High Impedance in Braided, Oiled Hair Before Lights Out
Priya, 34, BMI 41, presents for a diagnostic PSG. Her hair is in 12 tight box braids coated with heavy oil sheen, and she has a short, thick neck. At the 21:00 impedance check you record: F4-M1 14 kΩ, C4-M1 3 kΩ, O2-M1 5 kΩ, E1-M1 6 kΩ, E2-M2 7 kΩ, chin1 19 kΩ, chin2 21 kΩ, left leg 4 kΩ, right leg 6 kΩ. She tells you she can only fall asleep on her right side and that the back of her head is already sore from the electrodes. Lights out is scheduled for 22:30 and she is anxious about "failing" the test.
1. Which channels must be re-prepared before lights out, and how do you justify the decision to a colleague who says "anything under 20 kΩ is fine"?
Read the worked answer
Re-prep required: F4-M1 (14 kΩ) and both chin channels (19 and 21 kΩ).
Reasoning:
- The accepted performance target at the start of recording is roughly ≤5 kΩ per electrode. Above that, the electrode pair becomes an antenna for 60 Hz and for movement/sweat artifact, and the signal amplitude drops.
- Chin EMG is not a cosmetic channel. Scoring REM depends on demonstrating lowest chin EMG tone of the night, which means resolving low-amplitude activity. A 20 kΩ chin pair can attenuate the true signal enough that REM is missed or over-scored.
- The 14 kΩ frontal channel is also badly mismatched against C4-M1 at 3 kΩ; large inter-electrode impedance differences degrade common-mode rejection and worsen artifact.
- O2-M1 (5 kΩ) and right leg (6 kΩ) are borderline. Re-check them after the re-prep rather than assuming they are fine.
A 20 kΩ threshold is a myth; it is a tolerance for a channel you have already given up on.
2. Describe how you will get a stable F4 and chin placement without undoing the braids or shaving her, and what you will document.
Read the worked answer
Scalp (F4-M1): part between braids with a tail comb to expose a small patch of scalp; clean with the prep solution; gently abrade with grit paste; apply conductive paste; lay the hair back down and secure the strands with tape or gauze so the braids are not pulled. Avoid collodion where the patient cannot remove it easily. Do not cut braids.
Chin: do not shave a patient without consent. Clip hair close if necessary, degrease the skin, place two electrodes about 1 cm apart on the mandible below the lower lip, avoiding placement directly over a palpable arterial pulse.
Document: time, sites re-prepped, the before/after impedance values, technique used, patient tolerance, and any consent conversation. Numerical documentation is what lets the interpreting physician judge signal quality.
3. She can only sleep on her right side. How does that change your setup and monitoring plan?
Read the worked answer
Right-side sleeping puts pressure on the right-sided and occipital electrodes and compresses the arm/leg sensors on that side.
Plan:
- Give extra mechanical security to O2-M1, E2-M2, and the right leg lead (paste plus tape, strain relief loop).
- Re-check impedance with her lying in her actual sleep position, not just sitting up. Pressure and skin displacement routinely push a "good" electrode over 5 kΩ.
- Use a pillow arrangement that keeps the occipital area off hard contact points; a sore head is a compliance problem that turns into a 4-hour study.
- Re-verify flow, effort, and SpO2 once she is settled on her side, since position changes can unseat thermistors and oximeter probes.
Document the re-check and her position. A technically clean setup that drives the patient to sleep supine against her habit produces a non-representative night.