The electrocardiograph (ECG) and photoplethysmograph (PPG) are two methods of detecting heart rate (HR) and heart rate variability (HRV). A PPG sensor detects the pulse wave
as it travels through the vascular tree. HRV estimates from pulse wave variability are termed pulse rate variability (PRV).
PRV may be a poor ECG surrogate for measuring HRV when participants stand, perform slow-paced breathing, or have low HRV (Jan et al., 2019).
Consumer biofeedback devices incorporating PPG sensors have become increasingly popular for recreation and
self-regulation. Shown below are products by Elite HRV, HeartMath, myithlete, and Thought Technology Ltd.
BCIA Blueprint Coverage
This unit addresses III. HRV Instrumentation: A. Blood volume pulse (BVP).
Professionals completing this unit will be able to discuss blood volume pulse:
A. Source
B. PPG sensor
C. Signal
D. Placements
E. Tracking Test
F. Artifacts
This unit covers the Source of BVP, PPG Sensor, BVP Signal, PPG Sensor Placement, BVP Artifacts, and Tracking Test.
Please click on the podcast icon below to hear a full-length lecture.
Source of Blood Volume Pulse
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 (Peper, Shaffer, & Lin, 2010). Below is a BioGraph ® Infiniti BVP display. You can enlarge the video by clicking on the bracket icon at the bottom right of the screen. When finished, click on the ESC key.
PPG Sensor
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. The transmission technique places the light source and photodetector on the opposite sides of a digit (Matto, 2018).
The reflection technique places both light
source and photodetector on the same side of the tissue. In both methods, the intensity of the light reaching the
sensor varies with momentary shifts in blood volume (Shaffer & Combatalade, 2013). The illustration below is courtesy of Thought Technology Ltd.
The interval between successive peaks (A) is called the interbeat interval (IBI). The peak-to-trough difference (B) shows the relative blood flow (Matto, 2018).
A photodetector detects and converts light into a positive DC signal in transmission and reflectance modes. Graphics by minaanandag on fiverr.
Blood appears red because it reflects red wavelengths. More light is reflected, and the BVP signal increases when the volume of blood increases. (1) As blood surges, more light is reflected, and the BVP signal peaks as the volume of blood increases. (2) As the pulse wave travels through the vascular tree, it is reflected by the lower body and appears as a second smaller peak. (3) The dicrotic notch is the gap between the direct and reflected waves.
The ear is less prone to artifact than the finger due to less movement, stronger signal, and less risk of vasoconstriction due to temperature. Since the ear is closer to the heart than is the finger, there is less opportunity for the vascular tone rhythm to contaminate HRV frequency-domain measurements in the VLF, LF, and HF ranges (Lehrer, 2018b).
The thumb is an excellent site when a client's fingers are too small or have insufficient blood
flow to detect a strong pulse (Peper, Shaffer, & Lin, 2010). In the Flir infrared image below, the thumb is brighter than adjacent digits because of its greater perfusion with blood.
BVP Signal
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. An operational amplifier boosts the sensor’s DC output. 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 HR (beats per minute) by measuring the
interbeat interval (the period between successive heartbeats). Divide the time
interval between peaks by 60 seconds to calculate HR (Peper, Harvey, Lin, Tylova, & Moss, 2007).
Caption: Heart rate is derived from blood volume measures by calculating the interbeat interval and then transforming this information into beats per minute. For example, the interbeat interval of 0.80 seconds equals a HR of 75 beats per minute, whereas the interbeat interval of 0.93 seconds equals a HR of 64.5.
Clinicians may simultaneously monitor blood volume pulse, blood volume amplitude (relative volume of blood),
HR, and respiration during training to increase 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 HR of 73 beats per minute with a standard deviation of 10.1 beats.
Sampling Rate
Béres and Hejjel (2021) proposed a minimum 50-Hz PPG sampling rate to measure RMSSD and SDNN healthy young participants without interpolation. However, interpolation can reduce the required sampling rate to 10-20 Hz. The HeartMath Institute's Inner Balance PPG sensor samples the BVP signal at 125 Hz. Mind Media's NeXus-10 MKII samples from 128-256 Hz. Thought Technology Ltd.'s ProComp Infiniti samples at 256 Hz.
Limitations to Photoplethysmography
There are four main limitations to blood volume pulse. First,this blood flow index only describes blood volume under the sensor. The blood volume in
another area can be vastly different than in another.
