Glossary
RR interval
Also called R-R interval, RR intervals, interbeat interval, IBI, beat-to-beat interval
The time from one heartbeat to the next, measured in milliseconds between the R peaks of an ECG. HRV is calculated from a series of these intervals.
On an ECG ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG , each heartbeat makes a tall, sharp spike called the R wave. The RR interval is the gap between one R wave and the next. At 60 beats per minute the gaps average 1,000 milliseconds, but they are rarely identical. One might be 980 ms and the next 1,030 ms.
Researchers also use the wider term interbeat interval (IBI) for the time between consecutive heartbeats. Optical sensors in watches and rings time the pulse wave rather than the electrical R wave, so strictly they record pulse-to-pulse intervals. The idea is the same.
Every HRV number in your app starts as a list of these intervals. RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD , SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN and the other metrics are different ways of summing up how much the intervals change. If the list contains a missed beat or an extra one, the summary is off too. That is why the intervals are usually checked and cleaned first; the cleaned series is called NN intervals NN interval An RR interval between two normal heartbeats, after extra beats and measurement errors are removed. Most HRV metrics are calculated from NN intervals. More about NN interval .
The lesson How HRV is measured: from beats to a number follows the steps from raw intervals to a final value. How to measure HRV compares the sensors that record them.