Cite as: Kolli S, Singh M. POCUS spotlight: airway. ASRA News. 2021;46. https://doi.org/10.52211/asra080121.046.  

Introduction

In the last two decades, clinical application of ultrasound technology has expanded, not only for visualization of anatomical structures and identification of pathology, but also to facilitate medical procedures. Recently, the use of point-of-care-ultrasound (POCUS) has gained popularity for a variety of perioperative procedures and guidance in high-acuity settings. Another area of interest is the use of POCUS for evaluation of upper airway structures. This has proved to be a valuable and noninvasive tool for anesthesiologists, critical care and emergency physicians. Training and understanding of applied airway sonoanatomy will enable physicians to identify abnormal or difficult airway and perform procedures such as percutaneous cricothyroidotomy or tracheostomy, confirmation of endotracheal tube (ETT) placement, and performance of regional blocks for airway management.

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Training and understanding of applied airway sonoanatomy will enable physicians to identify abnormal or difficult airway and perform procedures such as percutaneous cricothyroidotomy or tracheostomy, confirmation of endotracheal tube placement, and performance of regional blocks for airway management.

Sonoanatomy of the Upper Airway

The important airway structures that can be visualized using ultrasound are tongue, hyoid bone, epiglottis, thyrohyoid membrane, thyroid cartilage, cricothyroid membrane, cricoid cartilage, trachea, and esophagus.¹ (Table 1, Figures 1-9) The POCUS airway scanning technique pertains to dividing the upper airway structures into suprahyoid and infrahyoid areas. A standard linear high frequency transducer (13-6 MHz) is sufficient to scan infrahyoid airway structures while the low frequency curved (3-8 MHz) is more suitable for tongue and deeper suprahyoid structures. Patient should be lying supine with the head extended and neck flexed in the sniffing position for scanning suprahyoid structures, and neck extended for infrahyoid structures. In this article, we will discuss the indications of POCUS airway technique with a specific focus on identification of the cricothyroid membrane and performance of airway blocks for facilitation of airway management in an awake patient. 

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Table 1: Sonographic Appearance of Airway Structures (Video 1)

Patient positioned supine with head and neck extended, use low frequency, curved transducer for suprahyoid structures and high frequency, linear transducer for hyoid and infrahyoid structures.

A-M – air-mucosal; TACA – (Thyroid Cartilage - Air line - Cricoid Cartilage - Airline)

Video 1. Transverse Scan of the Upper Airway

Figure 1. Anatomy of the neck anterior view (A) and lateral view (B) showing important airway structures.

Figure 2. Sagittal view of the suprahyoid structures using low frequency transducer, placed in the submandibular area behind the mentum.

MH -Mylohyoid muscle, GH – Geniohyoid muscle, GG – Genioglossus muscle, TS – Dorsal tongue surface, SLF – Sublingual fat.

Figure 3.  Transverse view of the hyoid bone with high frequency linear transducer placed over the hyoid bone.

Figure 4. Transverse view of the epiglottis with high frequency linear transducer over the thyrohyoid area.

SM - Strap muscle, PES - Pre-epiglottic space

Figure 5. Parasagittal view of the thyrohyoid membrane with high frequency linear transducer.

SM - Strap muscle

Figure 6. Transverse view of thyroid cartilage with high frequency linear transducer over the thyroid prominence.

SM - Strap muscle, FC - False Cords. Thyroid cartilage inverted V-shaped, marked by stars.

Video 2. TACA Technique

Figure 7. Transverse view of the cricothyroid membrane with high frequency linear transducer placed between the thyroid and cricoid cartilages.

SM – Strap muscle, CTM – Cricothyroid membrane, A-M – Air-Mucosal Interface

Figure 8. Transverse view of cricoid cartilage with high frequency linear transducer over the Cricoid cartilage. Hypoechoic arch shaped Cricoid cartilage marked with stars.

SM – Strap muscle, A-M – Air-Mucosal Interface

Figure 9. Transverse view of tracheal ring above suprasternal notch with high frequency linear transducer. Hypoechoic tracheal ring lined inside by hyperechoic A-M interface.

SM - Strap muscle, E - Esophagus to the left of trachea

Video 4. Detecting Esophageal Intubation

Identification of the Cricothyroid Membrane

The cricothyroid ligament, also called cricothyroid membrane (CTM), is composed of two components: one thick, median cricothyroid ligament and two lateral cricothyroid ligaments on each side, also called conus elasticus.

