Learning objectives.
By reading this article, you should be able to:
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Know the indications and limitations of ultrasound in airway management.
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Describe how to identify and mark the cricothyroid membrane with ultrasound.
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Understand the uses of airway ultrasound in patients with obesity, neck pathology, or who are pregnant.
Key points.
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Ultrasound has many advantages for the assessment and management of the airway.
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Ultrasound is superior to palpation for identifying the cricothyroid membrane.
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If identification of the cricothyroid membrane by palpation is uncertain or impossible, ultrasound should be used before induction of general anaesthesia.
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Ultrasound can help predict tracheal tube size and difficult laryngoscopy.
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Ultrasound can be used to confirm tracheal intubation as an adjunct to capnography.
Clinical scenario
A 70-yr-old male with a BMI of 28 kg m−2 was admitted for a diagnostic biopsy of a laryngeal tumour. He had previously been treated for laryngeal cancer with radiotherapy and had a temporary tracheostomy. On assessment of his airway, he had normal mouth opening and neck extension, the ability to protrude the lower jaw and was modified Mallampati class 1. It was not possible to see the vocal cords on surgeon-led nasoendoscopy because of mucus in the hypopharynx and pronounced swelling at the aryepiglottic fold. Postradiation fibrosis also made it impossible to palpate the trachea and the cricothyroid membrane. Tracheal intubation was planned using awake flexible bronchoscopy.
The cricothyroid membrane was identified and marked using ultrasound (US) and an infrared light was placed on the skin at this mark. Tracheal intubation was attempted with light sedation and topical airway anaesthesia with a 6.0 mm tracheal tube mounted on a disposable flexible bronchoscope. Using the infrared light, the bronchoscope was successfully advanced after several attempts into the trachea past a triangular tracheal stenosis, a sequela of his previous tracheostomy. Subsequent placement of the tracheal tube was straightforward. The biopsy confirmed recurrent malignancy. Some weeks later, the patient underwent a laryngectomy using the same technique for awake nasotracheal intubation.3
Airway management is one of the core responsibilities in anaesthesia and intensive care and is a major cause of complications and mortality.1,2 A thorough assessment of the airway is crucial for preventing complications and decreasing airway-associated morbidity, but it is often limited to bedside examination and tests that may not reveal conditions that are not easily visible. With the increasing prevalence of patients who are obese or have multiple comorbidities, there is a need for more accurate airway examination techniques. Although no test can definitively identify a difficult airway, modalities such as CT and MRI scans may reveal abnormalities in anatomy and pathology in relation to the airway, but they are more invasive, time-consuming and are not always feasible for everyday clinical use. More importantly, they do not provide point-of-care information to address questions such as:
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If I had or have to access the airway for emergency front-of-neck-access, can I identify the cricothyroid membrane?
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Can I identify the most appropriate level and location for a dilational tracheostomy and are anatomical structures such as blood vessels present that I need to avoid?
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Can I improve my prediction of a potentially difficult laryngoscopy beyond what is visible from bedside inspection?
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What size of tracheal tube would be optimal?
Endoscopy before induction of anaesthesia reveals point-of-care information about abnormalities in airway anatomy and pathology. Ultrasound is quick, readily available, safe and easy to learn for the assessment and management of the airway, and has the potential to answer the questions above. In this review, we discuss the sonographic challenges of the airway, indications, how to acquire and interpret images, and its application in emergency front-of-neck-access, tracheostomy insertion, prediction of difficult laryngoscopy, confirmation of tracheal intubation and its use in obstetric anaesthesia.
Sonographic challenges of the airway
The US image arises from the reflection of US waves resulting from the variation in acoustic impedance between tissues of different composition. The US that is reflected when it reaches an air-containing cavity is a crucial part of the image generation in airway US. Air is a very poor transducer of US, like bone and calcified cartilage, and it is not possible to see beyond the air–tissue border. Everything visualised in an air-filled space such as the trachea will be US artefacts. Parallel white or reverberation lines are often seen as US artefacts posterior to the air–tissue border. The transition from the tissue of the tracheal wall to the air inside the trachea is characterised by a significant shift in acoustic impedance and the air–tissue border will therefore be seen as a bright white line. This hyperechoic line delineating the air–tissue border is one of the hallmarks of airway US and is utilised in the identification of several important structures.
