Heart Rate Variability: What It Is & Why You Should Care
Level 4: All-Star
Welcome Players! Fun fact: “lub-dub” is the proper medical way to verbalize the sound of heart makes when it beats. When listening to a normal heart through a stethoscope, you will actually hear two “lub-dubs”. The first is the sound of the mitral & tricuspid valves closing, while the second closely following is the sound of the aortic and pulmonary valves closing.
Today we’re going to peel back the layers of pseudo-science surrounding the most popular form of physiological data that has hit the general consumer market: Heart Rate Variability (HRV). You’ve seen the advertisements; “increase your HRV, reduce stress, recover faster, perform better”. Somehow they manage to combine every buzzword in health & performance into one product. Impressive really just from a marketing standpoint.
We’re going to look at the research that tells the science of what HRV is, how it’s measured, how you can understand it’s data to improve your health & performance, and set the stage for future writings on proven-protocols to improve your HRV.
Contents:
Home is Where the Heart is
Respiration Wants to Party
Get Out Your Ruler
Predicting Stress
Tell Me What To Buy (No)
Coach’s note: This is our first Level 4: All-Star article. With increasing levels the depth of information will be greater, the technicality of the terms will be more finely delineated, and there will be abundant research associated with major statements. While it may be overwhelming for some of you at first, take your time, read the linked resources, and ask questions in the comments (!), because future articles and guides will rely on your understanding of this information in order to be executed properly. Don’t skip steps!
Home is Where the Heart is
Two of my favorite representations of how the human body works come from children’s cartoons. The first being this episode of The Magic School Bus, where little Ralphie is kept home from school with a fever so the class decides to take a field trip into his immune system. The second is Osmosis Jones, where Bill Murray stars as the host body of a white blood cell that works to keep the bad guys out. Both are fantastic and I might even go as far to say that they made a positive influence in sparking my early interest in human anatomy & physiology. But unfortunately I don’t have the cinematic expertise to make the following topics riveting and entertaining like they did. Nevertheless, they are important things to know, and so we’ll opt for directness. I’ll try to include pictures when applicable for the more visual folk.
A Heart and Lung Walk into a Bar: The Cardiovascular & Respiratory Systems
Your heart rate is measured as the amount of pulses (or beats) your heart completes over a defined period of time, a good benchmark being around 60 beats per minute. In that time most humans will take on average 12-15 breaths. Within the cycle of heart beats/breaths per min., there is an undulating micro-cycle happening where the rate of heart beats is increased during inhalation and decreased during exhalation. Your heart beats faster while inhaling and slower while exhaling. This physiological process is called Respiratory Sinus Arrhythmia, which is a measure of the variability between heart beats in synchrony with respiration.1
Why is this?
It is an optimization of oxygen diffusing into the blood with respect to the timing of when the blood pumped from the heart circulates within pulmonary capillaries in order to promote maximal uptake of oxygen into the blood during respiration.
Easy, right?
Ok maybe we need to unpack that a little bit.
Starting at [1], we see de-oxygenated blood get pumped by the heart to the capillaries of the the lungs [3]. Capillaries contain many alveoli, which are tiny sacs of air that allow for gas exchange between the air we inspire and our blood. When we inhale, oxygen (O2) is attaching to hemoglobin in our red blood cells, and when we exhale carbon dioxide (CO2) is being released from our red blood cells through the alveoli back into our respiratory system so that we can expire it.
Once the O2 is diffused into our blood in [3], it then has to flow back to the heart [5,6] in order to get pumped out to the rest of our body, tissues, and muscles [8]. We’re tempted to think that this process happens automatically, but in reality it takes time for our red blood cells to travel from the de-oxygenated heart to the lungs back to the heart to then to the rest of our body.
RSA optimizes this process ever so slightly (but significantly) so that while there is an influx of air in the bronchioles, AKA while you’re inhaling, the heart increases it’s cardiac output via quicker contractions, thus moving more blood through the capillaries in the time that they’re being flooded with oxygen.
Cardiac Output = [amount of blood pumped] X [frequency of pumps]
When there is no longer oxygen being brought into the system, the heart decreases output to allow for the red blood cells to fully release their binding of CO2.
Through this process RSA is able to to increase the efficiency at which our body binds oxygen to red blood cells, increasing the amount of oxygen the organs and tissues receiver per unit of time, and lower the number of red blood cells that end up repeating another cycle of travel without dispelling their CO2.
