[Influence of minimum current for peripheral nerve stimulation on the latency and success rate of sciatic blockade]

The efficacy of conduction anesthesia depends to a great extent on accurate application of the local anesthetic solution (LA) in close proximity to the nerve trunk. The problem with most peripheral nerve stimulators available in the past was that they did not provide a small enough electrical stimulus. Correct positioning of the needle could not be guaranteed if muscle contractions occurred. New current-controlled stimulators have recently been developed with low currents from 0.1 mA upwards. We studied the intensity of stimulus currents eliciting just-visible muscle contractions in the lower leg (m. triceps surae) to see how this influenced the latency and success rate of sciatic nerve block.

In a randomized study, 35 patients (18-68 years, ASA I and II) undergoing elective surgery were investigated. The block was performed by posterior approach using 30 ml 1% prilocaine. The stimulator was switched on with a frequency of 1 impulse/s and a stimulus current of 1 mA when the insulated needle was at a depth of 4 cm. In group 1 (n = 5) LA was injected when the first visible muscle contractions occurred at a current of 1.0 mA, but vanished if the current was diminished. In group 2 (n = 10) the needle was placed if a minimum current of 0.5 mA just triggered a muscular response. In group 3 (n = 10) the threshold current was 0.3 mA and in group 4 (n = 10) 0.1 mA. In all cases direct contact between the needle tip and the nerve was avoided.(ABSTRACT TRUNCATED AT 250 WORDS)