Impedance changes in chronic vagus nerve stimulator implantation in epileptic dogs

Computational modelling of the electrical fields generated by helical electrodes surrounding the vagus nerve suggested a current of 1.0 mA should depolarise enough axons to give a therapeutic effect in epilepsy. An assumption that fibrous tissue formation would form between the electrodes and the nerve in implantable vagus nerve stimulators (VNS) led to the recommendation that 1.5 mA should be used to account for the increased impedance this would cause. This increase in impedance has not been documented, meaning that the higher current might not be necessary and could produce avoidable adverse effects such as coughing. To test the hypotheses that impedance does not always increase following VNS implantation and that more adverse effects might be seen with lower impedance circuits, circuit impedance was measured in dogs with implantable VNS at implantation and compared to impedance at 3-6 months post-implantation and at non-standardised time points thereafter. Dogs were grouped by severity of their coughing and the impedance between groups at 3-6 months was compared. Mean impedance at first implantation (2219Ω, 95% CI 1894-2544) was no different from impedance at 3-6 months post implantation (2257Ω, 95%CI 1944-2570, p = .80). Only 2 of 17 dogs showed a > 50% increase in impedance (one at 3-6 and one at 7-12 months). Coughing severity was not influenced by impedance value despite similar stimulation currents. This work shows impedance does not consistently increase post implantation in dogs with VNS, suggesting that currents > 1.0 mA might not be routinely needed to achieve a therapeutic effect.