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 (Peper, Shaffer, & Lin, 2010).
Third, BVP and ECG methods may yield different HRV values with marked sympathetic activation. ECG values will be more accurate since they are not affected by vasoconstriction.
Finally, PRV may inflate HRV values and be a poor surrogate for ECG when participants stand or perform slow-paced breathing or have low HRV (Constant et al., 1999; Hemon & Phillips, 2016; Jan et al., 2019; Medeiros et al., 2011).
Advantages to Photoplethysmography
A photoplethysmograph can provide high-resolution feedback when
temperature feedback shows minimal change. A PPG sensor
is more sensitive to rapid blood volume changes. Blood volume pulse could easily
drop 50-60% in a patient who is a vascular responder (fingers cool when challenged by stressors). When a client plateaus (ceases to warm), a clinician could switch to blood volume pulse
biofeedback to increase hand-warming if the monitored hand is not significantly vasoconstricted.
Skin Preparation
Unlike the ECG recording, minimal skin preparation is required since the PPG sensor detects infrared light instead of an electrical potential.
Ask your clients to wash their hands so that dirt won’t occlude the sensor’s transducer window (Shaffer & Combatalade, 2013).
For PPG sensors that pass infrared light through the finger, instruct clients to avoid dark fingernail polish, which will block light transmission.
PPG Sensor Placement
Photoplethysmograph sensor attachment is critical since readings
are sensitive to limb position, 50/60Hz artifact, ambient light, movement,
and pressure.
For finger placements, attach the PPG sensor using a Velcro ® band or Coban™ tape to the palmar side of a larger finger (or thumb) and confine the sensor to only one finger segment.
Use the thumb when the fingers are
small or blood flow is compromised, such as when clients have cold hands (Peper, Shaffer, & Lin, 2010).
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). When displayed on a computer screen, the best location will produce the highest amplitudes and cleanest signals. A Mind Media BVP sensor is shown below.
Limb Position
Sensor position relative to the heart strongly affects blood volume pulse. If the PPG sensor is placed on a limb below the heart, BVP signal amplitude increases. We can take advantage of this phenomenon when signal amplitudes are weak (Lehrer, 2018b). If the limb is placed above the heart, the signal amplitude decreases. These changes appear to reflect venous filling (Peper, Shaffer, & Lin, 2010).
BVP Artifacts
Artifacts are false values produced by the client’s body (ectopic beats) and actions (movement), the environment (line current), and hardware limitations (light leakage).
Short-term values are proxies of 24-hour values. Never compare the short-term values shown below with 24-hour norms. Twenty-four-hour values are typically greater and can predict morbidity and mortality, while most short-term values cannot.
Cardiac Conduction Artifacts
Cardiac conduction artifacts include atrial fibrillation, premature atrial contractions,
and premature ventricular contractions. Click on the Read More button to review them.
Atrial fibrillation is a supraventricular arrhythmia, with HRs reaching 160 beats per minute (Tortora & Derrickson, 2021).
Cardiac conduction is chaotic in clients who experience this disorder.
Atrial fibrillation is a low-amplitude BVP signal (see the left side of the enlarged view) with a faster HR.
Premature atrial contractions (PACs) involve early atrial contraction, are characterized by abnormally-shaped P-waves, and result in calculating extra beats (Lehrer, 2018b).
Premature ventricular contractions (PVCs) can result in an extra heartbeat followed by a full compensatory pause (Clinical ECG Interpretation, 2018).
PVC artifacts are extra heartbeats that originate in the ventricles instead of the S-A node of the heart and can distort the BVP signal (Elgendi, 2012). Clinical Tips to Minimize Cardiac Conduction Artifacts
Since
atrial fibrillation, PAC, and PVC artifacts
cannot be prevented, they must be eliminated
by artifacting.
Cold Artifact
Cold artifact, produced by cold exposure or sympathetically-mediated vasoconstriction, can reduce or eliminate a pulse wave. Cold artifact may result in missed beats, resulting in artifactually-lengthened interbeat intervals (Shaffer & Combatalade, 2013).
Here is a low-amplitude BVP signal (Elgendi, 2012).
