SPECIAL ISSUE: Improve Client Care by Dispelling HRV Myths
Heart rate variability (HRV) indexes how efficiently we mobilize and utilize limited self-regulatory resources to maintain homeostasis. HRV plays a vital role in regulatory capacity, executive functions, health, and performance. We address common misconceptions regarding cardiovascular health, HRV measurement, and HRV biofeedback training. This review highlights the importance of context and individual differences in interpreting HRV. We debunk common misconceptions to improve HRV biofeedback training for health and performance.
Heart rate variability (HRV) has emerged as a significant biomarker for assessing cardiovascular health and autonomic nervous system function. Despite its growing application in clinical and research settings, misconceptions about HRV persist. This document aims to dispel these myths by providing evidence-based insights into HRV, clarifying its physiological underpinnings, measurement, and implications for biofeedback training. By demystifying these aspects, we endeavor to enhance the understanding and effective utilization of HRV in clinical practice, ultimately contributing to improved patient care and outcomes.
Cardiovascular Health
We cover two cardiovascular health myths: Variability is bad, stability is good, and a healthy heart is a metronome.
Myth 1: Variability Is Bad, Stability Is Good
Not all variability is healthy. Whereas HRV is desirable, blood pressure (BP) variability can endanger health. Blood pressure variability contributes to organ damage and cardiovascular risk in hypertensive patients, and drugs can potentially smooth antihypertensive action by reducing excessive variability (Parati et al., 2013). We require BP stability under constant workloads (Khazan et al., 2020).
Figure 1 explains the “variability is bad, stability is good” myth.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Higher HRV is beneficial for cardiovascular health, autonomic function, emotional regulation, physical performance, self-control, and potentially longer survival in chronic illnesses (Camillo et al., 2011; Conder & Conder, 2014; Dan et al., 2022; Giese-Davis et al., 2015; Grässler et al., 2021; Maier & Hare, 2017; Routledge et al., 2010; Schuit et al., 1999).
Clinical takeaway: Stabilize BP variability through behavioral and lifestyle interventions and medication while increasing HRV for greater health and performance.
Myth 2: A Healthy Heart Resembles a Metronome
“A healthy heart is not a metronome” (Shaffer et al., 2014).
The cardiovascular center can effectively modulate vagal tone when the time intervals between heartbeats significantly change across successive breathing cycles.
“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 et al., 2020).
Figure 2 shows healthy variability. The time intervals between successive beats differ.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
In contrast, Figure 3 shows low variability because the interbeat intervals (IBIs) are identical. This could represent a heart that may soon need a pacemaker.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Figure 4 addresses the “a healthy heart resembles a metronome” myth.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
HRV Sources
Research using tilt tables to provoke a sympathetic nervous system (SNS) response has fueled the myth that resting HRV contains a sympathetic component. A tilt table typically consists of a flat surface on which the patient lies, secured with straps to prevent falling. The table can be gradually tilted from a horizontal to a vertical position, allowing the patient to experience controlled weight bearing. Tilt tables are commonly used in clinical and research settings to study the body’s cardiovascular and autonomic responses to changes in posture. A key area of interest is how tilt tables influence SNS activation, which plays a crucial role in maintaining BP and heart rate (HR) during orthostatic stress (Kamiya et al., 2009). Figure 5 depicts a tilt table.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
However, respiratory sinus arrhythmia (RSA), the baroreceptor reflex, and the vascular tone rhythm are the most important sources of short-term (∼5-minute) HRV. They are exclusively produced by the parasympathetic nervous system (PNS; Hayano & Yuda, 2019; E. Vaschillo et al., 2002). Figures 6 and 7 show the three main parasympathetic HRV sources.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
RSA, HR speeding and slowing across each breathing cycle, is the primary and entirely PNS source of HRV (Khazan et al., 2020). Figure 8 illustrates RSA.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
We adapted Figure 9 with permission from Elite Academy. The upper waveform represents the breathing cycle, and the lower signals are heartbeats.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
In Figure 10, inhalation partially disengages the vagal brake, speeding HR. This is purely parasympathetic. Graphics inspired by Dr. Richard Gevirtz.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
In Figure 11, exhalation reapplies the vagal brake, slowing HR. Figures 10 and 11 illustrate the disengaging and reengaging of the vagal brake across each breathing cycle that contributes to RSA.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
The E. Vaschillo et al. (2002) two-closed-loop model described the HR and vascular tone (VT) baroreflexes as closed loops. They proposed that stimulating one closed loop activates its counterpart. We adapted Figure 12 from Vaschillo and colleagues.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Each baroreflex is a potential target for HRV biofeedback (HRVB) training. The HR baroreflex regulates acute BP changes to ensure stability. Although HRV promotes health, BP variability endangers it.
