pNN50

To get NN50, go through a recording beat by beat and count how many times one NN interval NN interval 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 differs from the next by more than 50 ms. Divide that count by the total number of intervals and you have pNN50, a percentage. If 30 out of 300 pairs cross the 50 ms line, pNN50 is 10%.

Like RMSSD 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 , pNN50 looks only at neighboring beats, so it picks up quick, beat-to-beat changes. These are mostly driven by the parasympathetic nervous system parasympathetic nervous system Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system , and pNN50 is closely correlated with both RMSSD and HF power HF power HF power The share of heart rate variability in fast rhythms of 0.15 to 0.40 Hz, the range of normal breathing. It mainly reflects vagal activity when breathing is 9 to 24 breaths a minute. More about HF power . The Task Force standards describe RMSSD, NN50 and pNN50 as highly correlated and recommend RMSSD over pNN50 and NN50 because it has better statistical properties.

Shaffer and Ginsberg add that RMSSD usually gives a better picture of breathing-linked heart rate changes, especially in older people, and that most researchers prefer it. So if a report gives you both, RMSSD is the one to follow. A pNN50 value also cannot be compared with an RMSSD chart: one is a percentage, the other is in milliseconds.

The lesson RMSSD, SDNN and lnRMSSD explained shows how pNN50 relates to the metrics most apps report.