Most anatomical structures of interest in airway US are superficial at a depth of 0.5–4 cm. Given this, an US machine with a standard linear high-frequency transducer in the range 3–15 MHz is adequate to scan most airway structures, although a curvilinear transducer in the range 3–10 MHz may also be used to obtain the optimal image of structures in the submandibular and supraglottic regions, mainly because of its wider field of view.4
Transcutaneous US can be used for scanning the airway from the tip of the tongue cranially to the mid-trachea caudally. It can identify anatomy and pathology not recognised by visual inspection. When placed under the chin, a curvilinear US probe in a longitudinal orientation can be used to view the floor of the mouth and the tongue from the symphysis of the mandible to the hyoid bone (Fig. 1). This image can help identify pathology at the base of the tongue and potentially impact subsequent airway management. More caudally, US can be used to view the vocal cords and pathology in the larynx. Although it is not possible to see beyond the air–tissue border, pathology such as a tumour or abscess inside the airway that is in contact with the anterior part of the airway can be identified and included in planning airway management. In addition, the cricothyroid membrane and the course of the trachea can be identified. The presence of overlying blood vessels can be checked and used to guide decisions for emergency front-of-neck access and elective percutaneous dilational tracheostomy. In the caudal part of the neck, it is possible to visualise the oesophagus, and this can support the correct placement of the tracheal tube by excluding oesophageal intubation. In what follows, specific techniques to facilitate viewing these anatomical structures and associated pathology on US will be discussed.
Fig 1.
Ultrasound of the floor of the mouth. (Left) Positioning of the ultrasound probe. (Right) Corresponding ultrasound image. The shadows of the mentum of the mandible (a) and the hyoid bone (b) are observed with the tongue (c) visible in between. Reproduced with permission from The Scandinavian Airway Management course, www.airwaymanagement.dk.
Localisation of the cricothyroid membrane
Indications
The success rate of cricothyroidotomy in an emergency setting when performed by specialist anaesthetists is low. This results, in part, from the inability to correctly identify the cricothyroid membrane, and so misplacement of the tracheostomy tube is a common complication.3,5 In patients who are obese or have poorly defined neck landmarks, the success rate for identification of the cricothyroid membrane by palpation is 0–39%.6, 7, 8, 9, 10, 11 Even in patients who are not obese and without any complicating factors, the success rate for identification of the cricothyroid membrane by palpation varies between 18% and 72%, with lower rates of success in females.8, 9, 10,12,13 Compared with these low rates of success, successful identification of the cricothyroid membrane by US in patients who are obese varies between 71% and 90%.6,7,9,11,14
Image acquisition and interpretation
As cricothyroidotomy is a technique of last resort when conventional airway management has failed, it is strongly recommended to identify the cricothyroid membrane before airway management to improve the success rate.15 In light of the low success rate for identifying the cricothyroid membrane by palpation in healthy patients who are not obese, even the routine use of US for identification can be considered. However, when identification by either visual means or palpation is not possible, we recommend the use of US, which has been shown to greatly increase successful identification of the cricothyroid membrane6,7,9,11,16 and reduce the likelihood of complications.17 It should also be considered in patients who have an anticipated difficult airway, regardless of the ease of palpation, as it may provide other useful information such as the skin-to-cricothyroid-membrane distance and the location of any overlying blood vessels (Fig. 2).
Fig 2.
Trachea and vessels. Transverse ultrasound image of the upper trachea just caudal to the cricoid cartilage. The anterior part of the trachea is observed centrally as a round structure. Blood vessels in the immediate vicinity of the trachea are marked with asterisks. Colour flow Doppler can be used to further evaluate and identify blood vessels around the trachea. Reproduced with permission from The Scandinavian Airway Management course, www.airwaymanagement.dk.
There are two main techniques for identifying the cricothyroid membrane by US: the longitudinal or ‘string of pearls’ approach and the transverse or thyroid cartilage–airline–cricoid cartilage–airline (TACA) method.18 Both of these are effective in patients in whom the palpation of landmarks is difficult and they can be performed in less than 1 min.7,14 The string of pearls technique allows good observation of the course of the trachea and is the method studied most. Conversely, the TACA technique can be of advantage in patients who have a short or decreased extension of the neck as the US probe may be difficult to place in a longitudinal direction. When marking the correct position of the cricothyroid membrane with either of these techniques, the neck should be in a fully extended position suitable for performing a cricothyroidotomy. The head can then be moved to the optimal position for the planned airway management strategy. In the event of failed airway management, it has been found that the marked position of the cricothyroid membrane is still accurate if the head is placed back in the same position as when the mark was made.19
Longitudinal technique: string of pearls (Fig. 3)
Fig 3.
Longitudinal or ‘string of pearls’ technique. The longitudinal or string of pearls technique for identifying the cricothyroid membrane and the interspaces between tracheal rings are described in the text. (Left) Positioning of the ultrasound probe. (Right) Corresponding ultrasound images. Blue, anterior part of tracheal ring; yellow, air–tissue border; green, anterior part of the cricoid cartilage; red, caudal end of the thyroid cartilage. Reproduced with permission from The Scandinavian Airway Management course, www.airwaymanagement.dk.