The VT baroreflex regulates resistance blood vessel diameter. A larger arteriole diameter means lower BP. Figure 13 illustrates arterioles.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Clinical takeaway: Recognizing that a healthy heart does not beat like a metronome and instead has variable rhythms helps better assess cardiovascular health, personalize stress management and exercise programs, and predict and monitor cardiac events. This leads to more effective and individualized client care.
HRV can be a health index (Shaffer & Ginsberg, 2017). Monitoring HRV can help assess stress levels and recovery in clients (H. G. Kim et al., 2018). By evaluating HRV, practitioners can tailor exercise and rehabilitation programs more effectively (Plews et al., 2013). Finally, variations in HRV can predict potential cardiac events and monitor chronic conditions, such as heart failure (Thayer et al., 2010).
HRV Measurements
We cover five myths: Very low-frequency (VLF) increases during HRVB training are sympathetic; low-frequency (LF) power measurements during rest contain a sympathetic component; high-frequency (HF) power measurements during slow-paced breathing are valid; we can directly compare HRV recordings of different lengths; and you should trust smartphone HRV app values.
Myth 3: VLF Increases During HRVB Training Are Sympathetic
The VLF band (0.0033–0.04 Hz) comprises rhythms with periods between 25 and 300 seconds. There is uncertainty regarding the physiological mechanisms responsible for activity within this band (Kleiger et al., 2005).
Excessive effort during HRVB training can trigger vagal withdrawal (parasympathetic inhibition). Contrary to popular belief, the fight-or-flight response (i.e., sympathetic activation) occurs in response to threats to our survival as opposed to mild stressors. Everyday stressors cause vagal withdrawal instead of sympathetic activation (Porges, 2010) as shown in Figure 14.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Clinical takeaway: Studies using pharmacological blockades (e.g., beta-blockers for sympathetic blockade and atropine for parasympathetic blockade) demonstrate that changes in HRV components can be linked specifically to either vagal or sympathetic modulation (Pomeranz et al., 1985). Interventions targeting autonomic dysfunctions need to address the specific imbalance. For instance, treatments enhancing parasympathetic activity (such as HRVB) may be more effective for HR increases resulting from vagal withdrawal. Conversely, if sympathetic activation is the cause, interventions might focus on reducing sympathetic activity (such as stress management; Thayer & Lane, 2007).
Myth 4: LF Power Measurements During Rest Contain a Sympathetic Component
The LF band (0.04–0.15 Hz) comprises rhythms with periods between 7 and 25 seconds and is affected by breathing from ∼3–9 breaths per minute (bpm). LF power increases during slow-paced breathing (SPB) and slow-paced contraction (SPC), likely reflecting baroreceptor and parasympathetic, but not sympathetic, activity.
Significant variability in sympathetic activity at rest among individuals suggests that the LF/HF ratio may not be a reliable marker for assessing autonomic balance in a resting state (Schlindwein et al., 2008). The controversy over the LF/HF ratio at rest stems from its inaccurate reflection of sympathetic activity and the complex interplay between sympathetic and parasympathetic influences. This ratio should be interpreted cautiously, especially in resting conditions (Reyes del Paso et al., 2013), in which there may be no sympathetic activity as shown in Figure 15.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Clinical takeaway: Unless HRV measurement protocols have been shown to increase SNS activity reliably (e.g., exercise or the Trier Social Stress Test; Kirschbaum et al., 1993), clinicians should not use the LF/HF ratio to assess autonomic balance.
Myth 5: HF Power Measurements During SPB Are Valid
The HF band (0.15–0.40 Hz) is influenced by breathing from 9 to 24 bpm (Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996). HF measurements are invalid during SPB. Slow breathing shifts the spectral power from HF to LF bands, complicating the interpretation of autonomic regulation. This inaccurately represents autonomic balance when using HF-HRV as a measure (Sasaki & Maruyama, 2014) and cannot serve as a proxy for vagal tone (Kromenacker et al., 2018) as shown in Figure 16.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Clinical takeaway: Only measure HF power when clients breathe at normal rates.
Myth 6: We Can Directly Compare HRV Recordings of Different Lengths
The recording period significantly affects both HRV time- and frequency-domain measurements. Cycle-length dependence prevents directly comparing 5- and 10-minute recordings because longer recording periods can capture more variability (Pomeranz et al., 1985) as shown in Figure 17.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Resting values obtained from brief monitoring periods can dramatically underestimate HRV, correlating poorly with 24-hour indices.
