Temperature and blood volume pulse are two primary measures of peripheral blood flow. Temperature
is a sluggish, tonic index of blood flow in skin arterioles. Blood volume pulse, in sharp contrast, is a rapid (phasic) index of change in arteriole blood flow. Clearly, these two measures complement each other. Temperature indexes average blood flow, while blood volume pulse tracks sudden changes.
This unit covers Descriptions of most commonly employed biofeedback modalities: Temperature, blood volume pulse, EKG and heart rate (III-A), Sources of artifact (III-B), and Structure and function of the autonomic nervous system (V-A).
Students completing this unit will be able to discuss:
- Descriptions of most commonly employed biofeedback modalities: Temperature, blood volume pulse, EKG and heart rate
A. Sensors and sensor placements
B. Characteristic signals
C. Signal processing and feedback displays - Sources of artifact
A. How to identify and correct extraneous biologic activity in recordings - Structure and function of the autonomic nervous system
A. The effects of commonly employed medications on autonomic activity
Temperature is detected using a thermistor, which is a transducer that
converts temperature into a resistance value. Thermistors are
temperature-sensitive resistors. Sensors used in temperature biofeedback
exhibit a negative temperature coefficient (resistance drops with rising
temperature).
A bead thermistor is the biofeedback industry standard temperature probe.
This sensor consists of a thermistor enclosed in an epoxy bead. The
smaller the bead’s mass, the faster the thermistor’s responsiveness to
changes in arteriole blood flow.
There is a time lag between change in arteriole diameter and a feedback
thermometer’s display of the new temperature. This phenomenon, called
thermal lag, consists of physiological and hardware components. Capillary
response and skin storage of heat may slow response for several seconds.
Thermistor sluggishness may add a delay of 3 s or more. To sum up,
thermal lag slows display of blood volume changes. By the time volume
change is shown as a change in temperature, it may have been "averaged" for at
least 5 s.
A thermistor’s speed is specified by its time constant. A time constant
is the period required for the thermistor to reach 63.2% of a final
value. You’re sitting in a 74 degree room. How long should a thermistor
with a time constant of 1 s take to register a hand temperature of 92
degrees F? The thermistor will reach 99.8% of your hand temperature
in 5 time constants, or 5 s. Time constants of 1 s or faster are
recommended in clinical work to minimize the time lag between the
temperature display and changes in blood vessel diameter.
Four precautions should be taken when attaching a thermistor. First, you
should securely attach the first 3-5 inches of the thermistor to your
patient’s skin using porous tape. This prevents the
stem effect in which body temperature is averaged with air
temperature, lowering readings (Montgomery, 2004).
Second, when a thermistor is placed on a digit like a finger, the tape
should be applied over the bead and cable (following the cable), not
around the circumference of the digit, which could reduce blood flow and
falsely lower readings.
Third, you should use only one tape layer since overwrapping could trap
heat and artifactually raise the temperature (blanketing effect).
Finally, you should tape the thermistor cable down to your patient’s
shirt or blouse (and possibly a reclining chair) with adequate slack to
prevent movement artifact (sensor decoupling from the skin and signals
produced by cable vibration).
The thermistor should be attached to a site on the hand or foot that is
well-supplied with blood vessels. The web dorsum, located on the back of
the hand (between the thumb and index finger), is one of several
acceptable sites.
Despite competing claims for the superiority of specific hand sites,
research using thermography (infrared imaging) has shown that no single
site is most responsive to stressors or relaxation exercises across a
majority of patients. Further, during an individual patient’s training
session, an initially responsive site may plateau (become less
responsive). A different site may then be more reactive. Since
temperature biofeedback to produce hand-warming can be overly specific
(warming could be confined to just the left index finger), several
thermistor sites should be monitored simultaneously to provide more
widespread vasodilation.
The room should be around 74 degrees F when measuring temperature. Rooms
below 68 degrees may produce a downward temperature drift. The patient
should be removed from drafts and cool surfaces and seated with
sufficient neck and knee support. Plants may be used to diffuse drafts.
