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Ultrasound

   

Jeff Gadsden, MD. Published by B. Braun Medical Inc.

Advantages

Ultrasound can monitor needle advancement and the spread of local anesthetic in real-time.

It is possible to see the needle in real- time and accurately guide the needle towards the target.

Ability to demonstrate local anesthetic spread on the screen image. If corresponding tissue expansion isn’t seen when injection begins, the needle tip is not where it is thought to be, and the provider should immediately halt injection, and relocate the tip of the needle.

Multiple injection techniques that were difficult, or indeed dangerous, to do in the era of nerve stimulation alone can now be performed as the nerves can be deposited at various points under US guidance.

Ultrasound may reduce the likelihood of systemic toxicity by allowing providers to use less local anesthetic.  Several studies have published large reductions in the volume required to obtain an equivalent block to standard, nerve-stimulation techniques.19-21

Adjacent structures of importance can be seen and avoided.  Examples include blood vessels (both large and small), pleura, and other nerve structures in the vicinity of the target structure. A useful adjunct to the visualization of structures on the ultrasound screen is the ability to measure the distance from skin to target using electronic calipers. This, coupled with needles that have depth markings etched on the side, confers a safety advantage by warning the provider of a “stop-distance”, or a depth at which he/she should cease advancement, reassess the needle visualization, and perhaps withdraw and start again.

Limitations

The ability of ultrasound to prevent nerve injury is likely not fool proof. 

Observing the needle tip in relation to the nerve is user-dependent, and the safety and precision can often be impeded by unfavorable echogenic characteristics of the tissue/needle interface.22

With current ultrasound technology, it is difficult to prevent or detect early enough an intraneural injection occurring on the ultrasound monitor. Krediet et al. performed sciatic blocks in cadavers, some of which were deliberately intraneural.23 Video clips of these blocks were then shown to both experts and novices in ultrasound guided regional anesthesia. The experts missed 16% of the intraneural injections, while the novices missed 35%.


Once injection has begun, even a miniscule amount of local anesthetic e.g. 0.1ml can produce neurological injury if intra-fascicular.24  However, the data indicates that the sensitivity of ultrasound to detect intraneural injection of 0.5 ml is only approximately 75%.23  Relying on the visual confirmation of tissue expansion may result in fasicular injury before expansion is detected by ultrasound. It is, in other words, probably too late.