Heart Rate Variability
What is Heart Rate Variability?
If your heart beats 60 times in one minute, we can say that on average, our heart beats at one beat per second. However, if we were to look at the intervals between each beat, a healthy heart (and nervous system) would show variation in the time between successive beats. It might be 1.1 second, followed by 0.93 seconds and so on.
This variation between your heart beats is called heart rate variability, or HRV, and a high HRV is a sign of good health.
Here is a screenshot showing heart rate varying over time:
The y-axis (vertical) is the heart rate in beats per minute, and the x-axis is time.
You can see that my actual heart rate varies between around 68 beats per minute and 79 beats per minute. This graph was taken with the subject at rest and relaxing.
What controls your HRV?
Your HRV is a measure of the time between successive beats of the heart. Maybe we need to ask, what controls your heart beat? Well, we should mention here that the heart has an intrinsic ability to beat using it own “pacemaker” tissues (the sino-atrial node, or SA Node) to start the wave of contraction. Biology students may have seen this when the heart of a rat or frog is removed, it continues beating for some time, outside of the body. However, what we are interested in here, is not the ability of the heart to beat, but what controls the rate of beating. The main controlling factor is the autonomic nervous system (autonomic, because it does not require conscious thought to function). In fact, your nervous system is responsible for controlling a lot more besides, including blood pressure, cholesterol levels, digestion, immune system, blood sugar, kidney function, liver function, metabolic activity, sleep and so on.
The autonomic nervous system consists of the sympathetic and parasympathetic systems. The parasympathetic influence on heart rate is controlled by the release of acetyl choline by the vagus nerve. Increased parasympathetic stimulation of the heart will slow down the heart rate.
The sympathetic influence is by the release of epinephrine and norepinephrine. Increased sympathetic stimulation of the heart increases the heart rate.
When an individual is at rest, the sympathetic and parasympathetic are both active, but the parasympathetic system is dominant, slowing down the heart rate. The two systems are in balance, but the balance is constantly changing.
When we are stressed or physically exerting ourselves, the sympathetic nervous system kicks in and increases our heart rate.
What does HRV tell us?
What researchers have found, is that HRV is an excellent overall indicator of nervous system health. If you have a high HRV, then your nervous system is working well. If you have a lot HRV, then you have some health problems (which you may or may not be aware of).
As far back as the 1960s, researchers noted changes in the HRV of distressed unborn babies, well before there was a change in heart beat rate (1). Since then, numerous studies have linked HRV to nervous system and heart health. A reduced HRV is seen as a predictor for death following a heart attack (2, 3) and even coronary atherosclerosis (4).
How can we measure HRV?
Fortunately there are some easy ways to measure HRV (and also to increase your HRV, which we’ll see in later posts).
To measure HRV, you a heart rate monitor, and some software to interpret the heart rate data. I personally use the Polar H7 heart rate monitor which you can get from Amazon.
SweetBeat for the iPhone or iPad.
If you are an Android user, there currently isn’t a version of Sweetbeat for Android. There is a new application for Android called My HRV Beta which will measure HRV when connected to the Polar H7 monitor. Be aware though, that on Android, you phone or tablet needs to have Bluetooth 4.0. I use this application on my Samsung Galaxy S3 with Android 4.3. (CyanogenMod).
I’ll show how to use the Polar H7 with Sweetbeat in a future post.
1. Hon EH, Lee ST. Electronic evaluations of the fetal heart rate patterns preceding fetal death: further observations. Am J Obstet Gynecol. 1965;87:814-826.