How to Measure Heart Rate Variability | Oura HRV Tracking

September 28, 2017

In our previous heart rate variability article, we concentrated on HRV basics:

  • Heart rate variability is a measure for the variability in heartbeat interval
  • Heart rate variability is highly individual and you shouldn’t compare your values with anyone else’s
  • Heart rate variability is higher when your rest-related parasympathetic nervous system regulates your heart rate
  • Heart rate variability is an indicator of your short and long term recovery status and readiness

But how can you track your heart rate variability? How easy is it? And can you measure heart rate variability at home? In search of answers, we dig deeper into the science and practice of HRV tracking.

What You’ll Learn In This Article

This is the second article in our heart rate variability blog series. You’ll learn:

  • What the common ways to track HRV are
  • What the challenges in HRV are
  • How Oura tracks HRV and how you benefit from it
  • How Oura’s HRV tracking correlates with ECG HRV tracking

How to Measure Heart Rate Variability – Common Methods

Let’s first look at the most common ways to measure heart rate variability: electrocardiogram (ECG) and photoplethysmography (PPG).

Electrocardiogram (ECG)

The golden standard method to determine heart rate variability is via electrocardiogram (ECG). You may have had your ECG recorded at a doctor’s office. Sometimes short-term HRV measurement with a specific ECG device is part of a personal training or coaching service.

ECG shows the electrical activity of your heart. Therefore, you need to have electrodes (adhesive pads) on your skin for the measurement. Moreover, the electrodes need to have a firm skin contact, so usually they’re attached on your skin with a sticker label.

When tracked through ECG, HRV is a measure of the variation in R-R intervals of a heartbeat. (Read more about this in our previous HRV article).

The HRV data you get through ECG is usually highly accurate and reliable. However, even the most reliable ECG devices are sensitive to motion artefacts, and require good analysis software that filters out extra variabilities and disturbances.

Moreover, ECG may not be that comfortable in the long run as sticking electrodes to your skin regularly can be rather burdensome. ECG isn’t therefore a method you would use frequently, but it offers a snapshot of your HRV in a specific, short timeframe.

Photoplethysmography (PPG)

Each contraction of the heart results in a blood volume pulse that propagates through the bigger arteries towards the small capillaries. We can all feel this – and count our heartbeats – by placing a finger (or two) gently on top of our arteries on our neck or the palmar side of our wrist.

This blood volume pulse signal can be tracked optically. Optical measurement is based on the absorption of certain wavelengths of light when reflected towards blood veins. In this case, we talk about photoplethysmographic (PPG) measurement.

In PPG recording, you don’t need electrodes with wires, but just a device that has a proper LED and a photo receiver. That’s why it’s a common method used in wearables, such as the Oura ring.

Oura utilizes infrared light, as do pulse oximeters in hospitals

Oura utilizes infrared light, as do pulse oximeters in hospitals. Infrared light travels deeper into our tissues, and enables more accurate measurements than other wavelengths of light, such as green light.

When HRV is tracked through PPG, it’s the inter-beat-intervals (IBI) that are tracked. In PPG, what marks a heartbeat and the start of a new interval is the steepest increase in the blood volume signal prior to its actual peak. (Read more about that in our previous HRV article).

ECG, PPG and HRV graphsAn illustration of interbeat intervals (ms) and PPG and ECG signals

The challenge with PPG is to find a measuring spot that provides accurate and reliable results. Firstly, the device should be close to an artery that produces a strong blood volume pulse signal.

Secondly, any motion during the recording can disturb the measurement. This means that the measuring device should be strapped so tightly that the light travels to your body coherently. In addition, excess light from the environment that could disturb the recording needs to be minimized. Inevitably, there’s a risk that wearing comfort can be compromised due to this.

Estimating the exact timing of a heartbeat can be trickier in PPG than in ECG, so the device and the analysis software need to be designed with great care. Notably, the optical tracking of HRV can have some limitations in special conditions such as arrhythmia.

Challenges in Heart Rate Variability Tracking

We already mentioned some of the challenges related to HRV tracking methods, but let’s analyze them a bit with what we know about HRV and its interpretation.

How to get accurate long-term data

You need to track your HRV with a method that provides accurate and reliable data. For ECG and PPG, this means, among other things, that the measuring device must be firmly fastened.

But in order to gain long-term data, you also need to track your HRV regularly, because you need to know your own HRV baseline before making any interpretations from the values. Remember, it’s the changes in your HRV data compared to your own baseline that you should be most interested in.

How to gain comprehensive data sets

HRV is very sensitive to changes within both your body and the surrounding environment. How much HRV data you get makes a difference – a snapshot such as one 5-minute average is not the same as a more comprehensive view, such as periodic averages throughout a night.

When there’s more than snapshots to interpret, you don’t need to rely only on single, momentary values, but you get a bigger picture of your HRV.

