In this chapter we will heavily rely on information explored on Airway Ultrasound as it will be the basis for this chapter. We will be making a recap on some key concepts.
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.
Ultrasound has a role in this critical aspect of care that can assist in clinical decision making. Ultrasound can identify the cricothyroid membrane for emergency airway access plus distinguish tracheal rings for ultrasound guided tracheostomy. Ultrasound can also be used to assist in airway blocks as we will see in this chapter.
Sensory Airway Innervation
The upper airway is divided into the nasal and oral cavities, the pharynx and larynx. Sensory innervation is provided by the trigeminal, glossopharyngeal and vagus nerves. As an alternative to and more challenging than topical anesthesia, three nerves can be blocked with the use of regional anesthesia for awake intubations and include the glossopharyngeal, superior laryngeal and the recurrent laryngeal block. The glossopharyngeal supplies sensory innervation to the posterior third of the tongue and vallecula and is not accessible with the use of ultrasound so will not be discussed here any further.
The superior laryngeal nerve provides sensation to the laryngeal structures above the vocal cords (posterior epiglotis, aryepiglottic folds and arytenoids ). It lies inferior to the greater cornu of the hyoid bone and then splits into the internal and external branches. The internal branch penetrates the thyrohyoid membrane continueing submucosally in the piriform recess. The external branch descends on the larynx. It is the internal branch of the superior laryngeal nerve that can be blocked with the use of ultrasound.
The recurrent laryngeal nerve provides innervation of the vocal cords and trachea and also provide motor supply to all intrinsic muscles of the larynx with the exception of the cricothyroid muscle. Blocking the recurrent laryngeal nerve will cause vocal cord paralysis and airway obstruction which is why this is not performed. A translaryngeal block achieves our goal of blocking only the sensory fibers of the nerve and is achieved by identifying the crycothyroid membrane. A 20 or 22 Gauge needle with syringe with local anesthesia is used and with continuous aspiration this membrane is perforated. Injection of local anesthesia (5ml of 4% lidocaine) and rapid withdrawal of the needle should follow. This results in coughing and helps disperse the solution to block the recurrent laryngeal nerve.
Conceptually we must first refresh ourselves with the bony anatomy of the larynx and its innervation which can be seen with the following illustrations. What we will be using on ultrasound is the location of these important bony structures to achieve appropriate targets.
Anatomy of the larynx and relevant structures. On the left the anatomy of larynx and its innervation. On On the right, surface anatomy of the larynx and its innervation on frontal and side view. Superior laryngeal nerve and its two branches, the internal (I) and external (E) laryngeal nerve. The recurrent laryngeal nerve can also be visualized (R). Image By Olek Remesz.
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).
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.
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 or midline sagital plane (label 1) and short axis planes or transverse 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 (Transv)
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 (Sagittal)
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. Larynx By Olek Remesz
Anatomical structures and clinical implications.
Ultrasound of the airway has various clinical implications improve airway management. Here our focus will be in the performance of regional nerve blocks of the airway as well as crycothyroidotomy for emergency airway procedures.
Identification of the Cricothyroid Membrane
Visualization and external palpation of the cricothyroid (CT) membrane may be difficult at times. This may impair our ability to perform a translaryngeal block for awake fiberoptic or lower the success rate of cricothyrotomy. Ultrasound can help in this regard as it greatly improves identification of the bony structures 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. Take a look at the images below for reference . 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 aid in understanding 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-muscosal interface. On blue, the thyroid, cricoid and first tracheal ring. 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.
US Procedures of the Airway
From airway blocks to tracheostomies, lets review what US can be used for.
1. Translaryngeal block with US
Before we go any further, make sure you have considered analgesia as part of your anesthetic plan when performing an awake fiberoptic.The aim here is to allow the procedure while at the same time provide optimal intubating conditions.
Now that we have identified the location of the CT membrane we can either mark its location or use of real time sonography to perform a translaryngeal block. The ultrasound probe starts at a midline long axis view or sagital view as in the SOP technique and then it is tilted from the midline while keeping the cricoid cartilage in view. The needle entry point should then be cranial to the cricoid cartilage. The needle is advanced using negative aspiration and the tracheal location of the tip of the needle is confirmed by the presence of air in the syringe.