Glossary
NN interval
Also called NN intervals, normal-to-normal interval
An RR interval between two normal heartbeats, after extra beats and measurement errors are removed. Most HRV metrics are calculated from NN intervals.
NN stands for normal-to-normal. The 1996 Task Force standards define NN intervals as the gaps between neighboring beats that both start in the sinus node, the heart’s built-in pacemaker. In practice, they are the RR intervals RR 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. More about RR interval left once the odd ones have been removed or corrected.
Not every recorded interval reflects your nervous system. An ectopic beat Ectopic beat A heartbeat that starts outside the heart's normal pacemaker and usually comes early. Ectopic beats are common, but they distort HRV unless removed or corrected. More about Ectopic beat starts somewhere else in the heart, arrives early and is often followed by a longer pause. A loose electrode or movement can make a sensor miss a beat or count one twice. Each of these leaves an interval that is much too short or much too long, and they bias HRV results.
Software handles this by finding the suspect intervals and either deleting them or replacing them with an estimate. The method used can itself shift the result, so two programs can give slightly different numbers from the same recording. You rarely see this step. When a reading looks strange, a noisy recording is one of the first things to rule out.
The lesson How HRV is measured: from beats to a number shows where this cleaning step fits.