purposes. The probe is moved down once the thyroid cartilage is identified (Start on the US image) as a triangular structure. It is then moved down. The cricothyroid membrane is encountered next followed by the C shaped cricoid. The probe is then moved back up to the CT membrane.
Lets have a closer look at the TACA technique based on still images to clarify the above concepts.
Below you will see an overlay of the bony structures on top of the ultrasound images as the movement of the probe slides down from the thyroid cartilage (position 1), to the cricothyroid membrane (position 2), then to cricoid (position 3) and finally back to position 2.
B mode of the TACA protocol. Start at position 1 with the thyroid, followed by the cricothyroid membrane, then cricoid at position 3 and back to position 2 at the cricothyroid membrane.
Tracheal placement and location of endotracheal tube
Ultrasound of the airway can also be used to confirm proper positioning of the endotracheal tube as an adjunction to standards of practice (visualization between vocal cords, end-tidal CO2, and endoscopic visualization of tracheal rings through the endotracheal tube). Here the goal is to evaluate proper tube placement by indirectly ruling out an esophageal intubation.
Endotracheal tube not in the esophagus.
We start with the ultrasound probe placed in a transverse position at the base of the anterior neck. The tracheal rings will be identified as C-shaped hypoechoic structures with deeper hyperechoic structures corresponding to the tissue-air border. The esophagus will be seen on the side of the trachea as an oval structure with an hyperechoic wall and hypoechoic center. The probe can be placed while the placing the endotracheal tube. An esophageal intubation will create an adjacent hyperechoic structure posterolateral to the trachea, also known as the 'double tract sign'. Ultrasound may not be able to identify esophageal intubation in the case of an esophagus lying directly posterior to the trachea. The trachea in this case will cause acoustic shadowing of the esophagus and thus will not be seen. If the esophagus cannot be observed on initial evaluation moving the probe towards the left will help reduced the acoustic shadow caused by the thyroid cartilage.
Identification of the esophagus. To help identify the esophagus, movement of the probe to the side of the neck (either side) may be necessary as illustrated on the video to the left and the corresponding image on the middle of the screen. E for esophagus. The right most image is taken on the side of the neck on the transverse view.
ETT in esophagus: Double barrel sign
Confirmation of the endotracheal tube (ETT) in the esophagus can be done with the double barrel sign. When interrogating the trachea with ultrasound there should only be one air-mucosa interface. The presence of a second air interface of a second air-mucosal interface implies that the ETT is actually in the esophagus.
Intubation of the esophagus. Left: an empty esophagus that expands as an ETT is advanced and appears as a double barrel sign. On the right, the double barrel (DB) sign creating a second air-mucosa interface which implies the ETT is in the esophagus. The ETT has already been placed in this clip.
ETT in the trachea
The following are US signs can also be used to confirm the presence of the ETT in the trachea as well as its location within the trachea. These include the 'snowstorm' and the the 'bullet' sign. The snowstorm artifact is created when the endotracheal tube advances into the trachea. The bullet sign is a single air-mucosal interface with increased posterior shadowing and increased diameter of the trachea. The air in the ETT balloon causes displacement of the walls of the trachea creating this artifact.
Bullet sign as seen in B mode ultrasound. On the left, the ETT cuff is inflated after the tube has been placed. On the right, the same clip but with two features. A cross section of a hollow ETT with its cuff being inflated appears in the image. This is then followed by the 'bullet' image overlay. The displacement cause by the ETT cuff creates a wider bony surface and thus casting a greater acoustic shadow. Images taken at the suprasternal notch.
Location of the ETT in the trachea
We can use the long axis or longitudinal view of the trachea to determine the location of the endotracheal cuff to evaluate its proper position. The optimal location of an endotracheal tube tip is between 5 +/- 2cm from the carina on neutral position. The cuff should then lie below the cricoid and above the lowest suprasternal tracheal ring that can be visualized. Bear in mind that due to different size tubes and manufacturers it would be prudent to check for bilateral lung sliding to confirm both lungs are ventilating. On the following clips we can identify the crycoid, the tracheal rings and its relation to the endotracheal cuff.
Inflated ETT cuff in long axis. Long axis view or longitudinal view of the trachea as the cuff is inflated. Both clips are display the same information. Right clip has an overlay of an ETT as the cuff is being inflating causing an anterior displacement of the bony structures of the trachea and in this case the cricoid is displaced and thus the proceduralist should advance the ETT. CT for cricothyroid membrane.
Tracheal US and ETT size
Optimal ETT Size
Ultrasound is a reliable tool to assess the infraglottic diameter of the upper airway. Ultrasound derived formulas to estimate the size of the endotracheal tube is superior to age and height-based formulas in estimating endotracheal tube size.
The correct size of tracheal tube was highly correlated with the subglottic diameter. This diameter is taken just below the cricoid cartilage. Linear models estimate endotracheal tube size with the following formula (keep in mind that while the internal diameter of a tube does not vary across manufacturers, the outer tube diameter does) :
When using US derived measurements to estimate the size of the ETT:
Neonate and infants: