10.1 BAEP Waveforms & Technique

10.1 BAEP Waveforms & Technique

Quick Answer: Brainstem auditory evoked potentials (BAEPs) are far-field responses to click stimulation that mark the auditory pathway from distal CN VIII through the brainstem. Intraoperatively you optimize insert-earphone clicks (often alternating polarity, high intensity, non-integer rate), record Cz–Ai (and related montages), and follow Wave V—and when visible Waves I and III—against the patient’s own baseline.

BAEPs (also called auditory brainstem responses, ABRs) are a core CNIM modality whenever the cochlea, vestibulocochlear nerve (CN VIII), or brainstem auditory pathway is at risk. Unlike cortical SSEPs or MEPs, short-latency BAEPs arise from few synapses in subcortical generators and therefore remain relatively stable under typical anesthetic regimens. The exam expects you to know which wave comes from where, how clicks are delivered, and how the montage is built so you can troubleshoot artifact versus true pathway change.

Why Technique Matters Before Interpretation

A “lost Wave V” is useless if the earphone fell out, wax blocked the canal, intensity was too low for pre-existing hearing loss, or stimulus artifact obliterated Wave I. Technique creates the baseline you will defend for the rest of the case. Establish identical stimulation and recording parameters for baselines and continuous monitoring so every later average is comparable.

Waves I–V: Generator Model Used in CNIM

Classic teaching maps successive vertex-positive peaks to ascending stations. Exact microanatomy is debated in research literature, but CNIM board content uses the following practical generator map:

Waves I, III, and V are the most consistent and clinically useful. Wave II may be absent in normals. Waves IV and V frequently fuse; when Wave V is poorly defined and Wave IV dominates the complex, follow the dominant peak consistently and use montage tricks (below) to clarify identity.

Near-field vs far-field framing

Scalp BAEPs are classic far-field potentials: generators are deep in the brainstem/nerve, yet electrodes on the scalp pick up volume-conducted activity. That is why amplitudes are small (sub-microvolt range after averaging) and why averaging with adequate sweeps is mandatory. Near-field recordings (for example, electrode near the cochlear nerve or electrocochleography) can supplement Wave I but are not always available; the standard far-field montage remains the CNIM workhorse.

Click Polarity and Alternating Clicks

A click is produced by a brief (commonly ~100 µs) rectangular pulse driving an earphone. Acoustic polarity matters:

• Rarefaction — initial negative pressure (diaphragm moves away), often yields slightly different Wave I morphology/latency than condensation in diagnostic labs.
• Condensation — initial positive pressure.
• Alternating — polarity flips each click so electrical stimulus artifact (and much of the cochlear microphonic) tends to cancel across averages.

In the diagnostic laboratory, alternating polarity is sometimes avoided because rarefaction and condensation responses can differ. In the operating room, alternating clicks are preferred precisely because high intensities and electromagnetic transducers create large stimulus artifact that can bury Wave I. Intraoperatively you compare each average to that patient’s earlier averages under the same polarity paradigm, so mixing polarities is acceptable when artifact reduction is the priority.

Practical polarity troubleshooting

If Wave I is obscured by artifact despite alternating polarity, check insert seating, stimulus cable routing away from recording leads, grounding, and whether intensity can be reduced slightly without losing Wave V. If morphology collapses when switching polarity modes mid-case, re-baseline rather than comparing apples to oranges.

Intensity and Rate

Intensity

Intraoperative BAEP stimuli are broadband clicks at high intensity so responses are maximized despite OR noise and possible conductive issues (fluid in the middle ear, packing, edema).

• Common targets: roughly 80–100 dB nHL (or equivalent peSPL/HL conventions used by the lab’s calibrated system).
• Pre-existing sensorineural loss may require still higher intensities within safe transducer limits.
• The contralateral ear usually receives masking white noise (often ~40–60 dB below the click intensity) to prevent cross-hearing when intensity is high.

Under-stimulation mimics peripheral Wave I delay or loss. Over-reliance on a single intensity without knowing preoperative hearing status is a common setup error.

Rate

Stimulus rate trades waveform clarity against update speed:

• Slower rates (about 5–12/s) resolve Peaks I, III, and V most cleanly.
• Many OR protocols use non-integer rates near ~11.1–21.1/s to avoid phase-locking with 60 Hz line noise while still acquiring usable Wave V quickly.
• Faster rates (toward ~30/s) can shorten average time but may degrade earlier peaks; use only when Wave V tracking is the priority and morphology remains identifiable.

Remember: changing rate mid-case changes latency norms—treat a rate change like a new baseline condition.

Transducers and Patient Setup

Insert earphones are preferred over headphones: better sterility management, less bulk in the sterile field or head holder, and more reliable coupling when the head is turned. Foam inserts must stay seated; document ear laterality carefully (left vs right stimulation channels). Blood, irrigation, or surgical draping can displace tubing—Wave I disappearance with preserved surgical context often means a technical earphone problem until proven otherwise.

Recording Montage and Filters

Standard montage logic

BAEPs are recorded with a vertex-positive convention:

• Active (non-inverting): Cz or Cz′ (slightly anterior/posterior vertex variants per lab protocol)
• Reference: ipsilateral ear or mastoid (Ai / Mi) for the classic Cz–Ai derivation that shows Waves I–V
• Additional channel: Cz–Ac (contralateral ear) often separates Waves IV and V when they are fused on Cz–Ai
• Optional: ipsilateral ear–contralateral ear or mastoid–mastoid channels to enhance Wave I

A practical multi-channel montage lets you confirm that a candidate Wave V is consistent across derivations before you call an alert.

Filters, averaging, and display

Typical BAEP bandpass settings emphasize short-latency activity (on the order of ~100–150 Hz high-pass to ~1500–3000 Hz low-pass, lab-dependent). Analysis windows are short (commonly ~10–15 ms). Because amplitudes are tiny, hundreds to over a thousand sweeps may be needed depending on noise. Display stacked trials so gradual Wave V latency drift is obvious against the pre-incision baseline.

What You Measure Continuously

Full interpeak cataloguing (I–III, III–V, I–V) is valuable at baseline and when diagnosing a change pattern, but continuous OR monitoring focuses on:

1. Wave V absolute latency and amplitude (peak to following trough)
2. Presence/absence of Wave I (stimulus delivery / peripheral integrity)
3. Presence of Wave III when reliable (pontine station)

Interpeak latency (especially I–V) separates peripheral delay (prolonged Wave I with preserved I–V) from central brainstem slowing (prolonged I–V).

Exam Focus Checklist

• Generator map I→V and why Wave V is the OR workhorse
• Alternating clicks → artifact/microphonic reduction in the OR
• High-intensity broadband clicks + insert phones + masking
• Non-integer rates and trade-offs with morphology
• Cz–Ai primary montage; Cz–Ac for IV/V separation
• Patient-as-own-control baselines with unchanged parameters