NN50
Also called NN50 count, NNxx
The number of pairs of neighbouring NN intervals that differ by more than 50 milliseconds. Dividing it by the number of intervals gives pNN50.
Unit: count
NN50 is the number of times, in a recording, that one normal-to-normal interval NN interval An RR interval between two normal heartbeats, in milliseconds, after extra beats and measurement errors are removed. Most HRV metrics are calculated from NN intervals. More about NN interval differs from the next by more than 50 ms. It is a plain count, with no unit.
Why this measure exists
Counting is the oldest way to measure beat-to-beat change. In a 24-hour recording of about 100,000 beats, the count of jumps over 50 ms gave the early Holter studies a number that reflected vagal activity Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone without any square roots, and the 1996 Task Force listed it as NN50 count. pNN50 pNN50 The percentage of neighbouring NN intervals that differ by more than 50 milliseconds. It tracks fast, vagally driven changes, like RMSSD, but as a share rather than a size. More about pNN50 , the count as a percentage, followed so that recordings of different lengths could be set side by side.
The formula
Go through the intervals in order, take the size of the difference between each one and the next, and count how many are larger than 50 ms. is the -th normal-to-normal interval in milliseconds and means “the number of”.
Worked example
NN50 step by step
Ten beats, every number shown
NN50 of these 10 intervals is 1 differences.
Step 4 of 4
Step 1 of 4 Start with the intervals
Beat 1 880 ms Beat 2 826 ms Beat 3 812 ms Beat 4 823 ms Beat 5 848 ms Beat 6 877 ms Beat 7 880 ms Beat 8 866 ms Beat 9 828 ms Beat 10 815 ms 10 intervals between 11 beats, in milliseconds.
Step 2 of 4 Subtract each interval from the next
Beat 2 − beat 1 −54 ms Beat 3 − beat 2 −14 ms Beat 4 − beat 3 +11 ms Beat 5 − beat 4 +25 ms Beat 6 − beat 5 +29 ms Beat 7 − beat 6 +3 ms Beat 8 − beat 7 −14 ms Beat 9 − beat 8 −38 ms Beat 10 − beat 9 −13 ms Plus means the heart slowed down for a beat; minus means it sped up.
Step 3 of 4 Mark the differences larger than 50 ms
Difference 1: −54 ms more than 50 Difference 2: −14 ms no Difference 3: +11 ms no Difference 4: +25 ms no Difference 5: +29 ms no Difference 6: +3 ms no Difference 7: −14 ms no Difference 8: −38 ms no Difference 9: −13 ms no The sign does not matter; only the size of the change.
Step 4 of 4 Count them
Differences larger than 50 ms 1 of 9 NN50 1 differences
With JavaScript on, the steps show one at a time and the beats can be changed.
A row of ten editable heartbeat intervals in milliseconds, then the calculation of NN50 as numbered steps: start with the intervals; subtract each interval from the next; mark the differences larger than 50 ms; count them. The final step reads NN50 1 differences. Back and Next buttons move between the steps.
The ten beats of the teaching recording hold 1 difference larger than 50 ms, the drop of 54 ms between beats 1 and 2 on the in-breath. Lengthen beat 7 by 40 ms and a second one appears.
What moves it
Breathing depth and rate, vagal activity and heart rate, as for RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD , and the length of the recording: twice as many beats gives roughly twice the count, which is why pNN50 is the form that gets reported. Shaffer and Ginsberg give 2 minutes as the shortest conventional epoch for it.
When it misleads
The count depends on recording length, so it cannot be compared across recordings of different lengths at all. It ignores the size of each jump beyond the threshold and sits at zero in many older adults, who still have measurable RMSSD. Premature beats add to it. For all these reasons the Task Force recommends RMSSD instead.
Where you will see it
The Task Force’s name is NN50 count. Kubios HRV prints NNxx, with the threshold set in its preferences (50 ms by default), beside pNNxx. Research papers mostly report pNN50; apps rarely show either.