Signal Artifact: Intrinsic, Extrinsic
or Pathological?
Simone de Lacy, RPSGT, Murcia, SP
ESST Meeting, Paris 2012
Signal artifact
• Signal artifact refers to extraneous signals
appearing within desired or pure signal
waveforms
• Some artifact is intrinsic (i.e. generated within
the body; physiological/ Pathological)
• Some artifact is extrinsic/environmental
• Artifact can be undesirable and low frequency
(LFF) and high frequency filters (HFF) can be
used to attenuate it but as a last resort.
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Intrinsic and Extrinsic Artifact
Intrinsic Artifact
Extrinsic Artifact
Physiological:
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Muscle: HF artifact
Eyes movements, Blinks,
Heart: ECG pickup
Sweat: LF artifact
Respiration: LF artifact
High impedance
Electrode ‘popping’
Mains interference
Polarity reversal
Over amplification
Incorrect referencing
Pathological:
Bruxism, excessive spindling,
seizures, parasomnias etc
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Intrinsic artifact examples
Artifact: Eye movement pick-up on EEG
Eyes open left to right
Eyes open up and down
Eye blinks
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Artifact: HF muscle activity on EEG and EOG signals as
well as increased Chin EMG
Jaw clenching during bio-calibration
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Artifact: ECG pick-up on single channel leg EMG
Cause:
Physiological: One electrode on L leg, one on R leg gives summation of EMG
activity on both legs but also acts as 2 lead ECG channel.
Solutions: Use 2 electrodes on each leg, separate L Leg EMG from R Leg
Alternatively, record using 2 electrodes on 1 leg only-but this may miss or
underestimate severity of PLMD
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Artifact: ECG pick-up on EEG Channels
Cause:
Physiological: M1 and M2 electrodes situated on soft tissue rather
than mastoid bone
Solutions: Reposition M1 and M2 electrodes
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Artifact:
Cause:
Actions:
Sweat artifact O2M1, LF artifact on EEG
Physiological: Patient too hot, sweat affecting impedance
1. Cool the patient down
2. Apply LFF to this channel
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Artifact: Cyclical, HF artifact all AC channels
Cause: Pathological: Cyclical muscle activity caused by bruxism
Actions: None necessary, Tech comment and print epoch for PSG report
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Artifact:
Cyclical bursts of faster EEG waveforms
Cause:
Pathological: Excessive sleep spindles, often seen in
benzodiazepine use
Actions: None necessary, Tech comment and print for PSG report
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Artifact: Atypical waveforms on ECG channel
Cause:
Pathological: intermittent and runs of PVCs, cardiac arrhythmia
Solutions: None necessary, Tech comment and print epoch for PSG report
NB: A prolonged run of PVC’s may be cause for emergency protocol
initiation especially if there is no mention of cardiac problems in the
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patient’s notes.
Artifact: Sudden HF artifact on all channels
Cause:
Pathological: Sudden spontaneous arousal from SWS as in night terror
Actions: None necessary, Tech comment and print epoch for PSG report
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Artifact: HF artifact superimposed on LF waveforms
during sleep on EOG/EEG channels
Cause: Pathological: Movement during N3 sleep as in sleep walking.
This is muscle activity on delta sleep.
Note patient is in N3 sleep before and after 14 second event
Actions: None necessary, Tech comment and print epoch for PSG report
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Artifact:
High amplitude spike and wave artifact on EEG
High muscle tone on Chin and Leg EMG
Cause: Pathological: Nocturnal Seizure
Actions: Follow protocol for seizures, Tech comment and print epochs for PSG
report
NB Prolonged seizures (>5mins) can be life threatening
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Artifact: Irregular, prolonged bursts of HF activity on Leg
EMG channel
Cause:
Actions:
Pathological: Loss of REM atonia as in REM Behaviour Disorder
Tech note and print epoch for PSG report
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Extrinsic Artifact
Artifact:
HF artifact on C3M2 & O1M2 Channels
Cause:
Poor electrode contact M2 (electrode common to both channels)
Solutions: 1. Reapply electrode
2. Apply 50Hz notch filter to M2
3. Reference C3 and O1 to M1 electrode
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Artifact:
Respiration artifact, LF artifact on O1M2
Cause:
O1 lead too tight, being pulled on inspiration, affecting impedance
Solutions: 1. loosen the lead from the bundle at the top of the head
2. Apply LFF to this channel
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Artifact: Inverted ECG signal
Cause:
ECG Electrodes correctly positioned but leads plugged into wrong
polarity port + : Solutions: 1. Invert signal display for this trace
2. Swap ECG leads into correct polarity port - : +
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Artifact:
Cause:
Solutions:
HF artifact on Leg EMG channel
Poor electrode contact, electrode probably displaced
Reapply electrode
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Artifact:
HF artifact on Chin EMG channel
(note patient is in Stage R so would expect very low amplitude chin EMG)
Cause:
Chin EMG gain set to high during bio-calibration or poor electrode
contact
Solutions: 1. Check bio-calibration signals
2. Reduce gain for Chin EMG
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3. Re reference chin EMG electrodes
Artifact: brief bursts of activity on Leg EMG channel
(note additional ‘intrinsic’ ECG pick up)
Cause:
Gain (sensitivity) set too high on Leg EMG channel
Solutions: 1. Reduce gain on leg EMG channel
2. Check bio-calibration signals
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Artifact:
Irregular abdominal effort signal
Cause:
Abdominal band too loose, gain turned up too much to compensate
Solutions: 1. Tighten Abdominal belt and then reduce gain
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Artifact:
Periods of airflow and effort cessation alternating with periods of
airflow with ‘paradoxical’ respiratory effort
Cause:
Abdominal band polarity reversed (piezo-electric belts)
Solutions: Reverse polarity of abdominal belt (see below)
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Artifact: poor signal with intermittent loss on thoracic
effort sensor
Cause:
Faulty thoracic effort sensor
Solutions: Replace effort sensor
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Artifact:
LF artifact on C3M2channel
Cause:
Air trapping or poor contact of C3 electrode causing electrode ‘popping’
Solutions: 1. Reapply electrode if necessary
2. Remove trace from recording view if C4-M1 giving good signal
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Artifact:
LF artifact on C3M2 and O1M2 channels
Cause:
Air trapping or poor contact of M2 electrode causing electrode ‘popping’
Solutions: 1. Reapply electrode if necessary
2. Re-reference C3 and O1 to M1
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Golden Rules:
• Keep your equipment in good working
• Check and replace any faulty electrodes/sensors
• Good skin preparation and careful application of
electrodes and sensors is paramount
• Always perform bio-calibration
• Re-check impedances prior to lights out
• Don’t wake the patient up to change electrodes
unless really necessary
• Use filters to attenuate artifact as a last resort