Healthy Habits: Measuring Heart Rate Variability

Stress is many things. It can be annoying, or life-destroying. It can be psychological, or physiological. We've covered definitions of stress before, so go read up if needed. The point is, whether you're an athlete looking to recover between workouts or a working professional looking to perform at your best, you can only control what you measure. Business 101. So if you want to avoid burnout and injury, you'll want a way to measure stress. Enter Heart Rate Variability (“HRV”).

Good news, it's a thing. Unbeknownst to you all mere mortals, top athletes and CEOs have been using expensive tools to quantify and optimize their stress for years. I'm about to lay it on you, for free.

NOTE: You can unlock the Stress Scan Habit in the Healthzilla app by either using the fingertip Stress Scan feature on the dashboard and the Activities Tab or by syncing HRV data from to the Apple Health app.

What is HRV?

In terms of clinical validation and mechanistic explanations of stress, the number one measure for stress across the board is heart rate variability (“HRV”). HRV as a number actually measures the time interval between heartbeats, which is small but meaningful. The variability of that time interval beats is controlled by your central nervous system ("CNS").

HRV is, therefore, a measure of your nervous system balance, which is the trigger for stress management through your body. Low HRV means high sympathetic bias, i.e. fight or flight, a state of stress. Useful to evade predators, less useful to relax after a day at the office. High HRV means high parasympathetic bias, i.e. rest and digest. Dangerous in the presence of saber tooth tigers, but useful pretty much most of the time.

Why does HRV even matter?

The fact is that HRV is a downstream measure for stress responses triggered by your body. It all goes through the CNS. So with one number, we can capture the state of those stress responses. Pretty nifty, right?

If it's not already apparent, the reasons you might want to manage your stress levels include:

Acute stress: How stress hits suddenly

If you’re working or working out super hard, and sleeping poorly, you’re putting your body under acute stress. Usually, the body recovers back to normal, assuming you allow it. However, research shows that HRV correlates to the body’s response to serious infection such as sepsis [1], endotoxins [2], and general inflammatory markers [3]. This is due to activation of the immune system to fight the infection. Large and sudden drops of 20% or more, not explained by your behaviors, could be a signal to back off and take it easy, before infection takes hold and you become symptomatic.

Chronic stress: How stress accumulates over time

Short-term acute stress can be good to keep you energized and alert to perform, but if you're unable to turn it off, you get into chronic stress territory. All the bad things happen here. Anxiety, depression, burnout, inflammation, weakened immune response, and even cardiac disease [4] [5] [6] [7] . Basically, you get sick and sad.

Recovery: Allowing your body time to manage stress

So if you'd like to avoid all that bad stuff, then the way to titrate appropriate levels of stress is to measure recovery. That shows up as a drop in your HRV, or at least return to baseline day-to-day. A steady buildup of lower HRV means you haven't recovered. Less is worse, remember.

Longevity: Keeping stress in check over time

If you're one of those guys that think you can sleep when you die, then your wish will come true sooner than you may think. To maintain your health well into advanced age, you'll want to now have chronic stress be a major storyline of your life. It burns the candle faster.

So if this one magic number encapsulates physiological and even psychological stress, how do we measure it, exactly?

How you measure HRV?

In recent years, as many professional athletes have adopted the technology with good results, we've started to see it trickle down into consumer-grade devices like the Apple Watch and Oura Ring. We are yet to see HRV available in lower-end devices around the $100 price point, though. Probably because even though you can use the same sensor as you do for regular heart rate, HRV requires a much higher sensitivity and lots of software to get a clean signal. So unless you already have that swag, what can you do?

Photoplethysmography, of course! Say that five times, really fast. Okay, we can just use PPG for short. As such a nerdy scientific name implies, it was developed in clinical settings to use a photosensitive cell to detect changes in blood volume. Especially handy for applications where connecting leads is a challenge, as you don't actually have to touch the sensor. You point a bright light onto the skin and measure how much light is reflected. The fun thing is that the amount of reflected light changes. Why? Because of the volume of blood under the skin changes with the beating of your heart. So if you plot that changing signal on paper, it looks like a heart-beat. Because it is! Actually, it's also how those wearable devices do it. That's that tiny greed laser you see sometimes.

The thing is, the same thing can be done using your phone camera and your fingertip! Who knew you could do science with your smartphone? If you put your fingertip on the camera lens and activate the flash, the capillary veins in your fingertip are illuminated. You can't see any of that with the naked eye. It just looks like E.T.'s finger. But when you read the frames into software, there is a subtle pulsating effect that reflects your heartbeat! All you need to do is tons of manipulation to the signal to remove noise and artifacts, and voila, it's like watching an ECG!

What's a good value for HRV?

