Abstract
Airway point-of-care ultrasound (POCUS) has the potential to make airway assessment and planning more accurate and complete. Studies have examined feasibility and usefulness of POCUS to predict and facilitate difficult airway management. This paper presents a brief overview of contemporary POCUS airway applications and current opinion on their place in clinical practice. Despite evidence, infraglottic airway assessment and its potential to improve patient safety remains relatively underused. A case series is presented to illustrate the translation of POCUS training to real life routine and acute practice. For each case, how ultrasound contributed to decision-making, preparedness, and successful management of challenging airways is highlighted. Understanding the airway’s true orientation and midline through imaging helps prevent failed front-of-neck access (FONA) attempts and supports deliberate, safe airway strategies. Wider adoption, structured training, and further research are needed to standardize POCUS airway techniques and evaluate their impact on patient outcomes.
Abbreviations
POCUS
Point of care ultrasound
IAIM
Indication acquisition image interpretation medical decision making
CICO
Cannot intubate cannot oxygenate
eFONA
emergency front of neck access
TACA
Thyroid cartilage, airline (cricothyroid membrane), cricoid cartilage, and airline (cricothyroid membrane)
CTM
Cricothyroid membrane
HMD
Hyoid mental distance
THM
Thyrohyoid membrane
DSE
Distance skin to epiglottis
DSVC
Distance skin to vocal cords
PES
Pre-epiglottic space
1
Before reading this article, we invite readers to reflect on these questions
-
•
What does your current routine airway assessment consist of?
-
•
Have you ever misjudged an airway that proved to be challenging?
-
•
Do you palpate the neck prior to induction of anesthesia, and if so, in which cases?
-
•
Have you ever doubted if the endotracheal tube was in the correct position?
-
•
When training, teaching, and practicing airway POCUS, is there a structured approach within your department’s curriculum?
-
•
What is your threshold of performing airway POCUS in your clinical practice?
2
Part 1. overview: when and how to do it
2.1
Overview
Despite advantages in technology, research and education, airway management always has been and always will be associated with both anticipated and unanticipated difficulty or failure. Audits and large data sets looking into complications continue to demonstrate that despite the general safe character of airway management, there is room for improvement in airway assessment and planning ,,,,,,.
Traditional bedside airway evaluation is not accurate enough to predict potential airway management difficulties ,. Nevertheless, it remains a part of guidelines and is embedded into our routine anesthesia workflow .
Over the past two decades, POCUS airway management techniques have been extensively studied. In the beginning, research and discussion of airway POCUS mainly explored and determined what part or structures of the airway can be visualized, and what type of parameters could be of use . Later, the focus changed to examining feasibility and indication of doing the assessment and linking these parameters to appropriate indications or applications for airway management.
More recently, studies have investigated the implementation of accurate assessment tools and their potential to improve outcome and patient safety. A systematic review and meta-analysis looked at diagnostic accuracy and predictive value of POCUS airway parameters per specific part of airway management . Several parameters demonstrated the potential to improve our current predictive capacity. Examples of predictive anatomic associations and difficulty included tongue thickness for face mask ventilation, distance from skin to vocal cords for direct laryngoscopy, and distance from skin to epiglottis, skin to hyoid bone, and hyoid-mental distance ratio for difficult intubation. POCUS was associated with better performance in percutaneous tracheostomy and identification of the cricothyroid membrane compared to palpation. POCUS confirmation of correct positioning of the endotracheal tube was demonstrated to be a reliable alternative to current radiologic methods.
A potential shortcoming of airway POCUS in both clinical practice and research is a low specificity in the description of the airway POCUS assessment. Using confirmation of a difficult airway as an outcome metric does not clarify what part of the airway management will be challenging, nor associate it with the airway POCUS findings. These limitations have delayed the progress for appropriate and personalized solutions based on the airway POCUS exam; further focus in this area may lower the need for to ad hoc decision making.