How to standardize measurement conditions

Since HRV is a very sensitive measure, its tracking should take place in as standardized conditions as possible. Otherwise, you may not be able to compare the tracked values.

With standardized conditions we refer to, to name a few, the time of day, prior activities and external stressors including ambient temperature, noise and presence of other people.

How to provide wearing comfort

For long-term measurement to take place, HRV tracking should be as comfortable as possible. If tracking becomes a burden, or feels uncomfortable, you most probably quit doing it regularly, or at least get annoyed – and that doesn’t do you any good!

Oura Heart Rate Variability Tracking

For Oura, combining precise and reliable body signal tracking with topnotch wearing comfort has always been a key priority. For HRV tracking, it’s also the only way to meet the listed requirements: to get accurate long-term data which goes beyond single snapshots, to measure in standardized conditions and to provide wearing comfort.

Consequently, Oura is a ring.

Heart rate variability tracking from a finger

A finger is one of the most optimal places for PPG-based HRV tracking. This is because it has the suitable arteries and capillaries for clear blood volume pulse signals, making the optical measurement more reliable and accurate.

A finger is one of the most optimal places for PPG-based HRV tracking

In addition, there are no moving parts between the joints in fingers, meaning that the ring sits firmly and doesn’t move. To ensure optimal fit, the Oura ring comes in 8 sizes, from 6 to 13.

Heart rate variability tracking during the night

From the finger, the Oura ring determines inter-beat-intervals by using invisible infrared light at a very high sample rate of 250 Hz. The measurement takes place throughout the night, and you don’t need to do anything except wear the ring.

Why track HRV during the night? For the sake of getting more than just snapshots, and in as standardized conditions as possible. Nocturnal tracking provides an excellent HRV measurement window where many of the environmental stressors aren’t present.

Nocturnal tracking provides an excellent HRV measurement window

The time when you sleep is also the period when your body is at rest. Remember that HRV is the indicator for especially rest-related parasympathetic autonomic nervous system. This is the reason why HRV is often measured first thing in the morning with other tracking methods as well.

Heart rate variability calculations

Oura calculates your HRV through the night by using a commonly used HRV formula called rMSSD (Root Mean Square of the Successive Differences). It also happens to be a parameter that mostly reflects the parasympathetic autonomic nervous system activity.

rMSSD indicates how much variation there is in your heartbeats within a specific timeframe. For Oura, this timeframe is 5 minutes – it’s the standard duration for measurements in research use.

Oura has designed the measuring method and analysis software with great care, paying close attention to possible variance in PPG data. The calculations are done in a way which filters out the possible inaccuracies.

An example of nocturnal heart rate variability visualization in Oura Cloud.

On the Oura app and Oura Cloud you see your average heart rate variability from all the 5-minute samples the ring collects throughout the night. Both also visualize your HRV during the whole night, letting you see all the 5-minute HRV values it has calculated. Hence, you will get a comprehensive set of HRV data each night.

Heart rate variability tracking with wearing comfort

A ring has a form factor that has been accepted and widely used by us humans for thousands of years. We often don’t even notice that we’re wearing one – even though rings sit quite firmly on our fingers.

The high wearing comfort proves to be especially important during night-time tracking. After all, it’s the functioning of your rest-related nervous system that HRV reflects, so we don’t want the measuring device to be an outside stressor!

Correlation Between Oura and ECG Heart Rate Variability Tracking

In 2017, Oura executed a validation study with the aim to quantify the accuracy of Oura heart rate and HRV measurement. The comparison was made by measuring nocturnal IBI data with the Oura ring and simultaneous R-R interval data with an ECG device. The sample consisted of 10 healthy individuals (3 female, 7 male). All subjects had the Oura ring on both hands, thus resulting in 20 nightly PPG recordings for analysis.

This study found a strong agreement (r = 0.984) between the nocturnal HRV values determined by the Oura ring and the ECG device. The reliability of the HRV measurement was further confirmed in short term (5 min) data, where the results showed high consistency between the methods (r = 0.84 ± 0.15).

Scatter plot on HRV derived from ECG and the corresponding value from the Oura ring in the development sample of 10 subjects, all wearing the Oura ring in both hands. Mean absolute deviation was 2.1 ms.

Further reading:

What Is Heart Rate Variability And What You Can Learn From It | Oura article
How To Easily Measure Your Heart Rate Variability | Coach Alex Fergus

If you’re interested in tracking your HRV but you don’t have the tools for it, have a look at the Oura ring in the Oura Shop. If you have an Oura ring, you can dig deep into your HRV data in the Oura Cloud.

Did you know that during your sleep, Oura tracks your nocturnal respiratory rate – the number of breaths you take each minute?

In this second part of our guest blog series, our prestigious visiting writer Dr. Benjamin Smarr tells you about chronotypes – our genetically shaped circadian preferences – and their effect on our everyday life.

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