An Introduction to Artifact Recognition Part 1
By Wayne Peacock, RPSGT
Artifact recognition is an essential part of
quality study is captured for the reviewing
the sleep technologist’s job. Timely, accurate
technologist and physician.
recognition and correction of several types
There are many types of artifact, some of
of artifact lead to the acquisition of a highwhich may be beneficial. For example, snoring
quality and artifact-free recording. In order
artifact seen in the chin electromyogram
to achieve this, the recording technologist
(EMG) may actually add value to the study by
must pay close attention to proper electrode
confirming the presence of snoring; muscle
and sensor application, ensure accurate signal
artifact during movement helps confirm
processing and maintain the integrity of the
patient movement.3 The task of the sleep
recording. Attention to these three things
technologist includes recognizing the artifact,
will determine the quality of the recording
determining its origin, and then deciding the
wayne peacock,
obtained.
most appropriate way to address the problem.
rpsgt
Artifact can affect every channel and
To determine the origin, begin with the patient.
biopotential recorded. In order to correctly handle
Check all electrodes and wires and work toward the
various types of interference, the recording technologist
amplifiers and eventually back to the computer.1 To put
must possess a working knowledge of what is being
it simply, follow the signal pathway.
recorded and how to best respond to each type of
Figure 1 shows a typical patient circuit.
artifact that may affect that channel. For the purpose
of this article it is assumed that the recording
Patient
Sensor
Headbox
Amplifier
AD Converter
Computer
technologist used proper 10-20 electrode placement for
Figure 1. A typical patient circuit1
the electroencephalogram (EEG), performed careful site
preparation, and secured to the skin quality electrodes
with relatively low impedances (<5kOhms). Impedances
Most artifacts may be recognized by an exaggerated
for both EEG and electrooculogram (EOG) should range
appearance, although some may be difficult to
from one to five kOhms.1 Even with detailed attention to
distinguish from physiological signals. An example
all of the aspects described above, a sleep recording may
of this is the appearance of sweat artifact in the EEG
still encounter intermittent artifact. Impedances should
channels. This artifact is generally identified by its
be checked periodically throughout the recording,
excessively slow frequency; however, the intruding waves
especially when signs of artifact are noted.2 Knowing
may be similar in their appearance to high-amplitude
how to deal with these occurrences will ensure that a
delta waves.3
ECG Artifact
ECG artifact is common and is usually seen in the
EEG, EOG or EMG channels. 3 See Figure 2. Strong
ECG voltages may be detected from almost any
location on the body. ECG artifact may be increased
due to impedance imbalance between the exploring
and referencing electrode. This should always be
considered. In some cases, as with obese patients,
some amount of ECG artifact will be present in spite
of low impedances. To reduce the probability of
ECG artifact, care should be taken to avoid placing
referencing electrodes (M1 and M 2) on soft, fatty tissue.
The technologist may find it advantageous to place
these electrodes slightly higher on the mastoid process,
just above the ear on the bony portion; they also may
be placed on the earlobes.3
If artifact remains, then the technologist may link
or “double reference” the two referencing electrodes.
This can be accomplished either with jumpers or by
using a derivation that allows the M 1 to be linked with
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A2Zzz 2008
• volume 17 • number 4
M 2 (M 1+M 2). 3 This method is called double referencing,
which was accomplished via the electrode selector
panel with analog systems. Many of today’s digital
recording systems have provisions allowing double
referencing, which may be as simple as the click of a
mouse.
Figure 2. ECG Artifact
Movement Artifact
Oximetry Artifacts
Figure 3. Movement artifact
If it does not disappear, then there may have been
a dislodging of electrodes during the repositioning.
Movement artifact may be minimized with proper electrode
application.
Figure 4. Oximeter probe displaced
Thick or painted nails also may present problems when
recording oximetry tracings. It is imperative for the
recording technologist to recognize poor or improper
tracings from the oximeter since this signal is significant in
scoring and diagnosing sleep related breathing disorders.
Often times this artifact may be corrected by simply
re-attaching the probe. When this does not correct the
problem it may be necessary to replace the probe or simply
trace the connections to ensure all cables are securely in
place.
This type of artifact is usually a generalized artifact in all
channels caused by the patient repositioning. This artifact
usually dissipates once the patient has settled.
See Figure 3.
Improper probe attachment or a faulty probe may be
the cause of oximetry signal artifact.3 Occasionally an
oximetry signal also may be lost temporarily due to large
movements. See Figure 4.
This article will continue in the next issue of A2Zzz with
examples of other common artifacts.
References
1. 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.
2. American Academy of Sleep Medicine. The AASM
manual for the scoring of sleep and associated events:
rules, terminology and technical specifications.
Westchester, Ill: American Academy of Sleep
Medicine; 2007. p. 19.
3. 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.
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.
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• volume 17 • number 4
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