Furthermore, the airway POCUS assessment must demonstrate a relevant difference in behavior or change of plan. In a time where video laryngoscopy (VL) is more and more the technique of first choice in an anticipated difficult laryngoscopy, intubation, or advanced airway case, the question is what a POCUS derived parameter suggesting difficult direct laryngoscopy will add. Perhaps the answer lies not in the quantitative measures but in the qualitative impact of understanding anatomy and knowing the possible scenario when airway management starts.
Besides planning, POCUS also plays a role in pre and postintubation care. The added value of confirmation of endotracheal tube position has been demonstrated, particularly in cases where waveform capnography is not convincing or not possible (e.g. cardiac arrest cases) ,. Use of POCUS to confirm endotracheal tube position and prevent unrecognized esophageal intubation is now part of widely endorsed airway management guidelines .
2.1.1
Education
The indication, acquisition, interpretation, and medical decision-making (I-AIM) framework aims to improve knowledge and integrate skills and transformation to clinical practice. This framework has been introduced for focused POCUS exams in anesthesiology and other medical fields, and is widely endorsed ,.To learn and understand the skill of image acquisition, expert opinion advises us not to rely on simulators, but to scan live models. With current recommended content for focused Airway POCUS, a minimum number of supervised scans has been suggested for different levels of competence: thirty supervised scans for level 1 exams, 20 supervised and interpreted scans for level 2 ,. Several teaching modalities are used and available that are consistent with the I-AIM framework. However, further investigation on effective training methodologies, competency assessment tools, and learning curves is still needed. Standardization of datasets and uniform use of educational concepts for airway POCUS may facilitate further development and use.
Lack of material, standardized curriculum or trained faculty have been mentioned as potential barriers to implement POCUS education . In a Canadian survey studying barriers to the overall use of ultrasound in the perioperative period, lack of material was the main finding . Besides implementing high quality education, another essential step to improve outcome is to facilitate optimal transfer of this training to real clinical practice. There is limited knowledge of the barriers to transfer of training and implementation of airway POCUS in workplaces where training facilities are present.
Based on the available literature, current modern airway assessment using focused airway POCUS suggests identifying landmarks and relevant anatomy for front-of-neck access (FONA) and confirmation of tube position. In addition to traditional assessment, other POCUS parameters that predict a difficult airway (direct laryngoscopy, intubation, or facemask) can be considered as part of a comprehensive airway exam. Other practical applications of airway POCUS include estimation of endotracheal tube size, determination of correct depth of tube, and facilitation of nerve blocks for awake tracheal intubation.
2.2
How to do airway POCUS
2.2.1
Probe selection and sono-anatomy
To scan the upper airway structures, high frequency transducers are chosen for more superficial infrahyoid structures, while the low frequency curvilinear probe is more suitable for deeper tissues such as tongue or some suprahyoid structures . Ultrasonography relies on reflection of soundwaves both sent and received by transducers. Distinct reflections result from the difference in acoustic impedance that waves encounter when they travel through tissues of different composition. Air, bone, and calcified cartilages are bad conductors. Air-tissue or air-mucosal interface borders reflect as a bright white hyperechoic line. Artefacts beyond the air mucosal interfaces are reverberation artefacts, as reflection does not occur in air-filled space posterior to the tissue borders. Hyperechoic structures give a strong echo and are more white or bright. Hypoechoic structures give a weak echo and are dark. Cartilages are visualized as hypoechoic structures, and more homogeneous than heterogeneously appearing hypoechoic muscles and connective tissues.
Description of anatomical airway tissues and sonographic appearance are in Table 1 .
