Upper Airway Ultrasound

Background

Upper airway ultrasound is a relatively novel technique that has multiple applications for the perioperative Anesthesiologist. Preoperatively, airway ultrasound can help identify patients who may have difficult front of neck access. Additionally this technique can be used to identify and mark the cricothyroid membrane, so that rapid front of neck access can be obtained in the case of impossible intubation and oxygenation after induction of anesthesia. Finally, upper airway ultrasound, in conjunction with lung ultrasound, can be used to identify tracheal intubation, endobronchial intubation and esophageal intubation.

Indication

Identification of cricothyroid ligament for front of neck access
Confirm tracheal intubation / rule out esophageal intubation
Identify difficult airway

Acquisition

Transducer

Linear high frequency

Patient Position:

Any position allowing for scanning front of neck, ideally supine with neck extended in the front-of-neck-access (FONA) position

Operator Mechanics

Operate probe with dominant hand
Place ultrasound machine on same side as operator to manipulate controls with non-dominant hand

Scanning Technique

Pre-intubation identification of cricothyroid ligament

Transverse “TACA” technique

Patient is positioned supine with neck extended (FONA position)
The linear probe is placed transversely over the front of the neck just below the chin
- Scan caudad until the Thyroid cartilage is identified as a hyperechoic triangular structure

With the thyroid cartilage centred on the screen, the probe is moved further caudad to identify the superior border of the cricothyroid ligament (AKA cricothyroid membrane)
- This is visualised as a hyperechoic midline structure or Airline
- With a pen, mark the superior border of this airline

Slide further caudad to identify the Cricoid cartilage, which appears as black ring with a posterior white lining
- With a pen, mark where the cricoid cartilage begins

If the probe is slid beyond the cricoid cartilage, tracheal rings will appear
Mark the midway point between the superior and inferior borders of the CTM and use ultrasound to confirm the presence of this Airline
- With the airline in view, mark the midline of CTM

Cricothyroid Membrane

Complete TACA Scan

Longitudinal “string of pearls” technique

Patient is positioned supine with neck extended (FONA position)
The linear probe is placed transversely over the front of the neck just below the chin
Scan caudad until tracheal rings are identified as a dark horseshoe shape with a posterior hyperechoic air-tissue interface
With a pen, place a mark over the midline
Rotate the probe 90º into the sagittal plane
- A series of hypoechoic tracheal rings (string of pearls) will appear with a posterior hyperechoic air-tissue interface
- Left on screen should correspond to cephalad tip of the probe
Scan cephalad to bring the cricoid cartilage into view
- This appears as a longer, larger “pearl” as compared to the tracheal rings
Continue to scan cephalad until the cricoid cartilage is at the rightmost part of the screen (caudad)
The cricothyroid membrane will be visualised as an air-tissue interface between the cricoid cartilage and the thyroid cartilage, which appears as heterogeneous hypoechoic structure cephalad to the CTM
- With a pen, place a mark to identify the middle of the CTM

Confirmation of endotracheal intubation

Airway
- Place probe transversely over anterior neck just superior to sternal notch
  - Pre-intubation scan may help to compare normal “empty airway” with post-intubation airway
- Scanning can be done peri-intubation or post intubation
- Avoid compressing neck with probe
- Trachea
  - Anterior c-shaped hypoechoic tracheal ring
  - Posterior hyperechoic tissue-air border with acoustic shadowing beneath
- Esophagus
  - Not always visualised
  - Posterior and often lateral to trachea
  - Appears as hyperechoic walled oval with hypoechoic centre
- Tracheal intubation
  - Appearance of single air filled structure
  - Increased appearance of artifact in trachea if done in real time = “bullet sign”
  - Intubator can shake ETT to see reverberations in trachea only
- Esophageal intubation
  - Appearance of a second “trachea” = “double tract sign”
  - Hyperechoic tissue-air interface will be present with acoustic shadowing posterior
- Endobronchial intubation
  - Difficult to determine position of ETT tip
  - Ultrasound operater may assist by communicating with intubator when ETT appears in trachea
Lung
- Presence of lung sliding bilaterally confirms endotracheal position of tube
- Lung sliding on one side may indicate endobronchial intubation
- Refer to module on lung POCUS

Interpretation

Identification of CTM as above
Confirmation of endotracheal intubation
- Airway
  - As above
- Lung
  - Refer to module on lung ultrasound
  - Absence of lung sliding on one side suggests endobronchial intubation, obstruction, or lung pathology

Medical Decision Making

Pre-intubation identification of cricothyroid ligament
- Challenging or impossible FONA
  - ?more conservative airway management
  - Involvement of ENT early
Confirmation of endotracheal intubation
- Place ETT at optimal position in trachea/avoid endobronchial or shallow intubation
- Reintubate in the case of esophageal intubation
- Intervene early

Pitfalls and Modifications

The sagittal scan of the CTM can often be challenging in the obese and those with small necks
- Transverse scanning usually works for most people

Comprehension Questions

1. Cephalad to caudad, what is the correct order of the following anatomic structures?

A. Cricoid cartilage, thyroid cartilage, CTM, trachea

B. Thyroid cartilage, cricoid cartilage, CTM, trachea

C. Trachea, thyroid cartilage, CTM, cricoid cartilage

D. Thyroid cartilage, CTM, cricoid cartilage, trachea

2. Label the following structures

A. Thyroid cartilage

B. Cricoid cartilage

C. CTM

D. Tracheal rings

3. What are two ways to confirm proper endotracheal tube position on ultrasound?

Answers

1. D

3. Presence of ETT in trachea, presence of bilateral lung sliding

References

Kristensen, M. S., Teoh, W. H., & Rudolph, S. S. (2016). Ultrasonographic identification of the cricothyroid membrane: Best evidence, techniques, and clinical impact. British Journal of Anaesthesia, 117, I39-I48. doi:10.1093/bja/aew176

Osman, A., & Sum, K. M. (2016). Role of upper airway ultrasound in airway management. Journal of Intensive Care, 4(1). https://doi.org/10.1186/s40560-016-0174-z

You-Ten, K. E., Siddiqui, N., Teoh, W. H., & Kristensen, M. S. (2018). Point-of-care ultrasound (POCUS) of the upper airway. Canadian Journal of Anesthesia, 65(4), 473-484. doi:10.1007/s12630-018-1064-8