Chapter 13
Ultrasound of the neck for airway management
Michael S. Kristensen1 and Wendy H. Teoh2
1Dept of Anaesthesia, Rigshospitalet, University Hospital of Copenhagen, Copenhagen, Denmark. 2Private Anaesthesia Practice, Wendy Teoh Pte. Ltd, Singapore.
Correspondence: Michael S. Kristensen, Dept of Anaesthesia, Center of Head and Orthopaedics, Section 3071, Rigshospitalet, University Hospital of Copenhagen, Blegdamsvej 9, DK-2100 Copenhagen, Denmark. E-mail: Michael.seltz.kristensen@regionh.dk
US is an important tool in the provision of safer airway management for patients. Using a high-frequency linear probe affixed to conventional bedside portable US machines, the patient’s upper airways from the mandible to the jugular notch can be depicted with US such that when the US beam penetrates to the luminal surface of the airway, as for example in the trachea, a strong echo arises yielding a distinct hyperechoic white line on the screen, the “tissue–air border”, and everything beyond that visualised on the screen is mere artefact. There are numerous indications for the use of US for airway management; only the most clinically valuable will be described in detail in this chapter. These include: US identification of the cricothyroid membrane in preparation for management of a difficult airway where emergency airway access via the anterior neck might become necessary, identification of the ideal interspace between tracheal rings for tracheostomy, and simultaneous scanning of the trachea and oesophagus just cranial to the jugular notch to observe whether a tube enters the airway or the oesophagus.
Cite as: Kristensen MS, Teoh WH. Ultrasound of the neck for airway management. In: Laursen CB, Rahman NM, Volpicelli G, eds. Thoracic Ultrasound (ERS Monograph). Sheffield, European Respiratory Society, 2018; pp. 172–183 [https://doi.org/10.1183/2312508X.10007517].
The use of US is an essential adjunct in the management of a difficult upper airway. US before initiation of airway management allows identification of airway structures including the larynx, cricothyroid membrane and trachea, which is essential for safe airway management, and allows visualisation of the oesophagus for detection of oesophageal intubation.
Basic description of US of the airway
A strong echo (i.e. a strong white line) will appear when the US beam reaches air [1]. This is the tissue–air border, and everything beyond this line is an artefact. This means that we can depict the tissue from the skin to the anterior luminal surface of the upper airway from the mouth to the midtrachea [2]. US for airway management should be performed at the point of care by the airway manager as an adjunct to the desired procedures. A standard linear high-frequency transducer is sufficient to scan the structures of the upper airway. The basic US appearance of the upper airways is shown in figures 1–4.
Figure 1. A curved low-frequency transducer and the area covered by the scan (left; blue line), with the resulting US image (middle). The following are indicated on the scan on the right: the shadow from the mentum of the mandible (green), the muscles in the floor of the mouth (purple), the shadow from the hyoid bone (orange) and the dorsal surface of the tongue (red). Reproduced and modified from [1] with permission.
Figure 2. A transverse midline scan over the thyroid cartilage in an 8-year-old boy (left) and the resulting US scan (middle). The following are indicated on the scan on the right: the thyroid cartilage (green), the vocal cords (orange), the anterior commisure (red) and the arytenoid cartilages (yellow). Reproduced and modified from [1] with permission.
Figure 3. A linear high-frequency transducer placed in the midsagittal plane and the scanning area (left; blue line), with the resulting US scan (middle). The following are indicated on the scan on the right: the thyroid cartilage (green), the cricoid cartilage (dark blue), the tracheal rings (light blue), the cricothyroid membrane (red), the tissue–air border (orange) and the isthmus of the thyroid gland (brown); below the orange line, only artefacts are seen. Reproduced and modified from [1] with permission.
Figure 4. The position of the transducer for a transverse scan just cranial to the suprasternal notch and on the left side of the patient’s trachea (left) and the resulting US scan (middle). The following are indicated on the scan on the right: the anterior part of the tracheal cartilage (light blue), the oesophagus (purple) and the carotid artery (red). Reproduced and modified from [1] with permission.
US for airway management: indications
US has a wide range of applications for safer airway management. The most important of the published indications are: 1) intrauterine (pre-natal) US to identify fetal airway abnormalities (e.g. lymphatic malformations or cervical teratoma) allowing planning of optimal airway management in the case of fetal airway obstruction [2], 2) scanning of the upper airway from the mandible to the trachea to screen for prediction of a difficult direct laryngoscopy [3, 4] and for evaluating pathology that can influence airway management [1], 3) scanning of the hyoid bone and thyrohyoid space to facilitate superior laryngeal nerve blocks [5], 4) scanning of the vocal cords for investigation of vocal cord palsy and other pathology and for verification of tracheal intubation, 5) identification of the cricothyroid membrane for pre-anaesthetic airway evaluation in preparation for elective or emergency front-of-neck airway access [1, 6–9], 6) identification and examination of the trachea for better planning of an elective or emergency tracheostomy [10], 7) scanning of the trachea/oesophagus/pleura for evaluation of the placement of a breathing tube and to determine whether it is in the trachea, in a main-stem bronchus or in the oesophagus [11], 8) scanning of the diaphragm/adjacent organs for evaluation of hemidiaphragmatic paralysis [12] and for evaluation of force of breathing before extubation [13], 9) scanning of the pleura to diagnose or rule out pneumothorax and to differentiate between different causes of hypoxia [2], 10) scanning of the gastric antrum to determine stomach content and aspiration risk [14, 15] and 11) scanning of the oesophagus and stomach to determine the correct placement of a gastric tube.
