Heart Rate Variability and Health

Some fitness watches, rings and other trackers track Heart Rate Variability. You may have seen that and wondered just what Heart Rate Variability is, how it differs from pulse rate, and what insight it can give you into your health. 

Heart rate is easy to measure by taking your pulse. 

When we are in a sympathetic state (SNS = fight or flight) due to acute stress, our pulse can go up to 100 or 150 or higher. During exercise, our heart rate also increases, ideally to 220 beats per minute, minus our age. 

A healthy resting heart rate, when we are in the parasympathetic state, (PSNS = rest and digest) is in the 60-90 range. Athletes tend to have resting heart rates that are lower, because they are so fit.

Heart rate varies considerably to accommodate different situations, in ways that are obvious to us. You might have thought that this variation in heart rate would be called Heart Rate Variability, but this is actually referred to as Pulse Rate Variability.

Heart rate variability (HRV) measures something much, much smaller than your pulse rate: It measures the time in between heart beats in milliseconds, and how much that small time interval varies.

Why is HRV important?

Your brain moves between sympathetic and parasympathetic states, in reaction to your circumstances, and passes information to your heart to speed up or slow down accordingly. When your heart is beating fast, there is less time between beats than when it is beating slowly. Also the time between beats isn’t exactly proportional to the number of beats per minute: it’s not like a metronome. Heart rate variability varies by tiny amounts because it is responding to information from many other body systems, not just the brain. This is why it can tell us about much more than just heart health.

People with high heart rate variability are usually less stressed and happier. A healthy HRV is 19-75 milliseconds, gradually reducing with age.

A high heart rate variability indicates that your autonomic nervous system easily moves between SNS and PSNS: your heart rate can increase in response to stress, then decrease once the stressor passes. It therefore indicates the ability to adapt, and so gives us a measure of resilience in the face of the changing circumstances. Reduced HRV signifies poor autonomic adaptability, and can be caused by simple fatigue, stress or overtraining.

It was in the 1970s that HRV was found to reflect this balance of SNS to PSNS. Since then studies have been exploring how much more we can learn from HRV, and found that low HRV is associated with a number of conditions including:

• Migraines

• High blood pressure

• Anxiety

• Diabetes

• Asthma

• Depression

• PTSD

Even more usefully, studies have shown that people with lower HRV were more likely to have certain future health concerns, including:

• High blood pressure

• Reduced recovery capability after cardiac by-pass surgery

• Tendency to arrhythmia episodes such as atrial fibrillation (A Fib)

Atrial fibrillation is a relatively common heart problem that can be intermittent, therefore hard to capture in a lab visit. It isn’t life-threatening itself, but is the cause of a third of all strokes. Atrial fibrillation is known to be associated with several chronic cardiovascular conditions including diabetes. Studies have assessed that wearable devices would be a cost-effective tool to monitor HRV, as a medical screening test in susceptible people.

Measuring Heart Rate Variability is more accurate in a cardiac lab, with more precise tests such as ECGs. But that can only offer a snapshot, in that circumstance. Wearable devices are a bit less accurate, but have the advantage that they can provide long term information, and tell us about how we respond in a variety of situations. But do note that different wearable devices measure it in different ways, so you can’t necessarily compare between them. 

Of course HRV doesn’t tells us everything: no test does. For example it is much better at showing acute stress due to adrenalin surges, than chronic stress due to ongoing high cortisol levels. We are learning more about what HRV can and can’t tell us, as the research progresses. For example, HRV shows promise as an early indicator of systemic inflammation. Also, monitoring HRV on personal devices can provide an early warning sign during strenuous exercise such as long distance runs or hockey games.

For most of us, we can use HRV as a screen test of how resilient many of our systems are, or whether they are starting to go off track.