11.1 Impedance Checks & Pre-incision Baselines

11.1 Impedance Checks & Pre-incision Baselines

Quick Answer: Impedance checks confirm the electrode–skin interface can support clean recording; pre-incision baselines—taken after anesthesia and positioning stabilize—are the reference for every later alert. Document and verbally share baseline quality with the team before incision; poor baselines make both false reassurance and false alarms more likely.

Signal change recognition begins before the knife. Domain II intraoperative tasks assume you can obtain usable data, know when that data is trustworthy, and tell the team what you can and cannot see. Impedance verification and baseline acquisition are the two checkpoints that convert setup into a defensible monitoring plan.

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Why Impedance Still Matters After Setup

Chapter 5 covered electrode types and typical impedance targets (commonly below about 5 kΩ for recording contacts, with pairs kept balanced). Intraoperatively, the question shifts from "how do I apply electrodes?" to "when must I recheck, and what does a failure mean for interpretation?"

High or mismatched impedance increases susceptibility to electromagnetic interference, reduces common-mode rejection at the differential amplifier, and can make genuine neural signals look small or noisy. Raising gain without fixing contact amplifies noise and signal together—it does not restore a bad interface.

When to check (or recheck) impedance

A single pre-induction impedance printout is not a lifetime warranty. Positioning for posterior spine, cranial pins, shoulder taping, and prone-to-supine transitions routinely degrade contacts that looked excellent on the stretcher.

Interpreting impedance problems in context

• One channel high: Suggests a local electrode, prep, or lead issue—re-prep and reapply that site before declaring a neural deficit.
• Many channels high together: Suggests systemic setup failure (gel dried, wrong reference, headbox disconnect) or widespread fluid/contamination.
• Balanced but noisy: Impedance numbers can look acceptable while a nearby Bovie, warmer, or power cord still couples 60 Hz—impedance is necessary but not sufficient for clean data.
• Sudden impedance jump mid-case: Suspect electrode displacement, dried paste, fluid under a surface electrode, or a partially unplugged connector—not an abrupt bilateral cord transection.

Corrective sequence stays the same as at setup: identify the bad contact, clean/abrade as appropriate, reapply, recheck impedance and live noise, then re-acquire a short confirmation average before resuming surgery-critical interpretation.

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Pre-incision Baselines: Timing and Purpose

Baselines are the stable waveforms against which you judge amplitude, latency, and morphology change. Alert frameworks (for example, teaching rules near a 50% amplitude drop or 10% latency increase for SSEPs) are meaningless without a patient-specific reference obtained under the same anesthetic and positional conditions that will exist during risk.

Obtain baselines after the monitoring environment is real

Ideal pre-incision baselines usually require:

1. Anesthesia regimen settled — especially volatile concentration, TIVA infusion rates, and neuromuscular blockade status relevant to MEPs/EMG
2. Final surgical position — including traction, pins, padding, and limb placement
3. Acceptable impedance and noise floor on critical channels
4. Reproducible traces — enough averages or trials that peaks are identifiable, not single lucky sweeps

Baselines taken in the holding area or before prone positioning often look better than what you will have after flipping and draping. Prefer the post-position, post-induction set that matches the operative state—even if amplitudes are smaller than the "pretty" pre-flip traces.

Multi-level and multi-modality baselines

For SSEPs, capture peripheral, subcortical, and cortical peaks when the montage supports them. A preserved Erb's point with lost cortical N20 localizes differently than global loss. For TcMEPs, document which muscles respond and at what stimulator settings. For BAEP, note wave I–V identifiability and laterality. Incomplete modality coverage at baseline must be stated aloud: "We have usable left PTN cortical SSEPs but no reliable TcMEP in the left AH" is actionable information; silence is not.

Setup electrodes → impedance OK
        ↓
Induce / position / settle anesthesia
        ↓
Recheck impedance & noise
        ↓
Acquire reproducible pre-incision baselines
        ↓
Verbal + documented handoff to surgeon/anesthesia
        ↓
Incision / continuous comparison to THAT baseline

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Documenting Baselines With the Team

Communication Domain IV overlaps here: baselines are both technical data and a shared decision point.

What to communicate before incision

• Which modalities are present, absent, or marginal
• Laterality and key generator sites (peripheral vs cortical, left vs right)
• Any expected limitation from neuropathy, prior injury, hearing loss, or dense paresis
• The alert criteria your lab will use relative to these baselines
• Whether anesthesia optimization is still needed (for example, volatiles still too high for reliable MEPs)

What to document

• Time of baseline acquisition
• Anesthetic snapshot (agents, approximate MAC or infusion rates, TOF if relevant)
• Position and any special conditions (hypotension for exposure, hypothermia)
• Waveform quality descriptors ("robust," "low amplitude but reproducible," "absent")
• Screenshots or saved traces per lab policy

If the surgeon asks "are we good to go?" answer with specificity: "Cortical SSEPs are baseline bilaterally; MEPs are present in all monitored myotomes; free-run EMG is quiet" is a monitoring statement. "Everything looks fine" without modality detail is not.

When baselines are poor, negotiate next steps before critical maneuvers: optimize anesthesia, increase averaging time, add a modality, or accept that monitoring sensitivity is reduced and document that shared understanding.

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Poor Baselines: Clinical and Legal Risk

Degraded baselines do not automatically cancel monitoring, but they change how you interpret change.

Risks of proceeding without a clear reference

False negatives: If baselines are already near the noise floor, a true surgical insult may not produce a clean "50% drop" you can prove—amplitude has nowhere reliable to fall from. You must communicate reduced sensitivity.

False positives: If you chase every noisy fluctuation as an alert without reproducibility checks, the team stops trusting monitoring. Require reproducible change relative to a stable baseline before escalating.

Re-baselining: When electrodes are replaced, anesthesia is intentionally optimized, or a major positional change occurs with team agreement, acquire a new working baseline and document why. Do not quietly overwrite the original pre-incision set without annotation.

Impedance discipline and honest baseline reporting are the foundation of every later section in this chapter: you cannot distinguish surgical from systemic change, or artifact from injury, if the starting reference was never trustworthy.