Clinical takeaway: Only compare HRV measurements of identical length. Longer recording periods can inflate HRV values compared with shorter epochs.
Myth 7: You Should Trust Smartphone HRV App Values
HRV time- and frequency-domain values obtained from smartphone apps may be invalid because of their failure to control artifacts as shown in Figure 18. Popular HRV apps may not perform even minimal data cleaning. They can be overwhelmed by abnormal beats such as premature atrial contractions. Significant but small differences in HRV metrics were observed when no artifact correction was applied in the seated position, and trivial differences were seen in the supine position (Himariotis et al., 2022). HRV smartphone apps without error correction can be valid for time-domain HRV indices, but at least 3 minutes are needed for accurate frequency-domain measurements (Chen et al., 2020).
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Clinical takeaway: Distrust values that exceed age-related norms by two or more standard deviations. For example, the 142-ms root mean square of successive differences (RMSSD) value for a 75-year-old male is 110 ms too high. Record data for at least 3 minutes. Manual data artifacting of smartphone app data within programs such as CardioPro and Kubios is the gold standard. This is crucial when cardiac arrhythmias (Pipitprapat et al., 2018) are present or when recording HRV during intense aerobic activity (Lee et al., 2017).
HRVB Training
We cover six myths: Try to relax; successful HRVB training always produces a 0.1 Hz LF peak; when slow-paced breathing increases HR oscillations, this signals greater vagal tone; the best sign of HRV training success is high resting LF power; HRVB asks clients to slow their daily breathing permanently; and slow-paced breathing is the best way to increase HRV.
Myth 8: Try to Relax
When clients try too hard to slow their breathing during HRVB practice, they can trigger vagal withdrawal as shown in Figure 19. Excessive effort may increase VLF power (García-González et al., 2000) and produce unwanted autonomic changes, such as increased skin conductance (Wegner et al., 1997) and decreased skin temperature (Zhang & Yan, 2001).
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Clinical takeaway: SPB instructions should encourage passive volition using words such as “allow.” When you observe VLF increases during an HRVB trial, look for respiration rates below 2.4 bpm (Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996) and ask your client about their perceived effort. Encourage them to breathe effortlessly (Peper & Tibbetts, 1994).
Myth 9: Successful HRVB Training Always Produces a 0.1 Hz LF Peak
If you train your client using a resonance frequency (RF) protocol, their target breathing rate might fall between 4.5 and 6.5 bpm for adults (Lehrer et al., 2000). The RF is the frequency at which a system, such as the cardiovascular system, can be maximally activated or stimulated. The peak frequency (e.g., the highest amplitude frequency) depends on your client’s breathing rate. Although breathing at 6 bpm yields a 0.1-Hz peak (6/60 = 0.1 Hz), 5 bpm creates a 0.08-Hz peak (5/60 = 0.083 Hz) as shown in Figure 20.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Clinical takeaway: Clinicians should not expect a 0.1-Hz peak if their client’s RF lies above or below 6 bpm.
Myth 10: When SPB Increases HR Oscillations, This Signals Greater Vagal Tone
Slowing breathing to the RF range increases RSA (E. G. Vaschillo et al., 2006). Increased RSA immediately exercises the baroreflex without changing vagal tone. Changing vagal tone or tightening BP regulation requires weeks to months of practice to produce necessary autonomic adaptations (Lehrer & Gevirtz, 2014).
HRVB can immediately increase RSA 4–10 times compared with a resting baseline (Gevirtz et al., 2016; E. Vaschillo et al., 2002) as shown in Figure 21.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Clinical takeaway: HRVB’s goal is not increasing RSA; that is a means to the end of increasing vagal tone. Although initial RSA increases signal that clients have effectively stimulated the baroreflexes, vagal tone increases may take 5 to 8 weeks (Deschodt-Arsac et al., 2020; Schumann et al., 2019). A combination of weekly supervised HRVB and 20-minute daily HRVB practice may be recommended (Lehrer et al., 2000).
Myth 11: The Best Sign of HRV Training Success Is High Resting LF Power
The best sign of HRV training success is high resting HF power when breathing at normal rates (Shaffer & Meehan 2020) as shown in Figure 22. The natural log of HF power is a proxy for vagal tone (Irwin et al., 2006). High resting LF power when breathing between 11 and 18 bpm may indicate that the vagal brake is stuck.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
In Figure 23, HF power increased from ∼100 μV to ∼300 μV from pretraining to posttraining baselines. HF power is a proxy for vagal tone when clients breathe normally without feedback.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Clinical takeaway: Assess client HRVB training success using HF power.
