Regular Sharp Deflections Time-Locked to the QRS
At 02:40 on a diagnostic study you notice sharp, regular downward deflections in F3-M2, C3-M2, and both chin channels. Each deflection lines up with the QRS complex on the EKG channel, which shows a rate of 78 bpm. The deflection amplitude in the chin channel is roughly equal to the background EMG. All impedances measure under 5 kΩ and there is no 60 Hz noise visible in the other channels. The patient is a 47-year-old man with a short neck, supine, with the chin electrodes placed along the jawline. He is asleep with stable SpO2 of 96% and no apneas.
1. Identify the artifact and describe the steps that confirm your identification.
Read the worked answer
EKG artifact (cardiac pulse artifact), not cortical or muscle activity.
Confirmation steps:
- Timing: measure the R-R interval (78 bpm ≈ 0.77 s) and check that the deflection occurs at a fixed latency after each QRS in every affected channel. Physiology that is time-locked to the heart is cardiac in origin.
- Distribution: it appears in channels whose active or reference electrode sits near a pulsating vessel (jawline, mastoid/ear reference) - consistent with a pulse artifact or a reference contaminated by cardiac field.
- Montage test: switch the chin to a bipolar derivation (chin1–chin2). If the artifact disappears, the source is the shared reference; if it persists, it is at the active electrode.
- Look at the raw EKG trace to confirm the QRS is genuine and not itself an artifact.
Rule out true periodic EEG activity (it would not be locked to the QRS) and 60 Hz (that would be continuous and higher frequency).
2. Why is filtering a poor solution here, and what is the correct fix?
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Filtering fails because EKG artifact is broadband: the QRS and T-wave energy spans roughly 0.5–40 Hz. The notch filter only targets 60 Hz, and tightening the high- or low-frequency filters to chase the artifact will distort the very EEG and chin EMG you need to score.
Correct fix - treat the source:
- Re-prep and move the offending electrode off the vessel (away from the facial artery along the mandible; off the mastoid if the reference is the culprit).
- Re-check impedance after repositioning.
- If a reference is contaminated, change to the other ear/mastoid or use a bipolar derivation.
- Re-verify with the patient in his sleeping position.
Never mask artifact by deleting or editing the raw data; if it cannot be fixed, it is documented and the physician interprets with that caveat.
3. Do you wake the patient to fix this? What exactly do you document?
Read the worked answer
Judgment call: the artifact is degrading data but is not a patient safety issue, so do not wake him. Wait for a spontaneous position change or a brief arousal, then re-prep. A short, careful intervention during an arousal costs far less sleep than waking a patient at 02:40.
Document:
- Onset time (02:40) and which channels were affected.
- Your identification of the artifact as EKG in origin.
- The intervention and time performed, plus post-fix impedance values.
- A note that affected data before the intervention should be interpreted with caution.
- That the patient was not awakened and no distress was observed.