Once a vessel is identified and centered on the screen, the ultrasound operator can compress with the probe to distinguish artery from vein. A vein should collapse more readily than an artery. The artery may also be found to be pulsatile. Color flow can also help, as the artery is pulsatile, whereas there will be continuous flow within the lumen of the vein. Augmentation is another technique that also uses color flow to differentiate artery from vein. The forearm is squeezed distal to the ultrasound probe; and with each squeeze, there should be enhanced color flow within the venous system.

After the desired vein is identified and centered on the ultrasound display, the depth of the image should be decreased until the vessel takes up as much of the screen as possible while still allowing identification of its anterior and posterior wall, as well as any surrounding neurovascular structures. (See image 3.) The ultrasound operator will then measure the distance from the skin to the vessel of interest. This distance will aid the operator who is placing the PIV, as the distance the catheter has to travel through the soft tissue to the vessel of interest can be calculated or estimated with some simple geometry. (See Pythagorean Theorem chart on page 22.)

As the vessel of interest should be centered on ultrasound display, the operator placing the PIV will aim the needle at the middle of the probe. While the catheter is being advanced, two sonographic findings may be visualized. In short axis, the actual catheter may not be visualized; however, reverberations caused by the ultrasound waves within the catheter may be seen as a phenomenon called the “ring down” artifact, which serves as a proxy for the catheter and helps determine the direction the catheter should be advanced or redirected. As the catheter approaches the vessel, it may push the vessel wall inwards toward the lumen, causing “tenting” of the vessel. (See image 4.) Once the needle punctures the vessel wall, resolution of “tenting” may be noted, and blood return should be seen.