pNN50

pNN50, short for the percentage of successive normal-to-normal (NN) intervals that differ by more than 50 milliseconds, is a time-domain metric used in heart rate variability (HRV) analysis. It is calculated from a sequence of NN intervals—the intervals between adjacent normal heartbeats—typically derived from ECG data after artifact and ectopic beat correction. The formula is pNN50 = 100 times the number of NN interval pairs with a difference greater than 50 ms divided by the total number of NN intervals minus one.

pNN50 is commonly estimated from short-term recordings (for example, 5 minutes) or longer Holter monitoring. Reliable calculation requires high-quality data with proper detection of R peaks, careful exclusion of artifacts or premature beats, and consistent preprocessing. The choice of data length, sampling rate, and preprocessing steps can influence the resulting value.

Physiologically, pNN50 reflects beat-to-beat variability and is influenced mainly by parasympathetic (vagal) modulation of the sinoatrial node. Higher pNN50 values generally indicate greater vagal tone and autonomic flexibility, while lower values suggest reduced vagal activity or relatively greater sympathetic influence. In healthy individuals, pNN50 tends to be higher than in older or clinically compromised populations, but exact values vary with age, health status, and measurement conditions.

Limitations include sensitivity to artifacts and ectopy, dependence on data length and preprocessing, and variability across measurement protocols. pNN50 is often interpreted alongside other HRV metrics such as rMSSD and SDNN, rather than as a standalone indicator of autonomic function.