Blood volume pulse (BVP) is an indispensable tool for evaluating and training clients for chronic pain, headache, hypertension, optimal performance, and stress. Although smartwatches use BVP to measure heart rate (HR) and heart rate variability (HRV), the BVP signal provides a wealth of additional information (Peper et al., 2007).
This post covers the Source of the BVP Signal, Skin Preparation, PPG Sensors, the BVP Waveform, Pulse Amplitude Variability, PPG Sensor Attachment, Cardiac Arrhythmias, Controllable Artifacts, Tracking Test, and Pulse Amplitude Norms.
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The Source of the Blood Volume Pulse Signal
Blood volume pulse (BVP) indexes rapid blood flow changes and mainly reflects blood flow and arteriolar tone. Sympathetic nervous system activation constricts cutaneous arterioles, reducing blood volume amplitude (BVA; peak-to-trough difference). Conversely, parasympathetic nervous system activation dilates arterioles, increasing BVA (Shelly, 2007).
Arterioles are almost microscopic (8-50 microns in diameter) that deliver blood to capillaries and anastomoses (Fox & Rompolski, 2022).
BVP detects the peak of the pulse wave using a photoplethysmograph (PPG) sensor. A PPG sensor measures relative blood flow through tissue using an infrared transducer. The intensity of the light reaching the sensor varies with brief blood volume shifts (Shaffer & Combatalade, 2013).
Skin Preparation
Only handwashing is needed since a PPG sensor detects infrared light instead of an electrical potential. Handwashing prevents dirt from occluding the PPG sensor’s transducer window (Shaffer & Combatalade, 2013).
PPG Sensors
A PPG sensor shines an infrared light through or off tissue.
Transmission PPG
In a transmission PPG, an LED and photodetector are placed on opposite sides of the tissue. This limits sensor placement to extremities like the digits and earlobe (Pereira et al., 2020).
The thumbs provide the best signal quality (highest energy and smallest ripples). Use the thumbs when a client's fingers are too small or have insufficient blood flow to detect a strong pulse (Peper, Shaffer, & Lin, 2010).
Fingers Versus Earlobes
Finger placements yield cleaner signals and greater measurement stability than earlobe placements, which are limited by fewer and weaker capillaries.
However, the earlobe is less prone to movement and vasoconstriction artifacts than the fingers. Since the earlobe is closer to the heart than the finger, less vascular tone rhythm contaminates frequency-domain measurements in the VLF, LF, and HF ranges (Lehrer, 2018).
Hands
The left and right hands are equivalent (Pribil et al., 2020).
Precautions
Instruct clients to avoid dark fingernail polish, which blocks light transmission.
Reflectance PPG
In a reflectance PPG, an LED and photodetector are placed on the same side of the tissue. This permits diverse placement sites like the ankle, forearm, wrist, and forehead (Pereira et al., 2020).
Transmission Versus Reflectance PPG Accuracy
Transmission PPG is generally more accurate than reflectance PPG because the detected light directly corresponds to blood volume changes. In contrast, reflectance PPG is affected by factors affecting backscattered light, such as ambient light, skin pigmentation, and tissue thickness (Elgendi, 2012). As a result, reflectance PPG signals are typically weaker and more prone to noise than transmission PPG signals (Allen, 2007).
How to Choose Between Transmission and Reflectance Methods
The choice between transmission and reflectance PPG depends on your specific application and measurement site. For example, transmission PPG is better suited for thin tissues like the fingertip or earlobe, while reflectance PPG is more suitable for thicker tissues like the forehead or wrist.
The BVP Waveform
Blood appears red because it reflects red wavelengths. More light is reflected, and the BVP signal increases when blood volume increases ①.
As the surge of blood ebbs, less light is reflected, and the BVP signal declines as the blood volume decreases. As the systolic pulse wave travels through the vascular tree, it is reflected by the lower body and appears as a second peak ②.
The dicrotic notch ③ is the gap between the direct and reflected waves. The dicrotic notch may be reduced or missing in atherosclerosis patients due to arteriole stiffness. Rigid arteriole walls do not reflect the systolic pressure wave as well as supple ones.
Pulse Amplitude Variability
A PPG sensor also detects pulse amplitude variability. Pulse amplitude variability (PAV) is the variability in pulse waveform amplitude. PAV indexes vascular tone regulation (Kamath & Fallen, 1993). PAV is reduced in atherosclerosis, hypertension, and peripheral artery disease (Wilkinson et al., 2000; Zong et al., 2003).
