Can Tension Be Seen as a Number? — On Heart Rate Variability

The device on your wrist tells you a number like a "stress index." More and more people bring me a screenshot of it and ask, "this is coming out low — is that all right?"

It is a good question. I use the words autonomic nervous system often in the clinic myself, and since none of it is visible, it is fair to find that frustrating. So today I want to say what that number actually is.

The heart does not in fact beat regularly

A healthy heart does not beat with the precision of a clock.

A pulse of 60 a minute does not mean it beats exactly once every second. 0.98 seconds, 1.03 seconds, 0.95 seconds… the interval between one beat and the next keeps wavering very slightly. Measuring the width of that wavering is heart rate variability.

Here is where many people have it the wrong way round. This wavering is not a fault. It is closer to the opposite.

The body has a nerve that pushes toward tension and a nerve that pushes toward release, both together. The two push and pull against each other moment by moment and so tune the heartbeat. It is also why the heart rate differs slightly between breathing in and breathing out. Wavering means the tuning is alive, and less wavering means the range of the tuning has narrowed.

As for how to measure and read this index, a standards document drawn up in 1996 by the European Society of Cardiology and others is still in use today. This is not a fashion that appeared out of nowhere; it is an index that has been studied for a long time.

Breath is a channel we can reach

What I find interesting in this index is somewhere else — breathe slowly and the value goes up.

In a study in people, the side that trained slow breathing for several weeks showed a rise in heart rate variability. There is also a review gathering studies which find that breathing slower than ten breaths a minute changes the autonomic measures.

Why this matters is that breathing is one of the few autonomic channels we can reach consciously. You cannot change your heartbeat at will, but you can slow your breath this very moment. And that change in the breath actually shows up in the heart's range of tuning.

It is why I ask about breathing so often in the clinic. When shallow breathing goes on for a long time, symptoms scatter in several directions. (When seemingly unrelated symptoms come together)

But this number wavers easily

There is one thing it is better to know going in.

Heart rate variability changes value easily depending on the conditions of measurement. Whether you measured it sitting or lying down is by itself enough to give a different value. Your breathing rate at the time, your age, and the medicines you take also move the value.

So "my number came out low" does not mean your body got worse today. It may be only that you breathed a little more shallowly than yesterday. What I mean is that there is no need to let one number unsettle you.

How far I trust a number

I neither ignore the numbers nor look only at them.

I read them as a flow within the same person. Instead of comparing with others, I look at that person's last month and this month, measured under the same conditions. The more easily an index wavers, the more the direction is worth having over a single measurement.

I listen to the story before the number. How long it takes to fall asleep, what mornings are like on waking, where the body reacts first under tension. These stories tell me more than one number does. (The exact meaning of "your tests are normal")

I do not make an improved number the goal. What I am trying to see is not an index but that person's day. Sleep coming more easily, mornings feeling less heavy, tension not lingering so long. (Not being able to fall asleep is not the same as sleep not coming)

One thing to add: a crushing pain in the chest, an episode of nearly losing consciousness, a pulse that is badly irregular even at rest — these are not things to judge by the number on your wrist. Cardiac testing comes first.

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Can tension be seen as a number?

In part, yes. It is true that the autonomic nervous system leaves its trace in the heart's wavering, and there is a way to measure it. The autonomic nervous system is not a vague phrase; it is something that can be measured like this.

But that number does not explain the whole person. So I use the number for reference, and make the judgement by looking at that person's story and body.

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References

• The international standards document on measuring and interpreting heart rate variability — Circulation, 1996
• A randomised controlled trial in which heart rate variability rose after slow breathing training — Cureus, 2022
• A review gathering the effects of slow breathing under ten breaths a minute on autonomic measures — Frontiers in Human Neuroscience, 2018

Written by Dr. Heo Ji-young (PhD in Korean Medicine Pathology, Kyung Hee University · former Research Professor of Herbology, Kyung Hee University)