Conversely, warm rooms may elevate temperatures. A cadaver’s hand
temperature will be 90 degrees in a 90 degree F room.
Thermistors attached to the hand should be heart level or lower since
temperature may drop if the hand is placed above the heart.
Use an alcohol or mercury thermometer as your reference to test feedback
thermometer accuracy. Place the thermistor next to the mercury
thermometer and compare room temperature values. If they are within
1
degree F, accuracy is acceptable.
The most common cause of malfunction is a damaged thermistor. If the
feedback thermometer varies from your reference thermometer by more than
1 degree F or fails the behavioral test, start your troubleshooting by
replacing the thermistor.
You can determine whether a temperature display mirrors the thermistor's
temperature by performing a tracking test,
during which you gently blow on the thermistor bead to warm it. The
temperature signal should increase while you blow and then decrease when
you stop. Below is a BioGraph ® Infiniti temperature display.
A tracking test checks the integrity of the entire signal chain from the
thermistor to the encoder, and the correct software selection of input
channels. When used with the ProComp Infiniti system, it ensures that the
thermistor is intact and snugly inserted into the right encoder
input socket; and the correct channel has been chosen for display.
For research purposes, a baseline period should allow temperature to
stabilize within 0.5 degrees F for 5 minutes. Baseline length will vary
with each subject between 15 and 45 minutes in a 74 degree F room.
Due to practical concerns, clinical baselines are often as brief as 5
minutes during training sessions. If the patient hasn’t stabilized before
the training session starts, warming during the session may reflect
adjustment to the room environment instead of self-regulation.
A feedback thermometer detects temperature indirectly. This device passes
a DC signal through a thermistor and back to a voltmeter. The voltmeter
measures the return voltage which is referenced to absolute temperature.
For example, if a temperature module’s output voltage were +4 VDC (volts
DC), this might correspond to a temperature of 100 degrees F.
Feedback thermometers use operational amplifiers to boost and process DC
voltages. A basic operational amplifier is a high-gain DC amplifier that
uses external feedback to add, subtract, or average thermistor signals.
Feedback thermometers use operational amplifiers to calculate absolute
temperature, average temperature, derivative temperature, and
differential temperature.
The DC signal that leaves the operational amplifier is filtered above 1
Hz to prevent contamination by 60-Hz and radio frequency (RF)
interference. A low pass filter selects signals below the 1 Hz cutoff
frequency for processing by an integrator.
The filtered DC signal is not rectified since it is already DC.
Rectification converts AC signals into DC signals to allow measurement of
signal strength. Like a rectified EMG signal, this voltage is then sent
to an integrator for quantification.
Next, a level detector checks whether the integrated voltage matches a
predetermined threshold value to control the feedback display. The
training threshold can be adjusted automatically by software or manually
by a clinician to progressively shape patient performance.
The feedback thermometer should be accurate to within ± I degree F when
monitoring temperatures from 65 to 100 degrees F. The feedback display
should have a resolution (should display change) of at least 0.1 degree F for clinical applications.
Peper and Olesen (1985) recommended
that clinicians use a portable infrared thermometer to rapidly scan
multiple sites on the same person or different individuals, monitor and
provide temperature feedback from sites that should not be touched by a
thermistor, and to covertly monitor temperature.
An infrared thermometer’s 0.5-s response time permits a clinician to
sequentially scan 10 different sites on the same patient in the time that
a thermistor with a 1-s time constant can measure the temperature at 1
site. While a clinician may be more concerned about the relative
temperatures of multiple sites than their absolute values, an infrared
thermometer’s accuracy of ± 1o to 2o C favorably compares with the ± 1o C
accuracy of clinical-grade thermistors. A Raytek MT4 Minitemp infrared
thermometer is shown below.
Shaffer et al. (2003) reported that
the Raytek MT4 Minitemp infrared thermometer achieved high concurrent
validity when compared with thermistor measurements. Infrared thermometer
values taken 0.5 s apart from the same site were highly reliable.
Finally, temperatures obtained from multiple locations on the same hand
may be directly compared.