PPG Sensor Attachment
For finger or thumb placements, attach the PPG sensor using an elastic band or Coban™ tape to the palmar side of a larger finger and confine the sensor to one segment.
Limb Position
Sensor position relative to the heart strongly affects BVP. The BVP signal amplitude increases if the PPG sensor is placed on a limb below the heart. If the limb is placed above the heart, BVP signal amplitude decreases. These changes may reflect venous filling (Peper, Shaffer, & Lin, 2010).
Controlling BVP Artifacts
Artifacts are false values produced by the client’s body (arrhythmias) and actions (movement), the environment (line current), and hardware limitations (light leakage). Graphic © Victor Correia/Shutterstock.com.
Use clean signals as a reference. Graphic courtesy of Daniel Matto.
Dr. Erik Peper recommends that the best way to recognize artifacts using your equipment is to create them intentionally. This won’t break your encoder box. See how they change the signal waveform and derived HRV statistics.
Dr. Richard Sherman advises clinicians to use themselves as test equipment by knowing their typical values, performing tracking tests to ensure a display mirrors their clients’ psychophysiological performance, and inspecting the raw signal to detect contamination by artifacts.
Inspect the raw BVP signal for cardiac arrhythmias, line interference, light, movement, pressure, and cold artifacts. Prevent artifacts before you record data so that you don’t have to remove them later. You should discard epochs (segments) containing 5% contamination.
Cardiac Arrhythmias
Cardiac conduction artifacts include atrial fibrillation, premature atrial contractions, and premature ventricular contractions.
Atrial Fibrillation
Atrial fibrillation (AFib) involves disorganized cardiac conduction by the upper heart chambers. AFib is a supraventricular arrhythmia, with HRs reaching 160 beats per minute.
Cardiac conduction is chaotic in clients who experience this disorder (Tortora & Derrickson, 2021). Graphic © Designua/Shutterstock.com.
AFib can contaminate the BVP signal. Visual inspection can detect its presence (Pereira et al., 2020). AFib appears as a low-amplitude BVP signal and faster HR.
Premature Atrial Contractions
Premature atrial contractions (PACs) involve early atrial contraction, are characterized by abnormally shaped P-waves, and result in calculating extra beats (Lehrer, 2018).
Premature Ventricular Contractions
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).
Since atrial fibrillation, PAC, and PVC artifacts cannot be prevented, they must be eliminated by artifacting.
Controllable BVP Artifacts
Inspect the raw BVP signal for line interference, light, movement, pressure, and cold artifacts.
Line Interference Artifact
Line interference (50/60 Hz) artifacts appear as ripples during downswings in the raw blood volume pulse signal if a PPG sensor cannot filter it out (Elgendi, 2012; Shaffer & Combatalade, 2013). Graphic reproduced from Elgendi.
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 artifacts.
Light Artifacts
Light artifacts occur when ambient light overloads a PPG sensor’s photodetector producing large peak-to-trough differences (Cherif et al., 2016; Shaffer & Combatalade, 2013). Graphic reproduced from Cherif et al.
Clinical Tips to Minimize Light Artifacts
1. Cover the PPG sensor with a baby sock, Coban™, a dark cloth, or Velcro ®.
2. Avoid direct illumination of the PPG sensor.
3. Instruct clients to restrict movement and verify compliance.
4. Examine the raw signal for artifacts.
Movement Artifacts
Sensor movement artifacts are 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 can distort the BVP waveform in different ways.
The display may show sudden changes in the raw BVP signal and HR.
HR may suddenly increase.
Clinical Tips to Minimize Movement Artifacts
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 artifacts.
Pressure Artifacts
Pressure artifacts can be caused by wrapping a restraining band too tightly. Clients 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 interbeat intervals (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 (Peper, Shaffer, & Lin, 2010).
Clinical Tips to Minimize Pressure Artifacts
1. Readjust the tightness of the restraining band.
2. Keep pressure off the PPG sensor.
3. Examine the raw signal for artifacts.
Cold Artifacts
Cold artifacts, produced by cold exposure or sympathetically-mediated vasoconstriction, can reduce or eliminate a pulse wave. Cold artifacts may result in missed beats, resulting in artifactually lengthened IBIs (Shaffer & Combatalade, 2013).
Linn and colleagues (2015) demonstrated that anger can significantly reduce blood volume amplitude.
Clinical Tips to Minimize Cold Artifacts
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.
