A (quick) overview of polysomnographic artefacts

1
A (quick) overview of polysomnographic artefacts

• Jo Tiete

Centre Hospitalier Luxembourg
2
polysomnographic artefacts

• 1
• Subject Induced Artefacts
• or
• Physiological
• Artefacts

3
Skin artefact

• Symptoms
• The skin is considered to contribute the greatest
  possible distortion of brain potentials.
• The factors that effect skin impedance are its
• thickness,
• the degree of cellular hydration,
• the number of sweat glands
• and hair follicles which provide low-resistance
  pathways into the skin.

4
Skin artefact

• Actions
• Proper preparation of the skin is the sleeptech
  most important concern.
• Degreasing with aceton/ether.
• Rubbing the skin (peeling), but with care for
  the subject and for the sleeptech! (blood born
  viruses).

5
Sweat artefact

• Symptoms
• Low frequency baseline swings that may affect one
  or more channels.
• Is due to transpiration and is a combination of a
  salt-bridge between electrodes and the skin.

6
Sweat artefact

• Actions
• Identify the electrode and eliminate it by making
  appropriate changes to the channels input
  selector or replace electrode.
• Cool the subject, by reducing the room T, use a
  fan or remove blankets.
• Place an absorbing towel under the subjects
  head.
• Use an anti-perspirant aerosol on the area around
  the electrode.
• As last resort, you can reduce the low-frequency
  filter.

7
ECG artefact

• Symptoms
• Is most prominent in subjects who are obese or
  have short, thick necks.
• Often when using ear reference montages (A1-A2)
  and if the input impedance is unbalanced.
• If EKG occurs on the EEG channels, you need to
  eliminate it to avoid misinterpretation.

8
ECG artefact

• Actions
• Reposition or move the mastoid reference (A1
  A2) electrodes to the ear lob.
• Link the 2 mastoids (A1lt-gtA2) with a jumper
  cable, doing so will most likely alter the
  amplitude of the waveforms.
• Lowering the high-freq filter is not a good
  practice!

9
Pulse artefact

• Symptoms
• It is a mechanical artefact that is picked up
  because the recording electrode is positioned
  over an artery.
• Monitoring EKG is useful.
• R phase of EKG wave occurs at the same point in
  each EEG slow wave.

10
Pulse artefact

• Actions
• Reposition the subjects head.
• Switch the amplifiers input configuration.
• Ad more electrolyte gel to the electrode.
• Reposition the electrode.

11
Cardio ballistic artefact

• Symptoms
• Cardiac pumping is detected on respiratory
  flow/effort channels during apneas.
• Related to pulse artefact.
• Also on oesophagial pressure channels but
  sometimes on any high impedance EEG channel.
• Mostly seen in thin/skinny individuals.

12
Cardio ballistic artefact

• Actions
• Repositioning the subjects head.
• Raising the subjects head off the bed with a
  roll of towels may eliminate it.
• Eliminate artefact by repositioning the
  oesophageal pressure catheter.

13
Pacemaker artefact

• Symptoms
• This artefact is due to an electrical pulse of
  the pacemaker.
• Can be periodic or intermittent and may appear in
  one or more channels.
• May look like spikes or sharp waves in the EEG.

14
Pacemaker artefact

• Actions
• Can not be eliminated without stopping the
  pacemaker.
• Monitor the ECG channel to identify.

15
Eye movement artefact

• Symptoms
• Eyes have a strong polar field (cornea-fundal
  cornea-retinal potential).
• Is of sufficient strength to introduce slow wave
  artefact into frontal temporal EEG signals.
• But useful for detecting stage 1 or REM.

16
Eye movement artefact

• Actions
• There is no correction for this artefact.

17
Eye blink artefact

• Symptoms
• Same process as eye movement artefact, but always
  in awake.
• Blinking artefact can pollute the EEG tracings
  but are useful because the frequency of eye
  blinks decrease with drowsiness prior to sleep
  onset.

18
Eye blink artefact

• Actions
• There is no correction for this artefact.

19
Unilateral REM

• Symptoms
• Subject has lost functionality of one eye.
• One EOG electrode is suspected of poor quality
  or high impedance.

20
Unilateral REM

• Actions
• Ophtalmic exploring (glass eye ?).
• Verify or change bad electrode, if electrode is
  the cause.

21
Movement artefact

• Symptoms
• High amplitude slow waves sometimes with
  amplifier blocking in the EEG channels.
• During a position change or limb movement or the
  characteristic head, neck and mandible movements
  seen in OSA.
• Cause electrode popping and amplifier blocking.

