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If the two values coincide, the total resistance is acceptable (impedance test) and the electric circuit is satisfactory. If the actual value is less than the set value, this indicates that the electrical resistance in the external circuit is too high. The cause must be found e. g. adhesive electrode too dry, skin resistance too high, broken cable.
After the impedance test, leave in the mA
position (current flowing at the tip of the needle indicated and a + sign in front of the current displayed) until the block is successful.
9.Set an actual current of approx. 1 to 2 mA and advance the needle further until the first muscles supplied by contractions occur in the nerve sought. Then, while reducing the current, advance the needle to the nerve until the recommanded thres- hold current. (For values see, for example, literature references 4, 7). The threshold current to be achieved depends on the pulse width and the geometry of the conductive tip of the needle.
Warning! When using Stimuplex® or Contiplex® needles, it must be noted that muscular contractions with a current of 0.2 to
0.5mA and a stimulus duration of 0.1 ms - with a current of
0.05to 0.2 mA and a stimulus duration of 1.0 ms - indicate satisfactory proximity to the nerve sought. Any further reduc- tion in the ”threshold currents” may cause direct contact of the needle with the nerve, resulting in the danger of a me- chanical nerve lesion.
If needles from other manufacturers are used, the threshold currents may be much higher. You would have to work out these yourself.
10.After clearly visible muscular contractions are seen using the desired threshold current, inject local anaesthetic.
The response to the electrical stimulation will reduce or disap- pear within a few seconds after an injection of 2 - 3 ml. If this effect is not definite, do not continue to search for the nerve at the same location. For safety reasons, a more peripheral site should be used.