Heart rate variability biofeedback (HRVB) aims to exercise the baroreceptor
reflex and the vascular tone rhythm to enhance homeostatic regulation, regulatory reserve, and executive functions (Gevirtz, 2021).
Lehrer and colleagues (2000, 2013) have published two descriptions of their HRVB RF protocol detailing their assessment and training approaches.
The client-practitioner relationship is the foundation of BFT and NFT. HRV biofeedback is most effective when clients and providers are mindful because this promotes self-awareness and a sense of agency. HRV biofeedback promotes self-regulation when practitioners provide effective coaching (Khazan, 2019).
Mindfulness guides the trial-and-error process underlying self-regulation by helping clients draw connections
between their actions, internal feedback, and results. Graphic retrieved from Pinterest.
Slow-paced breathing involves healthy breathing near an
individual's unique resonance frequency (RF) or 6 breaths per minute (bpm). HRV biofeedback doesn't train clients to increase power at 0.1-Hz if their optimal breathing rate is above or below 6 bpm.This Real Genius episode was drawn by Dani S@unclebelang.
HRV biofeedback stimulates the
baroreceptor reflex and vascular tone rhythm, increasing vagal tone and HRV.
Mindful low-and-slow breathing can amplify RSA. The graphic below was adapted from Grossman and Kollai (1993). RSA, shown as a change in heart rate from inhalation to exhalation, increases as the respiration rate approaches 6 bpm.
While your clients practice slow-paced breathing near their RF, this will increase peak-to-trough heart rate (HR) differences, RMSSD, and low-frequency (LF) power. Following weeks of practice, they should expand their peak-to-trough HR fluctuations, RMSSD, and high-frequency (HF) power when breathing at typical rates without feedback or pacing (Gevirtz, 2021).
BCIA Blueprint Coverage
This unit addresses V. HRV Biofeedback Strategies: A. How to explain HRV Biofeedback to a client, E. How to structure an HRV biofeedback training session, F. How to augment training with emotional regulation strategies, G. HRV biofeedback side effects and contraindications, and H. Practice assignments to promote generalization.
This unit covers Medical Cautions, Clinical Tips When You Start HRV Biofeedback Training, HRV Biofeedback Training, and Practice Assignments.
Please click on the podcast icon below to hear a full-length lecture.
Medical Cautions
Clinicians should examine ECG morphology for evidence of arrhythmias, ischemia, and prolonged QT intervals that could jeopardize patient safety (Drew et al., 2004) as part of an assessment preceding HRVB training. Clinicians should encourage their clients to consult with their physicians when there is evidence of conduction abnormalities. They should proceed with training if the physician considers it appropriate.
Atrial fibrillation is the most common cardiac arrhythmia and involves the
rapid, irregular contraction of the two upper atrial chambers.
A prolonged QT interval is associated with an increased risk of ventricular tachyarrhythmias, resulting in
cardiac arrest and sudden death. Graphic retrieved from Semantic Scholar.
Clinical Tips When You Start HRV Biofeedback Training
Critical elements of HRVB training include modeling, your relationship with your client, passive volition, monitoring HRV, selecting an effective display, teaching rhythmical skeletal muscle tension instead of paced breathing, monitoring excessive effort, engaging games and apps, pacing displays, resting heart rate (HR), and emotional self-regulation.
Consider an ECG sensor on the wrists or a PPG sensor on an earlobe or finger for clinical work.
A PPG sensor trades accuracy for ease of application. An ECG sensor may be required if there is significant vasoconstriction due to outdoor temperature or stress, movement, or standing (Constant et al., 1999; Hemon & Phillips, 2016; Jan et al., 2019; Medeiros et al., 2011).
Selecting an Effective Display
Provide an HRV training screen with respirometer and instantaneous HR waveforms.
Analog displays can provide your client with incredibly detailed and intuitive information. Encourage your client to
increase the peak-to-trough swings in heart rate during each breathing cycle (Lehrer et al., 2013; Lehrer & Gevirtz, 2014).