22
Movement artefact

• Actions
• Wrap electrodes and transducers together and form
  a neat bundle (poney tail) to reduce this type of
  artefact.
• Avoid suspending the head box over the beds
  headboard with all wires in bed.
• Better have the electrode wire bundle rest on the
  bed without the subject laying on the wires.

23
Tremor artefact

• Symptoms
• 4-8 Hz that can be confused with theta.
• In Parkinsons disease.
• Decrease with sleep onset but tremors may remain
  present during stage two or reappear during REM.
• Plethysmography belts can also pick up this
  artefact.

24
Tremor artefact

• Actions
• Repositioning the subjects head to avoid contact
  with the bed will reduce but not eliminate this
  artefact.
• Nothing that can be done to eliminate tremors
  detected by inductive plethysmography belts.

25
Body rocking Head banging artefact

• Symptoms
• Both disorders produce a periodic occurrence of
  movement artefacts.
• Sometimes with amplifier blocking which in most
  cases affects all EEG channels.

26
Body rocking Head banging artefact

• Actions
• There is no correction for this artefact.
• Bundle electrode wires.

27
Muscle artefact

• Symptoms
• Can appear in all channel.
• Can mimic cortical spikes If continuously
  present.
• Can be used to identify bruxism, movement
  arousals, vocalization, motion, tension, etc.
• Can render portions of the record unscorable.

28
Muscle artefact

• Actions
• There is no correction for this artefact.

29
Respiration artefact

• Symptoms
• Low frequency baseline sway in EEG.
• Occurs in-phase with the respiration tracing.
• Artefact is positional.
• Caused by body movements associated with
  respiration.

30
Respiration artefact

• Actions
• repositioning the subjects head or the electrode
  wires.
• Keep wires bundled and from under the subjects
  head or body.

31
Fish-mouthing artefact

• Symptoms
• False "breaths" recorded by thermal airflow
  probes.
• Misclassify obstructive apnoea as an obstructive
  hypopnoea.
• Produced by air being drawn into the mouth as the
  mandible retracts during a blocked inspiration.

32
Fish-mouthing artefact

• Actions
• There is no correction for this artefact.

33
Snoring artefact

• Symptoms
• Mostly, but not only, in EMG.
• Indicate an increase in respiratory drive.
• Sufficient to incorporate a secondary inspiratory
  musculature.
• Is considered as a physiological response to
  increased pCO2 levels.

34
Snoring artefact

• Actions
• There is no correction for this artefact.

35
Gurgle artefact

• Symptoms
• Appears as spindle-like activity in the EEG
  channels on each inhale or exhale. 
• Seen in comatose subjects. 
• Can affect the estimated airflow signal. 
• Can occur when CPAP machine is used with a
  humidifier in a cold room (condensation).

36
Gurgle artefact

• Actions
• Swallowing clear the airway, but comatose subject
  may require suctioning of the airway.
• When present in the estimated airflow channel of
  a CPAP you need to drain the condensed water from
  the hose. 
• It may also be helpful to insulate a portion of
  the CPAP tubing (decrease condensation). 
• Increase the room temperature in order to reduce
  condensation.

37
Genioglossal artefact

• Symptoms
• The subjects tongue is an electrically polarized
  mass that can introduce slow wave artefact into
  EEG channels. 
• Raising the tip of the tongue to the roof of the
  mouth can generate a potential of 100 micro volts
  at the vertex. 
• This artefact has been described as resembling
  intermittent, rhythmic delta.

38
Genioglossal artefact

• Actions
• There is nothing you can do to prevent this
  artefact.

39
polysomnographic artefacts

• 2
• Artefacts that arise
• from subject
• or
• the recording
• Equipment.

40
Bi-metallic artefact

• Symptoms
• Blocked Ionic flow between different metals that
  discharge suddenly.
• Oral appliances, dental fillings can produce an
  intermittent artefact that presents as sharp
  waves or spikes. (subject).
• If different metals, scratched silverchloride or
  gold electrode are used. ( environment).

41
Bi-metallic artefact

• Actions
• Record the discharges on a separate channel by
  positioning electrodes over the location of the
  dental fillings, or metal components of
  appliances to identify the artefact.
• Chloride the scratched (silver) electrode.
• Discard scratched gold electrodes.
• Use of  disposable  electrodes.

42
polysomnographic artefacts

• 3
• Recording Equipment
• or
• Environmental
• Induced Artefacts.

