Beyond heart rate variability: what your wearable monitor may not be telling you
Posted November 14, 2019
Heart rate is one of our innate physiological rhythms that fluctuates day and night. There are complex patterns beyond the pace of your sinus node (a cluster of cells in the heart that operates as a natural pacemaker); for example, how heart rate changes from beat to heat and how it synchronizes with other physiological rhythms such as circadian cycles, respiration, blood pressure and cortisol. Heart rate variability (HRV) has become increasingly popular in the past few decades as an important, understandable, and measurable function. Advances in technology have allowed us to track HRV on personal devices without medical equipment or an interpreter.
This is a good thing, right?
How we benefit from technology depends on how we use it. There are assumptions about HRV that may or may not be true. HRV is more complex than it seems and not always an omniscient reflection of our nervous system.
3 common assumptions are:
- HRV, the vagus nerve, and the parasympathetic influence are always interchangeable terms.
- Higher HRV is always better; parasympathetic tone is always better.
- There are specific ways to train the vagus nerve, my wearable monitor tells me so.
There is a lot we know about HRV, and much we don’t. Let’s review a few definitions.
What exactly is HRV?
HRV is the beat to beat change in heart rate–each beat is slightly faster or slower than the previous beat. The vagus nerve puts the brakes on the heart enabling it to slow down as needed, and its ability to do so depends on how active or toned the vagal response is. The vagus nerve wanders throughout the body from the brain to the face and ears, lungs and heart, stomach and intestines, and other organs. It contains parasympathetic fibers responsible for resting, digesting, and slowing of heart rate–the opposite of the fight or flight, or sympathetic, response. It can even affect how well we communicate with others (1).
Low HRV predicts sickness and death from all causes and throughout the literature is associated with everything from cardiac disease, asthma, fatigue, and chronic pain to depression, anxiety, and personality disorders. Generally, high HRV predicts health; low HRV predicts death. With few exceptions the rule is: the higher the better, the lower the sicker. High HRV demonstrates the nervous system’s adaptability and how stress-resilient we can be (2).
What is RSA resonance?
Respiratory sinus arrhythmia (RSA) is the oscillation of heart rate rhythms with breathing. Breathing techniques are frequently used to reduce the fight or flight response and rev up the relaxation and healing response. Beyond just slowing down respiration and heart rate to increase HRV, another goal is for HRV and breathing frequencies to resonate together. This may be the zone we feel and perform our best.
How is HRV measured?
The most common measure of HRV is using linear time (in milliseconds) from one R wave to the next, as on an electrocardiogram. A more detailed measurement is using frequency domain measures, as with RSA, (in absolute or relative power, in hertz), dividing heart rate oscillations into 4 frequency bands. There is a third way that involves non-linear measurements in relation to time and frequency domain measures of HRV (3).
The un-simple autonomic nervous system.
The nervous system has often been described as two sides toggling for domination: the gas versus brake, or stress versus relaxation (I’ve used this analogy many times myself to explain it). We conclude with good reason that we have too much stress, or sympathetic influence, and the parasympathetic needs toning with more relaxation. Handheld and wearable devices dictate when HRV is low and what you should do to improve it. The bummer truth is that we may be over-interpreting what HRV means for us at any given moment, HRV and vagal tone are not always synonymous, and higher HRV is not always better.
Drs. Junichiro Hayno and Emi Yuda published a paper in The Journal of Physiological Anthropology titled “Pitfalls of assessment of autonomic function by heart rate variability." They reminded the readers to slow down when jumping to conclusions about what we think we know about HRV based on a simplistic old framework of the autonomic nervous system.
Here are 4 problems they report in current assessments of HRV:
- HRV does not necessarily reflect the autonomic state of the entire body, just the heart. For example, when we eat, vagal activity goes to digestion while vagal activity to the heart is suppressed.
- Low HRV is related to illness, but also may be due to restricted movement during illness.
- The vagus nerve’s effect on heart rate can move to higher frequencies; meaning we can’t always find it in one frequency range because it moves around.
- Sympathetic activity, not just parasympathetic, influences RSA. Unexpectedly, medications that should block the sympathetic response do not always completely do so and the sympathetic response is reduced when the parasympathetic nervous system is cut (4).
