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Today electrical nerve stimulation is a very common way to locate nerves for peripheral nerve blocks in anesthesia. In general, the injection of a local anesthetic occurs after eliciting a just visible motor response triggered by nerve stimulation with a stimulation current threshold (SCT) between 0,3-0,5 mA. With these amperages, so is the general point of view, one places the needle close enough to get a reliable nerve block and on the other hand one is distant enough to avoid nerve injury. However, studies, which substantiate the predominance of this interval do not exist.
The aim of this study was to find out whether the use of a higher SCT for nerve sti-mulation than usually clinically used leads to a reduction in needle-nerve-contacts compared to lower SCT as routinely used.
Method: After eliciting a minimal motor response triggered by electrical nerve stimu-lation at the brachial plexus or the femoral nerve in 16 anesthetized pigs, a coloured synthetic resin was injected by the needle. The stimulation threshold to elicit this motor response was in randomized order either between 0,8-1,0 mA or 0,01-0,3 mA.
Post mortem the corresponding axillary and inguinal regions have been prepared and the localization of the synthetic resin representing the final position of the needle tip has been examined. Direct contact of the injectate to the epineurium of a nerve has been defined as position “needle tip close to nerve”. Did the injectate not have contact to the epineurium of a nerve, it was declared as “needle tip distant to nerve”.
Results: We performed a total number of 235 punctures. 91 of those accomplished punctures belonged to the low stimulation threshold group, 92 belonged to the high one. 52 injections served as control group and were performed with stimulation thresholds between 1,8-2,0 mA. All injectates of the high and low stimulation current threshold study groups could be classified as “close to nerve” positions (100%). 25 of the 52 control injectates (48%) were found with contact to the epineurium of a nerve, 27 of them (52%) were without it in a position “distant to nerve”. Additional three intra-neural injectates (classified as “close to nerve”) were found. One of them was detected after needle placement with a low SCT in the brachial plexus, two of them occurred after using high SCT, one in the axillary and one in the inguinal region (0,85 mA).
Conclusion: In this study, the use of higher stimulation current thresholds than routinely clinically used did not lead to a lesser number of synthetic resin-epineurium-contacts compared to the number after use of lower SCT. In pigs high and low stimulation thresholds lead to the same localizations of the needle tip.
Perspective: In this study in a porcine model the theorie, that a lower SCT is attended by an increased closeness to the nerve, seems after consideration of the control group to be approved. On the other hand the results are implying, that the difference in needle-nerve proximity after exact electrical nerve stimulation below 1,0 mA is small. Successful peripheral nerve blocks could be possible with the application of higher stimulation current thresholds than routinely used, too. Perhaps this would enhance the safety of the technique and reduce the time needed to carry it out. To test the transferability of these results at the application to the human being remains the assignment of clinical studies in the future.