They recommended that clinicians who perform psychophysiological profiles
and utilize temperature biofeedback incorporate an infrared thermometer
in their practice because it can supplement thermistor measurements
during assessment by mapping the distribution of skin temperatures across
both hands. This information could aid in selecting sites for thermistor
placement, evaluating patient response stereotypies, and monitoring sites
that are inaccessible due to disease or injury. An infrared thermometer
can also provide invaluable information during temperature training by
measuring the degree to which vasodilation has generalized across the
digits of trained and untrained hands.
Kabins et al. (in press), using a Raytek Raynger ST infrared thermometer, found that the web dorsum was warmer than the majority of the other sites on the left hand during the serial-sevens stressor and on the right hand during both baseline and serial-sevens stressor.
Baseline
Serial-Sevens Stressor
Use the infrared thermometer's laser guide to target the measurement
site. The red laser dot must be confined to the digit or web dorsum, or
else the measurement will be distorted by the temperature of other
surfaces. For accurate and repeatable readings, the infrared thermometer
should be within 3" of the target and oriented at a 90 degree angle with
respect to the target. A 3" guide can be used to consistently position
the infrared thermometer's barrel.
Blood volume is the amount of blood contained in an area. This measure
mainly reflects venous tone.
Blood volume pulse (BVP) indexes rapid changes in blood flow. It is calculated
as the vertical distance between the minimum value of one pulse wave and
the maximum value of the next. This measure mainly reflects blood flow
and arteriolar tone. Below is a BioGraph ® Infiniti BVP display.
Blood volume pulse is detected using a photoplethysmograph (PPG). This
device measures the relative amount of blood flow through tissue using a
photoelectric transducer.
An infrared (7000-9000o A) light source is transmitted through or
reflected off the tissue. Using the reflection technique, both light
source and photodetector are placed on the same side of the tissue. The photoelectric transducer (phototransistor) detects the light and converts
it into a positive DC signal. The intensity of the light reaching the
sensor varies with brief shifts in blood volume. More light is absorbed
when volume is greater, reducing the intensity of light reaching the
sensor. A PPG sensor can be placed at any site that has sufficient blood perfusion, including the earlobe, finger, and vaginal wall (Peper, Harvey, Lin, Tylova, & Moss, in press).
When inspecting the raw blood volume pulse signal, a
strong signal is a wave with a “sharp upswing and a longer downswing”
(Garber, 1986). The peak should be slightly rounded. Measurement units are arbitrary and proportional to the sensor’s voltage
output. The sensor’s DC output is boosted by an operational amplifier.
The DC signal is then routed to an integrator for quantification.
Each heartbeat briefly increases blood volume in the arteries and capillary beds. The blood volume pulse signal can be used to calculate heart rate (beats per minute) by measuring the interbeat interval (time period between successive heartbeats). Divide the time interval between peaks by 60 s to calculate heart rate (Peper, Harvey, Lin, Tylova, & Moss, 2007).
Caption: Heart rate is derived from measures of blood volume pulse by measuring the interbeat interval and then transforming this information into beats per minute. For example, the interbeat interval of 0.80 seconds is equal to a heart rate of 75 beats per minute, whereas the interbeat interval of 0.93 seconds is equal to a heart rate of 64.5.
Clinicians may simultaneously monitor blood volume pulse, blood volume amplitude (relative volume of blood), heart rate, and respiration during training to increase heart rate variability (HRV) as shown in the display below from Peper, Harvey, Lin, Tylova, and Moss (2007).
Caption: The data represent an average respiration rate of 7 breaths per minute with a corresponding heart rate of 73 beats per minute with a standard deviation of 10.1 beats
There are two main limitations to blood volume pulse. First, this index
of blood flow only describes blood volume under the sensor. Volume in
another area can be vastly different.
Second, blood volume pulse measurements are relative. Absolute values
cannot be compared across different individuals as with hand temperature.
However, values can be compared across a training session and relative
measures can be compared across individuals.