Clinical Tips to Minimize Cold Artifact
1. Maintain at least a 74° F (23° C) room temperature.
2. Use an earlobe or thumb placement or an ECG sensor. Earlobe blood flow may produce an adequate BVP signal when you can’t detect a signal from the fingers. The thumb is an excellent site when a client's fingers are too small or have insufficient blood flow to detect a strong pulse (Peper, Shaffer, & Lin, 2010).
3. Position the hand below heart level (Lehrer, 2018b).
4. Provide your clients several minutes to relax.
5. Allow your clients to place their hands in a sink filled with warm water or in front of a
space heater for several minutes.
6.
Examine the raw signal for artifact.
Light Artifact
Light artifact occurs when ambient light overloads a PPG sensor’s photodetector producing large peak-to-trough differences (Cherif et al., 2016; Shaffer & Combatalade, 2013).
You won't see it if your data acquisition system filters out the high-frequency component of the raw BVP signal before displaying it. The graphic below from Elgendi shows a 50-Hz peak and 100-Hz harmonic (left) and contamination of the raw BVP signal (right).
Clinical Tips to Minimize 50/60Hz Artifact
1. Use a 50/60Hz notch filter.
2. Place the encoder box 3 feet (1 meter) from electronic equipment.
3. Remove unused sensor cables from the encoder box.
4. Examine the raw signal for artifact.
Movement Artifact
Sensor movement artifact is the leading cause of BVP signal distortion and can eliminate the signal or result in
extra or missed beats (Elgendi, 2012; Shaffer & Combatalade, 2013).
Sensor movement can interfere with infrared light transmission by the PPG
sensor or allow contamination by ambient light.
Movement artifacts are colored red in this graphic by Couceiro et al. (2014).
Inspection of the raw BVP can detect movement artifacts. Click on the Read More button to see several examples of this artifact.
Below is a BioGraph ® Infiniti BVP display of
movement artifacts. Note the appearance of ripples and distortion in the shape of the waveform. You can enlarge the video by clicking on the bracket icon at the bottom right of the screen. When finished, click on the ESC key.
Movement
artifact can distort the BVP waveform in different ways.
Standing and sitting can produce blood
pressure-mediated upward and downward drifts. Arm movement above or below the heart can also generate drifts.
Repeated movements like finger tapping can create waveforms with ripples that resemble multiple notches.
The display may show sudden changes in the raw BVP signal and HR.
Below is a close-up view of sudden HR increases.
Clinical Tips to Minimize Movement Artifact
1. Firmly attach the PPG sensor to the client’s finger
with hands resting on the knees.
2. Firmly tape sensor cables to client clothing for strain relief and cover the sensor with a baby sock or dark cloth to minimize the entry of ambient light.
3. A Velcro ® band should hold the PPG sensor in place without suppressing the pulse (Peper, Shaffer, & Lin,
2010). Alternatively, a mechanical housing can secure the sensor to the finger.
4. Instruct clients to minimize movement and monitor compliance.
5. Examine the raw signal for artifact.
Pressure Artifact
Pressure artifact can be caused by wrapping a restraining band too tightly.
Patients may report throbbing when a
Velcro ® band is wrapped too tightly around a finger. Pressure reduces raw signal amplitude, resulting
in smaller values or a flat line, and may prevent detection of the peak of the pressure wave. Missed beats can lengthen IBIs and slow HRs (Shaffer &
Combatalade, 2013).
Excessive pressure can also be caused by resting too much weight (e.g., hand pressing sensor against a knee or table) on the PPG sensor. Pressure artifact reduces
raw signal amplitude, resulting in smaller values (Peper, Shaffer, & Lin, 2010).
Clinical Tips to Minimize Pressure Artifact
1. Readjust the tightness of the restraining band.
2. Keep pressure off the PPG sensor.
3. Examine the raw signal for artifact.
The graphic below from Elgendi (2012) shows multiple artifacts, including arrhythmia, EMG, low-amplitude, and movement, which can render an epoch unusable.
Tracking Test
You can determine whether the ECG or BVP signals respond to your client's breathing by observing whether their instantaneous HR speeds during inhalation and slows during exhalation.
Misconception: BVP and ECG HRV measurements are interchangeable.
