Table 1

The most widely used HRV measures used in the literature with short interpretation of their meanings

HRV measureUnitsDomainDescription
Mean of R–R intervals (mRR)msTime domainA measure of average R–R interval (60/heart rate).
Standard deviation (SD) of R–R intervals (SDNN)msTime domainA measure of variability of R–R intervals across the whole signal.
Root mean square of successive differencesmsTime domainA measure of shorter term variation through differences between adjacent R–R intervals.
SD of successive differencesmsTime domainA longer term measure of variability through SD of differences between adjacent R–R intervals.
SD of average R–R intervalsmsTime domainFor longer signals, mRR is calculated on segments, often 5 min, and SD of these values are calculated.
Number of R–R intervals over x msTime domainOften R–R intervals over 50 ms, count of longer intervals to determine variability.
Percentage of R–R intervals over x ms%Time domainAs above, normalised to the total number of intervals. Can be used when signal lengths vary.
Total power (TP)ms2Frequency domainTotal power in the frequency spectra up to 0.4 Hz. Can be measured from zero, from the start of the VLF band (0.003 Hz) or from the LF band (0.04 Hz).
Very low frequency power (VLF)ms2Frequency domainPower in the 0.003–0.04 Hz frequency band.
Low frequency power (LF)ms2Frequency domainPower in the 0.04–0.15 Hz frequency band. Often linked to combined levels of sympathetic and parasympathetic activity; however, this interpretation is widely debated.
High frequency power (HF)ms2Frequency domainPower in the 0.15–0.4 Hz frequency band. Linked to levels of parasympathetic activity and frequencies of respiration.
Normalised low frequency power (nLF) and normalised HF power (nHF)Frequency domainPower in each frequency band normalised to the total power.
Low frequency to high frequency (LF/HF) ratioFrequency domainRatio between LF and HF power bands. Often associated with sympathovagal balance in the literature; however, this interpretation is also debated.
Sample entropyNon-linearEntropy measures periodic variations in the R–R interval signals not detectable using means and SD.
Detrended fluctuation analysis exponent (α)Non-linearFinds long-term correlations in the signal, with the exponent giving a value of self-correlation of the signal.
Poincaré standard deviations (SD1, SD2)Non-linearPoincaré plots plot R–R intervals against the succeeding R–R intervals. With the SD in y=x representing longer term variation and in the perpendicular direction, short-term variation.
pNN50-Time domainProportion of consecutive R-R intervals that differ by more than 50ms.
Measure of parasympathetic activity.
  • HRV, heart rate variability.