Four HRV myths are Heart Rate Variability Is Bad, Stability Is Good, The Sympathetic Nervous System Plays a Major Role in Short-Term HRV, The Goal of HRV Biofeedback Is To Increase Low-Frequency Power At Rest, and Longer Exhalations Increase HRV.
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Myth 1: Heart Rate Variability Is Bad, Stability is Good
"We want a relatively stable average heart rate, but high variability of instantaneous heart rate" (Khazan, 2022).
"A healthy heart is not a metronome" (Shaffer, McCraty, & Zerr, 2014). When the time intervals between heartbeats significantly change across successive breathing cycles, this shows that the cardiovascular center can effectively modulate vagal tone.
"The complexity of a healthy heart rhythm is critical to the maintenance of homeostasis because it provides the flexibility to cope with an uncertain and changing environment . . . HRV metrics are important because they are associated with regulatory capacity, health, and performance and can predict morbidity and mortality" (Shaffer, Meehan, & Zerr, 2020).
Myth 2: The Sympathetic Nervous System Plays a Major Role in Short-Term HRV
We measure short-term HRV over ~ 5 minutes. Respiratory sinus arrhythmia (RSA), the baroreceptor reflex, and the vascular tone rhythm are the most important sources of HRV (Hayano & Yuda, 2019; Reyes de Paso et al., 2013; Vaschillo et al., 2002). These processes are parasympathetic, not sympathetic.
Respiratory sinus arrhythmia (RSA), HR speeding and slowing across each breathing cycle, is the most important HRV driver. The R-spikes are packed more closely together (faster HR) during inhalation and are more spaced out (slower HR) during exhalation. Graphic adapted from
Elite HRV.
The HR baroreceptor reflex, which exerts homeostatic control over acute BP changes, is second. The HR baroreflex regulates BP in response to HR changes. It has a ~5-second delay with a resonance frequency of ~ 0.1 Hz. Graphic © Alila Sai Mai/Shutterstock.com.
When HR speeds, BP decreases. When HR slows, BP increases. Graphic adapted from Evgeny Vaschillo.
Note. The bottom line represents respiration. A rising black bar is inhalation, and a falling black bar means exhalation. The following lines represent HR and BP. This diagram allows us to see the changes in HR and BP produced by breathing.
The vascular tone (VT) control system, which regulates arteriole (small muscular artery) diameter, is third. The VT control system regulates BP in response to arteriole diameter changes. When arterioles constrict, BP increases. When they dilate, BP decreases.
Summary of the Three PNS Resting HRV Sources
This has major implications for the controversial low-frequency/high-frequency (LF/HF) ratio.
The LF/HF Ratio
Let's revisit the LF band. The LF band (0.04-0.15 Hz) is comprised of rhythms with periods between 7 and 25 seconds, is affected by breathing from ~3-9 breaths per minute (bpm), and requires a recording period of at least 2 minutes (Task Force, 1996).
While there is disagreement regarding this band's activity sources, a sympathetic role during resting measurements appears unlikely (Hayano & Yuda, 2019). Unless you monitor participants during exercise or during a tilt table test, there should be no significant sympathetic contribution to LF power at rest.
Dr. Lehrer explains the LF band © Association for Applied Psychophysiology and Biofeedback.
Power is the signal energy contained within a given frequency band. The ratio of LF to HF power is called the LF/HF ratio. This ratio was initially based on 24-hour recordings, during which PNS and SNS activity contribute to LF power. PNS activity generates HF power. The intent was to estimate the ratio between SNS and PNS activity.
Calculating an LF/HF ratio from brief or resting recordings is controversial because there is usually no SNS activity during short-term resting measurements. Since the primary LF drivers (RSA, baroreflex activity, and vasomotor tone) at rest are parasympathetic, a PNS/PNS ratio cannot estimate autonomic balance (McCraty, 2013). From Laborde and colleagues' (2017) perspective:
. . . this view has been highly criticized (Billman, 2013; Eckberg, 1997). Among the most critical aspects is the loose relationship between LF power and sympathetic nerve activation, and the nonlinear and non-reciprocal relationship between sympathetic and parasympathetic nerve activity.
Myth 3: The Goal of HRV Biofeedback Is To Increase Resting LF Power
My colleague Dr. Donald Moss (2023) asks a deceptively simple question, "Why do we train HRV?"
His answer is that we train HRV to increase vagal tone to treat diverse disorders and enhance wellness and performance.