A photoplethysmograph can provide high-resolution feedback when
temperature feedback shows minimal change. This is because the PPG sensor
can be more sensitive to rapid changes. Blood volume pulse could easily
drop 50-60% in a patient who is a vascular responder. When a patient plateaus (ceases to warm), a clinician could switch to blood volume pulse
biofeedback to increase hand-warming if the hand is not very cold.
Photoplethysmograph (PPG) sensor attachment is critical since readings
are sensitive to limb position, 60-Hz artifact, ambient light, movement,
and pressure. For finger placements, use the palmar side of a larger
finger (or thumb) and confine the sensor to only one finger segment. For
temporal artery placement, lightly press your first or second finger to
detect a pulse between the corner of the eye and eyebrow (near the
hairline). The best location will produce the highest amplitudes and
cleanest signals when displayed on an oscilloscope screen.
Below is a BioTrace+ / NeXus-10 screen designed to train vascular headache patients to increase and decrease blood volume to increase vasomotor control.
Sensor position relative to the heart strongly affects blood volume
pulse. If the PPG sensor is placed on a limb elevated above the heart,
the signal increases. If the limb is placed below the heart, the signal
decreases. These changes appear to reflect venous filling.
Environmental noise sources like fluorescent lights can produce 60-Hz
artifact. Sixty-hertz noise appears as ripples during downswings in the
raw blood volume pulse signal. The best strategy is to eliminate major
60-Hz noise sources and reposition the patient to become a poorer 60-Hz
antenna.
When a PPG sensor is exposed to excessive direct light, baseline values
are abnormally large. You can determine whether values are higher due to
light artifact by covering the PPG sensor with dark cloth. If values
decline when covered, you have confirmed the presence of light artifact.
The best strategy is to reduce direct light striking the sensor.
You can minimize movement artifact by instructing the patient to sit
quietly and securing the sensor with an adhesive collar and a Velcro or
stretch terry cloth band. The band should hold the PPG sensor in place
without suppressing the pulse.
Inspection of the raw BVP can detect movement artifact. Clinicians can remove contaminated data using software and then recalculate heart rate and heart rate variability. Recordings from Peper, Harvey, Lin, Tylova, and Moss (2007) show the average heart rate before and after movement artifact removal. In the second recording, the contaminated segment is highlighted. Movement artifact raised heart rate 5 beats per minute.
Caption: Recording of the heart rate derived from the raw BVP during the baseline period. The average heart rate included a segment of containing a very rapid heart beat reflecting movement artifact rather than a true increase in average heart rate.
Caption: When movement artifact is excluded from the signal, the actual HR is estimated 61.05 beats per minute.
Excessive pressure can be caused by wrapping a restraining band too
tightly or resting too much weight on the PPG sensor. Patients often
report throbbing when a Velcro band is wrapped too tightly around a
finger. Pressure artifact reduces the amplitude of the raw signal
resulting in smaller values. You can remove pressure artifact by
readjusting the tightness of the restraining band or reducing the weight
resting on the sensor.
The PPG sensor can be insensitive to cold hands. If your patient’s hands
are very cold, blood volume pulse may be too weak to detect.
You can check whether the photoplethysmograph responds to changes in
blood flow by asking your patient to raise the monitored hand above
heart level and then return it to heart level. Blood volume pulse should
increase and then decrease. Below is a BioGraph ® Infiniti BVP display.
A tracking test checks the integrity of the entire signal chain from the
PPG sensor to the encoder, and the correct software selection of input
channels.
Following normal skin preparation for EMG,
active EKG electrodes (which can be identical to EMG
electrodes) should
be placed on the wrist over the radial artery on the left and right
hands. The reference electrode can be placed over the dorsal aspect
of the wrist on the right hand. This placement, described by Papillo and
Shapiro (1990), is an excellent way to detect the EKG signal since this
wrist-to-wrist configuration creates a large-surface area antenna.
A photoplethysmographic sensor provides an alternative method of detecting the R-wave of the EKG and
this approach has been popularized by several manufacturers. Below is the Freeze-Framer by the Institute of HeartMath.
A breathing harness can be placed snuggly over the abdomen (navel).