Measuring and interpreting HRV and vagal response is not as simple as it seems. There are still unanswered questions. Although it would be easier to divide things into categories of good and bad; we are more complex. Our system is dynamic and multi-dimensional, not just on or off, high or low. It is even more complex than a dimmer switch. There is an optimal range for humans and likely an optimal target for each individual within that range.
Why what we don’t know (probably) won’t hurt us.
Assumptions about HRV could be detrimental for people with serious illnesses or when determining invasive procedures for vagus nerve stimulation. However, they are not likely to be harmful to the rest of us day-to-day. Suggestions regarding non-invasive stimulation of the vagus nerve are generally good for us anyway, so using them regularly is recommended even before we fully understand exactly how it all works.
Popular recommendations generally fall into 3 general categories (with some overlap):
- Physical: slow diaphragmatic breathing, moderate to intensive exercise, rhythmic dance, singing, cold water therapy.
- Expressive: healthy communication, emotional expression, and expressive writing.
- Contemplative: mindfulness and meditation practices.
It seems that activities that are good for us in general are also good for HRV, but we still may want to update our understanding of the autonomic nervous system.
Allostasis: an updated framework.
It is healthy to have a robust stress response and to recover fully from it. Although reducing stress can also be helpful, our system’s ability to navigate challenges is even more important. The term homeostasis, meaning balance, lends itself to a feeling of precariousness and a just-rightness that we have to achieve each day. The term allostasis may help us move away from a bi-directional idea of balance toward a more multi-dimensional framework. Allostasis describes the process our body uses to adjust and adapt to maintain homeostasis. It is physically and emotionally advantageous to be able to navigate through a wide range of circumstances and adjust to new circumstances efficiently and effectively. We can learn to have more conscience control over this multi-directional adaptation by entraining our physiological rhythms. Enhancing allostasis helps us be durable and flexible.
Resonance: what we do know.
High resting baseline HRV is good for most of us and engaging in healthy lifestyle behaviors improves that number. There is also a growing body of evidence that HRV biofeedback targeting RSA (aligning HRV + respiration) directly seems to increase physical and emotional resilience. While observing RSA on a computer screen, you can learn to find your personal resonant frequency (see RFT on the image to the left). This usually includes lower and slower breathing patterns nearing 5.5 to 6 breaths per minute for most people. There is evidence that RSA training helps a variety of health problems, improves athletic performance and cognitive focus. Exactly why it works is not certain yet.
Dr. Richard Gevirtz and Dr. Paul Leher, who have studied HRV and biofeedback for several decades, suspect that there are a few underlying mechanisms. They have found evidence that blood pressure sensors in the aortic and carotid arteries (baroreceptors) become more sensitized to the system’s needs and therefore better adapted to maintain homeostasis. Parts of the brain including the brain stem’s nucleus tractus solitarius, the amygdala (the center controlling emotion) and into the insula (pain perception) may also be important players. Health gains could also be attributed to the increased efficiency of gas exchange in the alveoli that occurs with peak RSA oscillations (3). Perhaps it is a combination of these and other factors (5).
There is so much we don’t yet know about the autonomic nervous system, but don’t let that stop you from dosing yourself with daily physical and mental challenges, followed by full rest and recovery, breathing or meditation. Exercise, train, dance, and breathe intentionally. Let how well you feel and function overall be your guide, with or without validation from the latest technology.
1) Porges, S. (2011). The Polyvagal Theory: Neurophysiological Foundations of Emotions, Attachment, Communication, and Self-regulation. New York & London: WW. Norton & Company.
2) Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology (1996). Special report: heart rate variability standards of measurement, physiological interpretation, and clinical use. American Heart Association, Inc.
3) Shaffer, F., & Ginsberg, J. P. (2017). An Overview of Heart Rate Variability Metrics and Norms. Frontiers in public health, 5, 258. doi:10.3389/fpubh.2017.00258
4) Hayano, J., Yuda, E. Pitfalls of assessment of autonomic function by heart rate variability. J Physiol Anthropology: 38, 3 (2019) doi:10.1186/s40101-019-0193-2.
5) Leher, P.M. & Gevirtz, R. (2014). Heart rate variability biofeedback: how and why does it work? Frontiers in Psychology; 5: 756.