FRONT OF NECK · TUBE CHECK
Airway ultrasound
When to use it
- Find and mark the cricothyroid membrane before induction when the neck is hard to feel (obesity, previous surgery, radiotherapy, swelling). Ultrasound finds it far more reliably than fingers in these patients.
- Confirm the tracheal tube is in the trachea and not the oesophagus, when capnography is unavailable or unclear (for example in cardiac arrest).
- Help predict a difficult laryngoscopy (thick soft tissue in front of the hyoid, epiglottis and vocal cords), check vocal cord movement, and estimate tube size in children.
- Probe
- Linear, high frequency
- Position
- Supine, neck extended (pillow under shoulders)
- Depth
- 2–4 cm
How to scan
- Transverse ('TACA'): start at the thyroid cartilage (a bright triangle), slide down to the cricothyroid membrane (a bright air line), then to the cricoid (a dark arch), then back up to the membrane. Mark its centre.
- Longitudinal ('string of pearls'): start low on the trachea in the midline, turn the probe along the trachea and slide up. Tracheal rings look like dark pearls on a bright string. The first, bigger pearl is the cricoid; just above it is the membrane.
- Put a needle between probe and skin: its shadow marks the exact point. Mark the skin with a pen, with the head in the position you'll use in an emergency.
- Tube check: probe across the neck just above the suprasternal notch. A second air circle beside the trachea during intubation (the 'double tract sign') means the tube is in the oesophagus.
Longitudinal (parasagittal) midline view from the thyroid cartilage down the trachea: the 'string of pearls'.
- Strap
- strap muscles (infrahyoid)
- TC
- thyroid cartilage
- CTM
- cricothyroid membrane: the bright line between thyroid and cricoid cartilage
- CC
- cricoid cartilage, larger and darker than the rings
- T1, T2
- tracheal rings: the dark 'pearls' on a bright string
- AMI
- air-mucosa interface, with reverberation artefacts below it
Original schematic illustration © Nerve Blocks. Not to scale.
What you see and what it means
| On the screen | What it means |
|---|---|
| Bright line between thyroid and cricoid | Cricothyroid membrane: the front-of-neck access point |
| Dark ovals on a bright line | Tracheal rings, with air behind the bright line |
| Second air circle beside the trachea | Oesophageal intubation |
| Bilateral lung sliding after the tube | Tube in the trachea, above the carina (see Lung) |
Pearls
- Mark the membrane before you start, in difficult airways. You won't have time to scan once the oxygen saturation is falling.
- Neck extension moves the membrane: mark it in the same position you would use for a scalpel cricothyroidotomy.
- Practise on normal necks first, so the views are familiar when you need them.
Videos
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References
- Kristensen MS, Teoh WH, Rudolph SS, et al. A randomised cross-over comparison of the transverse and longitudinal techniques for ultrasound-guided identification of the cricothyroid membrane in morbidly obese subjects. Anaesthesia 2016;71:675–83. PubMed
- Kristensen MS, Teoh WH, Rudolph SS, et al. Structured approach to ultrasound-guided identification of the cricothyroid membrane: a randomized comparison with the palpation method in the morbidly obese. Br J Anaesth 2015;114:1003–4. PubMed