Myth 12: HRVB Asks Clients to Slow Their Daily Breathing Permanently
HRVB training may prescribe SPB practice for up to 20 minutes, one to two times daily. Clients breathe normally the rest of the time, adjusting their breathing to their workloads as shown in Figure 24.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
It would be counterproductive to curl a dumbbell throughout the day in weightlifting as shown in Figure 25.
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Clinical takeaway: Limit SPB breathing practice to once or twice a day for 20 minutes.
Myth 13: SPB Is the Best Way to Increase HRV
SPC of the muscles offers an alternative to SPB, which can be challenging (e.g., chronic pain) or medically contraindicated (e.g., kidney disease). In SPC, clients may contract wrists and ankles or wrists, core, and ankles for 3 seconds from 4.5 to 6.5 contractions per minute (cpm) with feet supported by a second chair. The contractions should be gentle (Meehan & Shaffer, 2023; see Figure 26).
Citation: Biofeedback 52, 2; 10.5298/1081-5937-52.02.08
Shaffer et al. (2022) reported that 1- and 6-cpm SPC increased several HRV metrics (e.g., the RMSSD, standard deviation of normal to normal, and LF power) to a greater degree than SPC at 12 cpm. Wrist-core-ankle SPC increased mean RSA (HR Max-HR Min) more than wrist-ankle SPC or a resting baseline condition (Meehan & Shaffer, 2023).
Clinical takeaway: SPC provides an alternative HRVB training option when SPB is difficult because of chronic pain or dysfunctional breathing or contraindicated by metabolic (H. J. Kim, 2021; Kitabchi et al., 2009) or respiratory acidosis (Epstein & Singh, 2001).
Conclusion
The exploration of HRV myths and their subsequent debunking serves to correct common misconceptions and highlight the intricate relationship between HRV and overall health. The evidence underscores the importance of accurate HRV interpretation and application in clinical and performance settings. As we advance our understanding of HRV through rigorous research and practice, it becomes crucial to integrate these insights into patient care. Doing so promises to refine diagnostic and therapeutic approaches, fostering a holistic understanding of cardiovascular health informed by the dynamic interplay of physiological systems.
Glossary
baroreflex: baroreceptor reflex that provides negative feedback control of BP and HR. Elevated BP activates the baroreflex to lower BP, and lower BP suppresses the baroreflex to raise BP.
cycle-length dependence: the phenomenon by which a faster HR reduces the time between successive beats and the opportunity for the IBIs to vary, resulting in lower HRV.
effortless contraction: SPC using about 25% of maximum effort
frequency-domain metrics: the absolute or relative power of the HRV signal within four frequency bands.
heart rate (HR): the number of heartbeats per minute.
heart rate baroreflexes: the closed loop encompassing the cardiovascular control center, HR control system, and BP control system.
heart rate variability (HRV): the beat-to-beat changes in HR involving changes in the RR intervals between consecutive heartbeats.
heart rate variability (HRV) biofeedback: the display of beat-to-beat changes in HR, including changes in the RR intervals between consecutive heartbeats to a client.
high-frequency (HF) band power: signal energy in the 0.15–0.40 Hz range that represents the inhibition and activation of the vagus nerve by breathing (respiratory sinus arrhythmia).
HR Max-HR Min: the average difference between the highest and lowest HRs during each respiratory cycle.
HRV time-domain indices: the calculation of the amount of variability in IBI measurements.
interbeat intervals (IBIs): the time intervals between the peaks of successive R-spikes (initial upward deflections in the QRS complex).
low-frequency (LF) band power: 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).
metabolic acidosis: an accumulation of acid or a significant loss of bicarbonate from the body. This condition lowers the blood pH below normal levels. It can result from kidney failure, diabetic ketoacidosis, lactic acidosis, or ingestion of toxins such as methanol.