4. Provide your clients several minutes to relax.
5. Encourage your clients to arrive early to acclimate during winter. A warm fireplace or shawl may be helpful.
6. Examine the raw signal for artifacts.
Tracking Test
Performing a tracking test, you can determine whether a heart rate display mirrors your client's breathing. Your client's HR should speed during inhalation and slow during exhalation.
Dr. Inna Khazan demonstrates BVP and respiration recording, artifacts, and a tracking test © Association for Applied Psychophysiology and Biofeedback. 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.
There Are No Blood Pulse Amplitude Norms
Like respirometers, flexible respiratory sensors, PPG sensors provide relative measurements that can be affected by the placement site, room environment, and skin characteristics. For example, the transmission method is influenced by a digit's vascular supply. The reflectance method is also vulnerable to factors affecting backscattered light, such as ambient light, skin pigmentation, and tissue thickness.
For this reason, Dr. Inna Khazan (2019) did not report pulse amplitude norms in her excellent "A guide to normal values for biofeedback" in Physiological Recording Technology and Applications in Biofeedback and Neurofeedback (pp. 2-6). Association for Applied Psychophysiology and Biofeedback.
Clinicians and researchers can compare pulse amplitude across consecutive conditions (e.g., anger versus recovery) but not across sessions (e.g., session 1 versus session 4 prebaseline).
Quiz
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Glossary
arteriole: an almost-microscopic blood vessel that delivers blood to capillaries and anastomoses.
artifacts: false signals.
atherosclerosis: a chronic inflammatory disease characterized by the buildup of plaque, primarily lipids and fibrous tissue, on arterial walls, leading to narrowed and less elastic arteries
atrial fibrillation (AFib): a form of supraventricular arrhythmia with a HR of up to 160 beats per minute.
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).
cardiac conduction artifacts: abnormal heart electrical activity, including atrial fibrillation, premature atrial contractions, and premature ventricular contractions.
Coban™ tape: a self-adhering elastic wrap.
cold artifacts: cold exposure or sympathetically-mediated vasoconstriction that can reduce or eliminate a pulse wave.
dicrotic notch: small downward deflection observed in the blood volume pulse (BVP) signal, indicative of the closure of the aortic valve after the main systolic peak.
epoch: recording period,
extra beats: ECG and PPG artifacts can shorten the IBI when signal distortion causes the software to detect nonexistent beats.
heart rate (HR): the number of heartbeats per minute, also called stroke rate.
heart rate variability (HRV): beat-to-beat changes in HR, including changes in the RR intervals between consecutive heartbeats.
high-frequency (HF) band: a HRV frequency range from 0.15-0.40 Hz that represents the inhibition and activation of the vagus nerve by breathing (respiratory sinus arrhythmia).
interbeat interval (IBI): the time interval between the peaks of successive heartbeats.
light artifacts: PPG artifact when light leakage increases BVP amplitude.
line interference artifacts: ECG and PPG artifact when 50/60Hz contamination of signals causes the software to detect nonexistent beats and shorten the IBI.
low-frequency (LF) band: a HRV frequency range of 0.04-0.15 Hz that may represent the influence of PNS and baroreflex activity.
missed beats: ECG and PPG artifacts can lengthen the IBI when signal distortion causes the software to overlook a beat and use the next good beat.
movement artifacts: ECG and PPG artifacts can shorten the IBI when signal distortion from movement causes the software to detect nonexistent beats.
P-waves: the depolarization of the atria, which precedes atrial contraction. It is the first positive deflection on the ECG tracing.
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.
premature atrial contraction (PAC): abnormally shaped P-waves that result in calculating extra beats and and distorting the BVP and ECG signals.
premature ventricular contraction (PVC): extra heartbeats that originate in the ventricles instead of the S-A node of the heart and can distort the BVP and ECG signals.
pressure artifacts: 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.
reflectance PPG: an LED and photodetector are placed on the same side of the tissue.
respirometer: flexible respiratory sensor.
systolic pulse wave: the upward surge in arterial pressure generated by ventricular systole, captured as the first and highest peak in a pulse wave analysis.
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.
transmission PPG: an LED and photodetector are placed on opposite sides of the tissue.
very-low-frequency (VLF): a HRV frequency range of 0.003-0.04 Hz that may represent temperature regulation, plasma renin fluctuations, endothelial, physical activity influences, and possible intrinsic cardiac, and PNS contributions.