Table 1
Description of anatomical airway tissues and sonographic appearance.
| Anatomical structure | Appearance | Considerations |
|---|---|---|
| Hyoid bone | Superficial hyperechoic inverted u-shaped structure | Hyoid bone also marks separation of scanning areas (supra vs infrahyoid) |
| Thyrohyoid membrane THM |
THM is a sonographic window in transverse view to visualize epiglottis
The membrane is a linear structure between strap muscles and PES |
Scan from hyoid bone to thyroid cartilage and THM lies in between |
| Strap muscles | Two hypoechoic structures inferior to skin and superior to PES | Small face sign refers to two eyes being strap muscles and mouth being the epiglottis |
| Pre epiglottic space PES | Hyperechoic collection of adipose tissue | |
| Epiglottis | Hypoechoic structure | Bordered by hyperechoic triangular pre-epiglottic space and posteriorly by a hyperechoic air/mucosa interface. Referred to as mouth of a face image. |
| Thyroid cartilage | Best visible in the young as calcification occurs with aging. Calcification creates a strong echo and acoustic shadowing posteriorly. | |
| Transverse view: | Triangular shaped hypoechoic structure | |
| Longitudinal view | Linear hypoechoic structure | The vocal cords that move during phonation are identifiable at this level in transverse view and can aid in identifying the cartilage. |
| Longitudinal view can be difficult to visualize due to prominence and calcification. For FONA landmarking, the lower border is relevant and if difficult to identify, focus on the big cricoid cartilage; the CTM area lies just cranial to the large cricoid cartilage. | ||
|
Cricothyroid membrane (CTM)
CTM lined by hyperechoic air-mucosa (A-M) interface. |
Hyperechoic white line is airline reflecting the air-mucosa border (Mucosal lining of the elastic cone and air underneath) | The CTM consists of two lateral cricothyroid ligaments, conjoining in the midline, also referred to as the elastic cone, on top of these lateral ligaments, the median cricothyroid ligament lies superiorly. |
| Superficial to this hyperechoic line, is the median cricothyroid ligament linking the hypoechoic thyroid and cricoid cartilages. | The elastic cone is attached to the cricoid cartilage and moves cranially to form the free upper border of the vocal ligaments. | |
| Transverse view Airline see http://airwaymanagement.dk/taca | Often hypoechoic muscles (strap/cricothyroid muscles) can be seen) in the transverse view | |
| Longitudinal view Airline see http://airwaymanagement.dk/pearls | ||
| Cricoid Cartilage | ||
| Transverse view | Arch like, hypoechoic structure anterior to a hyperechoic air mucosa line | Also view to measure transverse diameter for endotracheal tube size determination |
| Often hypoechoic muscles (strap/cricothyroid muscles) can be seen) | ||
| Longitudinal view | Round hypoechoic structure, anterior to the hyperechoic air mucosa interface | Both in transverse as longitudinal views, clearly bigger than the tracheal rings |
|
Vocal cords
Transverse view |
||
| True cords | Triangular shaped hypoechoic structures, outlined medially by hyperechoic vocal ligaments | True cords move to midline during phonation |
| False cords | Hyperechoic mucosal tissue, superior to true cords | Relatively immobile during phonation |
|
Tracheal rings
Suprasternal view |
Helps in preparing for tracheostomy
Skin to airway distance, identifying relevant vital structures that will bleed, identify optimal entrance point |
|
| Transverse | Hypoechoic horseshoe-shaped structure, with underneath a hyperechoic line (white air–tissue interface) and reverberation artefacts | Can be used for confirmation correct position and depth of tube |
| Longitudinal view | Hypoechoic small round structures, resembling “string of pearls” | |
|
Esophagus
Transverse suprasternal view |
Concentric layers appear as a “bulls’ eye.” | Introducing a tube or swallowing can be seen in this view |
| An ETT appears as double tract signal | ||
A description of POCUS airway for infraglottic use according to I AIM framework is given in Table 2 . The Longitudinal “string of pearls”-technique, as originally developed and described , can be seen in this video ( http://airwaymanagement.dk/pearls ), and the transverse “Thyroid-Airline-Cricoid-Airline, TACA , technique can be seen in this video ( http://airwaymanagement.dk/taca ).