NN intervals: the time between adjacent R-spikes originating at the sinoatrial node.
peak frequency: the highest amplitude frequency.
resonance: an amplification process in which an external force causes a closed-loop (negative feedback) system to oscillate with greater amplitude at its inherent RF.
resonance frequency (RF): the frequency at which a system, such as the cardiovascular system, can be maximally activated or stimulated.
respiratory acidosis: inadequate ventilation, leading to a buildup of carbon dioxide (CO2) in the blood. Increased CO2 results in a decrease in blood pH. It can be a result of conditions such as chronic obstructive pulmonary disease, severe asthma, or drug-induced respiratory depression.
respiratory sinus arrhythmia (RSA): the respiration-driven heart rhythm that contributes to the HF component of HRV. Inhalation inhibits vagal nerve slowing of the heart (increasing HR), whereas exhalation restores vagal slowing (decreasing HR).
RMSSD: the square root of the mean squared difference of adjacent NN intervals.
RR intervals: the time between adjacent R-spikes.
SDNN: the standard deviation of the normal (NN) sinus-initiated IBI measured in milliseconds.
slow-paced breathing (SPB): breathing in the adult 4.5–6.5 bpm range.
slow-paced contraction (SPC): wrist-ankle or wrist-core-ankle contraction in the adult 4.5–6.5 cpm range.
tilt table: a flat surface on which the patient lies, secured with straps to prevent falling. The table can be gradually tilted from a horizontal to a vertical position, allowing the patient to experience controlled weight bearing.
time-domain metrics: HRV indices that quantify the amount of variability in IBI measurements.
vagal tone: parasympathetic activity, which is estimated by the natural log of HF power.
vagus nerve: The parasympathetic vagus nerve decreases the rate of spontaneous depolarization in the sinoatrial and atrioventricular nodes and slows HR. HR increases often reflect reduced vagal inhibition.
Vaschillo’s two-closed-loop model: The HR and VT baroreflexes are closed loops; stimulating one closed loop activates its counterpart. Each baroreflex is a potential target for HRVB 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.
vascular tone (VT) baroreflex: negative feedback loop that regulates resistance blood vessel diameter with a 15-second delay and 0.03-Hz RF.
The “variability is bad, stability is good” myth. Real Genius episode by Dani S at Fiverr.com.
Healthy variability. Graphic by minaanandag at Fiverr.com.
Low variability. Graphic by minaanandag at Fiverr.com.
A healthy heart is not a metronome. Real Genius episode on WEBTOON by Dani S at Fiverr.com.
Tilt table graphic © Pepermpron/Shutterstock.com.
Three main parasympathetic HRV sources. Graphic created by authors.
The “sympathetic activity plays a major role in HRV” myth. Real Genius episode on WEBTOON by Dani S at Fiverr.com.
BioGraph Infiniti screenshot of RSA by authors. The upper waveform is respiration, and the lower is instantaneous HR.
The heartbeats are closer together during inhalation and further apart during exhalation. Illustration by minaanandag at Fiver.com.
HR speeds during inhalation. Real Genius episode on WEBTOON by Dani S at Fiverr.com.
HR slows during exhalation. Graphic by Dani S. at Fiverr.com.
Two-closed-loop model drawn by authors.
Arteriole graphic © Ali DM/Shutterstock.com.
The “VLF power increases during HRVB training are sympathetic” myth. Real Genius episode on WEBTOON by Dani S at Fiverr.com.
The “LF power and the LF/HF ratio are sympathetic” myth. HRV frequency graphic adapted from Dr. Richard Gevirtz by Dani S at Fiverr.com.
The “we should measure HF power during SPB” myth. Real Genius episode on WEBTOON by Dani S at Fiverr.com.
The “we can directly compare HRV recordings of different lengths” myth. Real Genius episode on WEBTOON by Dani S at Fiverr.com.
The “you should trust smartphone HRV app values” myth. Real Genius episode on WEBTOON by Dani S at Fiverr.com.
The “try to relax” myth. Real Genius episode on WEBTOON by Dani S at Fiverr.com.
The “successful HRVB training always produces a 0.1-Hz LF peak” myth. Real Genius episode on WEBTOON by Dani S at Fiverr.com.
The “when SPB increases HR oscillations, this signals greater vagal tone” myth. Graphic adapted from Gevirtz et al. (2016) by Dani S at Fiverr.com.
The “the best sign of HRV training success is high resting LF power” myth. Real Genius episode on WEBTOON by Dani S at Fiverr.com.
The spectral plots are courtesy of Dr. Inna Khazan. HF power increases from pretraining to posttraining baselines.
The “HRVB asks clients to slow their daily breathing permanently” myth. Real Genius episode on WEBTOON by Dani S at Fiverr.com.
Curling graphic © ra2 studio/Shutterstock.com.
The “SPB is the best way to increase HRV” myth. Real Genius episode on WEBTOON by Dani S at Fiverr.com.
Contributor Notes