Pretty cool, right?
Let’s try this again. What is RSA?
It is an optimization of oxygen diffusing into the blood with respect to the timing of when the blood pumped from the heart circulates within pulmonary capillaries in order to promote maximal uptake of oxygen into the blood during respiration.
Ok! Moving on!
Get Out Your Ruler
Now that we know what RSA is, let’s talk about how it relates to Heart Rate Variability (HRV).
They’re the same thing.
HRV is a measurement of the process which is RSA.
RSA is defined by the shortening of the R-R interval on an ECG during inhalation and lengthening of the interval during exhalation. Breathe in, more heart beats, shorter interval. Breathe out, less heart beats, longer interval.
The unit of HRV on the other hand, is not so easily defined. There are three ways you can look at the interaction of HRV: time-domain, frequency-domain, and non-linear.
Time-domain indices of HRV quantify the amount of variability in measurements of the interbeat interval (IBI), which is the time period between successive heartbeats.
Frequency-domain measurements estimate the distribution of absolute or relative power into four frequency bands.
Non-linear measurements allow us to quantify the unpredictability of a time series.
Don’t worry, we won’t be going super in-depth into these, but it’s important to understand that HRV is not a singular number on an objective scale where “higher is better”. It very much matters in what context you’re measuring and what you’re using as your identifier if you intend on producing changes.
We will touch on the 4 types of frequency domain-bands: ULF (ultra-low frequency), VLF (very-low-frequency), LF (low-frequency), and HF (high-frequency). These can be likened to using a prism to separate different wavelengths of visible light, where each of these frequencies are a component and can be associated with sympathetic or parasympathetic branches of the nervous system.
Metrics
Most importantly when talking about testing for HRV, is talking about how long the measurement lasts. To get an accurate predictor of lifestyle stress influenced by circadian rhythms, body temperature, metabolism, sleep-wake cycle, and nutrition, the gold-standard measurement for HRV is on a 24 hour time frame.
For a 24h testing period, the primary metrics used for HRV measurements is SDNN, or standard deviation of normal heart beats. This measurement is simply looking at how many times the heart beats during inhales/exhales with a standard deviation. SDNN has also been used as classifier of cardiac risk, where patients with SDNN values <50 ms are “unhealthy”, between 50-100 ms have “compromised health”, and >100 ms are “healthy”.2
RMSSD (root mean square of successive differences) reflects beat-to-beat variance in heart rate and primarily estimates changes in HRV that are influenced by the vagus nerve (part of parasympathetic nervous system). RMSSD is taken with a minimum 5-minute recording period, which is to say that it can’t be a minute but it also doesn’t have to be 10 min, meaning you can potentially get a good idea if someone is in an aroused or parasympathetic state.3
Another important metric to look at is the Heart Rate Max (HRmax) - Heart Rate Min (HRmin): The average difference between the highest and lowest HR’s during each cycle is sensitive to the effects of respiration rate independent of influence from the vagus nerve. This is important for when emphasis on the exhalation during slower respiration can produce higher amplitudes of RSA that are not inhibited by signals from the vagus nerve.
If you’re interested in going into further detail on how HRV is measured and calculated, this article does a good job at explaining the details.
Gold Standard of Measurement
But how is all this measured? Surely you can’t get super accurate data about electrical activity in the heart through a simple wristwatch, right?
Right.
The most accurate way to take measurement of HRV is to be hooked up to an ECG and tested for a 24h period. Outside of a hospital or research laboratory, this is hardly attainable.
This is where the recent push for developing accurate measurement systems for short time frame RSA measurements becomes useful for the general population. If we can take a 5 min HRV assessment, or even better, spend 10 minutes training HRV using a biofeedback tool (much more on this in future articles), we can make actionable steps towards improving our long term HRV with short-term positive modifications, all through the use of techniques as simple as slow-breathing.
The technology on wearables is rapidly improving, and I think in the next 5-10 years there will be drastic improvements in the amount of biometric data we can capture from a physiological standpoint.
While this article is not a compare/contrast of commercial wearables, in order of most accurate to lesser: chest straps → arm straps → wrist straps.
