Monitoring Setup, Procedures, and Problem Solving in the Operating Room
Setup discipline
Electrode placement follows the 10-20 system, with interelectrode distances measured rather than estimated. Subdermal needle electrodes for recording, corkscrew electrodes for transcranial stimulation, and surface or needle pairs over the muscle belly for MEP recording. Target impedances are low (commonly ≤5 kΩ, ideally 1–3 kΩ); high impedance invites 60 Hz noise and unreliable traces. Peripheral stimulation sites are over the nerve where it is superficial: median and ulnar at the wrist, tibial at the ankle, peroneal at the fibular head. Establish and save a reliable baseline before incision, and document the anesthetic and physiologic conditions at that moment.
Systematic troubleshooting
Localize the problem by scope. One channel abnormal with neighbors intact → suspect that electrode (impedance, dislodgement, sweat, drying). One entire limb's responses lost → think peripheral: tourniquet, limb malposition, nerve compression, stimulation failure. All evoked responses lost while ECG artifact persists → suspect the stimulator, its connections, or stimulation settings. Everything lost including ECG → amplifier, headbox, cable, or power. Other classic confounders include electrocautery artifact (suspend acquisition), pneumocephalus after posterior fossa surgery (delayed BAEP/SSEP changes), and systemic hypotension or hypothermia.
Traps: chasing a change in a channel that never had a reproducible baseline; adjusting filters to hide a real deterioration; failing to check the ground and reference when noise appears.
Rule of thumb: "One channel = electrode; one limb = peripheral; everything = machine." Work outward from the most localized explanation and confirm before notifying the team, but never delay reporting a genuine global loss.