Lehrer and colleagues' (2020) systematic review and meta-analysis of 58 papers concluded:
A significant small to moderate effect size was found favoring HRVB, which does not differ from that of other effective treatments. With a small number of studies for each, HRVB has the largest effect sizes for anxiety, depression, anger and athletic/artistic performance and the smallest effect sizes on PTSD, sleep and quality of life. We found no significant differences for number of treatment sessions or weeks between pretest and post-test, whether the outcome measure was targeted to the population, or year of publication. Effect sizes are larger in comparison to inactive than active control conditions although significant for both. HRVB improves symptoms and functioning in many areas, both in the normal and pathological ranges. It appears useful as a complementary treatment.
Increasing LF power is a means to an end, not the goal itself. Increased LF power during HRV biofeedback training means that we are stimulating our interconnected baroreflexes (HR and BP) at an optimal rate to increase RSA.
Vaschillo's Two Closed-Loop Model
Vaschillo’s two closed-loop model explains how HRV biofeedback procedures like slow-paced breathing (SPB) and slow-paced contraction (SPC) can increase HRV. Vaschillo et al. (2002) described the HR and VT baroreflexes as closed loops and proposed that stimulating one closed loop activates its counterpart. Graphic adapted from Vaschillo et al. (2002).
Each baroreflex is a potential target for HRV biofeedback training. SPB and SPC at ~ 6 bpm/cpm can stimulate the HR baroreflex, separately or synergistically. SPC at ~ 1 cpm can activate the VT baroreflex.
We train clients to increase LF power using SPB or SPC to increase RSA and HRV instantly.
HRV biofeedback can immediately increase RSA 4-10 times compared to a resting baseline (Vaschillo et al., 2002). Graphic adapted from Gevirtz et al. (2016).
However, increasing vagal tone (parasympathetic firing) can require months of practice. Think of SPB or SPC as exercising a "muscle."
Athletes don't become "shredded" after a single workout or a week of workouts.
Instead, months of practice may be needed to increase vagal tone, measured by the natural log of HF power when breathing at typical rates of 16-20 bpm (Gevirtz, Lehrer, & Schwartz, 2016).
The graphic below shows HF power in blue for pre-training and post-training baselines, during which the client breathed at 15 and 14 bpm. Dr. Inna Khazan generously provided the spectral plots.
Note that HF power increased from 100 μV (pre-training) to 300 μV (post-training).
We shouldn't expect to see increased LF power when breathing at these rates if the vagal brake is working properly.
A short refresher. Inhalation disengages the vagal brake, speeding HR. Graphics inspired by Dr. Richard Gevirtz.
Exhalation reapplies the vagal brake, slowing HR.
Elevated LF power when breathing at 16-20 bpm means our client is not releasing their vagal brake.
Measuring VLF, LF, and HF
The processes contributing to HRV operate at different speeds and generate different frequencies. Frequency-domain measurements quantify the amount of HRV signal power within each of four frequency bands (ultra-low-frequency, very-low-frequency, low-frequency, and high-frequency).
In the graphic below, adapted from Dr. Richard Gevirtz, ultra-low-frequency activity is red, very-low-frequency activity is green, low-frequency activity is yellow, and high-frequency activity is white. Ultra-low-frequency activity is best measured over 24 hours or longer (Kleiger et al., 2005).
Dr. Khazan (2023) cautions that VLF and HF measurements are only valid when breathing at normal rates during resting baselines. Likewise, LF values are only meaningful during SPB or SPC (~ 4.5 to 6.5 bpm/cpm for adults) exercises.
Don't compare HRV time-domain metrics like the RMSSD or SDNN obtained during resting and SPB conditions. The different breathing rates and tasks preclude valid comparisons.
A Real-World Example
A European colleague concluded her undergraduates are fitter than Americans because their HRV values exceed US norms. She overlooked one critical detail. Her HRV measurements were taken during HRV biofeedback, the US norms during resting baselines.
Although LF power confirms that we are stimulating our two closed-loops at an optimal rate during HRV biofeedback, the log of HF power at rest verifies that our clients have increased parasympathetic activity. The goal of HRV biofeedback is to increase vagal tone.
Myth 4: Longer Exhalations Increase HRV
There have been no definitive findings regarding the best inhalation-to-exhalation (IE) ratio when breathing within the RF range. In theory, a 1:2 ratio would increase cardiac vagal activity (CVA) more than a 1:1 ratio if extended exhalation prolongs the parasympathetic slowing of the heart (Laborde et al., 2022).
Methodological Challenges in Following Pacing Displays
Studies comparing the effects of IE ratios during RF breathing have three methodological challenges that involve following a pacing display precisely.
