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The editors would like to acknowledge that this chapter is abridged from a chapter originally written by Drs. John J. Finneran IV and Brian M. Ilfeld.
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Nerve Stimulation Technique
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For this technique, an insulated needle concentrates electrical current at the needle tip, while a wire attached to the needle hub connects to a nerve stimulator—a battery-powered machine that emits a small amount (0–5 mA) of electric current at a set frequency (usually 1 or 2 Hz). A grounding electrode is attached to the patient to complete the circuit. When the insulated needle tip is placed in proximity to a motor nerve, specific muscle contractions are induced, and local anesthetic is injected. Most practitioners inject local anesthetic when current between 0.2 and 0.5 mA results in a motor response. For most blocks using this technique in adults, 30–40 mL of anesthetic is usually injected with gentle aspiration between divided doses.
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Ultrasound imaging has overwhelmingly become the dominant modality taught for nerve localization and needle guidance in recent years. Ultrasound may be used either alone or combined with other modalities such as nerve stimulation.
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The optimal transducer varies depending upon the depth of the target nerve and approach angle of the needle relative to the transducer. High-frequency transducers provide a high-resolution picture with a relatively clear image but offer poor tissue penetration and are therefore used predominantly for more superficial nerves. Low-frequency transducers provide an image of poorer quality but have better tissue penetration and are therefore used for deeper structures. Transducers with a linear array offer an undistorted image and are therefore often the first choice among practitioners. However, when a steep needle trajectory relative to the long axis of the transducer is required, linear array transducers will poorly visualize the needle. For deeper target nerves that require a more acute angle between the needle and the transducer, a curved array (curvilinear) transducer will maximize returning ultrasound waves, providing the optimal needle image. Nerves are best imaged in cross-section, where they have a characteristic honeycomb appearance (short-axis). Needle insertion can pass either parallel (in-plane) or not parallel (out-of-plane) to the plane of the ultrasound waves. In-plane technique is more frequently utilized as the entire shaft of the needle can be visualized as it approaches the target nerve and navigates surrounding structures. Unlike nerve stimulation alone, ultrasound guidance allows for a variable volume of local anesthetic to be injected, with the final amount determined by what is observed under direct ultrasound visualization. Generally, the goal will be a circumferential spread around the target nerve, and this technique usually results in a smaller injected volume (10–30 mL) of local anesthetic.
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Continuous Peripheral Nerve Blocks
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Also termed perineural local anesthetic infusion, continuous peripheral nerve blocks involve ...