The Effect of Injectate Conductivity on the Electric Field with the Nerve Stimulator Needle: A Computer Simulation

BACKGROUND: Electrical neural stimulation is commonly used to localize neural structures and place local anesthetic for regional anesthesia. The sharp tip of the stimulating needle gives rise to an electric field which is highly localized. The electrostatic effect of the injected solution on the field distribution and strength has not previously been modeled. METHODS: The three-dimensional electric field around a 0.7 mm diameter, un-insulated, hollow needle with a 30 degrees bevel was calculated in silico using a hybrid finite difference/impedance network method to solve the Laplace equation. The surrounding tissue was assumed to be electrically uniform. A sphere of injectate centered on the bevel was modeled as a region of conductivity differing from that of the bulk tissue. RESULTS: The electric field strength was highly concentrated at the needle tip and decayed rapidly with distance r approximately as 1/r1.7. It was demonstrated that the electric field in the immediate vicinity of the needle tip was greatly reduced in the presence of solutions with conductivity greater than that of the surrounding tissue. A 1.5 mm radius region of conducting solution (equivalent to saline or local anesthetics) reduced the field by 31%. The same volume of a relatively insulating injectate, such as dextrose solution, led to a field enhancement of approximately 15%. CONCLUSIONS: The electric field magnitude in the vicinity of the needle tip decayed more slowly with distance than predicted by Coulomb’s law. This was independent of the presence of injectate. The near instantaneous abolition of muscle twitch with injection of small volumes of local anesthetic is consistent with an electrostatic effect, rather than a pharmacological or mechanical one. The change in field strength depended upon the volume of the injectate and its conductivity relative to that of the surrounding tissue. In this simulation, even tiny volumes of injectate lead to significant changes in field and therefore threshold current, which may have clinical implications.