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The trachea is identified first by palpating the sternum and placing the US probe for a transverse scan on the anterior neck of the patient just cranial to the suprasternal notch. It will be visible as a dark horseshoe-shaped structure with a posterior hyperechoic and white air–tissue border (Fig. 3, row 1).
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The right border of the US probe is positioned over the midline of the trachea by moving the probe to the patient's right side. The trachea will appear truncated into half on the US image (Fig. 3, row 2).
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The left end of the US probe is turned 90° into the sagittal plane while the right end of the US probe is kept in the midline of the trachea. This leads to a characteristic longitudinal view of the tracheal midline where the anterior part of the tracheal rings will be visible as numerous hypoechoic and dark rings, superficial to the hyperechoic and white air–tissue border, resembling a string of pearls (Fig. 3, row 3).
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Subsequently, the US probe is moved cranially while keeping it longitudinally in the midline until the cricoid cartilage comes into view. The cricoid cartilage appears as a dark, more anterior, elongated and larger ring compared with the tracheal rings. As the US probe is moved even further cranially, the caudal part of the thyroid cartilage can be seen as a usually calcified structure (Fig. 3, row 4).
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In this position, the air–tissue border between the cricoid cartilage and the thyroid cartilage will be the cricothyroid membrane, and can be marked by stabilising the US probe with one hand while the other hand is used to slide a needle in transverse orientation from the cranial end of the US probe without the sharp tip touching the skin. The needle is moved to where its shadow is seen in the middle of the caudal border of the thyroid cartilage and the cranial border of the cricoid cartilage.
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The skin at both ends of the US probe can be marked with a pen to identify the longitudinal midline of the trachea. The probe is then removed, and the position of the needle can be marked on the skin with a pen indicating the middle of the cricothyroid membrane in the transverse plane.
The longitudinal or string of pearls technique can be viewed on http://airwaymanagement.dk/pearls.
Transverse technique: TACA (Fig. 4)
Fig 4.
Transverse or thyroid–airline–cricoid–airline technique. The transverse or thyroid–airline–cricoid–airline (TACA) technique for identifying the cricothyroid membrane are described in the text. (Left) Positioning of the ultrasound probe. (Right) Corresponding ultrasound images. Red, thyroid cartilage with a distinct triangular shape; yellow, air–tissue border; green, anterior part of cricoid cartilage with a distinct horseshoe shape. Reproduced with permission from The Scandinavian Airway Management course, www.airwaymanagement.dk.
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The thyroid cartilage is identified by placing the US probe anteriorly on the neck in a transverse scan at the level where the thyroid cartilage is estimated to be located. The probe is moved in a cranial or caudal direction until a hyperechoic triangular structure appears, representing the thyroid cartilage (Fig. 4, row 1).
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The US probe is moved caudally to identify the cricothyroid membrane. The cricothyroid membrane is seen as a hyperechoic and white line that arises from the air–tissue border between the lumen of the trachea and the mucosal lining from the posterior aspect of the cricothyroid membrane (Fig. 4, row 2).
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The US probe is then moved further caudally to identify the cricoid cartilage, which appears as a ‘black lying C’ with the air–tissue border visible underneath (Fig. 4, row 3).
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Finally, the US probe is moved back slightly cranially until the centre of the cricothyroid membrane is identified (Fig. 4, row 4).
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The skin can be marked with a pen on both ends of the US probe to identify the location of the cricothyroid membrane in the transverse plane. The centreline function of the US machine, if available, can be used to accurately identify and mark the cricothyroid membrane in the sagittal plane. Both the cranial and caudal borders of the cricothyroid membrane can be identified by their highly characteristic shapes.
The transverse or TACA technique may be viewed on http://airwaymanagement.dk/taca.
Tracheostomy
Indications
It can be challenging to assess the trachea and select the optimal location for a tracheostomy. This is especially the case in patients who are obese, have neck pathology or have previously undergone radiotherapy or surgery on the neck. In particular, radiotherapy leads to not only difficult palpation, but may also cause distortion of the anatomy and tracheal deviation. Ultrasound can be used to determine the skin-to-lumen distance, identify any aberrant or overlying blood vessels to avoid bleeding, identify the thyroid isthmus that normally overlies the first to third tracheal rings, identify the most appropriate intratracheal ring space and guide the selection of the optimal tracheostomy tube size. Furthermore, US can guide clinical decision-making on whether to perform a bedside dilational percutaneous tracheostomy or schedule a surgical tracheostomy in the operating theatre. The usefulness of US for tracheostomy has been previously demonstrated for the identification of the tracheal midline and overlying blood vessels and in patients who have tracheal deviation in order to perform a surgical tracheostomy.20
Image acquisition and interpretation
The same US techniques as described for the identification of the cricothyroid membrane can be used for the identification of the optimal site for tracheostomy, and it may then be marked in the same way. The longitudinal approach will give a better overview of the tracheal rings, whereas the transverse method may provide a superior overview of the overlying blood vessels.
