Your Guide To Understanding Heart Rate Variability
And how Welltory Uses It To Help People Change Their Lives
Biofeedback, biohacking, quantified self data. The recent uptick in the popularity of these trends shows that a lot of people wish there were a way to check in with their bodies — to look under the hood of the car and find out what can be tweaked to make things run more smoothly.
Wouldn’t be great if there was a quick and easy way to do just that? There is!
It’s called heart rate variability (HRV) — a non-invasive way to assess the body’s systems, physiological stress levels, risk of getting sick, and much more.
HRV was originally used in the 1960s to measure astronaut health. Space scientists needed a way to figure out how people’s bodies respond to drastic changes in surroundings and monitor recovery upon their return to Earth.
Recent advances in technology have made HRV available to people outside space crafts. HRV has been adopted by cardiologists who use it to track recovery after strokes, therapists who use it to monitor the physiological effects of depression, and endocrinologists who use it to assess people’s risks of developing diabetes. It’s even used by NHL players and other high-performance athletes to optimize their workout loads.
So why aren’t all of us using heart rate variability to stay on top of our game?
The truth is there are still a ton of challenges on the road to making variability mass market-friendly. As a wellness app based on heart rate variability analysis, Welltory has encountered just about all of them and we’re happy to share our experience with you.
In this article, we’ll talk about how variability works, how it’s analyzed, the solutions we’ve come up with on our journey to make variability accessible to everyone, and where we expect this journey to take us in the future. Let’s start with the basics.
So, what is heart rate variability?
Heart Rate Variability is the Variation in Time Between Heartbeats
Your heart does not beat evenly. For example, if your average heart rate is 60 beats per minute, your heart will not beat every second on the dot. Instead, it will look something like this:
This difference in time intervals is called variability [6].
Your Heartbeat Varies Because Your Body Is Always Adapting to Changes
Your heartbeat responds to anything that happens to you: a jog, a cup of coffee, or even an upsetting post on your newsfeed.
When conditions change, the body works to maintain homeostasis — a stable internal environment inside the body. Your autonomic nervous system kicks in to do this work. It takes control of the body’s reactions and processes like breathing and digestion, sending signals to your heart so it can adapt to the changes faster [2, 3].
Signals sent by the autonomic nervous system play tug of war with your heartbeat, speeding it up and slowing it down. The result is that your heart never beats evenly, varying its rhythm in response to everything.
The autonomic nervous system has two parts responsible for controlling your heartbeat: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS) [4].
The PNS is the heart’s brake pedal
Your heart has a sinoatrial node (SA) and an atrioventricular node (AV). Here is where they’re located:
The Vagus nerve, one of the central nerves of the parasympathetic nervous system, runs through the SA and AV nodes, like this.
When the PNS is activated, the impulses are generated less frequently and transmitting the signal from the SA to the AV takes longer [2,4].
When the PNS is active, the heart beats less frequently and each beat lasts longer.
The SNS is the heart’s gas pedal
The walls of the heart are made of muscle cells, or cardiac myocytes. They look like this:
These cells contract. When they contract, the heart contracts.
The nerves shown below belong to the sympathetic nervous system (SNS). They connected to cardiac muscle cells [4]
When the SNS is activated, it boosts the production of ATP (an energy source) in the cardiac muscle cells, which means that the heart beats stronger. The cells in the heart activate, which means the impulses generated more quickly [2,4].
SNS makes the heart beat more frequently and each beat is stronger.
The SNS & PNS work together to help your body adapt to external changes in the environment. For example, when you work out, your muscles need more oxygen to perform. The SNS sends signals to increase the heart rate to deliver more blood to the muscles. When the workout is over, the PNS slows the heart rate down.
People Analyze Variability to See How Well the Body Copes With Pressure
When we analyze the tiny differences in the intervals between heartbeats, what we’re doing is tracing the signals that are making the heartbeat vary back to their source — the SNS and the PNS.
Knowing which systems in your body are active can give us information about how stressed the body is and how well it’s coping with pressure.
