Chapter 7 – Ultrasonography for Airway Management

Abstract

Ultrasonography is an established modality in medical imaging and is evermore entering clinical practice. This chapter provides an introduction to the principles of clinical ultrasonography. It describes the use of airway ultrasonography for identification of the cricothyroid membrane, the trachea and for confirming correct tracheal intubation. Bedside ultrasonography by the anaesthetist has a much higher success rate than palpation for identifying the cricothyroid membrane, especially in patients with neck pathology. It should be applied before initiation of airway management and not be delayed until airway problems are apparent. The role of lung ultrasonography for identification of normal ventilation and pathology is described. Gastric ultrasonography for assessing the starvation status of a patient is described.

Chapter 7 Ultrasonography for Airway Management

Wendy H. Teoh and Michael Seltz Kristensen

Ultrasonography is an indispensable tool in the hands of the anaesthesiologist to identify and mark the trachea and the cricothyroid membrane before embarking on further airway management or induction of anaesthesia in patients with predicted difficult airways, morbid obesity or pathology of the neck. Evaluating the ability to identify the trachea and the cricothyroid membrane is a fundamental part of airway examination before induction of anaesthesia, just as identification of predictors for difficult intubation or mask ventilation. We know from the literature that clinical methods have a disappointingly low success rate especially in the morbidly obese and patients with pathology of the neck (8–39%), whereas ultrasonography reaches 80–100% success in these patients, and also improves success with cricothyroidotomy.

In this chapter, we describe in detail a systematic approach that will enable the clinician to identify and mark the trachea and the cricothyroid membrane before induction of anaesthesia, thus being prepared to undertake a front of neck airway access should it become necessary during subsequent airway management. This procedure is also beneficial for elective tracheotomy. With ultrasound, we can depict the airway from the tip of the tongue to the mid trachea, and at the pleural level (Table 7.1). There are numerous indications for ultrasonography in the management of the airway (Table 7.2): we describe the most important of these and for the rest we refer to the Further Reading. Ultrasound of the stomach is of increasing interest and is also discussed.

Table 7.1 Anatomical structures relevant for airway management and visible with ultrasonography

Mouth

Tongue

Oropharynx

Hypopharynx

Hyoid bone

Epiglottis

Vocal cords

Thyroid cartilage

Cricothyroid membrane

Cricoid cartilage

Trachea

Oesophagus

Lungs

Pleurae

Diaphragm

Gastric antrum

Reproduced with permission from www.airwaymanagement.dk

Table 7.2 Clinical applications for airway ultrasonography

Evaluation of pathology that can affect the management of the airway

Diagnosing sleep apnoea

Determine the nature and volume of stomach contents

Predict the optimal single-/double-lumen-tube-diameter

To guide blockade of the recurrent laryngeal nerve

Identify the cricothyroid membrane for cricothyroidotomy

Identify trachea for tracheotomy

Confirmation of tracheal, endobronchial or oesophageal intubation

Ruling out/diagnosing pneumothorax

Identification of vocal cord palsy

Diagnosing pleural or pulmonary disease

Predicting successful extubation/weaning from ventilator

Major applications for airway ultrasonography. Only those in bold are described in this chapter, for the rest please see Further Reading.

Reproduced with courtesy from www.airwaymanagement.dk

Airway-Ultrasound Technical Aspects

Tissue/Air Border

When the ultrasound wave travels through tissue and reaches air, a strong white line, the tissue/air border, appears because air has an extremely high resistance to ultrasound (Figure 7.1). Everything beyond that line is only 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 mid trachea.

Figure 7.1 This shows the white (hyperechoic) appearance of the tissue/air border, the black (hypoechoic) appearance of cartilage and the white appearance of the calcified part of the thyroid cartilage.

(Reproduced with permission from The Scandinavian Airway Management course www.airwaymanagement.dk.)

Cartilage

Cartilage transmits ultrasound well and thus appears hypoechoic (black). The cricoid cartilage and tracheal rings normally remain cartilaginous throughout one’s life (Figure 7.1), whereas the thyroid cartilage starts to calcify early in life, and thus becomes gradually less penetrable for ultrasound waves.

Lung sliding

When the transducer is placed over an intercostal space, the two ribs bordering the intercostal space are visible as two hyperechoic (light) lines with an underlying shadow (Figure 7.2). Lying a bit deeper between the two ribs, a hyperechoic horizontal line is seen representing the visceral and parietal pleura and called the pleural line. A horizontal motion of the pleural line, called lung sliding, can be seen, synchronous with the patient’s breathing or ventilation. The motion represents the movement of the visceral pleura. (See video at http://www.airwaymanagement.dk/ultrasonography-in-airway-management.) When M-mode scanning is applied, the corresponding characteristic image is called the seashore sign because the area under the pleural line looks like a sandy beach and the parallel lines above look like waves.

