7 Do the Math
As previously mentioned, depth has been shown to play a role in the successful placement of ultrasound-guided catheters. However, simply recognizing the depth of a vessel is not enough to successfully cannulate a vein. There are several other factors that must be considered. The depth of the vein, the point of catheter insertion, and the angle of entry all play a role in deciding the necessary length of the catheter to be used. Table 1 shows several important values derived from trigonometric functions for catheter insertion at a 45º angle. A prospective study published in The American Journal Of Emergency Medicine showed that although it did take more time to place longer catheters, the rate of failure was only 14 percent versus 45 percent in the short catheter group.8 This highlights the importance of choosing catheters of the correct length for each cannulation, especially when dealing with deeper veins. This may reduce the incidence of failure and extravasation, which are complications associated with ultrasound-guided peripheral lines.
8 Be Cognizant of Compression
Veins, unlike arteries, lack muscular walls, which allows for easy compressibility. To verify the target vein, compression with the ultrasound probe is encouraged. However, when attempting to successfully place the catheter, be cognizant of the inherent compression that the probe itself may have on soft tissue. In areas with potentially excessive soft tissue, such as the medial bicep, it is important to use minimal compression as this gives a more accurate determination of depth. In the figures, there is not enough compression to collapse the vein but clearly enough to compress soft tissue. What looks like a superficial vessel may in fact be much deeper than initially thought when applying little to no pressure (see Figure 9). This may explain why some ultrasound-guided IVs are prone to extravasation and failure after seemingly successful placement. Understanding this concept and using the appropriate-length catheter may remedy any problems encountered. It is also important to note that excessive compression of the vein may also create a new challenge when attempting cannulation. Many times, the catheter will puncture the posterior wall, leading to extravasation.
9 Look Before You Leap
After finding a vein you wish to cannulate, it is important to align the vessel in the middle of the screen while stabilizing the probe in that position. While some transducers have a marker in the center of the probe, others do not, and you must rely on your own judgment of where midline may be. To ensure your entry point is both midline and directly on top of the vessel, you can use the plastic catheter cover to compress the skin at the suspected midline. If the tissue directly above the vessel and the vessel itself compress, then you know you will enter in the appropriate midline location. If compression occurs lateral to the vessel, then you can adjust accordingly. Once you are comfortable with your chosen entry point, you may use the catheter to puncture the skin and cannulate the vein.