Written by Clay Smith
The average angle of the trachea with a patient lying horizontally was 25° in men and 14° in women. A good cannula angle would be between 45-60°. The cricothyroid membrane (CTM) was lower than the midpoint of the neck in all men and half of women.
Why does this matter?
In a can’t intubate, can’t ventilate situation, it’s imperative to gain emergency front-of-neck access (a.k.a. eFONA). This may mean needle or surgical cricothyrotomy. But often this procedure is not successful. The authors wanted to determine the angle of the trachea with respect to horizontal when patients were supine and the minimum angle that a cannula would pass through the CTM.
This was a prospective study of 50 patients, 30 men and 20 women, who were undergoing chest CT and consented to extend the scan up to the chin for this anatomic study. They all had a towel under the neck to mimic the position during a cric. The major finding was that the trachea angles down, from cranial to caudal, at about 25° for men and 14° for women. Of note, the CTM was below the midpoint of the neck in all the men and about half the women. The absolute minimum angle that a cannula would pass through the cricothyroid membrane (CTM) was over 45 degrees in all patients, a little higher in men than women. In reality, you would not want to pass a needle at the minimum angle; you’d want to pass it at the best angle. From my vantage point, around 45-60° looks to be the sweet spot. Remember, you want the angle of the cannula to correspond with the downsloping angle of the trachea. Also, when feeling for the CTM, it is caudal to the midpoint in men and half of women. It is not right in the middle of the neck. I have altered an image from the article to illustrate this.
Emergency Front-of-Neck Airway Rescue Via the Cricothyroid Membrane: A High-Resolution Computed Tomography Study of Airway Anatomy in Adults. Anesth Analg. 2021 Jul 1;133(1):187-195. doi: 10.1213/ANE.0000000000005583.