2 Continuous Peripheral Nerve Blocks
General Approaches to Continuous Catheter Placement
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.