A Guide to HRV: Introduction to the Autonomic Nervous System

The autonomic nervous system is a very complicated system of neurological networks that runs through your entire body and is really responsible for controlling all of the things that you usually don't have to think about controlling: our blood pressure, our heart rate, our temperature control, digestion. It's crucial for day-to-day functioning.

The autonomic nervous system can get knocked out of balance in lots of different ways. It can happen as a result of illness, it can happen as a result of trauma, so a car accident or a concussion. When we think about what happens to the body when the autonomic nervous system goes wrong — that's sometimes called dysautonomia, which is dysfunction of the autonomic nervous system — there are a number of symptoms that can be associated with that.

When someone reports having a racing heart rate, they feel like their heart is beating really hard in their chest. That's one thing that we hear a lot that typically turns out to be dysautonomia. People will talk about shortness of breath and air hunger, feeling like they can't get enough air in their lungs. We also have a lot of reports of dizziness and headache, especially with postural changes. As your heart rate changes and modulates very erratically, it can lead to things like feelings of panic or feelings of anxiety, but you don't know why. It can affect your ability to sleep in a healthy manner.

The autonomic nervous system has two main parts. There is the sympathetic nervous system, which we sometimes call our fight-and-flight nervous system, which manages our responses to threat, our responses to fear, our responses to something that requires immediate action. And then there is the parasympathetic nervous system, which we call the rest-and-digest portion of the autonomic nervous system. This is the part of the nervous system that calms you down. Depending on which part of the nervous system is affected, you can have very, very different symptoms.

Many things can trigger a dysautonomic reaction: stressful situations, hot weather — specifically hot and humid weather — and dehydration. People who menstruate will often experience worse dysautonomia symptoms in their premenstrual period. This is just due to the hormonal fluctuations. Poor sleep can also be a big trigger for dysautonomia symptoms. When people haven't slept enough or they've slept badly, it can lead to that fight-flight part of our nervous system kicking into overdrive and making you feel what's sometimes called "tired and wired".

Heart rate variability is a metric that can really give us a good proxy of how well our autonomic nervous system is working in a balance of our fight-flight nervous system and our rest-and-digest nervous system. It is a measurement that specifically tells us the average amount of time between each heartbeat that you have. If your heart is beating at a certain rate — let's say your heart is beating at 80 beats per minute — that doesn't mean that every distance between each heartbeat is exactly the same. Sometimes they'll be quick, heartbeats in sequence — boom, boom — and then other times there will be a longer distance, so one heartbeat and then another heartbeat. That variability in beat-to-beat distance tells us what heart rate variability is.

HRV tends to be a very good general marker of overall health, and so many things that affect your health can affect your heart rate variability. There are many studies showing, for instance, that just before you become actively infected with a pathogen, your heart rate variability will drop, because your body is detecting the pathogen before you even develop symptoms. You can also experience worsening heart rate variability when you've exerted yourself too much with a complex chronic illness, and that's the beginning of a crash.

We've seen trends in many studies showing that when your heart rate variability is tending generally low — especially low for you, in a personalized way — that means you're going to be having a harder day with worse symptoms. And when your heart rate variability is tending generally high, you're more likely to be experiencing fewer symptoms and having a better day.

Another part of heart rate variability that I think is important to talk about is what we call the stability of your heart rate variability. Someone who maybe isn't having the same highs, but whose heart rate variability is steady — it's not going up and down day to day, hour to hour, minute to minute, it's staying where it is — is more associated with stable symptoms and stable health. I tend to not like to throw out numbers for heart rate variability that are good or bad, because what's most important is that we understand what is normal for you, and we go from there.

Heart rate variability is a very sensitive metric. You want as controlled a heart rate variability measure as you can possibly have. That's why measures taken first thing in the morning when you first wake up — or even better, because it gives you a longer-duration measure, when you're actually down and asleep, when your body isn't responding to too many environmental triggers — are probably the best time for us to get a solid understanding of what your heart rate variability is doing.

The evidence is still emerging, but it really does seem that we can use HRV in our daily lives to modify activities and modify behaviors in ways that can promote better health and fewer symptoms overall. HRV can be used to evaluate whether certain medications are working well. HRV can be used to evaluate the level of activity that is best for you to avoid crashing. And it can be generally used as an aid to pacing and an aid to good behavioral health management of a complex chronic illness.

In the following videos in this series, we're going to talk about various aspects of your lifestyle, how they affect your nervous system and your overall health, and then different tips and tricks that you can use to manage them.