While client preference and success should guide your selection of feedback displays, you might first try feedback of LF power and peak-to-trough differences.
The concentration of signal power around 0.1 Hz in the LF band corresponds to the Institute of HeartMath's concept of coherence, in which a client produces a "narrow, high-amplitude, easily visualized peak" from 0.09-0.14 Hz (Ginsberg, Berry, & Power, 2010, p. 54; McCraty et al., 2009).
The BioGraph ® Infiniti screen shown below is
designed to increase the percentage of power in the LF
band. This display shows the dynamic change in the
distribution of HRV amplitude as a client breathes effortlessly and
cultivates positive emotion.
Some clients may prefer a display of the synchrony (alignment of peaks and troughs) between respirometer and instantaneous HR signals. Synchrony is crucial because it amplifies the resonance effects of slow-paced breathing (Vaschillo et al., 2002). The NeXus-10 ® BioTrace+ screen below displays the degree of synchrony between respiration and heart rate. The flower opens as synchrony increases.
Teach Slow-Paced Contraction Along With Slow-Paced Breathing
Slow-paced contraction (SPC) offers an alternative to slow-paced breathing, which is sometimes challenging (e.g., chronic pain) or medically contraindicated (e.g., kidney disease). SPC may be helpful for clients who breathe dysfunctionally or who cannot slow their breathing to the adult resonance frequency range (4.5 to 6.5 bpm).
In SPC exercises, clients briefly contract and relax skeletal muscles (wrist and ankles or wrist, core, and ankles) at the same 4.5 to 6.5 cpm rates as they breathe normally. For example, for 6 cpm, a display would prompt them to contract their muscles for 4 s and relax for 6. Contraction force should be moderate, but not maximal, to ensure a smooth rhythm and minimize fatigue.
The video below shows 6-cpm SPC. Observe the recruitment of the core muscles, including the rectus abdominis.
The video below shows 6-cpm SPC. The model breathed slowly without instructions. The SPC entrained the breathing rhythm.
Like slow-paced breathing, continuous muscle contraction generates the largest heart rate oscillations and stimulation of the baroreceptor reflex to increase heart rate variability.
Maximum-Minimum heart rate for each breath indexes respiratory sinus arrhythmia (RSA). The peak frequency is the HRV frequency with the greatest power. In the screen captures below, SPC stimulated the baroreceptor reflex at the intended frequency (0.2 Hz for 12 cpm and 0.1 Hz for 6 cpm).
Choose an ECG sensor using a chest or upper torso placement, shown respectively.
Alternately, select a PPG sensor attached to an earlobe. Graphic courtesy of the Institute of HeartMath.
While reclining with feet supported by a chair, your clients can rhythmically contract their hands, core, and feet for 3 seconds at their resonance frequency (i.e., optimal stimulation rate) to increase heart rate variability (Vaschillo et al., 2011). Although the original Vaschillo protocol only contracted wrists and ankles with legs uncrossed, we have observed greater RSA using wrist, core, and crossed-ankle contraction as shown below.
Monitoring Excessive Effort
Excessive effort (active volition) during HRVB training can increase SNS activation, reduce vagal tone, and promote overbreathing. Clinicians can detect effort by monitoring respirometer, HR and HRV, accessory muscle SEMG, and capnometer waveforms.
Respirometer
Breathing effort can disrupt a smooth breathing waveform and create inflection points.
HR and HRV
If training effort produces vagal withdrawal, HR may increase with very-low-frequency (VLF) band power, and HRV may decrease.
In the FFT spectral plot, VLF power is colored gray.
The BioGraph ® Infiniti screen below provides respiratory
and SEMG biofeedback to teach rhythmic breathing while maintaining
relaxed accessory muscles.
Note the overuse of accessory muscles during clavicular breathing.
