11.3 Artifact Identification & Systematic Troubleshooting

11.3 Artifact Identification & Systematic Troubleshooting

Quick Answer: Artifacts are electrical or mechanical contaminants that look like—or hide—neural signals. Learn the signatures of electrosurgery, 60 Hz interference, and electrode faults, then troubleshoot in a fixed order: confirm the finding, verify the recording chain, quiet the environment, check physiology/anesthesia, and only then attribute change to surgery.

Intraoperative recognition fails when noise is mistaken for injury or when real injury is dismissed as "just artifact." Domain II expects a disciplined approach that any competent technologist can explain—no secret vendor algorithm required.

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Common Artifact Signatures in the OR

Electrosurgery (Bovie / bipolar / monopolar)

Active electrosurgery injects broadband, high-amplitude noise into nearby electrodes and often saturates amplifiers. Typical behavior:

• Noise appears only while the pedal is down (or while bipolar is active)
• Traces may flat-line or show hash across many channels
• Averaging during active Bovie contaminates the average with non-neural energy

Response: Pause acquisition/averaging during heavy electrosurgery; resume when the field is quiet. Do not interpret a Bovie-contaminated average as an SSEP alert. If the surgeon must coagulate continuously during a critical window, say so: monitoring sensitivity is temporarily reduced.

Line-frequency (60 Hz) interference

In North America, 60 Hz (and harmonics) couples from AC power, warming blankets, blood warmers, surgical lights, microscopes, and poorly routed cables. Appearance:

• Rhythmic sinusoidal contamination
• Often worse with high or unbalanced electrode impedances
• May preferentially affect long lead runs parallel to power cords

Response priority (source first):

1. Recheck and balance impedances
2. Separate electrode leads from power cords and electrosurgery cables
3. Unplug or relocate unnecessary powered devices near the head/recording sites
4. Verify ground/reference integrity and headbox connections
5. Consider stimulus rate choices that avoid exact locking to line frequency when your lab protocol allows
6. Notch filter last — a 60 Hz notch can reduce artifact but may distort EP morphology near the notch; treat it as temporary damage control, not a substitute for finding the source

Electrode and cable artifacts

Other environmental noise

C-arm, intraoperative MRI/CT, ultrasonic aspirators, drills, and somatosensory stimulators from other teams can inject interference. Time your averages between bursts of equipment use when possible. Document unavoidable downtime.

Artifact? → Does it track a device (Bovie on/off, warmer, C-arm)?
         → Is it one channel (electrode) or all (systemic/power)?
         → Fix source before changing interpretation rules

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Systematic Troubleshooting Sequence

Use the same order every time so you do not skip technical causes or delay surgical communication.

Step 1 — Reproduce and characterize

Is the change present on the next average/trial? Unilateral or bilateral? Which peaks/muscles? Did it begin abruptly or drift? Characterization prevents treating a single noisy sweep as cord injury.

Step 2 — Technical integrity (recording and stimulation chain)

• Impedance check on affected channels
• Visual inspection of electrodes, tape, and connectors
• Confirm stimulator is delivering (peripheral twitch or known peripheral potential)
• Confirm correct montage, polarity, and that the intended channel is displayed
• Swap a suspect lead with a known-good spare if available

If technical failure explains the change, fix it, re-baseline if needed, and document—do not leave a false surgical alert hanging.

Step 3 — Environmental / artifact control

• Ask whether electrosurgery is active
• Move cables; silence unnecessary devices
• Wait for imaging bursts to finish
• Avoid interpreting during known noise epochs

Step 4 — Physiologic and anesthetic review

• MAP, temperature, EtCO₂, hematocrit/oxygenation as available
• Recent boluses, volatile changes, neuromuscular blocker redosing
• Correlate with Section 11.2 bilateral/systemic patterns

Step 5 — Surgical correlation and communication

• What maneuver is occurring?
• Communicate the pattern and your differential
• Request pause, reversal, or inspection as appropriate
• Re-acquire after intervention and report recovery or persistence

This sequence is deliberately generic. Vendor auto-alerts, proprietary SNR scores, or black-box "significance" flags may assist workflow but are not a substitute for understanding impedance, averaging, artifact signatures, and clinical correlation. On the CNIM exam, defend the physiologic and technical reasoning—not a brand-name feature.

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Averaging, Filters, and Gain: Tools With Side Effects

More averages improve signal-to-noise for random noise but help less against continuous 60 Hz or Bovie hash that is time-locked or overwhelming. Do not "average away" electrosurgery.

Filter changes can clean a display while altering latency/amplitude appearance. If you change filters mid-case, document it; large unexplained morphology shifts after a filter tweak are technical until proven otherwise.

Gain/display scale changes how big a wave looks, not how big it is. Always judge alerts from measured amplitude relative to baseline, not from a zoomed screenshot.

Rejecting bad sweeps (artifact rejection) is appropriate when your system and protocol support it—rejection should remove obvious non-neural contaminants, not silently discard the very averages that show a real surgical change. Know your lab's rules.

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Documentation During Troubleshooting

Annotate:

• Time artifact began and ended (for example, "Bovie 10:12–10:18—averaging paused")
• Impedance values before/after electrode replacement
• Devices relocated or unplugged
• Anesthetic/vital-sign interventions
• Surgical pauses and whether signals recovered

Postoperative questions often hinge on whether a flat period was technical downtime or a true neurologic event. Your annotations are the record.

Worked mini-scenario

Lower-extremity cortical SSEPs become unreadable during pedicle work. Free-run EMG shows intermittent spikes. Steps: (1) confirm Bovie is active—pause averages; (2) when quiet, impedance OK bilaterally; (3) MAP stable, no volatile change; (4) during screw stimulation, triggered EMG threshold is low on one screw—communicate possible breach. The "lost SSEP" during Bovie was artifact; the EMG finding was the surgical signal. Separating those threads is the skill this section trains.

Master artifact literacy and a repeatable checklist, and you will protect patients from both missed injury and unnecessary alarm fatigue—exactly what intraoperative signal-change recognition demands.