Four Heart Rate Variability Myths

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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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).

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; Vaschillo et al., 2002). None of these processes are sympathetic. Respiratory sinus arrhythmia (RSA), HR speeding and slowing across each breathing cycle, is the primary and entirely parasympathetic source of HRV. The baroreceptor reflex, which exerts homeostatic control over acute BP changes, is the second most important and entirely parasympathetic source of HRV. The vascular tone (VT) control system regulates arteriole (small muscular artery) diameter.

The LF/HF Ratio

This has major implications for the controversial LF/HF ratio. Let's revisit the LF band. The low-frequency (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). 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 based initially on 24-hour recordings, during which both 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 short-term measurements correlate poorly with 24-hour values. Moreover, the SNS contribution to LF activity varies profoundly with testing conditions (Lehrer, 2012). For example, when LF is calculated during resting conditions, the primary contributors are PNS activity and baroreflex activity—not SNS activity. Therefore, a 5-minute resting baseline cannot estimate autonomic balance (McCraty, 2013).

. . . this view has been highly criticized (Eckberg, 1997; Billman, 2013). 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 (Billman, 2013). Laborde et al. (2017).

The Goal of HRV Biofeedback Is To Increase Low-Frequency Power At Rest We train clients to increase low-frequency (LF) power using slow-paced breathing to increase high-frequency (HF) power during normal breathing (i.e., 12-16 bpm). 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, vagal tone (parasympathetic firing) increases take time. Think of slow-paced breathing or slow-paced contraction 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 (Gevirtz, Lehrer, & Schwartz, 2016).

The graphic below shows HF power in blue during a pre-training baseline, HRVB training, and a post-training baseline. Note the greater HF power concentration post-training compared with pre-training during which the client breathed at typical rates. Dr. Inna Z. Khazan generously provided the spectral plots.