Once your client has mastered slow-paced breathing, games can motivate practice and speed skill
acquisition. Biofeedback software allows clients to challenge themselves with increasing game difficulty levels without triggering parasympathetic withdrawal. This is crucial for transferring self-regulation skills to everyday life.
Click the Read More button to see screens from
Zukor's Drive, Zukor's Sport, the Garden Game, BioGraph Infiniti, Inner Balance, Elite HRV, HRV4 Training, and Camera HRV.
Pacing Displays
Pacers can guide breathing and RSMT using animation and sound. Software may integrate a pacer into an HRVB display or be separate. Assign practice with breathing pacers and then gradually fade them.
You may use a computer, pad, and smartphone apps that provide auditory or visual pacing. Try them out to find the apps that offer the adjustability and ease of use best for your clients.
Consider
Coherence Coach and EZ-Air Plus for computers.
These apps are available for both Android and Apple platforms.
Use a stopwatch app that continuously loops when teaching RSMT instead of slow-paced breathing. For 6 contractions per minute (cpm), select 10 seconds with no pause and instruct your client to simultaneously contract their hands and feet for the first 3 seconds of each cycle.
Clients can increase RSA and HRV time- and frequency-domain measurements during their initial training session. After four 30-minute sessions, many clients correct dysfunctioning breathing and increase vagal tone and HRV. However, they may require extended training
and practice (e.g., 10 or more sessions) to achieve maximum health and performance gains (Lagos et
al., 2011).
This section covers session structure, resting pre- and post-baseline measurements,
how to select a starting respiration rate, training introduction, training success indicators, training difficult indicators, training segment review, promoting mindful breathing, session review, comparing pre- and post-session values, HRV biofeedback training elements, and how many sessions are required.
Session Structure
Resting Pre- and Post-Baseline Measurements
Resting baselines measure your clients’ psychophysiological activity without feedback or paced breathing. You might instruct your clients to “sit quietly and breathe normally for the next few minutes.” As appropriate, you might monitor HRV, breathing (depth, pattern, and rate), autonomic activity (skin conductance and temperature), BP, and end-tidal CO2.
Compare pre-baseline to post-baseline changes within a session to show within-session learning. Evaluate pre-baseline changes across sessions to see the combined effect of clinic training and home practice.
Since your clients will breathe at typical rates (~12-16 bpm) during pre- and post-baselines, do not expect increases in LF power. Instead, look for increased HF power, HRV time-domain metrics (e.g., RMSSD), and hand temperature, and reduced skin conductance level.
We train clients to increase LF power during slow-paced breathing to increase HF power during baselines when they breathe at typical rates.
How to Select a Starting Respiration Rate
Select an initial respiration rate for an animated pacing display based on your client's RF. Since shaping is crucial to ensuring client motivation and success, choose a respiration rate within 1 or 2 breaths per minute of their baseline mean.
Training Introduction
Click on the Read More button to see sample introductory instructions.
A healthy heart is not a metronome. As you inhale, your heart rate speeds, and as you exhale, your heart rate
slows. This rhythmic speeding and slowing of your heart produces heart rate variability, which is vital to your
health, performance, and resilience against stressors. The purpose of heart rate variability biofeedback training is
to teach you to increase the healthy speeding and slowing of your heart by breathing effortlessly at the rate that
is best for you and by increasing your ability to experience positive emotions like feelings of
appreciation and gratitude.
Adopt a passive attitude in which you trust your body to breathe itself. Allow your attention to settle on your waist. Let your exhalation continue until your body initiates your next breath. Your inhalations should be no deeper than if you were smelling a flower. Allow your breathing to follow the yellow ball effortlessly.
Allow
your stomach to gradually plop out as you inhale and then slowly draw inward as you exhale. As you practice, we will
adjust the speed of the pacing display. Let it guide your inhalation and exhalation. Allow
your stomach to gradually plop out as you inhale and then slowly draw inward as you exhale. As you practice, we will
adjust the speed of the pacing display.
