Insulated beveled needles are commonly used in combination with a nerve stimulator for single nerve blocks (Fig. 2-2
). The negative electrode of the nerve stimulator is connected to the insulated needle while the positive electrode of the nerve stimulator is connected to an electrocardiogram electrode serving as a ground electrode. There are different sizes of needles and, for some sizes, different gauges. Although the most appropriate angle at the tip remains the object of some debate (15° vs. 30°), the size and the gauge of the needle for a given block should be chosen according to the approach, the patient population (e.g., adult vs. pediatric, larger patients vs. smaller patients). Thus a 22-gauge, 2.5-cm needle is indicated for an interscalene block in adults, whereas in children a 25-gauge, 2.5-cm needle is preferred for the same block. The use of a longer needle (up to 5 cm) may also be indicated in morbidly obese or very muscular patients. Although some experts prefer the use of a 5-cm needle to perform an interscalene block, as a rule of thumb, for less experienced practitioners, the shortest recommended needle is generally safest and should be preferred. Insulated needles are usually available in lengths from 2.5 to 15 cm. In addition, for the placement of perineural catheters for continuous nerve block techniques the use of an insulated introducer Tuohy needle is frequently preferred (Fig. 2-3
). Although the use of a stimulating stylet for the placement of a perineural catheter was described as early as 1951, the use of a stimulating catheter (Fig. 2-4
) has only recently been introduced clinically.
In a few cases, it is possible to use noninsulated needles to perform peripheral nerve blocks, including transarterial (axillary) and paresthesia techniques (interscalene and axillary), and field blocks (median and radial nerve blocks at the wrist and fascia iliaca block). For the fascia iliaca block, the use of a Tuohy needle in adult patients facilitates the performance of the block by allowing a better feeling of the needle going through the fasciae lata and iliaca.