[A bipolar electrode with a radial asymmetrically aligned electric field. A new principle of nerve localization in regional anesthesia and pain therapy].

Electrostimulation is a common method of nerve localization. To improve the nerve-stimulating needle, we developed a new bipolar nerve-stimulating electrode with a more convenient extension of its electric field. Figure 1a shows the new nerve-stimulating needle; in Fig. 1b a longitudinal section is depicted. A hollow cannula (1) is surrounded by an isolator (5) that separates the internal electrode (1) from the external electrode (6). The surface of the latter is also covered by an isolator (8). Two cables, (2) and (7), connect the electrodes to a nerve stimulator. Syringes and tubes can be connected to the needle by using the attached part (4). Figs. 2a and b show top- and side views of the needle's sharp end. Except for small parts of the internal (1) and external electrode (2), both electrodes are covered by isolation (3). This arrangement leads to the radially asymmetric extension of the electric field. Studies with animal nerve-muscle preparations showed that the density of the electric field is higher, and thus electrostimulation can be carried out with up to 75% lower impulse amplitude as compared with insulated monopolar electrodes. The main advantage of the new arrangement of electrodes, however, is the dependency between revolution of the nerve-stimulating needle on its axis and the evoked muscle response. Muscle potentials measured by electromyography and respective degrees of needle torsion are shown in Fig. 3 (black circles: bipolar radially asymmetric electrode, white circles: insulated monopolar electrode).(ABSTRACT TRUNCATED AT 250 WORDS)