Local anesthetic dose and volume used in ultrasound-guided peripheral nerve blockade

Ultrasound guidance has made a significant impact on the practice of peripheral nerve block. The ability to visualize the block needle, nerve, and local anesthetic injectate facilitates greater precision in terms of perineural needle placement than was afforded by either nerve stimulation of landmark-based approaches. Closer perineural needle placement facilitates bathing nerve structures in local anesthetic with smaller volumes than those used conventionally. The use of comparatively small doses of local anesthetic may limit the occurrence of systemic local anesthetic toxicity.^14-18 Recent reports of local anesthetic myotoxicity^70-72 provide further impetus to reduce the local anesthetic dose in peripheral nerve block to the absolute minimum. The obvious caveat in this brave new world of sighted regional anesthesiologists is that placing needles adjacent to 3-dimensional nerves using 2-dimensional imaging may lead to a high likelihood of needle nerve contact. Contact between needles and nerves does not necessarily imply injury.⁷³ However, whether the wrong needle (cutting) makes contact with a nerve or the right needle (short bevel/blunt) is placed within a nerve, nerve injury may occur. Placing a needle directly adjacent to nerves should not result in nerve injury; however, regional anesthesiologists and their residents must strive to avoid intraneural needle and injectate placement. Nerve architecture itself plays an important role in determining the MEAV. In the brachial plexus, the ratio of neural to non-neural tissue decreased from a proximal (interscalene/supraclavicular) value of approximately 1:1 to a distal (midinfraclavicular/subcoracoid) value of approximately 1:2.⁷⁴ At the same time, from proximal to distal, the neural tissue is surrounded by an increasing amount of epineural connective tissue. This is even more prominent in the lower limb in the popliteal region. The non-neural tissue serves as a reservoir for lipophilic local anesthetics; therefore higher volumes are needed.⁷⁵ Ultrasound guidance seems to have resulted in a reduction in the dose of local anesthetic used to achieve successful peripheral nerve block in a variety of settings. As the next generation of anesthesiologists emerge trained in the practice of ultrasound-guided regional anesthesia, the true impact of ultrasound on regional anesthesia practice will be fully appreciated. Although the accumulated evidence seems compelling, whether the work of the aforementioned researchers will translate into a change in clinical practice remains to be seen.