Impedance as guidance for electrode placement in intraoperative monitoring of nerve fibers

Abstract

Electrodes for intraoperative monitoring should be reliable and should not disturb the surgeon. Since any wiring could complicate the handling of other medical instruments, new developments of autonomic, wireless electrodes are preferred. This new generation of electrodes will not only stimulate the nerve, but will monitor the action potentials as well. A failure to read nerve potentials may be indicative not just of damaged nerves, but may also result from bad electrode-nerve contact. To resolve this, we have developed electrodes which are equipped with impedance measurement features, facilitating simple connectivity checks. Due to energy constraints, the system works in the time domain using a rectangular voltage wave excitation. The voltage at the electrode is sampled and transmitted via RF link to the host computer. The frequency range covered by the excitation and sampling is between 100 Hz and 50 kHz, which is sufficient not only for detecting contact failure, but also for detecting thick layers of connective tissue between the electrode and the nerve fibre. In either instance, the surgeon can be warned of a bad electrode placement.