A mathematical model of peripheral nerve stimulation in regional anesthesia
A common technique in regional anesthesia uses peripheral nerve stimulation (PNS) to position the tip of a needle in close proximity to a target peripheral nerve, where the submerged, uninsulated portion of the needle shaft serves as the electrode. The safety and efficacy of the PNS technique may be improved with a better understanding of the interactions between the stimulus current from the needle electrode and the nerve. However, the relevant data are difficult to obtain in vivo.
A computational model was developed to explore needle-nerve interactions in the context of nerve localization guided by PNS. The needle electrode was represented by a series of equally spaced current points, with the total delivered stimulus distributed among the current points to approximate the isopotential needle surface. The skin surface and a planar inhomogeneity, or boundary, in tissue conductivity were included. The nerve was modeled as a single myelinated fiber positioned below the boundary in conductivity.
An interactive software package was designed based on the model framework. The software was used to explore the effects of the boundary in conductivity on the distribution of needle stimulus current, the resulting potential field, and how the current thresholds for nerve activation change with needle position. The latter current-distance data were generated for a bare, uninsulated needle and a needle fully insulated except at its tip, for both stimulus polarities. When the electrode was attached to the cathode and positioned close to the nerve fiber, suprathreshold current amplitudes above a critical value resulted in block of the nerve impulse.
Model data were compared to clinical and experimental data available from the literature. Despite some limitations, the model provided results consistent with past work as well as with established conventional wisdom. Therefore, the simulation software appears to be a valid tool for studying PNS and nerve localization; it can be used to explore various needle designs and test novel nerve localization protocols.