Digital palpation (DP) methods to identify the CTM are highly unreliable, and a recent systematic review concluded that ultrasound-guided CTM identification has enhanced accuracy, higher success, and lower complication rates compared to DP, especially in patients with a difficult airway.² The Difficult Airway Society (DAS) recommends that emergency front-of-neck access should be performed with the patient placed in the extended neck position.³ The sniffing position (head extension and neck flexion) or neutral position have been shown to shift skin structures away from the true CTM in more than 50% of patients.⁴ Marking the mid-point of CTM prior to induction of anesthesia should be done with the head and neck extended to prevent any displacement of the skin structures during airway manipulation.⁵

How I Do It

With the patient in the supine position, with the head and neck extension, place a high-frequency linear transducer at the level of thyroid cartilage in the transverse position. (Table 2) Identify the hypoechoic inverted V-shaped thyroid cartilage and slide the transducer caudad to identify the CTM lined by hyperechoic air-mucosa (A-M) interface. Slide the transducer further caudally until the cricoid cartilage is identified as an arch-shaped hypoechoic structure with hyperechoic A-M interface posteriorly. Having identified the cricoid, now move the transducer back cephalad to the mid cricothyroid membrane and mark it. This is called the TACA (Thyroid cartilage-Airline-Cricoid cartilage-Airline) technique, described by Kristensen et al.⁶ (Video 2) From this position the transducer can be rotated 90 degrees to get a longitudinal view of the thyroid and cricoid cartilages with the CTM in between.

The CTM also can be identified with the transducer placed longitudinally in the front of the neck above the suprasternal notch in the sagittal plane. The trachea can be identified by identification of hypoechoic tracheal rings resembling a “string-of-pearls” lined posteriorly by a bright A-M interface. With the transducer in a sagittal plane, slide in a cephalad direction, the cricoid cartilage can be identified as a larger and more superficial hypoechoic structure, compared to the tracheal rings. Further cephalad movement of the probe enables identification of the lower end of the thyroid cartilage in the cephalad position, the hyperechoic CTM in-between, and the cricoid cartilage caudally. Adjusting the transducer to bring the CTM to the center with one hand, use the other hand to slide a hyperechoic marker (metallic probe or a blunt needle) between the transducer and the patient’s skin from above. The marker is seen as hyperechoic dot with an acoustic shadow below, which can be adjusted to position midway between the inferior end of thyroid cartilage and the superior end of cricoid cartilage. At this point, the transducer is removed and the marker overlies the mid-point of CTM, where the cricothyroidotomy may be performed. (Video 3)

Table 2. I-AIM Framework for Airway Ultrasound ⁷ 

Patient positioned supine with head and neck extended, use linear high frequency transducer.

CTM – cricothyroid membrane; ETT – endotracheal tube; TACA – (Thyroid Cartilage - Air line - Cricoid Cartilage - Airline)

Figure 10: Midsagittal scan of the neck with high frequency linear transducer over the suprasternal notch sliding up to thyroid cartilage.

Airway Blocks for Awake Fiberoptic Intubation

Awake fiber-optic intubation is the gold-standard technique in the management of a known difficult airway. Patient discomfort during the procedure can be mitigated by sedation and various techniques that achieve airway anesthesia. Topical application of local anesthetics by nebulization, spraying, and application at specific anatomical landmarks to block the afferent neural transmission from the oropharynx and larynx have been described.⁸ (Table 3) In the setting of the COVID-19 pandemic, it is important to prevent aerosol generation such as coughing or bucking during awake intubation.⁹ Techniques have been described to minimize aerosol generation during awake fiberoptic intubation, such as the use of a negative pressure tent, and may be used on a case-by-case basis.¹⁰ Nonetheless, ultrasound-guided airway blocks may supplement airway topicalization.

Table 3. Nerve Supply of the Airway

CN - cranial nerve, V1 - ophthalmic, V2 - maxillary, V3- mandibular branches of trigeminal nerve

To achieve complete airway anesthesia for orotracheal intubation, three types of regional blocks are needed: glossopharyngeal (oropharynx), superior laryngeal (larynx above the cords) and trans-laryngeal (larynx and trachea below the cords).⁸ (Figure 11)

Figure 11: Sagittal section of the upper airway showing the sensory nerve supply

Figure 12: Nerve supply of Larynx

Figure 13: Endotracheal tube in the Trachea: Bullet Sign.  ETT – Endotracheal tube

Figure 14: Transverse scan of the neck showing endotracheal tube in the esophagus. The double tract sign. ETT – Endotracheal tube

Glossopharyngeal Nerve Block

Ultrasound-guided glossopharyngeal nerve block has been described in the chronic pain practice to treat glossopharyngeal neuralgia.¹¹ The nasal cavity, nasopharynx, oropharynx, and tongue can be easily anesthetized by topicalization without elicitation of coughing, gagging, or aerosol generation. Hence, we do not recommend specific glossopharyngeal and trigeminal nerve blocks for this purpose.