Few things to establish. First, HRV isn't actually a unit of measurement. It's an overall representation of different ways to analyze those time intervals between beats. In fact, there are a whole host of measures for HRV, derived from time and frequency domain analyses. The main one you need to know about is the standard deviation of those time intervals, also known as SDNN. Some apps do use other numbers like RMSSD of LF/HF, so those aren't comparable with SDNN at all. Apple made a decision to use SDNN on the Apple Watch and Healthkit, so that's probably the winner here.

Perhaps even more important than what number you have access to is WHEN to measure. Like resting heart-rate, the sensitive nature of HRV makes it hard to isolate what exactly is affecting the result. Maybe you had a heavy lunch. Maybe you had a bad meeting. Maybe you just ran up some stairs. Or had coffee, or whatever. So measure it when you wake up before you get out of bed! That is the most clinically valid measure and allows you to then look at trends.

The trend is really the main event here, not the specific number you get. So let's break that down.

So what's a normal HRV value?

Even harder to gauge than RHR, because to be frank there isn't as much research for HRV yet. Devices like Apple Watch that measure HRV have only been in the market a few years. What we do know about HRV (sdnn calculation) is that if you're close to 0, that isn't good. It means your heart pumps like a metronome, which isn't normal. If you're closer to 100, that's good. Between the two, it's harder to judge.

What can I do with HRV?

Well, since it's far more sensitive than resting heart rate, the same advice applies. It can be a very accurate measure of what state your body is in. The trend is more important than the absolute number. If much lower than usual, say this week on average, then take it easy. Maybe go for a walk rather than a hard run. If high, then go out and conquer the world.

Average HRV values for a healthy population from a 2015 study. Data source: Voss et al. 2015 [8].

How do you get higher HRV?

So higher the better, pretty much. High-level answer: exercise [9]. There is research that shows some temporary changes through stretching, and some results through lifestyle interventions incl. diet and meditation, and some anecdotal evidence that shows different stressors like cold showers and sauna can boost your HRV [10] [11] [12]. I’m sure we’ll see a lot more research on HRV in the future.

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References

1. Ahmad, S., Tejuja, A., Newman, K.D. et al. “Clinical review: A review and analysis of heart rate variability and the diagnosis and prognosis of infection.” Crit Care 13, 232 (2009).

2. Jan, Badar U et al. “Relationship of basal heart rate variability to in vivo cytokine responses after endotoxin exposure.” Shock (Augusta, Ga.) vol. 33,4 (2010): 363-8. doi:10.1097/SHK.0b013e3181b66bf4. Pubmed.

3. Marsland, AL et al. “Stimulated production of proinflammatory cytokines covaries inversely with heart rate variability.” Psychosom Med. 2007 Nov;69(8):709-16. Epub 2007 Oct 17. Pubmed.

4. Dantzer, Robert et al. “From inflammation to sickness and depression: when the immune system subjugates the brain.” Nature reviews. Neuroscience vol. 9,1 (2008): 46-56. doi:10.1038/nrn2297. Pubmed.

5. Raison, Charles L et al. “Cytokines sing the blues: inflammation and the pathogenesis of depression.” Trends in immunology vol. 27,1 (2006): 24-31. doi:10.1016/j.it.2005.11.006. Pubmed.

6. Sloan, Richard P et al. “RR interval variability is inversely related to inflammatory markers: the CARDIA study.” Molecular medicine (Cambridge, Mass.) vol. 13,3-4 (2007): 178-84. doi:10.2119/2006–00112.Sloan. Pubmed.

7. Soares-Miranda L et al. “High levels of C-reactive protein are associated with reduced vagal modulation and low physical activity in young adults.“ Scand J Med Sci Sports. 2012 Apr;22(2):278-84. doi: 10.1111/j.1600-0838.2010.01163.x. Epub 2010 Jul 6. Pubmed.

8. Voss A, Schroeder R, Heitmann A, Peters A, Perz S (2015) Short-Term Heart Rate Variability—Influence of Gender and Age in Healthy Subjects. PLOS ONE 10(3): e0118308. https://doi.org/10.1371/journal.pone.0118308.

9. Sloan, Richard P et al. “Aerobic exercise attenuates inducible TNF production in humans“. Journal of Applied Physiology 2007 103:3, 1007-1011. Pubmed.

10. Day, John. “17 Ways to Increase Your Heart Rate Variability and Life Expectancy”. Drjohnday.com.

11. Farinatti PT et al. “Acute effects of stretching exercise on the heart rate variability in subjects with low flexibility levels.“

    J Strength Cond Res. 2011 Jun;25(6):1579-85. doi: 10.1519/JSC.0b013e3181e06ce1. Pubmed.

12. Justin Lawler (2016). “Heart Rate Variability — What, Why & How?”. Medium.

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