Researchers Use Two Standard Methods to Analyze Variability
Time-domain analysis & frequency-domain analysis. They’re based on analyzing each interval between heartbeats, otherwise known as RR-intervals [5, 6].
Time-Domain Analysis
Time-domain analysis shows how heart rate variability changes over time and is used to estimate sympathetic and parasympathetic activity. The main time-domain parameters are RR, SDNN, pNN50, rMSSD, HRVi, and TINN.
For example, all the intervals can be grouped in bands depending on how long they are: 700–800 ms, 800–900 ms, etc. Then intervals are counted to see how many of them fall within each band, and researchers use this information so estimate how stressed the body’s systems are.
Frequency-Domain Analysis
Frequency-domain analysis shows which process prevails in your body right now, stress or recovery.
To extract spectral parameters, the length of each R-R interval is transformed into waves to measure their frequency. Then waves are divided into low-frequency (LF), high frequency (HF) and very low frequency (VLF) bands.
The share of each power spectrum is then assessed to estimate stress levels and recovery processes in the body. The sum of all spectra (referred to as Total Power) estimates how much the body is capable of.
Analyzing all of these parameters simultaneously can give doctors and coaches a ton of insights about the body, how it’s coping with pressure, and how various changes (like an increase in workout loads or a move to a zero-gravity environment) have affected the body’s systems [5, 6].
So why aren’t we all using heart rate variability?
HRV Is Useful and Reliable, but Interpreting the Results is Too Much Work for the Mass Market
Unfortunately, HRV interpretation doesn’t have one-size-fits-all solutions. It is highly variable between individuals and depends on many different factors: a person’s past measurement results, time of day, the way your nervous system typically responds to stressors, and much more.
Thus, numbers that would indicate extremely high stress for one person might be completely par for the course for someone else.
Because of this, HRV has traditionally been confined to fields like space medicine and professional sports, where a whole lab & team of researchers can afford to focus on one person.
The point of Welltory is exactly this — to give people who don’t have access to labs & researchers personalized HRV interpretations that are relevant to their daily lives in order to help them manage stress, reach peak performance when they need to be at their best, and take their bodies into account when they plan their day.
Welltory Gives Personalized HRV Interpretations Without Labs & Researchers
To give our users personalized interpretations of heart rate variability, we used data to automate processes that previously had to be done by hand.
Welltory’s main advantage is our extensive database of over 2 million heart rate variability measurements enriched with people’s lifestyle and health data (a total of over 2 billion data points).
We used big data technology to separate people into 4 different nervous system types, testing the results against 5,000+ clinical assessments completed by our users. The result was the world’s first self-learning heart rate variability algorithm that adapts to people’s nervous system type over time. Our algorithm also takes into account factors like age and gender, time of day, and results from past measurements.
To make the app even more user-friendly, we replaced the standard list of parameters with health scores that are easy to understand and supplemented the scores with data-driven recommendations that help people get more done today & feel better tomorrow.
To Make HRV Analysis Easier, Welltory Groups People into 4 Nervous System Types
Previous research into heart rate variability has found that the heart can be controlled in different ways. For some people, the brain plays a role in heart regulation. For others, the heart auto-regulates. The vast majority of people are somewhere in between.
These differences in regulation type mean that the same heart rate variability numbers can mean different things for different people. Thus, most researchers split people into different regulation types when running HRV assessments [7, 8, 9].
At Welltory, we used big data technology and insights from previous researchers to classify people into 4 different regulation types.
Brain control
This is when the brain’s subcortical structures are impacting heart function. They stimulate the hypothalamus and trigger reflexes, both of which activate the sympathetic nervous system. Parasympathetic nervous system activity is suppressed via reflex regulation and lower structures of the brain stem.
In short: The relaxing effects of the parasympathetic nervous system are suppressed and the sympathetic nervous system stimulates the heart. This stimulation comes from two key structures — the subcortical structures and the hypothalamus.
Moderate brain control
This is when the hypothalamus is primarily responsible for heart regulation. It triggers reflexes that activate the sympathetic nervous system. Depending on the situation, these reflexes can activate the parasympathetic nervous system as well.
So the sympathetic nervous system is constantly active, and is periodically balanced out by the parasympathetic nervous system.