The computer can help you learn slow, effortless breathing. The pink tracing shows your heart rate, while the
violet tracing shows the movement of the sensor around your stomach. As you gradually learn low-and-slow breathing,
the two tracings should resemble smooth, repeating ocean waves.
Start recording data for 3 minutes. At the end of the training segment, ask: ”How was the speed of the pacing
display? Should we change it? Should we adjust the inhalation and exhalation lengths? What did you experience as you
practiced breathing effortlessly?"
Training Success Indicators
Training Difficulty Indicators
Training Segment Review
Fit an entire 3-minute segment on the screen to review it together. If your client succeeded, you might point out where
they succeeded.
Promote Mindful Breathing
Before starting the next segment, you might ask: "What were you doing when the display became wavelike and regular?
What happened when the display became more jagged and irregular?" If accessory SEMG exceeded 2 microvolts, point
this out on the display, ask them if they felt the heightened breathing effort, and encourage them to "let their
shoulders relax and allow themself to breathe."
Reassure them that it's normal for the
tracings to be choppy when people start training and that they will gradually become more wavelike as their breathing
becomes more rhythmic and regular.
Instead of overwhelming them with corrections, ask them to experiment with one or two changes at a time. For example,
"Effortless breathing is rhythmic like ocean waves. Allow your stomach to gently expand and contract as you follow
the pacing display."
Session Review
After your client has completed six 3-minute training segments, take a 3-minute post-baseline without feedback. After
the post-baseline, ask your client how they felt and what they learned during the training session. Display the entire
session on one screen and highlight where they succeeded and where they need more work.
Comparing Pre- and Post-Session Values
Without pacing or feedback, your client should breathe at typical rates during the pre- and post-
baselines. HF power, RMSSD, and hand temperature may increase, while the skin conductance level may decrease.
The graphic below shows HF power in blue during a pre-training baseline, HRVB training, and a post-training baseline.
The y-axis shows power in each band. HF power increases from ~100 μV during the pre-training baseline to ~300 μV during the post-training baseline. This change is important because increased HF power can signal greater vagal tone.
Also, note the greater LF power concentration post-training compared with pre-training during which the client breathed at typical rates. Inna Khazan generously provided the spectral plots.
HRV Biofeedback Training Elements
How Many HRVB Sessions Are Required?
Many clients start to breathe more effortlessly and increase HRV during their initial training session. There can be a several-week lag between increased HRV and improved health or performance. Clients require this time to consolidate their learning and transfer enhanced skills to the diverse settings of their lives.
Practice is the bridge between the clinic and everyday life.
Increased RSA immediately “exercises” the baroreflex without changing vagal tone or tightening blood pressure regulation. Those changes require months of practice (Gevirtz, Lehrer, & Schwartz, 2016; Lagos et al., 2011).
Assign 20 minutes of practice twice a day with HRV monitoring. After each practice session, they can fill out a diary online and send you their IBIs. Khazan (2013) provides an excellent selection of logs in The Clinical Handbook of Biofeedback that you can adapt.
How much will your clients actually practice? You may have to settle for 10 minutes once a day or
“as needed.” Lehrer et al. (2020a) concluded that the frequency of home practice length did not impact effect size. The practice of resonance frequency breathing as needed may have produced most of their gains (p. 125).
As recommended by Gevirtz, include starting and ending hand temperatures since successful HRVB practice may produce hand-warming.
Explain the purpose of the exercise, demonstrate the skill in the clinic, and confirm that they can correctly perform it
and agrees to practice it. To build a collaborative relationship and empower your client, encourage them to find ways to
improve the exercise or develop a different one.
Overview
This section addresses practice in diverse settings,
aerobic activity, monitor HR, monitor HRV, emotional self-regulation, home HRV practice, advanced HRV assessment, and monitor finger temperature.
Invite clients to monitor their HR during daily activities and emotional states to increase mindfulness of stressors and their physiological responses to these challenges.
