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TECHNICAL CORNER: low-pass (high frequency) filters
By Mary Ellen Wells, MS, RPSGT, REEGT, RNCST
QUESTION
What is a low-pass filter and when should it be used?
ANSWER
The concept of filtering data has troubled new and seasoned
technologists for as long as the polysomnogram has been used in
clinical practice. To understand the practical aspects of using the
low-pass filter, let’s begin by briefly describing what the low-pass
filter is and how it works. First, the terms “low-pass” filter and
“high frequency” filter are synonymous. Second, filters (e.g., air,
water, e-mail) are typically designed to block things. For polysomnography, the high frequency filter (HFF) is designed with
a manually set frequency “cut-off ” to help block relatively fast or
high, interfering frequencies.
The HFF works by attenuating (i.e., reducing) the amplitude
of frequencies close to and above a specified level, sometimes
eliminating them completely. At the cut-off frequency, amplitude
is reduced to 70 percent or 80 percent, depending on the equipment manufacturer. For example, using a 35Hz HFF will allow
frequencies below 35Hz to pass through (hence “low-pass”) and
display on the screen without being altered; frequencies of 35Hz
and above will not be allowed to pass through the filter without
amplitude reduction. Although there are differences between
analog and digital HFF filters, the basic concepts described are the
same for both.
When should a low-pass (high frequency) filter be used?
The sleep laboratory is full of signal interference. Examples
include signal interference from cellular telephones, lamps, radios,
television signal transmissions and other diagnostic equipment.
Bioelectric interference also can be generated by the patient in
the form of heart pulses, body movements, sweat and activity
from muscles close to recording electrodes. Differential amplifiers
eliminate most major forms of interference by common mode
rejection. However, the recording is still susceptible to other forms
of interference. Selectively using filters helps reduce or eliminate
the remainder of the unwanted signals, achieving a recording most
reflective of the true biologic signals of interest.
The HFF should not be used to correct artifact from improper
electrode application or improper polysomnographic recording
techniques. These, and many other types of artifacts including
60Hz artifact, should be corrected at the source and not by use
of the HFF. It is important to note that failing to filter a channel
enough should have minimal effects on the recording when proper
Mary Ellen Wells, MS, RPSGT,
REEGT, RNCST
Mary Ellen Wells, MS, RPSGT, REEGT,
RNCST, has been in the sleep field since
1999 and is a clinical assistant professor
in the Department of Allied Health Sciences at the University of North Carolina
at Chapel Hill School of Medicine.
patient preparation and equipment setup are used. On the other
hand, over filtering a channel may be detrimental to the recording and could filter out the true signals from the patient. HFFs
eliminate or reduce fast activity regardless of its origin.
Using an HFF in combination with a low frequency filter (LFF)
creates an upper and lower limit so that only the band of frequencies you need are allowed to go through. All physiologic variables
recorded during polysomnography fall within a frequency band
or range, so you should set filters just outside of this range. For
example, if you are only interested in frequencies less than or equal
to 25Hz, then you should set the HFF to an interval above 25Hz,
such as 30Hz. This will ensure that you will not lose important
data close to 25Hz.
Figure 1 shows the HFF settings for routine recordings that are
recommended in The AASM
Manual for the Scoring of Sleep
Recording
HFF
and Associated Events.1
EEG
35 Hz
EOG
35 Hz
How do you know when
“low” is too low?
There are instances when the
ECG
70 Hz
AASM’s HFF recommendaRespiration
15 Hz
tions may not be appropriate.
Snoring
100 Hz
Setting the HFF too low will
cause a loss of valuable data,
FIGURE 1. High frequency
filter settings recommended and too low is different per
channel type. To determine
by the American Academy of
the appropriate HFF setting,
Sleep Medicine.
the most important questions
to ask for each channel are: What physiologic variable am I trying
to record (e.g., EEG, respiration), and within what frequency band
does it usually fall? The following explanations provide additional
details about variables specific to each HFF setting.
EMG
100 Hz
EEG
Neurophysiological activity in the brain ranges from 0.25Hz to
as high as 2,000Hz. In routine polysomnography, sleep stages are
usually recognizable within the range 0.5-16Hz, with 16Hz being
the upper limit for sleep spindles. Decreasing the HFF to 15Hz
may reduce the appearance of sleep spindles, yielding unrecognizable stage N2 epochs as seen in Figure 2. Also, when the HFF
is set too low, muscle activity may be misleading, appearing as
normal brain activity.
A higher frequency (beta) provides useful clinical information.
Beta ranges from more than 13Hz to 30Hz. Benzodiazepines,
barbiturates, antidepressants and many other psychotropic agents
may cause increased beta activity in ranges of 13Hz to 20Hz or
more. A HFF setting of less than 25Hz to 30Hz can remove
significant beta frequency patterns.
A final issue for the HFF with regard to EEG is seizure activity
in the form of spikes, which range from 13.3Hz to 50Hz. HFF
settings less than 50Hz may attenuate seizure activity. The AASM
recommends raising the HFF to 70Hz if seizure activity is suspected.
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FIGURE 2. The top image demonstrates typical stage N2 sleep. The bottom image demonstrates how decreasing the HFF
attenuates the spindles seen in the top image, making it difficult to identify the sleep stage.
EOG
The fastest eye movements occur during rapid eye movement
(REM) sleep and have a frequency of more than 1Hz. The sharpest REM slopes last 50 to 200 milliseconds. Muscle artifact is of
concern in the EOG due to the close proximity of active muscles;
therefore, a HFF setting of 30Hz to 35Hz will reduce muscle
artifact while recording the fastest eye movements.
EmG
Muscle activity is very fast and produces high frequency waveforms, even above 1,000Hz. The HFF should be set as high as
possible (usually 70Hz to 100Hz) or turned off to provide a faithful representation of EMG potentials.
ECG
In adults most of the diagnostic information in the ECG is
below 100Hz. The fastest component of the ECG may be as fast
as 0.04 seconds or about 25Hz. A HFF setting of 30 to 35Hz produces a stable, relatively artifact-free ECG. However, this setting
may be too low for diagnostic purposes. A HFF setting of 70Hz is
recommended to produce a wider bandwidth for a more detailed
ECG analysis.
Respiration
For respiratory channels, very slow frequencies are of primary
interest. Therefore, the HFF can be fairly low (about 15Hz).
However, if snoring is recorded within the primary respiratory
channels, the HFF should be raised to allow for faster frequencies
up to 100Hz.
Snoring
Snoring produces very fast frequencies (like EMG). Using HFF
settings similar to the EMG settings (i.e., 70Hz to 100Hz, or
turned off ) should adequately represent snoring.
REFERENCE
1. 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. 
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