3.1 Montage, 10-20 System & Electrode Application

Why the Montage Matters for the RPSGT Exam

Sleep Study Preparation and Performance is the largest domain on the Registered Polysomnographic Technologist (RPSGT) examination at 27.3% of scored items per the Board of Registered Polysomnographic Technologists (BRPT) exam blueprint (tied with Treatment and Intervention, also 27.3%). More questions test how you derive a montage, measure head sites, and achieve clean impedances than test any single scoring rule. The RPSGT exam is a 175-item computer-based test (150 scored plus 25 unscored pretest items) delivered in 180 minutes, with a scaled passing score of 350 on a 200-500 scale.

A study can only be scored as accurately as it was acquired, so the BRPT expects you to build a recording that meets AASM technical specifications before the patient ever falls asleep.

A montage is the specific set of derivations (electrode pairs) displayed on the recording. Each channel shows the voltage difference between an exploring electrode and a reference electrode. Because PSG channels are differential amplifiers, a clean reference is as important as the exploring site, and unbalanced impedances destroy common-mode rejection.

The Recommended PSG EEG Montage

The AASM recommends three frontal, central, and occipital EEG derivations referenced to the contralateral mastoid:

Backup derivations F3-M2, C3-M2, and O1-M2 mirror the left hemisphere so the study stays scoreable if a primary electrode fails mid-night. Mastoid references are labeled M1 (left) and M2 (right); the older 10-20 labels for these sites are A1 and A2. The AASM also specifies a minimum EEG sampling rate of 200 Hz (desirable 500 Hz) so spindle and slow-wave morphology survive digitization.

Worked Example

Suppose C4 pops free at 1:30 a.m. and cannot be re-secured without waking the patient. You switch staging to the C3-M2 backup derivation. Because the backup mirrors the same region on the opposite hemisphere, spindle detection and arousal scoring continue uninterrupted — provided you logged the change with a timestamp so the scorer knows which channel was active. This is exactly why the AASM mandates a full backup chain rather than a single C-channel.

The International 10-20 System

The International 10-20 System is a standardized method for placing scalp electrodes so recordings are reproducible between technologists, nights, and labs. The name refers to the spacing: each electrode sits at 10% or 20% of the total measured distance between defined skull landmarks. Because every site is a percentage of the individual's head, the system self-scales to infants and adults alike.

Bony Landmarks

• Nasion — the depression at the bridge of the nose, between the eyes
• Inion — the bony bump at the back of the skull (external occipital protuberance)
• Preauricular points — the depressions just in front of each ear canal
• Vertex (Cz) — the true top of the head, found where the front-to-back and ear-to-ear lines cross

Naming Convention

• Letters indicate the brain region: F = frontal, C = central, O = occipital, P = parietal, T = temporal, Fp = frontal pole
• Odd numbers (1, 3, 5) are on the left hemisphere; even numbers (2, 4, 6) are on the right
• z denotes a midline electrode (Fz, Cz, Pz)

Core Measurement Steps

1. Measure the anterior-posterior distance from nasion to inion over the vertex.
2. Mark Fpz at 10%, Fz at 30%, Cz at 50%, Pz at 70%, and Oz at 90% along that line.
3. Measure the transverse distance between the two preauricular points through Cz; true Cz is where both measurements intersect.
4. Measure the circumference and place the temporal/lateral chain at the matching 10% and 20% increments.

Using a flexible tape and a skin-marking pencil, the technologist must measure rather than estimate. A misplaced central electrode shifts spindle amplitude; a misplaced occipital electrode weakens the posterior dominant rhythm needed to identify wake and stage N1, both of which propagate into sleep-stage scoring errors.

Common Traps

• Eyeballing Cz instead of crossing both measured lines — the most common placement error.
• Reversing odd/even numbering (left vs. right) — mislabels the whole montage.
• Forgetting that F4 references M1 (contralateral), not the same-side mastoid; same-side referencing reduces the captured signal and invites ECG contamination from the nearby carotid.

Electrode Types, Application & Impedance

Electrode Types

• Gold cup electrodes — durable, low-noise, used for EEG/EOG; filled with conductive paste
• Silver/silver-chloride (Ag/AgCl) — stable low-offset potential, excellent for slow signals such as DC-coupled recordings
• Adhesive snap (disposable) — common for electrocardiogram (ECG) and limb electromyogram (EMG)

Application Technique

1. Locate and mark the site using 10-20 measurements.
2. Prep the skin with a mild abrasive prep gel to remove dead skin and oils, lowering impedance at the electrode-skin interface.
3. Apply conductive paste in the cup, place the electrode, and secure with a gauze square or collodion (a fast-drying adhesive used for long studies and pediatrics).
4. Strain-relieve lead wires and bundle them so head movement does not pop electrodes.

Impedance Targets

Impedance is the opposition to alternating current at the electrode-skin interface, measured in kilohms. High or unequal impedances cause 60 Hz (mains) artifact, electrode pop, and signal attenuation.

The AASM standard is concrete: re-prep and reapply any electrode that exceeds 5 kΩ rather than accepting a noisy channel. Equally important, impedances must be balanced — a 4 kΩ exploring electrode paired with an 18 kΩ reference produces 60 Hz contamination even though one electrode is technically in range, because the mismatch defeats the amplifier's common-mode rejection.

Worked Scenario

During impedance check the C4 site reads 12 kΩ. You re-abrade the scalp, add fresh paste, and re-seat the cup; it now reads 3 kΩ, matching its M1 reference at 3 kΩ. The pair is now both low and balanced. Always resolve impedance problems before lights out — fixing a popped occipital electrode at 2 a.m. risks waking the patient and losing data, and the BRPT explicitly tests the principle that prevention beats mid-night repair.