Heart rate variability (HRV) is measured by detecting the interbeat
interval between successive R-waves.
The SDNN is the standard deviation of the N-to-N interval. The N-to-N interval is the normalized interbeat interval. The SDNN measures how these intervals vary over time and is expressed in milliseconds (ms).
The SDNN is more accurate when calculated over 24 hours than during the shorter periods monitored during biofeedback sessions. This measure is a frequently-used medical index of heart rate
variability that is used to estimate cardiac risk (Shaffer & Moss, 2006).
In
the BioGraph ® Infiniti display below, the boy morphs into a superhero as
SDNN increases.
The pNN50 is the percentage of adjacent N-to-N intervals that differ from each other by more than 50 milliseconds. This may be a more reliable index than the SDNN for the brief samples used in biofeedback. At least a 2-min sample is required to calculate an accurate pNN50.
HR Max – HR Min is the difference between the highest heart rate and the lowest heart rate within each cardiac cycle, measured in beats per minute.
Heart rate variability can be displayed on a computer monitor along with
the excursion of the abdominal strain gauge. The BioTrace+ / NeXus-10 display below shows patients how heart
rate changes during inspiration and expiration.
The Freeze-Framer's display provides an intuitive way of teaching clients to increase HRV coherence.
Shaffer and Moss (2006) describe
HRV training: "Therapists use either the
electrocardiogram (EKG) or photoplethysmograph (PPG) to monitor the
frequency bands that comprise heart rate variability. They also use a
respiratory strain gauge to measure abdominal or chest expansion and
contraction during each respiratory cycle and respiration rate. Relaxed
breathing produces a biofeedback display showing a smooth sinusoidal
line, depicting exhalation and inhalation, and a parallel smooth
sinusoidal line showing heart rate variation. Anxious thoughts, on the
other hand, produce a jagged irregular respiration signal and a jagged
irregular variation in heart rate. Producing this coherence––or
smoothly organized and regular variation––of the respiratory and heart
rate displays is also a training goal for biofeedback."
When the display is properly adjusted, your patient should see how heart
rate changes during inspiration and expiration. In healthy patients, it
should increase during inspiration and decrease during expiration. In
patients who hyperventilate or show a thoracic breathing pattern, there
may be minimal heart rate variability and the heart rate trace may be
out-of-phase with the abdominal strain gauge trace.
Heart rate variability is produced by the interaction of multiple
regulatory mechanisms that operate on different time scales (Moss,
2004).Spectral analysis separates
heart rate variability into its component rhythms that operate within
different frequency bands. The Task Force of
the European Society of Cardiology and the North American Society of
Pacing and Electrophysiology (1996) has divided heart rate
oscillations into the following bands:
The ultra low frequency (ULF) band (below
0.0033 Hz) represents very slow-acting biological processes
and is too gradual to train using conventional biofeedback.
The very low frequency (VLF) band (0.005-0.05
Hz) may represent sympathetic activation.
Worry and rumination increase the power of this waveform.
The low frequency (LF) band (0.05-0.15 Hz) may represent the influence of both the parasympathetic and sympathetic branches and blood pressure regulation via baroreceptors or blood pressure receptors (Lehrer, 2007). Meditation and slow breathing, like the
“tanden breathing” practiced by Rinzai Zen monks, increase the power of
this waveform.
The high frequency (HF) or respiratory band
(0.15-.40 Hz) represents the inhibition and activation of the
vagus nerves by breathing at normal rates (Moss, 2004).
In HRV training, the clinician teaches clients to gradually slow respiration to their personal resonant frequency, from 4.5-7.5 breaths per minute, to increase the amplitude around .1 Hz. This maximizes RSA because it combines the influence of baroreceptors and the parasympathetic system. The BioGraph ® Infiniti screen shown below is designed to increase the percentage of power in the low frequency band. This display shows the dynamic change in the distribution of RSA amplitude as a client breathes effortlessly and cultivates positive emotion.
Now that you have completed this module, summarize the specifications
recommended for a feedback thermometer. Explain when blood volume pulse
feedback could complement temperature biofeedback and why.
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