Prediction of difficult laryngoscopy
Indications
There is currently no definitive clinical predictor of difficult laryngoscopy or tracheal intubation, and the unanticipated difficult airway remains a challenge in airway management. Ultrasound has been suggested to aid in the prediction of difficult laryngoscopy or tracheal intubation, and currently more than 40 different parameters which are measurable using US have been suggested.21,22 Some of the parameters most often investigated with the highest accuracy for the prediction of difficult laryngoscopy or tracheal intubation are the distance from the skin to epiglottis (sensitivity up to 80% and specificity up to 80%), skin to hyoid bone (sensitivity up to 71% and specificity up to 71%) and the hyomental distance (sensitivity up to 61% and specificity up to 88%).23,24 These US measurements can be considered as a supplement to standard airway assessment in patients who have an increased a priori risk of difficult laryngoscopy. As the overall incidence of difficult laryngoscopy in the general population is low, the positive predictive value of these US predictors will be limited. They can potentially be used to rule out difficult laryngoscopy in cases of doubt after standard airway assessment.
However, US prediction of difficult laryngoscopy is limited by extensive clinical and methodological heterogeneity between studies.23 Standardised positioning and scanning techniques are missing for the evaluation of each US variable, and there is significant variability in cut-off points for the same measure among studies. The cut-off, for example, for the US distance from the skin to epiglottis ranged between more than 1.6–2.8 cm which may further explain substantial differences in predictive performance.21, 22, 23, 24 Moreover, the investigated outcome for the vast majority of studies was not difficult intubation but difficult direct laryngoscopy. Although correlated, conclusions and predictions about difficult intubation based on difficult laryngoscopy should be interpreted with caution. As such, their predictive accuracy in larger populations is unclear and their role in large scale clinical application remains uncertain.
Tracheal tube size
Indications
The diameter of the trachea can be reliably measured with US to choose the correct size of tracheal and tracheostomy tubes, and has been found to be superior to clinical assessment in paediatric and adult patients.25,26 This can be especially helpful in the paediatric patients where a recent systematic review has revealed US to have an accuracy of 92% and 82% in determining the correct cuffed or uncuffed tracheal tube size, respectively, compared with 56% and 50% for age-based estimations.27 Ultrasound may similarly be useful in adult patients where it has been shown to have a high accuracy in the assessment of the tracheal diameter.25 It might be relevant in the selection of double-lumen tracheal tubes where difficulties with smaller than expected tracheal diameters can be encountered or in patients who have had previous tracheostomy and can have potential tracheal stenosis.28
Image acquisition and interpretation
The tracheal diameter is measured with a transverse scan at the level of the cricoid cartilage and it is identified in a similar way to the transverse or TACA technique previously described for the identification of the cricothyroid membrane. At this level, the transverse diameter of the airway lumen is measured which then correlates to the outer diameter of the tracheal tube (Fig. 5).
Fig 5.
Selection of tracheal tube size. Transverse ultrasound image at the level of the cricoid cartilage. The anterior part of the trachea is seen centrally as a round structure. The transverse diameter of the airway lumen is measured across the midline of the trachea as indicated by the red line. Reproduced with permission from The Scandinavian Airway Management course, www.airwaymanagement.dk.
Confirmation of tracheal intubation
Indications
Waveform capnography remains the gold standard for the confirmation of tracheal intubation, but US can be considered as an adjunct and has been found to have a very high accuracy with a sensitivity of 98.7% and specificity of 97.1%.29 There are many ways to confirm tracheal intubation with US, with the most often described method being to exclude oesophageal intubation29, 30, 31 as recommended in the recent consensus guidelines for preventing unrecognised oesophageal intubation.32
One of the advantages of this particular technique is that it enables the identification of tracheal and oesophageal intubation without the need for ventilation. This may be of interest in time-critical cases where patients have limited respiratory reserves so oesophageal intubation may be recognised before ventilation is initiated or in patients with a high risk of pulmonary aspiration where ventilation on a tracheal tube placed in the oesophagus might thus be undesirable. In both scenarios, tracheal reintubation can be performed before ventilation of the patient and correct placement of the tracheal tube may subsequently be confirmed with waveform capnography. Furthermore, confirmation of tracheal intubation with US may be used in situations where capnography might be unreliable, such as very low cardiac output states or severe bronchospasm, as its use is independent of the circulatory status of the patient.