The BVP method may not be useable if participants are vasoconstricted and software cannot detect the peak of the pulse wave. It may inflate HRV estimates. PVR may be a poor proxy for HRV when participants stand or perform slow-paced breathing or have low HRV. Discount studies that report that PVR and HRV measurements are correlated if they don't report their measurement conditions (e.g., paced breathing) and limits of agreement (LOA; Shaffer, Meehan, & Zerr, 2020).
What should you do if the BVP signal is too weak to detect the peak of the pressure
wave?
Shift the PPG sensor to the thumb or earlobe if placed on a finger. If you cannot record from the thumb
because it is vasoconstricted, allow your client to warm the digit using relaxation, dipping the hands in a warm
basin of water, or placing them in front of a space heater. If none of these options work, use the ECG method.
Glossary
blood volume pulse (BVP): the phasic change in blood volume with each heartbeat. It is the vertical distance between the minimum value (trough) of one pulse wave and the maximum value (peak) of the next measured using a photoplethysmograph (PPG).
cold artifact: cold exposure or sympathetically-mediated vasoconstriction that can reduce or eliminate a pulse wave.
electromagnetic interference (EMI) artifact: ECG and PPG artifact that is generated when cell phones transmit an artifactual voltage.
extra beats: ECG and PPG artifact that shortens the IBI when signal distortion causes the software to detect nonexistent beats.
light artifact: PPG artifact when light leakage increases BVP amplitude.
line interference artifact: ECG and PPG artifact when 50/60Hz contamination of signals causes the software to detect nonexistent beats and shorten the IBI.
missed beats: ECG and PPG artifact that lengthens the IBI when signal distortion causes the software to overlook a beat and use the next good beat.
movement artifact: ECG and PPG artifact that shortens the IBI when signal distortion from movement causes the software to detect nonexistent beats.
photoelectric transducer: phototransistor that detects infrared light transmitted by a PPG sensor and converts it into a positive DC signal.
photoplethysmographic sensor: a photoelectric transducer that transmits and detects infrared light that passes through or is reflected off tissue to measure brief changes in blood volume and detect the pulse wave.
pressure artifact: reduction in the amplitude of the BVP signal due to a tight restraining band or resting too much weight on the PPG sensor.
pulse rate variability (PRV): a proxy of HRV derived from the BVP signal.
tracking tests: checks of whether the biofeedback display mirrors client behavior. BVP amplitude should increase and then decrease as a hand is raised above the heart and then dropped below the heart.
Test Yourself
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Visit the BioSource Software Website
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BCIA has accredited each course, and they combine affordable pricing ($150) with industry-leading content.
Essential Skills
Blood volume pulse
1. Explain the blood volume pulse signal and biofeedback to a client.
2. Explain PPG sensor attachment to a client and obtain permission to monitor her.
3. Explain how to select a placement site and demonstrate how to attach a PPG sensor to minimize light and movement
artifacts.
4. Perform a tracking test by asking your client to raise the monitored hand above the heart and then it.
5. Identify common artifacts in the raw PPG signal, especially movement, and explain how to control for them and remove
them from the raw data.
6. Explain the major measures of heart rate variability, including HR Max - HR Min, pNN50, SDNN, and SDRR.
7. Explain why we train clients to increase power in the low-frequency band of the ECG and how breathing at 5-7 breaths
per minute helps them accomplish this.
8. Demonstrate how to instruct a client to utilize a feedback display.
9. Describe strategies to help clients increase their heart rate variability.
10. Demonstrate an HRV biofeedback training session, including record keeping, goal setting, site selection, baseline
measurement, display and threshold setting, coaching, and debriefing at the end of the session.
11. Demonstrate how to select and assign a practice assignment based on training session results.
12. Evaluate and summarize client/patient progress during a training session.
Assignment
Now that you have completed this module, how would you explain how a PPG sensor detects a heartbeat? How do you control
movement artifact?
Acknowledgment
This unit draws heavily on graphics published in Didier Combatalade’s Basics of Heart Rate Variability Applied
to Psychophysiology, published by Thought Technology Ltd. Didier is the Director of Clinical Interface at
Thought Technology Ltd and is a gifted educator and writer, and generous colleague.
References
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https://doi.org/10.1016/j.bspc.2021.102589
Berntson, G. G., Quigley, K. S., & Lozano, D. (2007). Cardiovascular psychophysiology. In
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Premature ventricular contractions (PVCs) can result in an extra heartbeat followed by a full compensatory pause (Clinical ECG Interpretation, 2018).