Because some things we don’t know is:
at what time frame are they taking measurements
what metrics are they taking into account when calculating the HRV ‘score’
what types of processing they’re using to reduce noise from the signal captured in the sensor
what algorithm they’re using to covert the metrics/data gathered into HRV score
whether or not they scrub for artifacts in the recordings
This is not to demonize commercial wearables, just to provide context as to what environment these measurements are taken in. If you’re trying to use a wearable technology for day-to-day changes in HRV, you’ll be at the mercy of error.
What they are great for is tracking long term changes in large populations of people. Let’s say you decided to go on a vegan diet, or went from long distance running to strength training, over a period of 3-6 months your HRV trends can be reflective of the results of your habits.
So by now we should know what RSA is, how it relates to the measurement of HRV, some of the more common metrics used to define that measurement, and which tools are best equipped to provide accurate measurements. Let’s go put these tools to good use!
Predicting Stress: How Do You Measure It?
You can’t.
You should’ve seen that one coming. Although you can’t measure stress directly, what you can do is measure some of the things that seem to be impacted, either positively or negatively, in response to stress and use that as an indicator.
Stress can be defined in a few different ways:
“A response to change in order to maintain the state of stability or homology that the body has maintained against the stimulus to break the mental and physical balance and stability of the body.” - Hans Selye
“A maladaptive state in which the sympathetic nervous system is overactivated, causing acute or chronic physical, psychological, and behavioral impairment.” -Kenneth Hambly
“The body’s response to physical, mental, or emotional pressure.” - National Cancer Institute
What science has had a difficult time putting in to words is that stress is anything that knocks human physiology out of a balanced state.
The parasympathetic and sympathetic nervous systems are a lot less like see-saws and a lot more like driving a boat. If you’re going straight in calm waters, it will only take minimal use of the motors and you might need to make minor corrections (activations) in steering to maintain course. In the case of the body there’s even cyclical variations in these activation patterns that will bias towards one direction for a small period of time before giving way to the other. In an undisturbed environment, things are great.
But if there’s a storm and the seas are rough, all hell is breaking loose. You’re constantly adjusting the direction and speed to avoid capsizing. You might even be stuck going away from your intended direction to get to safety. If you’re only spending an hour or two navigating a storm before making it out, it won’t be long before you correct course and are on your way again. The boat is able to handle short-term chaos, and perform well at that. Back to the open seas!
But if you’re chronically stuck in that storm, rapidly adjusting direction on end and constantly increasing/decreasing the motor to withstand the changing seas with no predictable rhythm or rest, well that’s when things start to break down. The engines can run hot, the hull of the boat can become damaged from repeated impact, the gears that control the rudder can be knocked out of alignment. Spend too long in a storm and you’ll find many ways things can go wrong.
This is the impact of stress. Short-term stress = good. Ignites our sympathetic nervous system (SNS), prepares our body to be fully active in a fight or flight situation. We’re alert, aware, focused, ready to take on a challenge.
Once that challenge passes, we should be getting a reduction in sympathetic signals and an increase in parasympathetic signals to bring us back into a balanced state before we experienced the stressor.
HRV as an indicator of stress then reflective of our our bodies ability to respond highly to a stressor, and then subsequently restore itself back into a balanced state. How the heart responds to these increases in stress gives us a window into the nervous system as a whole.
Studies that compare psychological exams of standardized stress questionnaires find that the more stress the participants responded on paper, the worse their HRV measurements were. 4
And that is just psychological stress. As put here:
Many physical conditions and lifestyle habits can affect HRV results, including physiological factors (e.g., breathing, circadian rhythms, and posture), non-modifiable factors (e.g., age, sex, and genetic factors), modifiable lifestyle factors (e.g., obesity, metabolic syndrome, physical activity, smoking, and drinking), and other factors [e.g., medication (e.g., anticholinergics, stimulants, and beta-blockers)]5
What measurements in HRV can tell us, through the use of metrics like SDNN and different frequency-domain measurements (remember VLF, LF, and HF?), is which part of the autonomic nervous system may be contributing, or not contributing to the current state of physiology. For example, the HF band is a measure of parasympathetic nervous system (PNS) activity as it reflects the activity of the vagus nerve, and LF reflects the activity of the SNS.6 7
The purposeful use of something like this cannot be understated.
For starters, the ability to possibly evaluate and monitor physiological stress has major implications for performance. Total work load can be dynamically adjusted based on the individuals ability to tolerate stress. High HRV values? Can tolerate greater demands. Lower HRV values? They are already stretched thin and should be given time/resources to return to optimal performing state.
