Table of Contents:
Introduction: So far, there are no techniques available to completely prevent neurologic impairment after peripheral nerve blockade. Although these complications do happen unfrequently with nerve stimulator guided regional anesthesia, they come along with a high clinical relevance. Needle-nerve-contact while needle positioning is discussed as a common risk factor for nerve injury. The conductive electrode area of the applied isolated stimulation needles may differ. Aim of this study was to determine whether the use of needles with small emitting surfaces for electrical nerve stimulation is associated with a lower incidence of needle-nerve-contacts compared to needle-tips with larger emitting surfaces.
Methods: In six anaesthetized pigs the brachial plexus was surgically exposed bilaterally and underwent a measurement of needle-nerve-distance eliciting a minimal motoric response with 0,3 to 0,8 mA using either a Stimuplex® A (standard needle with a medium conductive surface, n = 2025) or a Stimuplex® D (small conductive surface, n = 1800) cannula or a Tuohy-tip needle (StimucathTM, large conductive surface, n = 2025). The incidence of needle-nerve-contacts for the different needles was compared using Fishers Exact Test. Differences between the needle-nerve-distances were assessed by Friedman´s Test and confirmatory post hoc pairwise testing.
Results: A total of 5850 stimulation attempts were examined. For a current intensity of 0,3 to 0,5 mA there was a high incidence of needle-nerve-contacts for all of the examined needle types. Besides, for low amperages there were needle-nerve-contacts without stimulation success for all tested needle designs. For the Tuohy-needle the frequency of unseccussful stimulations was highest, followed by the Stimuplex® D and the Stimuplex® A. The occurance of failed stimulation attempts was significantly different between the needles (p < 0,0001 for 0,3 mA). For 0,6 to 0,8 mA the incidence of needle-nerve-contacts was lowest for the Stimuplex® A and there was a significant difference between the needles (Stimuplex® A vs. Stimuplex® D p < 0,005 for 0,6 mA, p < 0,0001 for 0,7 mA, p < 0,005 for 0,8 mA, Stimuplex® A vs. Tuohy p < 0,0001 for 0,6 mA to 0,8 mA). The incidence of needle-nerve-contacts for the Tuohy-needle was higher compared to the Stimuplex® D.
Conclusions: The use of stimulation needles with small conductive areas is not associated with a lower incidence of needle-nerve-contacts. However, the significant difference in the rate of needle-nerve-contacts with and without stimulation success between the tip configurations imply different stimulation properties of the individual designs. The Tuohy-design seems to be particularly prone to needle-nerve-contacts and stimulation failure in spite of immediately touching the target nerve. Regardless of the cannula design, stimulation above 0,5 mA is associated with more safety and leads to equivalent needle-nerve-proximity in comparison to lower thresholds. With higher current amplitudes the ideal threshold moreover varies dependent on the used needle type.