Table 2
I-AIM framework for recommended indications. All refer to infrahyoid views.
| Indication | Acquisition of the image | Image interpretation | Medical decision making |
|---|---|---|---|
| Cricothyrotomy |
Linear probe approach (string of pearls)
The CTM refers to the area between the thyroid and cricoid cartilages, just superficial to the hyperechoic air mucosal interface Transverse approach (TACA) Probe is moved from hypoechoic triangular shaped thyroid cartilage to the big round hypoechogenic cricoid cartilage. Using these views, midline can also be assessed and drawn. Between these cartilages, is the CTM. At the level of CTM, muscles on both sides are seen, but there is no cartilage completely covering the hyperechoic air mucosal interface. |
Understand the anatomical position of the airway
Landmarking level and depth of CTM and midline of the airway, |
Determining optimal entrance point for cricothyrotomy
The mark of the location of the membrane can be used to mark for transtracheal local anesthesia |
| Tracheostomy |
Linear probe
Transverse suprasternal view Round hypoechogenic structure with hyperechoic lining Longitudinal appearance is string of pearls: hypoechoic round tracheal rings covering a hyperechoic line (mucosa of anterior tracheal wall) |
Landmarking depth of trachea and midline of the airway.
Identifying anatomical position (like deviation) relevant structures that can cause complications like major vessels or thyroid gland |
Determine optimal entrance point for tracheotomy
Determine if percutaneous or surgical procedure is appropriate |
| Confirmation of tube position |
Linear probe
Transverse view Move from glottis to suprasternal tracheal view |
Bulls’ eye appearance of esophagus changes when a tube is passed
Double tract image refers to two hyperechoic lines next to each other (one in the trachea and one in the esophagus |
Double tract signal means esophageal intubation. Tube needs to be removed.
Additional confirmation of correct tube position can be done by POCUS lung and look for bilateral lung sliding. |
A list with the stepwise approach to prepare for eFONA is shown in Fig. 1 .
Short guide to image acquisition POCUS for cricothyrotomy. Source: Bruijstens, Cricourse Radboudumc, Department of Anesthesiology.
Publications on airway assessment and management research confirm that assessment in general, and infraglottic assessment in particular and their correlation with clinical outcomes were relatively underreported in the literature . Over the past years, the role of imaging for FONA has been topic of many studies. A systemic review demonstrated that POCUS assisted identification of the CTM is more accurate compared to palpation and that time needed to identify landmarks is comparable to palpation in difficult anatomy . Despite growing evidence, infraglottic airway assessment in general is often not a routine part of airway assessment in all patients.
The next section of this article presents a case series written from an observational and educational perspective to demonstrate why imaging is valuable and how POCUS can complement airway assessment and facilitate procedures through anatomical knowledge, identifying the midline and direction of the rotated/deviated infraglottic airway. We will discuss how POCUS can directly influence your airway management options and plan.
3
Part 2. Clinical cases to illustrate transfer of training to clinical practice and lessons learned
3.1
Cases
An adult male with dynamic airway pathology presented for an elective thyroidectomy via a cervical approach. Palpation of landmarks for front-of-neck-access (FONA) was not straightforward. An eight-month- old CT scan revealed posterolateral tracheal deviation. Before induction, a pre-emptive ultrasound assessment of the neck was performed with the neck in extension. The trachea could not be identified using the longitudinal approach but thyroid and cricoid cartilages and the cricothyroid membrane (CTM) were readily visualized with the transverse approach. Landmarks for cricothyrotomy were marked out and connected; the vertical line was found neither in nor parallel to the midline of the neck due to significant deviation and rotation of the larynx due to the large goitre ( Fig. 2 a–c). Tracheotomy (by surgeon) was deemed not a realistic FONA option in case of airway crisis, due to the inability to identify the trachea which was located deep and the large goitre. It was agreed that if FONA was necessary, a scalpel-bougie cricothyrotomy would be performed by the trained anesthesiologist.
