Most of us routinely insert central venous catheters under real-time ultrasound guidance. The technique is time-tested, and there is robust evidence that it is safer and more reliable compared to the landmark-based approach. However, peripheral venous access can, at times, be even more challenging in critically ill patients. Quite often, access may be difficult due to thrombophlebitis or edema. It is not unusual for poorly experienced operators to struggle with the insertion of peripheral IV lines causing avoidable pain and discomfort to the patient. Would ultrasonography enable visualization of veins that are invisible to the naked eye and enable ease of insertion?
The importance of using an effective tourniquet cannot be overemphasized for ultrasound-guided cannulation of peripheral veins. Many monitors have a built-in, sustained, cuff inflation mechanism on the NIBP module to enable venipuncture. The other option is to inflate a manual blood pressure cuff to slightly lower than the systolic blood pressure. A high-frequency linear probe is most suited to visualize peripheral veins. The initial scan may be on the short axis view just distal to the cubital fossa, where the cannula will not get bent or displaced with arm movement. If veins are not readily visible at this location, move the probe distally along the forearm.
Once you see a vessel of reasonable size, you need to ensure that it is, in fact, a vein. This may occasionally be more difficult than it may seem. If the inflation pressure is too high, arteries may be compressed on minimal probe pressure, making the distinction difficult. Pulse wave Doppler helps to distinguish between arteries and veins but may not be absolutely confirmatory if the inflation pressure is set close to the systolic pressure. However, a good way to differentiate is using what I call the “deflation test.” When the cuff is deflated, the artery becomes more prominent, while the vein collapses.
I would strongly recommend cannulation in the long axis view, using an in-plane technique. This enables precise guidance; besides, it is possible to visualize the cannula from the site of puncture all along the soft tissue through to the vein. The stylet of the cannula projects beyond the plastic tip by about a millimeter. With the in-plane technique, it is possible to ensure that the cannula is also within the vein (not just the stylet) and guide it along under vision, without transfixing the vein. This prevents the possibility of the stylet being inside the vein while the cannula lies outside.
The first step is to search for a suitable vein in the short axis view (Fig 1).
Fig 1. Short axis view. A: Artery; V: Vein
Once you identify an optimally sized vein, the probe is turned around to obtain a long axis view of the target vein (Fig 2). A sufficiently good length of the vein, without tortuosity, should be visible to enable ease of insertion.
Fig 2. Long axis view of the vein
In practice, the cannula is inserted just distal to the probe in the long axis to enable an in-plane technique (Fig 3).
Fig 3. The direction of insertion of the cannula on the long axis view
Puncture the skin and look for movement on the ultrasonographic image; pass the cannula a millimeter at a time and follow it right from under the skin through to the vein (Fig 4).
Fig 4. The cannula (arrow) is seen within the vein on the long axis
Do ensure that you do not lose sight of the cannula at any time. At times, the cannula may seem to be in the vein, when it actually lies outside. This can be prevented by ensuring indentation of the wall of the vein as it makes contact.
How do you confirm that the cannula is in the vein? You could view the cannula inside the lumen on the long axis. However, the definitive method is by performing a bubble test with 10-20 ml of saline (Fig 5).
Fig 5. The bubble test. Opacification is seen within the right ventricle (RV)
The bubble test is carried out by rapid injection of 20-30 ml of saline into the IV line while watching for echocardiographic opacification of the right atrium and the ventricle. Do not forget to record the clip; it helps to review the loop if the opacification is not clear-cut on the initial view.