Live HRV from your pulse oximeter

How to take a morning reading

• Measure soon after you wake up, before coffee, food or exercise.
• Lie on your back or sit, with the hand wearing the sensor resting still.
• Breathe as you usually do. Don't talk or move during the reading.
• Use the same position, time and length every morning, so one reading compares with the next.
• The first 15 seconds settle the signal and are left out of the numbers.

What the numbers are

RMSSD is the root mean square of successive differences: how much each beat-to-beat interval differs from the one before, in milliseconds. It mostly reflects the beat-to-beat changes that follow your breathing.

SDNN is the standard deviation of all the intervals in the reading: the overall spread, slow changes included.

HRV differs a lot between people and with age, so your own trend says more than any comparison. Sleep, alcohol, a hard workout, a cold or a stressful day can each move it for a morning or two.

Pulse wave, not ECG

Chest straps and ECG time the heartbeat itself. A finger oximeter times the pulse arriving at your finger, so this is strictly pulse rate variability (PRV). A review of studies comparing the two found PRV agrees well with ECG-based HRV at rest, tends to read a little higher for short-term measures such as RMSSD, and agrees much less during movement. So compare readings from this page with each other, not with a chest strap.

A study of athletes found a 60-second RMSSD agreed closely with a 5-minute one at rest, while 10- and 30-second readings drifted further. One minute is the shortest option here for that reason.

Which oximeters work

Built and tested with the Wellue Checkme O2 Max. Other Wellue O2 devices that show a live pulse wave in the ViHealth app may work too: if no wave appears within a few seconds, your device doesn't stream it. Bluetooth in the browser works in Chrome and Edge on a computer or Android phone, not on an iPhone. Close ViHealth first, since the oximeter talks to one app at a time.

Common questions

What is a good RMSSD?

There is no single target. Values vary widely between people, fall with age, and depend on how and when you measure. Take readings the same way for a couple of weeks and watch how your own numbers move.

Why is my number different from my watch or chest strap?

Each device times beats differently and measures at different moments, such as overnight versus one minute in the morning. A finger pulse also reads a little higher than ECG. Compare like with like.

It says the reading was too noisy. What now?

Movement, a loose sensor or cold hands break up the pulse wave. Rest your hand on something, warm it if it's cold, check the sensor sits snugly, and measure again.

Is my data sent anywhere?

No. The pulse wave goes from the oximeter to this page and is analysed in your browser. Nothing is uploaded unless you save readings in an account.

Sources

1. Schäfer A, Vagedes J (2013). How accurate is pulse rate variability as an estimate of heart rate variability? A review on studies comparing photoplethysmographic technology with an electrocardiogram. International Journal of Cardiology 166(1):15-29.
2. Esco MR, Flatt AA (2014). Ultra-short-term heart rate variability indexes at rest and post-exercise in athletes: evaluating the agreement with accepted recommendations. Journal of Sports Science and Medicine 13(3):535-541.