The Double Line Sign

Ultrasound Visualization of the Endotracheal tube

The Endotracheal tube (ETT) is generally not clearly visualized in the trachea due to air artifacts . However when visualized it is seen as a double line. This has been termed as Kannapur's "Double Line Sign"(parallel lines close together, posterior margin not visualized).

Visualization is done with the help of a vascular ultrasound probe (13-6 mHz) that is placed along the length of the trachea. To ensure better visualization the cuff of the endotracheal tube is filled with 5 to 8 ml of saline.

Utility of the Double Line Sign
1. To visualize the endotracheal tube in the trachea after intubation
2. To accurately re-position the endotracheal tube prior to trachestomy so as to avoid cuff puncture,  abutment of the needle against the ETT, and passage of the guidewire into the Murphy’s eye of the ETT.
3. To position the tube after intubation so as to achieve bilaterally equal ventilation.

Reference 
Kannapur Anand Shankar, Nakra Monish, R. Ramprasad, Srivastava Vikas: Ultrasound imaging of saline-filled endotracheal tube cuff for accurate repositioning of tube during percutaneous dilational tracheostomy. Intensive care Medicine, Jan 2016 (Online First).

To read more please follow the link: USG visualization of endotracheal tube