First, researchers must confirm that participants breathed at the target rate. If participants breathed faster than 6.5 bpm, the findings might not apply to RF breathing. Following a pacing display at RF rates can be challenging. For example, Laborde et al. (2021) reported that participants breathed 6.48-6.55 bpm following a 6-bpm video pacing display based on Thought Technology's EZ-Air software. Also, Steffen et al. (2021) found that participants in the 6-bpm condition breathed at 12 bpm.
Second, they must ensure that participants breathed at the assigned IE ratios to manipulate the independent variable successfully.
Third, they must ensure that participants breathed at the same rate in each IE condition to avoid confounding.
Researchers should monitor respiration rate (RR) and IE ratio to confirm a participant's breathing rate and ratio. Real-time displays of the respiration waveform and RR can guide participants and alert researchers when it is necessary to repeat a trial.
A Critical Look at IE Ratio Studies
Eight reviewed studies disagreed regarding IE ratio effects. Three studies (Cappo & Holmes, 1984; Edmonds et al., 2009; Klintworth et al., 2012) found no IE ratio effect. One study (Lin et al., 2014) reported an advantage for equal inhalations and exhalations. Four studies (Bae et al., 2021; Laborde et al., 2021; Strauss-Blasche et al., 2000; Van Diest et al., 2014) found an advantage for longer exhalations than inhalations. Serious methodological problems compromised these studies. Only one (Laborde et al., 2021) confirmed that their participants breathed at 6 bpm. None confirmed successful breathing at assigned IE ratios. Two (Klintworth et al., 2012; van Diest et al., 2014) confounded IE ratio with order. One (Linn et al., 2014) confounded the IE ratio with RR in their data analysis.
Original and replication Truman Center studies (Meehan et al., 2018; Shaffer & Meehan, 2022; Zerr et al., 2015) were the only ones to date that confirmed breathing at 6 bpm and assigned IE ratios (1:1 and 1:2). Since power analysis confirmed that the sample sizes were sufficient to detect IE ratio effects, we concluded that a 1:2 ratio did not increase time-domain, frequency-domain, or nonlinear HRV metrics compared to a 1:1 ratio.
Perspective
Many clients prefer longer exhalations, and experienced professionals encourage clients to exhale longer than they inhale. Since we have not found any randomized controlled trials addressing this issue, the jury is still out on whether this pattern is healthier. However, two Truman Center studies provide overwhelming evidence that the IE ratio does not increase widely used time-domain, frequency-domain, or nonlinear HRV measurements.
Summary
The verdict is "false" for all four HRV myths:
(1) HRV is good, and equal interbeat intervals are bad.
(2) The parasympathetic nervous system generates short-term HRV.
(3) HRV biofeedback aims to increase the natural log of HF power at rest during normal breathing.
(4) Longer exhalations do not increase popular HRV metrics.
Quiz
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Glossary
baroreceptor reflex: the baroreflex provides negative feedback control of BP. Elevated BP activates the baroreflex to lower BP, and low BP suppresses the baroreflex to raise BP.
frequency-domain measurements: HRV metrics that quantify absolute or relative power distribution into four frequency bands.
high-frequency (HF) band: an HRV frequency range from 0.15-0.40 Hz that represents the inhibition and activation of the vagus nerve by breathing (respiratory sinus arrhythmia).
LF/HF ratio: the controversial ratio between LF- and HF-band power.
low-frequency (LF) band: an HRV frequency range of 0.04-0.15 Hz that may represent the influence of PNS and baroreflex activity (when breathing at the RF).
natural log of high-frequency power: the logarithm to the base e of HF power is a proxy for vagal tone during resting baselines.
power: the signal energy found within a frequency band.
respiratory sinus arrhythmia (RSA): the respiration-driven heart rhythm that contributes to the high frequency (HF) component of heart rate variability. Inhalation inhibits vagal nerve slowing of the heart (increasing HR), while exhalation restores vagal slowing (decreasing HR). ultra-low-frequency (ULF) band: HRV frequency range below 0.003 Hz. Very slow biological processes, including circadian rhythms, core body temperature, metabolism, the renin-angiotensin system, PNS, and SNS contributions.
vagal tone: parasympathetic activity, which can be approximated by the natural log of HF power when breathing at normal rates.
vascular tone (VT) control system: the closed loop encompassing the cardiovascular control center, vascular tone control system, and blood pressure control system.
very-low-frequency (VLF) band: a HRV frequency range of 0.003-0.04 Hz that may represent temperature regulation, plasma renin fluctuations, endothelial, physical activity influences, intrinsic cardiac, and PNS contributions.
References
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