Your heart rate variability (HRV) is a measure of the variations in time elapsed between your heartbeats, and is representative of your heart’s ability to respond to physiological and environmental stress stimuli.1,2,3,4,5
Contrary to what might seem logical at first glance, a healthy heart rhythm is not perfectly even. When your heartrate is “monotonously regular,” your HRV will be low. Low HRV is associated with autonomic nervous system (ANS) impairment,6 which reduces your body’s ability to cope with stressors, be they internal or external.
As such, HRV is a good stress indicator. When your HRV is high, the intervals between heartbeats is “large and irregular,” and this is an indication of augmented resilience against stress.7 High HRV is also indicative of greater cardiovascular fitness.8
Some experts now believe HRV may actually be one of the most important biomarkers of cardiovascular health, even more so than standard testing like cholesterol, C-reactive protein and even blood pressure.9
Indeed, as detailed in Dr. Thomas Cowan’s guest article about the real cause of heart attacks, published in 2014, increasing our understanding of the role of the autonomic nervous system in the development of ischemia (inadequate blood flow to the heart) — and how we can use HRV as a measure of risk — could revolutionize the prevention and treatment of heart disease. We’re inching ever closer to that as time goes on.
HRV and Your Autonomic Nervous System
As you probably know, you have two distinct nervous systems:
- The central nervous system (CNS), which controls conscious muscle and nerve functions
- The autonomic nervous system (ANS), which controls the unconscious function of your internal organs
Your ANS is crucial for optimal health, as it controls things like breathing, heart rate, sweating, digestion and the general functioning of your internal organs. If your ANS is dysfunctional or blocked, you’re bound to experience health problems.
There are any number of factors that can cause your ANS to not function properly. Examples include but are not limited to the following. To determine whether one or more of the factors listed below are affecting your ANS, you would perform a stress or challenge test to see how the suspected factor affects your HRV.
ANS Imbalance May Be a Key Factor in Your Heart Disease
Your ANS is further divided into two branches, namely your:
1.Sympathetic nervous system (SNS) — This system, also known as “fight or flight,” is centered in your adrenal medulla and uses adrenaline to ready your body for action in response to perceived threat. A series of biochemical responses are thus triggered, including glycolytic pathways that accelerate the breakdown of glucose for a rapid energy boost.
2.Parasympathetic nervous system (PNS) — This branch, known as the “rest and digest” arm of the ANS (relaxation response), is centered in your adrenal cortex. Its chemical mediators include acetylcholine, nitric oxide and cyclic guanosine monophosphate (cGMP).
Your PNS plays a central role in alleviating the stress response.10 When your HRV is low and your PNS is inhibited, you’re more vulnerable to the detrimental effects of future stress.11
Your vagus nerve, which is part of the parasympathetic chain, modulates your heart activity, slowing your heartbeat and relaxing your heart when your PNS is activated. When your SNS is activated, your vagus nerve will accelerate your heartbeat and cause constriction in your heart muscle.
When you’re healthy, these two branches of your ANS — the SNS and PNS — are well-balanced yet in a “ready” state. According to Cowan, an imbalance in your SNS and PNS is responsible for the vast majority of heart disease.
HRV Measures Your ANS Status
By monitoring your HRV, you can get a clear idea of your ANS status in real time. Again, the reason for this is because your ANS controls your heart rhythm. Research12 published in 2004 shows the parasympathetic activity of patients with ischemic heart disease is one-third lower, on average, than that of a healthy individual. As explained in the abstract:13
“About three quarters of myocardial ischemic events are triggered by the autonomic nervous system. The pathognomonic constellation is a combination of an almost complete withdrawal of tonic vagal activity with increased sympathetic activity.
The reduction of tonic vagal activity, which is characteristic for ischemic heart disease, and the acute withdrawal of vagal drive preceding the onset of ischemia are not dependent on coronary artery disease.
In this paper, the pathophysiological steps that lead from sympathetic-parasympathetic imbalance to myocardial ischemia shall be discussed. A considerable increase of aerobic glycolysis within the myocardium as a result of the autonomic imbalance is of special importance in this process.”