Monitor HRV
Apps like Elite HRV allow you to take HRV snapshots with limited artifact correction.
Institute of HeartMath Lock-In ® Technique Instructions.
Try to focus your attention on the area around your heart. Maintain your heart focus and, while breathing,
imagine that your breath is flowing in and out through the heart area. Breathe casually, just a little deeper than
normal.”
Now try to recall a positive emotion or feeling that makes you relaxed and comfortable. Find a positive
feeling like appreciation, care, joy, kindness, or compassion. You can recall a time you felt appreciation or care.
This could be the appreciation or care you feel towards a special person, a pet, a place you enjoy, or an activity
that was fun for you.
If you can’t feel anything, it’s ok. Just try to find a sincere attitude of appreciation or care.
Continue to think of this positive feeling or emotion.
Acceptance and Commitment Therapy (ACT) and compassion meditation may also aid emotional self-regulation.
Home HRV Practice
Portable HRV biofeedback devices like the Institute of HeartMath's Inner Balance allow personal training whenever your clients want.
Advanced HRV Assessment
The Institute of HeartMath's emWave Pro Plus provides HRV assessment using their automated deep breathing protocol. The software reports commonly-used HRV metrics referenced to age-related norms.
The emWave Pro Plus also reports resting metrics for training sessions ranging from 1 to 99 minutes.
Monitor Finger Temperature
Encourage your clients to monitor their hand temperature using inexpensive alcohol thermometers to see whether their practice resulted in warming or cooling.
accessory muscles: the sternocleidomastoid, pectoralis minor, scalene, and
trapezius muscles, which are used during forceful breathing, as well as clavicular and thoracic breathing.
apnea: breath suspension.
baroreceptor reflex (baroreflex): a mechanism that provides negative feedback control of BP. Elevated BP activates the baroreflex to lower BP, and low BP suppresses
the baroreflex to raise blood pressure.
capnometer: an instrument that monitors the carbon dioxide (CO2) concentration in an
air sample (end-tidal CO2) by measuring the absorption of infrared light.
clavicular breathing: a breathing pattern that primarily relies on the external
intercostals and the accessory muscles to inflate the lungs, resulting in a more rapid respiration rate, excessive
energy consumption, and incomplete ventilation of the lungs.
coherence: a narrow peak in the BVP and ECG power spectrum between 0.09 and 0.14
Hz.
diaphragm: the dome-shaped muscle whose contraction enlarges the vertical diameter
of the chest cavity and accounts for about 75% of air movement into the lungs during relaxed breathing.
end-tidal CO2: the percentage of CO2 in
exhaled air at the end of exhalation.
frequency-domain measures of HRV: the calculation of the absolute or relative power
of the HRV signal within four frequency bands.
high-frequency (HF) band: the HRV frequency range from 0.15-.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 R-spikes (initial upward
deflections in the QRS complex). This is also called the NN (normal-to-normal) interval after removing artifacts.
low-frequency (LF) band: the HRV frequency range of 0.04-0.15 Hz that may represent the influence of PNS and baroreflex activity when breathing at the RF.
metabolic acidosis: a pH imbalance in which the body has accumulated excessive acid and has
insufficient bicarbonate to neutralize its effects. In diabetes and kidney disease, hyperventilation attempts
to compensate for abnormal acid-base balance, and slower breathing could endanger health.
mindfulness: a nonjudgmental focus of attention on the present on a
moment-to-moment basis.
peak frequency: the HRV frequency with the greatest power.
pulse oximeter: a device that measures dissolved oxygen in the bloodstream using a
photoplethysmograph sensor placed against a finger or earlobe.
resilience: adapting effectively to stressors, threats, and trauma.
resonance frequency: the frequency at which a system, like the cardiovascular
system, can be activated or stimulated.
reverse breathing: the abdomen expands during exhalation and contracts during
inhalation, often resulting in incomplete ventilation of the lungs.
slow-paced breathing (SPB): diaphragmatic breathing between 4.5 to 6.5 bpm.