Superior Laryngeal Nerve Block

Superior laryngeal nerve is a branch of the vagus nerve. It divides into internal and external branches at the level of hyoid bone. The internal branch passes immediately inferior to the greater cornu of hyoid bone and enters through a foramen in the thyrohyoid membrane. (Figure 12) Sonographic visualization of the superior laryngeal nerve may not be easy and can be inconsistent even with locating the superior laryngeal artery. Hyoid bone, thyroid cartilage, and the thyrohyoid membrane are easy to identify with ultrasound, and depositing local anesthetic in the target tissue plane around the thyrohyoid membrane reliably blocks the internal branch of superior laryngeal nerve.¹² (Figure 5)

How I Do It

As with any regional anesthetic technique, calculate toxic dose of the local anesthetic and prepare the room with appropriate monitors and local anesthetic systemic toxicity (LAST) protocol. With the patient in the supine position, with neck extended, the skin is prepped with chlorhexidine. Place a high-frequency linear transducer in the midline sagittal plane, and identify the hyoid bone (hyperechoic with a dropout shadow below), thyroid cartilage, and thyrohyoid membrane connecting the two. Slide the transducer laterally approximately midpoint between the anterior midline and the level of greater cornu of the hyoid bone postero-laterally. Insert a 22-gauge, 50-mm needle in-plane to puncture the thyrohyoid membrane and inject 2 ml of 2% lidocaine. Repeat on the other side. In patients with short necks, use transverse view or a transducer with a small footprint such as the hockey-stick probe.

Translaryngeal Injection

This block may induce coughing as the local anesthetic can initially irritate the tracheal mucosa; therefore, the patient should be warned, and the practitioner should consider using this injection to anesthetize the subglottic airway on a case-by-case basis. Identify the CTM, as described in the section above, in the sagittal plane. Slide a 22-gauge, IV cannula between the transducer and the patient’s neck from cranial end. (Video 3) Adjust the cannula by sliding and aligning it to the center of the CTM. Slowly withdraw the needle until it just disappears from the screen. At this point, remove the probe and tilt the needle upright, perpendicular to the skin, maintaining the entry point for the needle. (Video 3) Attach a 5-cc syringe with 2% lidocaine, and advance the 22-gauge IV cannula with gentle negative pressure until air is aspirated. Once trachea is entered, withdraw the needle and advance the sheath to prevent injury to the posterior wall of cricoid and trachea. Inject the local anesthetic to anesthetize the trachea, vocal cords, and larynx. The catheter can be left in place for later use, in case of an unable to intubate and ventilate scenario.

This block also can be performed under real-time ultrasound. After identifying the CTM in the sagittal plane in midline, the probe is moved slightly to a parasagittal position and tilted, keeping the CTM in view. The needle is inserted out-of-plane through the CTM, visualizing the needle tip as it enters the airway. Once air is aspirated, confirm the needle tip is in the airway and inject the local anesthetic.

Endotracheal Tube Confirmation

ETT confirmation with POCUS is highly accurate for both adult and pediatric patients.^13,14 The accuracy remains consistent regardless of the ETT size or transducer type,¹⁴ with relatively short learning curve for using POCUS.¹⁵  ETT placement can be reliably detected during resuscitation with low cardiac output and low end-tidal carbon dioxide.¹⁶  The transducer is placed in transverse position in the suprasternal notch. Observe for movement in the trachea appearing like a snowstorm as the ETT passes across the probe, called the “snowstorm sign.” If the ETT enters the esophagus, the unique “double tract sign” is seen.¹⁷ (Video 4) If only one operator is present, postintubation check with twisting the ETT side to side, using the color doppler while moving improves the diagnostic accuracy. Inflation of the ETT cuff with saline also may improve the ability to identify and confirm ETT location. Adding the lung-sliding sign to this method improves diagnostic accuracy of endotracheal intubation.

Other Clinical Uses

POCUS for upper airway has been described to assess and predict difficult mask ventilation and difficult laryngoscopy using skin-hyoid distance,¹⁸ while the epiglottis to vocal cord space has been shown to correlate with Cormack-Lehane intubation grade. It also has been studied for screening obstructive sleep apnea¹⁹ and evaluation of vocal cords and epiglottis.²⁰ Airway ultrasound is utilized for identification of tracheal rings for correct location and safe placement of percutaneous tracheostomy avoiding pre-tracheal blood vessels in both elective and emergency situations.^21,22

Conclusion

POCUS airway is a non-invasive, reliable, and relatively simple technique that is reproducible to obtain real-time dynamic images of the upper airway anatomy. Application of POCUS airway has been useful to perform elective and emergency procedures such as confirmation of endotracheal intubation and cricothyrotomy, as well as facilitating upper airway regional techniques. Further research and innovation are under way to examine other potential uses such as assessment of difficult laryngoscopy or screening for risk of obstructive sleep apnea.

Sree Kolli, MD, EDRA, is a clinical assistant professor in the department of General Anesthesiology and Pain Management at the Anesthesiology Institute of Cleveland Clinic in Cleveland, OH.

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Mandeep Singh, MBBS, MD, MSc, FRCPC, is an assistant professor in the department of Anesthesiology and Pain Management at Women’s College Hospital, University Health Network, and University of Toronto in Toronto, Canada.

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