43
50 or 60 Hz Artefact

• Symptoms
• 50 (60Hz) interference was very common on analog
  amplifiers (bad common mode rejection).
• Source is environmental, but can also indicate
  malfunction or improper installed equipment. 
• Electromagnetic radiation from the buildings
  electric wiring and from devices powered with
  these wires.
• At the frequency of the alternating current.

44
50 Hz Artefact

• Actions
• Check impedance and balance impedance between
  electrodes, specially Ref. or ground.
• Apply gel or place fresh electrode(s).
• Verify if electrode cables are not lose and
  bundle them.
• All conductive surfaces (antennas!) and electric
  appliances near subject should be grounded to the
  same ground.
• Keep bio-signals and power leads far away.
• Better a non-motorized wooden than a metal bed.
• Dont make loops in power cords.
• Use a  Fahraday cage .
• As a last resort, you can use the 50/60Hz notch
  filter.

45
Salt-bridge Artefact

• Symptoms
• When excessive amount of electrode gel spreads
  between two electrodes forming a conductive
  pathway.

46
Salt-bridge Artefact

• Actions
• Switch to a backup electrode.
• Cleaning the skin surface between the electrodes.
• Remove the electrodes, clean the skin then,
  reapply the electrodes.

47
Amplifier-blocking artefact

• Symptoms
• Excessif amplification leads to signal clipping
  or amplifier blocking.
• Pen deflection is maximal for an amount of time.
• No interpretation possible on one or more
  channels.

48
Amplifier-blocking artefact

• Actions
• Reduce the channels sensitivity.
• Wait till channels set.

49
Electrode-popping artefact

• Symptoms
• Is usually intermittent.
• Can be mistaken as spike trains, sharp waves and
  even K-Complexes.
• The cause is most often a loose electrode or
  broken electrode wire (corrosion).

50
Electrode-popping artefact

• Actions
• Switch the input to a backup electrode. 
• Repair or replace the electrode who suffer from
  high impedance.
• Change the broken or corroded electrode lead.

51
Supply line artefact

• Symptoms
• Resemble as a spike and may affect one or all of
  the channels.
• Referred to as a power surge.
• Power supply is unstable or is placed under heavy
  load by the current demand (on/off switch).

52
Supply line artefact

• Actions
• Inexpensive commercial filters are available and
  recommended.
• Unplug devices that share the polygraphs power
  source.

53
Static electricity artefact

• Symptoms
• Surrounds us and on a polygraphic recording.
• An environmental artifact that can appear on
  differential channels.
• Isolated slight baseline sway or as a spike. 
• It may also appear as a spike on any DC channel.

54
Static electricity artefact

• Actions
• Increase the relative humidity in room.
• Use commercial aerosol sprays designed to
  neutralize static charges (on the blankets).
• Use proper grounding techniques.

55
Aliasing artefact

• Symptoms
• results when frequencies from separate sources
  combine to produce an interference pattern at
  lower frequencies.
• Typical sources include hospital paging systems,
  telemetry, television and radio signals.
• In digital recordings when too low sampling rate
  are used.

56
Aliasing artefact

• Actions
• Proper grounding reduces aliasing from
  electromagnetic radiation. 
• Shielding the laboratory may be required.
• Appropriate sampling rate (gt 2 x value of the
  highest frequency) is needed to properly
  reconstruct the original waveforms in digital
  recording systems.

57
Mobile phone artefact

• Symptoms
• When mobile phone is near headbox and not
  switched off.
• Intermittant, when cellular contact operators
  antenna or visa versa.

58
Mobile phone artefact

• Actions
• Switch off cellular or keep away from headbox.

59
Telemetry artefact

• Symptoms
• Generated by the local propagation of radio waves
  (intensive care).
• Occur in periodic bursts.
• Frequently present in Inductive plethysmography
  belts.

60
Telemetry artefact

• Actions
• Balance and / or lower electrode impedances.
• Change old or corroded Inductive plethysmography
  belts.
• Shielding the laboratory may be required.

61
Photonic artefact

• Symptoms
• Applies to transducers that utilize photonic
  technology (oximetry, transcutaneous CO2 and
  photoplethysmography).
• introduced by any device that emits near-IR to IR
  light (camera infra-red light, ambient light).
• Degrade accurate reading of subjects pulse or
  SAO2.

62
Photonic artefact

• Photonic transducers should be shielded from
  exposure to extraneous photons.
• Commercial wraps for shielding oxymetry probes
  are available.

63
From your  Artefact Buster  Thank you for
your attention. Email tiete.jo_at_chl.lu