In athletics this is easy. Athlete training and competition volume can be monitored objectively through the use of GPS trackers and accelerometers which can then be correlated with physiologic responses based on biometric data.
Harry Rice runs 3200m total distance in practice with an average speed of 19mph and 30 start-stop sequences. His average HRV values are X. If he had a particularly tough practice where he ran 4200m total distance, and his HRV values the next morning measured 20% lower than X, then that information could be used to reduce Rice’s total workload either the next day, or through progressive volume reduction over the course of the days leading up to the game. In either case, the data allows for improvement strategies to be created in real time.
In the business workplace, this too can be of great value. Often-times professional sports is seen as the highest-stress career to have, but a fast-paced trading floor or high-pressure sales environment can enact comparable responses from a human nervous system. Take this study that finds a significant activation of the sympathetic nervous system and changes to HRV while opponents played chess! That’s right, a serious game of chess can be a physiological stressor.
Rebecca Bradley is an entrepreneur. Her schedule is unpredictable, has periods of extreme mental effort interspersed with occasional downtime. She often has to make critical time-management decisions and balances taking care of her health with being productive. Rebecca tracks some HRV metrics and uses the insights to gauge her ability to make positive efforts towards work or if she needs to delegate and allow her body the time it needs to get back to optimal capacity. Day’s where her HRV metrics have measured low are given less caffeine, greater sleep opportunity, and more nutrient-dense food. Day’s where it’s high means more time for work, greater leniency on supplements and nutrition, and full-steam ahead.
Tell Me What To Buy (No)
I know you may be convinced already to go out and buy the most expensive, accurate HR/HRV monitor you can get. But pump the brakes, we’re not going to do that just yet. Let’s summarize:
We learned that HRV is a measurement of RSA, our physiologic ability to increase heart rate during inhalation to promote maximum oxygen saturation into the blood, and then decrease heart rate during exhalation for subsequent CO2 release. We know that it’s a measurement of time between successive heartbeats, usually from the R-R interval, and that those measurements are taken ideally over the course of 24h along with spectral analysis allowing for different frequency bands to be established. Those bands are related to input from the sympathetic and parasympathetic nervous system, while different statistical analyses’ of the R-R intervals over time can lend insight as to the influence of the autonomic nervous system. We also recognize the importance of HRV is the attempt to measure our current ability to tolerate stress, both as a predictor of past stress incurred and to inform decisions on future stress tolerance. Stress itself has no negative or positive connotation, simply being a shift away from a point of homeostasis and balance between the SNS and PNS. The greater amplitude with which our heart (and body) can activate the SNS and then subsequently return to baseline can be measured through HRV, providing a valuable insight to our physiologic response. The best way to measure this is in a research lab or hospital, but commercial products are quickly closing the technological gap. HR or HRV monitors worn around the chest will be more accurate than those worn around the arm/wrist, but in either case they should be used more for tracking general trends over time vs. specific effects of interventions until the accuracy is improved.
If there’s anything you don’t understand, or want clarification on a point, leave a comment! You can also request certain topics to touch on next week as we continue to dive further into HRV measurements and taking advantage of data to improve your Health & Performance.
Next week’s article will take a look at what current research supports in lifestyle activities and modifications. Things such as sleep, nutrition, emotional wellness, and community interactions will all impact HRV to a great degree. For sustainability purposes you should look to establish good habits in the larger pillars before optimizing smaller processes.
Put otherwise, it’s not going to matter what your HRV measurements are if you eat like shit, have a sedentary lifestyle, and neglect the opportunity for quality sleep. We’ll see great similarities between things that impact HRV negatively and things that disrupt quality sleep. If you haven’t read the 2 pieces on sleep (Why We Sleep) (Sleep for Success), you should do that soon. All the information we talk about is related!
Following how lifestyle choices impact HRV, we’ll get down to what are specific training tools you can take advantage of to increase your performance edge and improve your resiliency to stress. Here we’ll be getting our first exposure to the intricacies of breath work, a personal favorite of mine.
Just got through the first newsletter. This was really digestible. I listen to many Whoop Podcast episodes about the science behind the wearable products but I never deeply understood what all the metrics truly meant in regards to health, wellness and performance.