One of my ultrasound studies in a standing position 1-G. Left IJ, anterior is up in images. This is Zone I between the SCM and carotid artery. Legend: double arrow- sternocleidomastoid muscle, left arrow- carotid artery, star- internal jugular vein, upper photos relaxed breathing, lower photos valsalva.
Review anatomy. Valsalva to delineate IJ position. Note the deformation of the sternocleidomastoid during valsalva and deformation of the IJ by the carotid artery, which is high pressure and minimally deformable. Also, this is not a mild valsalva- this is a significant strain to open the IJ.
In the female subject on the right, note location of her IJ which is more lateral- thus significant anatomic variability.
Also note the significant difference in thickness of the SCM- more on that shortly.