Continuous Peripheral Nerve Blocks

Published on 06/02/2015 by admin

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2 Continuous Peripheral Nerve Blocks

André P. Boezaart

Acute pain medicine is a subspecialty of anesthesiology, and the capability to administer continuous nerve blocks (neuraxial, paraneuraxial, and peripheral) is a growing and essential skill of the acute pain specialist. Continuous nerve blocks provide analgesia over a continuum of hours to weeks and allow the clinician to control the spread, density, and duration of the nerve block, putting him or her firmly in control of the patient’s analgesic requirements. These advances stimulated the ongoing development of continuous peripheral nerve blocks, the subject of this chapter. Research into reversible yet long-acting local anesthetics has been ongoing for many decades, but to date no effective long-acting drug is available—likely because long-lasting undesired side effects of the block will accompany the long-term desired effects of the block.

Advances in perineural techniques focus on improving catheter placement, thus reducing the diminishment of analgesia after the initial bolus injection. There are three primary techniques for placing perineural catheters: the nonstimulating catheter technique, the stimulating catheter technique, and the ultrasonography-guided technique. Most physicians use all three techniques in combinations that depend on the location of the block and the clinical situation; only a few use a single technique exclusively. The most popular and perhaps most effective way of placing a perineural catheter is under ultrasonographic guidance with or without nerve stimulation needle placement using a stimulating catheter.

General Approaches to Continuous Catheter Placement

Nonstimulating Catheter Technique

With the nonstimulating catheter technique, an insulated needle (usually a Tuohy needle) is advanced near a nerve with nerve stimulator or ultrasonographic guidance. Once the physician is satisfied with the position of the needle tip, saline or local anesthetic is injected through the needle to expand the potential perineural space, and a typical (usually multiorifice) epidural catheter is advanced through the needle. This technique is relatively easy to perform and usually provides an adequate initial or primary block, but the success rate of the secondary block—the block that develops as a result of the local anesthetic’s infusing through the catheter after the initial local anesthetic bolus through the needle has worn off—is variable, depending on which nerve or plexus is being blocked.

Stimulating Catheter Technique

During stimulating catheter placement, an insulated needle (typically a Tuohy needle) is placed near the nerve to be blocked under nerve stimulator or ultrasonographic guidance; no bolus injection is made at the time of needle placement. The next step is to place a catheter with an electrically conductive tip through the needle; electrical stimulation is now performed through the catheter. If a bolus injection is made to expand the perineural space, 5% dextrose in water is used rather than saline or local anesthetic; the latter two will impair the nerve stimulation needed for correct catheter placement using this technique. This technique has more steps than a nonstimulating method. The primary success rate with this technique equals that of the nonstimulating technique, but in theory it has a higher secondary block success rate because of more precise catheter placement. Numerous formal outcome comparisons (nonstimulating vs. stimulating catheters) have been completed, and the findings show analgesic and even surgical outcomes significantly better with use of stimulating catheters. For optimum results, the stimulating catheter should be placed to block the entire region (limb) where the pain originates—for example, the brachial plexus in the case of shoulder surgery or the sciatic nerve in the case of ankle surgery (combined with a saphenous nerve block). Conversely, if only one of a number of nerves that innervate the area (limb) where the pain originates is blocked, such as the femoral nerve after major knee surgery, there seems to be no difference between the analgesic and surgical outcomes of stimulating and nonstimulating catheters. This is especially true if effective multimodal analgesia is also used.

Technique Details

Nonstimulating Catheter Technique

An insulated stimulating needle is directed near the peripheral nerve to be blocked with a stimulator current output of 1.5 mA, or under ultrasonographic guidance. The final needle position is confirmed by (1) observing an appropriate motor response with the nerve stimulator current output set at 0.3 to 0.5 mA, with a frequency of 1 to 2 Hz and a pulse width of 100 to 300 µsec; or (2) demonstrating the needle to be near the nerve by ultrasonography. When ultrasonography is used, it is customary to inject a small volume of fluid through the needle to demonstrate its spread around the nerve—so-called hydrodissection and doughnut sign formation. The needle is often attached to a syringe by tubing from a side port (Fig. 2-1). This arrangement allows the physician to aspirate for blood or cerebrospinal fluid during needle placement and thus minimize unintentional intravascular or intrathecal injection; however, this can give potentially dangerous false-negative results because the suction produced by needle aspiration causes the surrounding tissue to obstruct the needle tip, thus allowing injection of local anesthetic into the intravascular or intrathecal space. Ultrasonography theoretically protects against missing the obstruction, although this depends on the operator’s skill.

Figure 2-1. Side-port device used during catheter placement for infraclavicular block. A,

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