Figure 7.2 Scan of the right lung and pleura with A-mode in the small upper screen image and M-mode visible in the lower larger part of the screen. Transducer is placed so that two ribs are seen with the pleural line just deep to the ribs. Lung sliding is seen, indicating that the lung is in contact with the parietal pleura and is ventilated.

(Reproduced with permission from The Scandinavian Airway Management course www.airwaymanagement.dk.)

Lung Pulse

Every heartbeat pushes the lungs a tiny bit; this is visible on ultrasonography as a small double motion at the pleural line synchronous with the pulse. In the breathing or ventilated lung this is difficult to see because the lung sliding will be dominant. But in a lung that is not being ventilated, it becomes easily visible (Figure 7.3).

Figure 7.3 Scan of the right lung and pleura with A-mode in the small upper screen image and M-mode visible in the lower larger part of the screen. Transducer is placed so that two ribs are seen with the pleural line just deep to the ribs. Lung pulse is seen, but no lung sliding, indicating that the lung is in contact with the parietal pleura but is not ventilated.

(Reproduced with permission from The Scandinavian Airway Management course www.airwaymanagement.dk.)

Localisation of the Cricothyroid Membrane and Trachea

The success rate of anaesthesiologists attempting to perform lifesaving cricothyroidotomy is unsatisfactorily low despite it being the ubiquitously recommended procedure when ventilation and oxygenation with non-invasive methods fail. 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 cricothyroidotomy. In order to improve the success rate of emergency cricothyroidotomy it has been recommended to identify the cricothyroid membrane before induction of anaesthesia in all patients. If identification by inspection and/or palpation is not possible, then it can be performed with the help of ultrasonography, which greatly improves the success rate. Ultrasound guidance not only reveals the location of the cricothyroid membrane, but also the depth of tissue between the skin and the airway lumen. Studies have shown that if the midpoint of the cricothyroid membrane is identified with ultrasonography and marked with a pen, even if the patient’s head and neck position changes (e.g. is subsequently manipulated as in a failed intubation attempt), then the original marking will still be accurate and correctly sited after the patient’s head is brought back to the original position in which the marking was performed. Ultrasonography has been shown to improve cricothyroidotomy success rate in human cadavers and in clinical case reports.

Two techniques have been described for the systematic, stepwise identification of the cricothyroid membrane:

The longitudinal string of pearls technique
The transverse TACA – Thyroid–Airline–Cricoid–Airline – technique

The string of pearls technique is the most well published and it has proven its superiority over palpation in a cadaveric study that demonstrated its ability to increase success and limit tube misplacement in cricothyroidotomy. The same technique can be used to identify the best interspace between tracheal rings for tracheostomy placement. We recommend the longitudinal technique as the first to learn and as the initial technique to use, such that every anaesthesia department dealing with difficult airways on a regular basis should have the expertise to apply it. On occasions when one encounters a patient with a very short neck, or flexion deformity of the neck, that leaves no space to place the ultrasound transducer in the longitudinal position, the transverse TACA technique may be the only successful technique. When both the longitudinal and the transverse techniques are applied in tandem, the cricothyroid membrane can be identified in close to 100% of cases.

Longitudinal, String of Pearls Technique

1. Standing on the patient’s right side, 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 as a horseshoe-shaped dark structure with a posterior white line (Figure 7.4, first row).
2. The transducer is slid towards the patient’s right side (towards the operator), so that the right border of the transducer is positioned at the midline of the trachea. The ultrasound image of the tracheal ring is thus truncated into half on the screen (Figure 7.4, second row).
3. The right end of the transducer is maintained over the midline of the trachea, while the left end is rotated 90° upwards 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 7.4, third row). In patients with a short neck most of the tracheal rings may be behind the sternal bone.
4. 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 to the other tracheal rings. Further cephalad, the distal part of the thyroid cartilage can be seen as well (Figure 7.4, fourth row). The longitudinal midline of the airway can now be depicted by marking the skin at the midpoint of each end of the transducer with a pen.
5. 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 ultrasound 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 7.4, fourth row).
6. Now the transducer is removed, and the needle marks the centre of the cricothyroid membrane in the transverse plane and this can be marked on the skin with a pen. The midpoint of the cricothyroid membrane is at the cross section of the two lines.