Image acquisition and interpretation
To view the oesophagus, the US probe is placed transversely just cranial to the suprasternal notch and slightly to the patient's left side. The trachea will be visible, with the oesophagus appearing posterior and just lateral to the trachea as a round or oval multilayered structure with a hyperechoic and light border and hypoechoic and dark centre (Fig. 6). Ideally, the oesophagus is identified in the awake patient where it can be seen to expand and compress during swallowing, which in addition may cause artefacts to appear in the centre of the oesophagus owing to the movement of air through its lumen. Identification of the oesophagus can be aided with slight flexion and rotation of the head towards the patient's right side. The tracheal intubation can then be visualised in real time, where a brief flicker may be observed on the tracheal air–tissue border when the tracheal tube is passed into the trachea. In the case of oesophageal intubation, the oesophagus will appear similar to the trachea with a more uniform and round shape as well as a hyperechoic and white air–tissue border which will obscure the view behind (Fig. 6). This is also known as the double trachea sign.33
Fig 6.
Ultrasound of the oesophagus. Transverse ultrasound image just cranial to the suprasternal notch. Top row: the oesophagus is observed as an oval and multilayered structure with a brighter border and dark centre. Middle row: the oesophagus during swallowing of fluid where it is a round dark structure. Unlike in oesophageal intubation, a clear air–tissue border is not seen and the area posterior to the oesophagus remains visible. Bottom row: the oesophagus when intubated is seen with a clear air–tissue border and the area posterior to the oesophagus is now not visible because of the acoustic shadow cast by the air-filled lumen. Blue, anterior part of tracheal cartilage; yellow, air–tissue border; orange circle, oesophagus. Reproduced with permission from The Scandinavian Airway Management course, www.airwaymanagement.dk.
Obstetric anaesthesia
Obstetric patients constitute a special group in relation to airway management. The incidence of failed tracheal intubation is up to eight times that of non-obstetric patients, at least partly because of increased airway oedema, and they also have an increased risk of pulmonary aspiration secondary to the anatomical and physiological changes of pregnancy.34 Importantly, the accuracy of identification of the cricothyroid membrane by palpation is low in obstetric patients.35 Ultrasound has been shown to improve identification of the cricothyroid membrane, and although taking slightly longer than palpation, was generally performed within 30 s.35 In view of the recognised increased risks related to airway management in obstetrics, it seems justified, in the opinion of the authors, to identify the cricothyroid membrane using US if it is not easily palpable before induction of general anaesthesia in the non-emergency setting. Although evidence is lacking, it might even be considered for emergency and time-limited category 1 Caesarean sections if resources permit and when US is readily available such as during preoxygenation if an additional anaesthetist not otherwise occupied with patient management is available.
Other uses of ultrasound for airway management
There are several other uses for US in airway management, but they are beyond the scope of this review.33 Of these, gastric US represents a non-invasive and point-of-care technique to stratify the risk of pulmonary aspiration by evaluating the presence, character and the volume of gastric contents.36
Conclusions
Ultrasound is a non-invasive, fast, readily available and easy-to-learn tool for the assessment and management of the airway that can be used dynamically before, during and after airway procedures. It is particularly validated in the successful identification of the cricothyroid membrane, and has the potential to support clinical decision-making and difficult airway management in perioperative, emergency, and intensive care.
Declaration of interests
The authors declare that they have no conflicts of interest.
MCQs
The associated MCQs (to support CME/CPD activity) will be accessible at www.bjaed.org/cme/home by subscribers to BJA Education.
Biographies
Robin Lohse MD is a consultant anaesthesiologist at Copenhagen University Hospital, Rigshospitalet. His research interests include airway ultrasound, which he teaches on national and international courses.
Wendy H. Teoh FANZCA is a consultant anaesthesiologist at Wendy Teoh Pte. Ltd, private anaesthesia practice in Singapore. She has authored several publications in the area of airway management and ultrasound.
Michael Seltz Kristensen MD is a consultant anaesthesiologist at Copenhagen University Hospital, Rigshospitalet. He is on the board of Directors of EAMS and SAM and chair of The Scandinavian International Airway course, www.airwaymanagement.dk. He has authored numerous publications on airway management and ultrasound and has presented on airway ultrasound globally.
Matrix codes: 1B02, 2A01, 3A01
References
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