pNN50
pNN50 is the percentage of pairs of adjacent normal heartbeat intervals that differ by more than 50 ms. Like RMSSD, it reflects fast beat-to-beat changes of the heart rhythm controlled by the vagus nerve: the more often neighbouring intervals differ markedly, the stronger the parasympathetic influence on the heart.
How pNN50 is calculated
For each pair of neighbouring NN intervals the absolute difference is calculated. The number of pairs in which this difference exceeds 50 milliseconds is called NN50; dividing it by the total number of pairs gives pNN50.
Physiology and limitations
The vagus nerve lengthens the heartbeat interval during exhalation and shortens it during inhalation. When vagal tone is high, these swings easily exceed 50 ms and pNN50 is high; when the sympathetic system dominates or the heart rate is fast, successive intervals become similar and pNN50 approaches zero.
The fixed 50-ms threshold is a limitation: at a fast heart rate, when intervals themselves are short, differences rarely reach 50 ms even if regulation is normal. This makes pNN50 strongly dependent on heart rate and less sensitive than RMSSD at low variability, where it often shows a "floor" of 0 %. For this reason the Task Force recommends RMSSD as the preferred measure of short-term variability, with pNN50 as an additional index.
Interpretation
Low pNN50 (close to 0 %)
- Weak vagal regulation, sympathetic predominance or a fast heart rate
- Stress, lack of sleep, illness, fever, alcohol
- Ageing — pNN50 declines markedly from early adulthood
- Persistently low values with low RMSSD: heart failure, after myocardial infarction, diabetic autonomic neuropathy
High pNN50
- Strong vagal tone: young people, endurance athletes, rest and sleep
- Slow breathing during the recording
- Falsely high values: extrasystoles, atrial fibrillation, artefacts
There are no universally accepted short-term norms for pNN50, because it depends heavily on age, heart rate and recording conditions. It is best followed as a personal trend and interpreted together with RMSSD, which carries the same information more robustly.
Discuss the results with a doctor if pNN50 and RMSSD remain very low over a long period at rest, especially with palpitations, breathlessness, chest discomfort, dizziness or known heart disease.
References
- Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation. 1996;93(5):1043–1065. doi:10.1161/01.CIR.93.5.1043
- Umetani K, Singer DH, McCraty R, Atkinson M. Twenty-four hour time domain heart rate variability and heart rate: relations to age and gender over nine decades. J Am Coll Cardiol. 1998;31(3):593–601. doi:10.1016/S0735-1097(97)00554-8
- Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Front Public Health. 2017;5:258. doi:10.3389/fpubh.2017.00258
This article is for general information and does not replace a consultation with a doctor. HRV values depend on the device, recording length and conditions.
Published on 11 June 2019