slow-paced contraction (SPC): the simultaneous contraction of the wrists, core, and ankles to increase RSA.
synchrony: the phase relationship between two signals in which their peaks and
valleys are aligned (e.g., HR and respirometer expansion reach their maximum and minimum values simultaneously).
thoracic breathing: a breathing pattern that primarily relies on the external
intercostals to inflate the lungs, resulting in a more rapid respiration rate, excessive energy consumption, and
incomplete ventilation of the lungs.
time-domain measures of HRV: indices like SDNN that measure the degree to which
the IBIs between successive heartbeats vary.
torr: the unit of atmospheric pressure, named after Torricelli, which equals 1 millimeter of mercury
(mmHg) and is used to measure end-tidal CO2.
trapezius-scalene placement: active SEMG electrodes are located on the upper trapezius and scalene
muscles to measure respiratory effort.
ultra-low-frequency (ULF) band: the HRV frequency range below 0.003 Hz. Very-slow
biological processes may contribute to this band, including circadian rhythms, core body temperature, metabolism, and the renin-angiotensin
system. PNS and SNS contributions are possible.
very-low-frequency (VLF): the HRV frequency range of 0.003-0.04 Hz that may represent temperature regulation, plasma renin fluctuations, endothelial and physical activity influences, and possible intrinsic cardiac, PNS, and SNS contributions.
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Assignment
Which of the two screens below shows better HRV performance? What is the basis for your judgment?
Check the mean respiration rates to confirm that this client breathed around 6 breaths per minute in each session. She
did. Examine the values for HR Max - HR Min and LF amplitude. Both were higher in the second session. The greater HR Max
- HR Min was also visually evident in the peak-to-trough differences, which were larger in the second session. Look at
the distribution of signal power at 0.1 Hz. Signal energy was concentrated at and around this dominant frequency in the
second session. Inspect the synchrony of the HR and respirometer tracings. The peaks and valleys of these tracing were
more closely aligned in the second session. Finally, compare hand temperatures. Her hands were almost 1 degree F warmer
in the second session.
These recordings were from a chronic pain patient who received treatment in an interdisciplinary chronic pain
program.
References
Bax, A., Robinson, T., & Goedde, J. (2007). The Cousins relaxation exercise increases heart rate variability
[Abstract]. Applied Psychophysiology and Biofeedback, 32(1), 52.
Carter, R., Cheuvront, S. N., Wray, D. W.,
Kolka, M. A., Stephenson, L. A., & Sawka, M. N. (2005). The influence of hydration status on heart rate variability after exercise heat stress. Journal of Thermal Biology, 30(7), 495-502. https://doi.org/10.1016/j.jtherbio.2005.05.006
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. PMID: 10491338
Gevirtz, R. N., Lehrer, P. M., & Schwartz, M. S. (2016). Cardiorespiratory biofeedback. In M.S. Schwartz & F. Andrasik (Eds.). Biofeedback: A practitioner’s guide (4th ed.). The Guilford Press.
Grant, J., Wally, C., & Truitt, A. (2010). The effects of Kargyraa throat-singing and singing a fundamental note
on heart rate variability [Abstract]. Poster presented at the meeting of the Biofeedback Foundation of Europe, Rome,
Italy.
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
Kabat-Zinn, J. (1994). Wherever you go, there you are: Mindfulness mediation in everyday life.
Hyperion Books.
Lehrer, P. M. (2007). Biofeedback training to increase heart rate variability. In P. M. Lehrer, R. M. Woolfolk,
& W. E. Sime (Eds.). Principles and practice of stress management (3rd ed.). The Guilford
Press.
McCraty, R., Atkinson, M., Tomasino, D, & Bradley, R. T. (2006). The coherent heart. Institute of HeartMath.
McCraty, R., Atkinson M, Tomasino, D., & Bradley, R. T. (2009). The coherent heart: Heart-brain interactions,
psychophysiological coherence, and the emergence of system-wide order. Integral Review, 5(2), 10-115.