References
Aldini, M. (2016). Issues in heart rate variability (HRV) analysis: Motion artifacts & ectopic beats. Blog post: https://www.hrv4training.com/blog/issues-in-heart-rate-variability-hrv-analysis-motion-artifacts-ectopic-beats
Allen J. (2007). Photoplethysmography and its application in clinical physiological measurement. Physiological Measurement, 28(3), R1–R39. https://doi.org/10.1088/0967-3334/28/3/R01 Andreassi, J. L. (2000). Psychophysiology: Human behavior and physiological response. Lawrence Erlbaum and Associates, Inc.
Béres, S. & Hejjel, L. (2021). The minimal sampling frequency of the photoplethysmogram for accurate pulse rate variability parameters in healthy volunteers. Biomedical Signal Processing and Control, 68, 102589. https://doi.org/10.1016/j.bspc.2021.102589
Berntson, G. G., Quigley, K. S., & Lozano, D. (2007). Cardiovascular psychophysiology. In J. T. Cacioppo, L. G. Tassinary, & G. G. Berntson (Eds.). Handbook of psychophysiology (3rd ed.). Cambridge University Press.
Cherif, S., Pastor, D., Nguyen, Q.-T., & L’Her, E. (2016). Detection of artifacts on photoplethysmography signals using random distortion testing. 38th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC). http://dx.doi.org/10.1109/EMBC.2016.7592148
Combatalade, D. (2010). Basics of heart rate variability applied to psychophysiology. Thought Technology Ltd.
Constant, I., Laude, D., Murat, I., & Elhhozi, J.-L. (1999). Pulse rate variability is not a surrogate for heart rate variability. Clinical Science, 97(4), 391–397. https://doi.org/10.1042/cs0970391.
Elgendi, M. (2012). On the analysis of fingertip photoplethysmogram signals. Current Cardiology
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Fox, S. I., & Rompolski, K. (2022). Human physiology (16th ed.). McGraw-Hill.
Hemon, M. C., & Phillips, J. P. (2016). Comparison of foot finding methods for deriving instantaneous pulse rates from photoplethysmographic signals. Journal of Clinical Monitoring and Computing, 30(2), 157–168. https://doi.org/10.1007/s10877-015-9695-6.
Jan, H.-Y., Chen, M.-F., Fu, T.-C., Lin, W.-C., Tsai, C.-L., & Lin, K.-P. (2019). Evaluation of coherence between ECG and PPG derived parameters on heart rate variability and respiration in healthy volunteers with/without controlled breathing. Journal of Medical and Biological Engineering, 39, 783-795. https://doi.org/10.1007/s40846-019-00468-9
Kamath, M. V., & Fallen, E. L. (1993). Power spectral analysis of heart rate variability: A noninvasive signature of cardiac autonomic function. Critical Reviews in Biomedical Engineering, 21(3), 245–311. PMID: 8243093
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Lehrer, P. M. (2012). Personal communication.
Lin, I. M., Fan, S. Y., Lu, Y. H., Lee, C. S., Wu, K. T., & Ji, H. J. (2015). Exploring the blood volume amplitude and pulse transit time during anger recall in patients with coronary artery disease. Journal of Cardiology, 65(1), 50–56. https://doi.org/10.1016/j.jjcc.2014.03.012
Peper, E., Harvey, R., Lin, I., Tylova, H., & Moss, D. (2007). Is there more to blood volume pulse than heart rate variability, respiratory sinus arrhythmia, and cardio-respiratory synchrony?Biofeedback, 35(2), 54-61.
Peper, E., Shaffer, F., & Lin, I-M. (2010). Garbage In; Garbage out—Identify blood volume pulse (BVP) artifacts before analyzing and interpreting BVP, blood volume pulse amplitude, and heart rate. Biofeedback, 38(1), 19-23. https://doi.org/10.5298/1081-5937-38.1.19
Pereira, T., Tran, N., Gadhoumi, K., Pelter, M. M., Do, D. H., Lee, R. J., Colorado, R., Meisel, K., & Hu, X. (2020). Photoplethysmography based atrial fibrillation detection: A review. npj digital medicine, 3(3). https://www.nature.com/articles/s41746-019-0207-9#Fig1
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Shelley K. H. (2007). Photoplethysmography: beyond the calculation of arterial oxygen saturation and heart rate. Anesthesia and Analgesia, 105(6 Suppl), S31–S36. https://doi.org/10.1213/01.ane.0000269512.82836.c9
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BCIA 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.
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