pNN20

pNN20 is the share of successive NN intervals NN intervals 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 that differ from each other by more than 20 ms, as a percentage.

Why this measure exists

The 50 ms line in pNN50 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 is a convention. In older people, in patients with heart failure and in anyone with a fast heart, the gaps between beats jump 50 ms less often, so pNN50 counts few of them. Mietus and colleagues re-examined the whole family in 2002 on Holter recordings and found that thresholds well below 50 ms, 20 ms or less, separated healthy people from heart-failure patients, sleep from waking, and young from elderly better than pNN50 did. The Task Force does not list pNN20; it is a later refinement of the same idea.

The formula

$$\mathrm{pNN20} = \frac{\#\left\{\,i : \left|\mathrm{RR}_{i+1}-\mathrm{RR}_i\right| > 20\ \mathrm{ms}\right\}}{N-1}\times 100$$

Count the successive differences larger than 20 ms, divide by the number of differences, and turn it into a percentage. $\mathrm{RR}_i$ is the $i$-th normal-to-normal interval in milliseconds and $N$ the number of intervals, so there are $N-1$ differences.

Worked example

pNN20 step by step

Ten beats, every number shown

The ten intervals, in ms (300 to 2000)

Beat 1 Beat 2 Beat 3 Beat 4 Beat 5 Beat 6 Beat 7 Beat 8 Beat 9 Beat 10

pNN20 of these 10 intervals is 44.4 %.

Step 5 of 5

Back Start at step 1

1. Step 1 of 5 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.

2. Step 2 of 5 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.

3. Step 3 of 5 Mark the differences larger than 20 ms

   Difference 1: −54 ms	more than 20
   Difference 2: −14 ms	no
   Difference 3: +11 ms	no
   Difference 4: +25 ms	more than 20
   Difference 5: +29 ms	more than 20
   Difference 6: +3 ms	no
   Difference 7: −14 ms	no
   Difference 8: −38 ms	more than 20
   Difference 9: −13 ms	no

   The sign does not matter; only the size of the change.

4. Step 4 of 5 Count them

   Differences larger than 20 ms	4 of 9

5. Step 5 of 5 Turn the count into a percentage

   4 ÷ 9 × 100	44.4 %

   pNN20 44.4 %

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 pNN20 as numbered steps: start with the intervals; subtract each interval from the next; mark the differences larger than 20 ms; count them; turn the count into a percentage. The final step reads pNN20 44.4 %. Back and Next buttons move between the steps.

For the teaching recording pNN20 is 44.4 %, against a pNN50 of 11.1 %: four of the nine differences are larger than 20 ms, three of them between 20 and 50 ms, which only the lower threshold counts.

What moves it

The same drivers as pNN50 and RMSSD 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 : breathing, vagal activity 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 , heart rate, age, posture, sleep. Because the bar is lower, it keeps moving in people whose pNN50 has already reached zero.

When it misleads

It is still a threshold count, so it ignores how far each difference lies beyond 20 ms, and in young people with large swings it saturates near 100 % instead of near 0 %. Timing resolution Timing resolution Sampling rate How many times a second a sensor measures the signal, in hertz (Hz). It sets how precisely each beat can be timed: about ±1 ms at 1000 Hz, ±4 ms at 250 Hz and ±20 ms at a 25 Hz camera. More about Sampling rate matters more at a low threshold: a sensor that times beats to the nearest 10 or 20 ms blurs the line. Artifacts Artifacts Artifact An error in a heartbeat recording, such as a missed, extra or mistimed beat, often caused by movement or poor sensor contact. Artifacts can badly distort HRV numbers. More about Artifact raise it, as they do every beat-to-beat measure.

Where you will see it

Kubios HRV reports pNNxx with whatever threshold is set in its preferences; a few apps and research papers report pNN20 beside pNN50. It is a percentage and cannot be compared with RMSSD or SDNN 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 values.