Previously I did a quick voiceover podcast about how to assess airways using chest x-rays. This time let’s take a look at how to assess airways using ultrasound. There are 2-3 methods that ultrasound can be used in assessing the airway. One is in locating the cricothyroid membrane and the other is using dynamic ultrasound while the patient is being intubated to indirectly confirm proper tracheal placement..(Yes, you should still use ETC02!)
It’s easy for those of us have done ultrasound now for a while to forget what it’s like when he first look images on the screen. Sometimes they can look like a snowstorm or a fuzzy old TV with a poor signal (from back in the day, before the blue screen of death). Don’t lose hope, continue scanning and continue to label images and study anatomy. Before you know it, your brain will be able to reconstruct the images on the screen to make more sense. The way that I study this was by scanning a lot, printing out images, and labeling every aspect of that image. I also uses a lot of different anatomy books to make sure that I had the basics down. Find yourself a good ultrasound mentor as well.
In 2011 Kundra and colleagues from India documented the use of ultrasound and finding landmarks to identify the cricothyroid membrane. Since then this has been validated by other studies and is acceptable practice.
There have been other studies validating this method and more info can be found published by Kristensen and colleagues.
There is even a great Phillips Tutorial online by doctors Stone and Chan describing this. Which can be found here:
Finding the CTM
When finding and confirming the Cricothyroid membrane there are 2 methods. The TACA method and the Beads on a String Method
The TACA Method of Finding the CTM:
When holding the probe in the transverse orientation and using the linear probe cartilage will appear as hypoechoic, which means that it will be dark in appearance. Initially hold the probe over the thyroid cartilage which will be a triangular structure and bright or hyperechoic. Moved down caudally towards the patient’s feet to where the cricothyroid membrane is. This will appear as a hyperechoic and bright white line because of an air mucosal interface you will also see parallel white lines that are referred to his reverberation artifact. As you continue to move caudally and towards the patient’s feet he will then see a dark ring-like structure and this is the cricoid cartilage. Then moving back up towards the head he’ll be right where the cricothyroid membrane is. See the picture below.
String of Pearls Method
Another method to visualize the cricothyroid membrane is to use the string of pearls technique. The way to do this is to hold the probe transversely cephalad to the suprasternal notch so that you can see the trachea. This will look like a horseshoe that is a dark structure with the posterior white line. Next, move the transducer to the right so that the right border of the probe is at the midline. Then rotate the probe 90° in the sagittal plane so that it is along the long axis of the neck. In doing this you will see the “beads on a string or string of pearls” which is the dark tracheal rings (which btw I totally flubbed in the video and called it cricoid rings, there is only one cricoid ring and then several tracheal cartilages D’oh!).
Due to the air mucosal interface of reverberation artifact is generated and it’ll appear to look like a series of parallel bright white lines that occur regularly spaced intervals deep to the air mucosal level. As you slide the whole probe upwards on the patient’s neck the “beads on a string” will terminate cephalad with a larger oval-like shape that is hypoechoic and dark. This is the cricoid cartilage. As you move the transducer up even more cephalad the cricoid membrane will be identified. If you continue to move the probe pointer up then he will see the thyroid cartilage. Note that he will actually see the membrane itself but know that it’s bordered by the dark cricoid cartilage inferiorly and the dark thyroid cartilage superiorly.
Visualizing tracheal intubation with Ultrasound
The other way that ultrasound can be used to assess the airway would be during intubation. The person assisting intubation will hold the transducer at the level near the cricothyroid membrane and slightly to the patient’s left to visualize the trachea and the esophagus. In the foamfrat video you can see where my esophagus is when I take a drink of Coke and you see the bright bubbles reflecting sound waves. Usually the esophagus lies slightly to the left and posterior to the trachea. In this orientation you can see the trachea as a large horseshoe ring type structure and as the ET tube is passed through the trachea you will see a snowstorm affect and then something called the bullet sign. You can see this below. If while intubating you suddenly have 2 rounds structures then the endotracheal tube is most likely in the esophagus. See below.
Abbasi S et al. Direct ultrasound methods: a confirmatory technique for proper endotracheal intubation in the emergency department. Euro J of EM 2015, 22:10-16
This is just a quick intro and not all inclusive. It’s amazing what uses we are discovering for ultrasound and it’s an exciting time to be involved in EMS.
Let me know if you have any questions.
- Cynthia Griffin D.O.
The ideas expressed in the video and this blog are mine alone and not that of any employer.
The mobile US console used in the video is my own personal Terason Ultrasound machine. Yup, I love ultrasound that much!
I’ll be speaking at AMTC in the next few weeks about cricothyroidotomies and complications. If you are there, please stop by and say hi.
Kundra P et al. Ultrasound of the Airway. Indian Journal of Anaesthesia, 55(5) sept-Oct, 2011: 456-462
Kristensen MS et al. Ultrasonographic identification of the cricothyroid membrane: best evidence, technique, and clinical impact. British J of Anaesthesia, 117(S1):i39-i48 (2016)