There are some ways to assess HRV. It can be described using time-domain methods (statistical and geometrical methods) and frequency-domain methods. Time domain methods answer the question: “How big is HRV?”.  Frequency domain methods answers the question: “What is it (HRV) determined by?”.

Statistical methods – indices:

Heart rate (HR), mean RR (mRR) – indices that characterize the heart rate and mean value of R-R interval. They reflect the level of functioning of CVS.

SDNN – standart deviation of the N-N interval (N-N means “normal-to-normal” R-R interval, when it is assumed normal rhytm which is generated by sinoatrial node (it is manifested by normal P-Q-R-S-T complexes in ECG). It characterizes integral HRV without its division into sympathetic and parasympathetic components.

RMSSD – the square root of mean squared differences of successive NN interval. “HRV-index” = 20*Ln(RMSSD). pNN50% – the part (percent) of successive NN intervals, in which the value of difference exceeds 50 ms. These three indices (RMSSD, HRV and pNN50%) reflect “beat-to-beat” variability, characterizing mainly parasympathetic activity of ANS.

Geometrical methods:

I. Based on histogramm building

There are two approaches concerning the choice of the value of width of histogramm pocket.

The first approach is based on 50 ms width of pocket. Usually calculated indices are:

Mo – mode – the most frequent value of R-R interval (the highest column in the histogramm).

AMo – amplitude of mode – number of R-R intervals included in the pocket corresponding to mode. Usually measured in percents (%).

SI – stress-index (s. index of strain of Roman Bayevskiy). SI = AMo(%) / (2*Mo*Var); [Mo] = s; Var = RRmax – RRmin; [Var] = s.

The higher AMo and SI, the higher is the level of strain of ANS (or, in other words, the higher is the level of stress).

The second approach is based on 1/128 s width of pocket.

Heart rate variability triangular index (HRVTi) – is ratio between N (total count of R-R intervals) and n (number of R-R intervals included in the pocket corresponding to mode). The higher is HRVTi, the higher is HRV, espesially conditioned by parasympathetic regulation.

WN1 – width of histogramm which is taken at the level of 1% of N

WN4 – width of histogramm taken at the level of 4% of N

WN5 – width of histogramm which is cut on the level of 10% of N

II. Scatter–plot

Scatter – plot is the plot based on two axises. The first axis (horizontal) corresponds to current R-R interval (RRi). The second axis corresponds to the next (following) R-R interval (RRi+1). We can assess the shape of cloud, its width (W), length (L), ratio between length and width (L/W). The higher is the square of the cloud, the higher is HRV. The higher is the ratio L/W, the higher is the level of neuro-emotional strain (or stress).

Like this article? Why not share it...



Purchase the clinical-grade CardioMood for a simple one-off price of £239 / €278 / $296

What’s included:
  • CardioMood smartband
  • Black silicon strap
  • 1m / 39″ USB Charging cable
  • iOS and Android mobile Apps
  • Access to the Web platform to access your data from any computer
  • Comprehensive Manual
  • Full support and assistance
Smartband features:
  • 2 single channel optical wave PPG sensors (up to 525 nm, 19 bits)
  • 3-axis motion sensor (acquisition noise 1.3 mg RMS, range ±16 g full scale)
  • Battery life up to 1 week
  • Size: 25mm x 37mm x 9mm
  • IP66 water resistant
  • Bluetooth 5.0

Designed and manufactured in Geneva, Switzerland 🇨🇭

(If you require more than 5 CardioMood devices, please contact us at

Sign Up

recieve 10% off your first purchase