As a general rule, the worse your ischemia is, the lower your parasympathetic activity will be.14 As noted in the 2004 study above, a vast majority of ischemic events are preceded by a drastic reduction in parasympathetic activity in conjunction with increased sympathetic activity, brought on by strenuous physical activity or emotional shock, for example.15
In stark contrast, people with normal parasympathetic activity who experience an abrupt increase in sympathetic activity do not suffer from ischemia.
So, to summarize, in order for you to have a heart attack, you must experience both a drastic decrease in parasympathetic activity and an increase in sympathetic activity. In the absence of those two simultaneous factors, you’re unlikely to have a heart attack.
Vagus Flow Nearly Stops Before and During Ischemic Events
This flies in the face of conventional thought that says heart attacks are simply the result of one or more blocked arteries. But as noted in the 2004 study above, “the acute withdrawal of vagal drive preceding the onset of ischemia are not dependent on coronary artery disease.” In other words, your likelihood of having a heart attack is not dependent on having blocked arteries.
Instead, your heart attack risk is primarily dependent on the functioning of your ANS. Studies have also shown your risk of actually dying from a heart attack is significantly increased if your HRV is low. In fact, low HRV has been shown to be a “strong and independent predictor of mortality after an acute myocardial infarction.”16
An earlier study17 by the same author, this one published in 1997, looked at the role of ANS activity before and during transient ischemic events, using HRV measurements. As noted in this paper:
“With two exceptions, all ischemic episodes were preceded by an acute almost complete suppression of respiratory sinus arrhythmia. During the entire ischemic episode, HRV stayed at this reduced level, and preceding the end of the ischemia, it increased again.
This suppression of intrinsic heart period variations reflects an almost complete withdrawal of modulated vagal outflow immediately before and during ischemic episodes.”
The take-home message here is that while we all experience times of excess sympathetic activity (fight-or-flight mode), thanks to the stresses of daily life, shocks to your system only become dangerous to your health when your parasympathetic activity (relaxation response) is suppressed for a long period of time.
Factors that decrease vagal activity include high blood pressure, smoking and diabetes, while physical and emotional stress decrease your parasympathetic activity.18 All of these factors, then, contribute to heart problems and raise your risk of a cardiac event.
On the flip-side, factors that upregulate your PNS include dietary nitrates that stimulate nitric oxide production and foods or supplements that stimulate acetylcholine production, such as eggs and other choline-rich foods.19
HRV as an Objective Indicator of Stress and Mental Health
HRV does more than measure and predict your heart health, though. As explained in a review paper20 published in Psychiatry Investigation in 2019, SNS hyperactivation caused by chronic stress can cause “physical, psychological, and behavioral abnormalities,” and HRV can be used as a psychological stress indicator and an objective assessment tool of mental health.
The paper reviews 37 publications where HRV reactivity was used as an objective psychological stress measure. According to the authors:21
“In most studies, HRV variables changed in response to stress induced by various methods. The most frequently reported factor associated with variation in HRV variables was low parasympathetic activity …
Neuroimaging studies suggested that HRV may be linked to cortical regions (e.g., the ventromedial prefrontal cortex) that are involved in stressful situation appraisal.”
The Three-Stage Stress Response Model in Clinical Practice
The Psychiatry Investigation paper also presents a three-stage stress response model, and explains how HRV can be used in a clinical setting to assess a patient’s overall health and disease risk:22
“The first stage is the ‘alarm reaction stage,’ in which the body reacts to a stressor with the fight-or-flight response and activates the SNS. The second stage is the ‘resistance stage,’ in which the body adapts to the stressor. During this stage, the PNS restores many physiological functions to normal, while the body focuses its resources against the stressor.
Although the outward appearance of the organism seems normal, blood glucose, cortisol, and adrenalin levels remain elevated. If a stressor continues beyond the body’s capacity to cope, the organism exhausts its resources, making it susceptible to disease or death. This ‘exhaustion stage’ is reached when the acquired adaptation or resistance is lost.