In short: The sympathetic nervous system stimulates the heart and the relaxing effects of the parasympathetic nervous system are relatively suppressed.
Moderate auto control
The heart operates autonomously via impulses sent by the sinoatrial node (SA). Its activity is equally modified by the sympathetic and parasympathetic nerves controlled by the brain stem. So sympathetic and parasympathetic activity is in balance.
In short: The heart receives an equal number of stimulating and relaxing signals.
Auto control
The heart operates autonomously via impulses sent by the sinoatrial node (SA) and its activity is only modified by the parasympathetic nervous system.
In short: the heart receives primarily relaxing signals.
Different Nervous System Types Let Us Determine Baseline and Contextual States for Users
Everybody’s heart rate variability has a really wide range. It can fall within any of the following zones:
- Green — your numbers are showing that the condition you’re in is safe for your health
- Red — your numbers are showing that you’re stressed, but it’s not doing any damage to your body.
- Blue — your numbers are showing that you’re inching closer to a breaking point and risk getting sick or otherwise damaging your body
Every person has a predominant regulation type — the one that takes over when they’re calm and feel fine. We call this HRV range people’s baseline state.
For example, here is the baseline state for two people — Alex and Max. Both of them fall within the green zone, but they have different regulation types.
Here is how their heart rate variability measurement results will change depending on what happens to them
Alex’s baseline state is worse, because his measurement results fall outside the red zone almost daily.
This doesn’t mean Alex is sick or is having heart problems. But it does mean that his body is not as good at coping with stress. His stress resilience is lower.
Max’s measurement results almost never leave the red zone. Even when he’s sick, he might still fall on the border between the red and blue zones:
In our example, Max’s measurement results when he is focused are better than Alex’s when he’s calm (1). But a result that shows Alex is moderately stressed would be interpreted as high stress for Max (2):
Our Solution Works! The Proof — Hundreds of Thousands of Happy Users
By separating people into different nervous system types, Welltory has taken the first step in creating a personalized heart rate variability algorithm that brings people relevant insights about their bodies without manual processing.
This approach to personalized HRV interpretation has a ton of potential, since it eliminates the need for expensive equipment and qualified labor, making preventive health tracking available to people all over the world. All they need is a smartphone!
The best part is that it actually works. At Welltory, we see hundreds of thousands of people use our app to track their health and use personalized recommendations to get more done, recover properly, and feel better by taking their bodies’ needs into account. They leave us glowing reviews in app stores and give us a constant flow of feedback that will help us improve our algorithms in the future.
What’s more, Welltory is part of a growing movement called quantified self — a community of preventive health aficionados who use self-tracking and digital biomarkers to boost their productivity, stress resilience, and overall health.
Together, we are all working to break new ground in preventive health. And as more and more data becomes available, the opportunities in this sphere will be limitless.
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Appendix
[1] Genesis and regulation of the heart automaticity. https://www.ncbi.nlm.nih.gov/pubmed/18626064
[2] Autonomic and endocrine control of cardiovascular function. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4404375/
[3] A physiologist’s view of homeostasis. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4669363/
[4] Physiology of the Autonomic Nervous System. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1959222/
[5] Time and frequency domain methods for heart rate variability analysis: a methodological comparison. https://www.ncbi.nlm.nih.gov/pubmed/7568644
[6] Heart Rate Variability: New Perspectives on Physiological Mechanisms, Assessment of Self-regulatory Capacity, and Health risk. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4311559/
[7] Heart rate variability: standards of measurement, physiological interpretation and clinical use. Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. https://www.ncbi.nlm.nih.gov/pubmed/8598068
[8] Shlyk, N.I. and Sapozhnikova, E.N. “Analysis of Heart Rate Variability and Dispersive Mapping of ECG of Participants of Parallel Researches Mars-500 With Different Dominant Types of Vegetative Regulation.” Vestynik Udmurskogo Universiteta. 2012
[9] Typoloagical characteristics of the functional state of regulatory systems in schoolchildren and young athletes (According to Heart Rate Variability Data). https://link.springer.com/article/10.1134/S0362119709060103
