18
An Introduction to Artifact Recognition Part 2
By Wayne Peacock, RPSGT
T
his article supplements the information presented in the
previous issue of A2Zzz.1 More examples of several types of
common artifact are provided here. Again, it is important for sleep
technologists to remember that the timely, accurate recognition
and correction of artifact lead to the acquisition of a high-quality
and artifact-free recording. In order to achieve this, the recording technologist must pay close attention to proper electrode and
sensor application, ensure accurate signal processing and maintain
the integrity of the recording. Attention to these three things will
determine the quality of the recording obtained.
50/60 Hertz Artifact
One of the most common types of signal artifact is 50/60 Hz
frequency, which is often associated with a power source.2 See
Figure 1. The presence of this artifact can often indicate high or
poor impedances. It also may be affected by excessive electromagnetic fields in the lab environment or leakage current from
nearby electrical equipment. The primary method of eliminating
50/60Hz interference is by common mode rejection.2 Common
mode rejection applies when impedances are low and equal, and
both electrodes are producing 50/60 Hz interference. This method
should minimize or completely eliminate 50/60 Hz interference.
The patient–ground connection is also key in dissipating excessive
50/60Hz artifact.3
Occasionally these methods will not work, such as when
50/60Hz artifact appears in leg EMG channels of a patient who
is diabetic or has poor skin condition. In these cases a 50/60
Hz notch filter may be applied.2 This filter should not be used
routinely on EEG and EOG channels, which are typically set
below the 50/60 Hz bandwidth.4 The appearance of this type
of artifact in these channels may indicate poor impedances or a
damaged EEG wire. In these cases the application of the 50/60
Hz filter merely masks the problem. To correct this artifact on the
EEG/EOG leads, the technologist must identify the source of the
problem. Electrode reapplication or replacement also may correct
the problem.
Muscle Artifact
Muscle artifact often may appear in the tracing on the EEG,
EOG and occasionally the ECG channels. See Figure 2. This
artifact is generally caused by localized muscle activity close
to the electrode placement.2 This activity may appear similar
to the 50/60 Hz activity previously mentioned; however, it is
distinguished by its irregular frequency. This artifact will usually
disappear upon sleep onset and may be diminished by asking the
patient to relax and lie quietly. If the source of the artifact is identifiable, then the derivation may be changed to a backup configuration. This artifact also may be recorded from the M1 and M2
electrode when they are placed too high on the mastoids or too far
back.2 In this case the electrode collecting the artifact should be
repositioned.
Slow Frequency Artifacts
Slow frequency artifact seen in the EEG or EOG channels is
usually the result of perspiration or direct pressure to the electrode. See Figure 3. Perspiration allows for chemical changes in
the electrolyte interface between the electrode and the patient’s
skin, which causes slow-oscillating wave patterns commonly
known as “sweat artifact” or “sweat sway.”2 A similar pattern also
may be seen if a patient is lying on an electrode; this is commonly
referred to as “electrode popping.” Dirty or faulty electrodes also
can cause electrode popping. Popping artifacts on EMG channels can produce sharp, almost spike-like waves due to the higher
filter settings. Often it is difficult to distinguish between sweat
and popping artifact, although high-amplitude sweat artifact may
cause pen blocking. 2
Body movement also may cause slow frequency artifact. Generally slow frequency artifacts appear synchronous with the breathing pattern of the patient. This may be due to slight movements
associated with breathing. These slow artifacts are commonly
referred to as “respiratory artifact.”2 See Figure 4. Although this
artifact is usually associated with respiration, its underlying cause
most often can be attributed to either a chemical or mechanical
instability of the electrode. Slow wave artifact may, in many cases,
be a combination of all of the above causes.
When the artifact is isolated to a particular side of the body, as
with positional artifact, changing the input derivation to the opposite side of the body may solve the problem. If the problem is
sweat artifact and it is affecting all channels, the technologist may
attempt to cool the room or add a fan if available. Another common technique is to place a towel between the patient’s head and
pillow. This may help eliminate some of the artifact. As a last resort, raising the low frequency filter in the EEG/EOG channel(s)
from 0.3Hz to 1Hz can eliminate slow frequency artifacts.2
References
1.
Peacock W. An introduction to artifact recognition part 1.
A2Zzz. 2008;17(4):20-21.
2.
Butkov N. Recording quality and artifacts. In: Butkov N,
Lee-Chiong T, editors. Fundamentals of sleep technology.
Philadelphia: Lippincot Williams & Wilkins; 2007. p.
351-365.
3.
Butkov N. Polysomnography recording systems. In:
Butkov N, Lee-Chiong T, editors. Fundamentals of sleep
technology. Philadelphia: Lippincot Williams & Wilkins;
2007. p. 263.
Wayne Peacock, RPSGT
Wayne Peacock, RPSGT, has been in the
sleep field for 10 years and is the manager of the Sleep Disorders Center and
Neurodiagnostic Department at Baptist
Hospital in Pensacola, Fla.
4. American Association of Sleep Technologists. American
Association of Sleep Technologists technical guideline for
standard polysomnography: update 3-2008. Available to
members online at www.aastweb.org. 
A2Zzz 18.1 | March 2009
19
Figure 1. 60 Hz is shown on the RLEG channel and chin.
A2Zzz 18.1 | March 2009
 Continued on Page 20
Figure 2. Increase muscle artifact
 Continued from Page 19
20
Figure 3. Slow frequency artifact
Figure 4. Slow frequency and respiratory artifact, loss of CPAP trace
A2Zzz 18.1 | March 2009