Medeiros, R. F., Silva, B. M., Neves, F. J., Rocha, N. G., Sales, A. R. K., & Nobrega, A. C. (2011). Impaired hemodynamic response to mental stress in subjects with prehypertension is improved after a single bout of maximal dynamic exercise. Clinics,66(9), 1523–1529. https://doi.org/10.1590/S1807-59322011000900003.
de Oliveira, R., Araújo, L. F., de Figueiredo, R. C., Goulart, A. C., Schmidt, M. I., Barreto, S. M., & Ribeiro, A. (2017). Coffee consumption and heart rate variability: The Brazilian Longitudinal Study of Adult Health (ELSA-Brasil) Cohort Study. Nutrients, 9(7), 741. https://doi.org/10.3390/nu9070741
Shaffer, F., Bergman, S., & Dougherty, J. (1998). End-tidal CO2 is the best Indicator of
breathing effort [Abstract]. Applied
Psychophysiology and Biofeedback, 23(2).
Shaffer, F., Bergman, S., & White, K. (1997). Indicators of
diaphragmatic breathing effort [Abstract]. Applied Psychophysiology
and Biofeedback, 22(2), 145.
Shaffer, F., Greve, E., & Reinagel, K. (1996). Predictors of inhalation volume [Abstract]. Biofeedback and
Self-Regulation, 21(4), 352.
Shaffer, F., & Moss, D. (2006). Biofeedback. In Y. Chun-Su, E. J. Bieber, & B. Bauer (Eds.). Textbook of
complementary and alternative medicine (2nd ed.). Informa Healthcare.
Vansickle, J., Putnam, R., Harris, M., Garcia, M.,
Bell, B., Nelson, M., & Lowery, L. (2020). Caffeine intake does not negatively affect heart rate variability in physically active university students: Preliminary findings. Current Development in Nutrition, 4(Supplement 2), 1770. https://doi.org/10.1093/cdn/nzaa066_025
Vaschillo, E. G., Vaschillo, B., Pandina, R. J., & Bates, M. E. (2011). Resonances in the cardiovascular system caused by rhythmical muscle tension. Psychophysiology, 48, 927–936. https://doi.org/10.1111/j.1469-8986.2010.01156.x
Wally, C., Korenfeld, I., Brooks, K., Carrell, D., Lau, D., Peterson, J., Schafer, M., Truitt, A., Fuller, J.,
Westermann-Long, A., & Korenfeld, D. (2011). Chanting “Om” increases heart rate variability by
slowing respiration [Abstract]. Applied Psychophysiology and Biofeedback, 36, 223.
Watso, J. C., & Farquhar, W. B. (2019). Hydration status and cardiovascular function. Nutrients, 11(8), 1866. https://doi.org/10.3390/nu11081866
Zerr, C., Kane, A., Vodopest, T., Allen, J., Fluty, E., Gregory, J., DeBold, M., Schultz, D., Robinson, G., Golan, R., Hannan, J., Bowers, S., Cangelosi, A., Korenfeld, D., Jones, D., Shepherd, S., Burklund, Z., Spaulding, K., Hoffman, W., & Shaffer, F. (2014). HRV biofeedback training raises temperature and lowers skin conductance. Applied Psychophysiology and Biofeedback, 39(3), 299. https://doi.org/10.1007/s10484-014-92549
Zerr, C., Kane, A., Vodopest, T., Allen, J., Hannan, J., Fabbri, M., Williams, C., Cangelosi, A., Owen, D., Cary, B., & Shaffer, F. (2015). Does inhalation-to-exhalation ratio matter in heart rate variability biofeedback? Applied Psychophysiology and Biofeedback, 40(2), 135. https://doi.org/10.1007/s10484-015-9282-0
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The QT interval signals the depolarization and repolarization of the ventricles. Graphic © Designua/Shutterstock.com.
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