When assessing the severity of a patient’s stress level in a clinical setting, HRV results should be interpreted with this three-stage process in mind. At each stage, stress causes changes in physiological function, which are reflected in HRV changes.
Due to the variety of potential stressors and individual stress responses, it is essential to understand the overall autonomic context and examine a patient’s medical and psychological history when interpreting the relationship between HRV and stress.”
HRV and Inflammation
Your HRV can also give you an idea of your inflammation levels. Inflammation, of course, is a hallmark of cardiovascular disease, but most other chronic diseases involve elevated inflammation levels as well. As noted in a 2013 review paper in Frontiers of Physiology:23
“Many experimental and clinical studies have confirmed a continuous cross-talk between both sympathetic and parasympathetic branches of autonomic nervous system and inflammatory response, in different clinical scenarios.
In cardiovascular diseases, inflammation has been proven to play a pivotal role in disease progression, pathogenesis and resolution.
A few clinical studies have assessed the possible inter-relation between neuro-autonomic output, estimated with heart rate variability analysis, which is the variability of R-R in the electrocardiogram, and different inflammatory biomarkers, in patients suffering from stable or unstable coronary artery disease (CAD) and heart failure.
Moreover, different indices derived from heart rate signals’ processing, have been proven to correlate strongly with severity of heart disease and predict final outcome.”
As explained in this paper, both the SNS and PNS are powerful modulators of inflammation. When the two are properly balanced, they promote an anti-inflammatory landscape.
HRV and Your Overall Health
Low HRV has been linked to a range of diseases, either by promoting inflammation or via other mechanisms. It’s also been shown to be a reliable predictor of disease progression.24
Diabetes, for example, is a risk factor for heart disease and stroke, and being able to detect the early signs of complications can go a long way toward lowering your risk of death. HRV can be a valuable tool in this regard.
As reported in a 2018 paper,25 “A systematic review of those with diabetes concluded that HRV can help to predict cardiac morbidity and mortality, and that it can be used at an early stage to indicate the future risk of complications.”
This paper also points out that “a reduction in HRV predicts macrovascular disease, for example carotid artery atherosclerosis,” and “is associated with a significantly increased risk of death from cardiovascular disease.”
Interestingly, the authors also review a number of studies looking at how various foods and diets affect your HRV, concluding that tracking your HRV may be a helpful tool “when examining the impact of what we eat.”
How to Check Your HRV
So, how do you check your HRV? Harvard University offers the following suggestions:26
“The gold standard is to analyze a long strip of an electrocardiogram … But over the past few years, several companies have launched apps and heart rate monitors that do something similar.
The accuracy of these methods is still under scrutiny, but … the technology is improving substantially … The easiest and cheapest way to check HRV is to buy a chest strap heart monitor … and download a free app (Elite HRV is a good one) to analyze the data.
The chest strap monitor tends to be more accurate than wrist or finger devices. Check your HRV in the mornings after you wake up, a few times a week, and track for changes as you incorporate healthier interventions.”
Keep in mind that using a chest strap heart monitor in combination with a cellphone app exposes you to Bluetooth EMF frequencies, which makes it less than optimal.
The HRV4Training app27,28 claims to accurately calculate HRV even without an external sensor or chest strap (although it can also be integrated with Apple Watch and the Oura ring), and has research29 that backs it up. With this app, you can simply use your phone camera to get a measure of your HRV.
By collecting your HRV data at different times over the course of weeks, you can start to get a picture of how your body responds to stress, and identify the activities or situations that raise and lower it.
It’s also really useful for athletes, as it can tell you whether you’ve sufficiently recovered or are pushing yourself too hard. A high HRV is a sign that your body is handling the stress well, whereas a low HRV indicates your body is under stress, which can make you more prone to ill health if you keep pushing.
How to Improve Your HRV
If your HRV is consistently subpar, don’t fret. There are many ways to improve your HRV, and most are inexpensive or free. Strategies that can improve your HRV include:30,31
|Rest and sleep
||Mindfulness and other forms of meditation
||Avoiding excess work and extensive commutes, and minimize work-related stress
|Spending time outdoors, in nature
||Optimizing your omega-3 level