The authors consider indications (5)–(7) the most important and clinically useful, and these are covered in this chapter. Indication (9) is also important for airway management, and the technique is described in another chapter of the Monograph [16]. For detailed descriptions of the other indications, the reader is referred to [2] and the accompanying video links [17].
Localisation of the cricothyroid membrane
The success rate of anaesthesiologists attempting to perform lifesaving cricothyrotomy is unsatisfactorily low, despite it being the ubiquitously recommended procedure when ventilation and oxygenation with noninvasive methods fail [18–22]. The inability to identify the cricothyroid membrane by external visualisation or palpation is an important contributor to this low success rate, and misplacement is the most common complication when attempting cricothyrotomy [23–25]. In order to improve the success rate of emergency cricothyrotomy, it has been recommended that the cricothyroid membrane should be identified before the induction of anaesthesia in all patients [6, 26]. If identification by inspection and/or palpation is not possible, this can be performed with the help of US, which greatly improves the rate of successful identification [7, 26, 27]. Additionally, US guidance reveals not only the location of the cricothyroid membrane but also the thickness of the tissue that has to be penetrated to gain access to the airway, and improves the success rate of cricothyrotomy in human cadavers [8].
Two techniques have been described for systematic, stepwise identification of the cricothyroid membrane: 1) the longitudinal “string-of-pearls” (SOP) technique [1], and 2) the transverse thyroid–airline–cricoid–airline (TACA) technique [28].
The SOP technique is the most well published and has proven its superiority over palpation in a cadaveric study that demonstrated its ability to heighten success and limit tube misplacement in cricothyrotomy [8, 27]. This technique can also be used to identify the optimal interspace between tracheal rings for placement of a tracheostomy tube. We recommend this technique as the first to learn and as the default technique, so that every anaesthesia department dealing with difficult airways on a regular basis should have the expertise available to apply this method. On occasions where one encounters patients with a very short neck, or with flexion deformity of the neck that leaves no space to place the US transducer in the longitudinal position, we recommend the transverse TACA technique to identify the cricothyroid membrane, as in these subsets of patients this may be the only successful technique [28]. Achieving a 100% success rate of identifying the cricothyroid membrane was possible when the longitudinal SOP technique was applied in tandem with the transverse TACA technique [28].
Thus, US-guided localisation of the cricothyroid membrane fills the void of very poor results caused by inaccurate localisation by visualisation or palpation; it is easily learned and should be considered if not routinely, then at least before embarking on the management of anticipated difficult airway situations.
Performing the longitudinal SOP technique
The longitudinal SOP technique is carried out as follows [29]. The sternal bone is identified and the transducer is placed transversely on the patient’s neck, just cephalad to the suprasternal notch to visualise the trachea (a horseshoe-shaped dark structure with a posterior white line) (figure 5a). The transducer is then slid towards the patient’s right side (towards the operator), so that the right border of the transducer is positioned in the midline of the trachea, and the US image of the tracheal ring is thus truncated in half on the screen (figure 5b). The right end of the transducer is maintained over the midline of the trachea, while the left end is rotated 90° into the sagittal plane, resulting in a longitudinal scan of the midline of the trachea. A number of dark (hypoechoic) rings will be seen anterior to the white hyperechoic line (air–tissue border), akin to a string of pearls. The dark hypoechoic “pearls” are the anterior part of the tracheal rings (figure 5c). The transducer is kept longitudinally in the midline and slid cephalad until the cricoid cartilage comes into view (seen as a larger, more elongated and anteriorly placed dark “pearl” compared with the other tracheal rings. Further cephalad, the distal part of the thyroid cartilage can also be seen (figure 5d). The longitudinal course of the midline of the airway can be marked with a pen. While still holding the transducer, the other hand is used to slide a needle (as a marker, for its ability to cast a shadow in the US image) between the transducer and the patient’s skin until the needle’s shadow is seen midway between the caudal border of the thyroid cartilage and the cephalad border of the cricoid cartilage (figure 5d). The transducer is then removed, and the needle marks the centre of the cricothyroid membrane in the transverse plane, which can be marked on the skin with a pen. For a video demonstration of this technique, see [30].
