Background: Successful peripheral nerve blocks require accurate placement of the injection needle tip before local anesthetic application. In this investigation, we experimentally reconstructed polarity-dependent (anode and cathode) stimulation maps using ex vivo and in vivo animal models.
Methods: A novel ex vivo configuration (muscle-nerve composite) was first used to probe both cathodic and anodic stimulation characteristics. The electrophysiology (compound nerve action potential, CAP) of rat sciatic nerve was recorded at varying stimulation (monopolar electrode) distances and intensities. We repeated this methodology with an open dissection rat model that was more analogous to the clinical setting. Resultant data from the current sweeps were plotted as a 3-dimensional distance-stimulus-CAP map. These plots depict the minimum stimulation currents required for nerve activation and describe the expected electrophysiological outcomes as a function of distance and input stimulus intensity. The stimulation maps provide positional information relevant to clinical procedures such as nerve localization during regional anesthesia.
Results: Cathodic stimulation produced a complex biphasic electrophysiological response. The CAP amplitude (with fixed current) increased as the electrode moved closer towards the nerve, but decreased upon close proximity or nerve contact. This phenomenon was dependent upon stimulation intensity and was observed in both ex vivo and in vivo models. Anodic stimulation produced a monotonic relationship, with the CAP increasing with closer electrode-to-nerve distances. Minimum extraneural activation thresholds were found to be 0.34 ± 0.11 mA (mean ± sd) and 0.63 ± 0.12 mA for cathode and anode stimulation, respectively. Intraneural thresholds were substantially lower, 0.12 ± 0.03 mA and 0.32 ± 0.09 mA, for cathode and anode, respectively.
Conclusion: Cathodic stimulation may produce conduction block at close tip-to-nerve distances. In contrast, anodic stimulation elicited output characteristics that were predictable and more suitable for nerve localization. We believe anodic stimulation is a viable option at near-nerve distances, despite the increased current requirements. This hypothesis is a paradigm shift in stimulation nerve localization, which conventionally has been cathode based. The hypothesis should be clinically validated.