Measuring HRV: Optical (PPG) vs ECG/EKG

Wearable devices often detect HRV for health and fitness metrics. Here we compare devices that use PPG vs ECG.

Heart rate variability (HRV) has gained traction in clinical science and more recently in sports/performance as an immensely useful and informative metric that can predict cardiac risk, track sleep, recovery, and stress, and for some as an overall measure of general fitness and health. Wearable devices using PPG and/or ECG each have their advantages which will be discussed here.

PPG or Optical HR Sensor

Photoplethysmography (PPG) is the method of illuminating the skin and the microvascular bed beneath, measuring the reflected light to estimate the change in blood volume. Outside clinical use, it is also often called Optical Heart Rate sensor.

PPGs found their way into sports and fitness tracking as it was relatively simple to implement, all that was needed was a miniature LED light and a photodiode to capture the reflected light. These days the majority of smartwatches or wrist wearable devices use PPG sensors, and in comparison to ECG on smartwatches, PPGs do not require the user to hold on to the smartwatch during measurement, allowing them to take part in the activity they wish to continuously track. HRV measurements with PPGs use the peak of the pulse detected to measure inter-beat-intervals IBI.

HRV measurements with PPGs however are not without their drawbacks, they tend to be more prone to motion artifacts, less effective when worn loosely, affected by ambient light, and because of these, they have a tendency to miss individual beats or produce false beats, reducing the accuracy of HRV measurements that rely on accurate counting of the beats and inter-beat-interval (IBI). They are also less capable of discerning abnormal beats which are not included in the Standard Deviation of NN (N here represents normal beats without irregular beats) Intervals SDNN HRV measurements. SDNN HRV is the clinical standard for HRV assessment for cardiac health. They are also much more power intensive than ECGs and in many cases may not be suitable for continuous HRV monitoring beyond a couple of minutes, depending on the device.

Although PPG is less effective at measuring HRV for the same given sampling period than ECG. PPG can be used to monitor a user for more extended periods of time than an ECG smartwatch, it counters its lower accuracy by having a longer sampling time. Many PPG devices will monitor the user over their sleep, as it minimizes motion and ambient light, to produce a general HRV measurement for health and fitness assessment.

As most wearable devices use PPG, at Terra we have plenty of integrations with these devices! and read this to see which wearable samples HRV and how often.

ECG/EKG

Electrocardiogram ECG (sometimes also known as EKG for Electrokardiographe) is perhaps the most widely known form of heartbeat detection and measurement. ECG is the technology most used in the clinical setting for detecting overall heart rate, it is also used to detect atrial fibrillation (AF or Afib), which is the irregular beating of the heart.

ECGs tend to provide a more accurate measurement of HRV, they are not prone to motion artifacts or ambient light, however, it does require continuous electrical contact, which motion can interrupt. Whilst ECG requires very low power to operate and therefore a more suitable continuous monitoring tool, in the scenario of smartwatches, however, as it requires holding on to the watch, continuous monitoring of HRV may not always be viable for smartwatches.

As HRV is calculated using the inter-beat-interval (IBI), i.e. the time between each beat, the sharper more defined peak of the ECG signal, and the lower likelihood of false/missed beats provide a more accurate HRV value than PPG. ECG is the preferred mode of HRV assessment in the clinic, though there is increasing clinical acceptance of PPG[1]. However as most ECG wearables require the user to hold on to the device, the short duration of the measurement is insufficient for useful or reliable HRV assessment.

Terra has experience with ECG devices and here is a handy article discussing ECG wearables

See this article for how HRV specifically is measured and sampled

Conclusion

ECG and PPG both have their advantages and drawbacks for measuring HRV, in fact, many new devices try to implement both sensing options into their devices. No matter which method you use to measure your HRV, Terra will be here to ensure your health and fitness application is integrated with these products!

References

[1]Lu G, Yang F, Taylor JA, Stein JF. A comparison of photoplethysmography and ECG recording to analyse heart rate variability in healthy subjects. J Med Eng Technol. 2009;33(8):634–41. doi: 10.3109/03091900903150998. PMID: 19848857.