HRV measure | Units | Domain | Description |

Mean of R–R intervals (mRR) | ms | Time domain | A measure of average R–R interval (60/heart rate). |

Standard deviation (SD) of R–R intervals (SDNN) | ms | Time domain | A measure of variability of R–R intervals across the whole signal. |

Root mean square of successive differences | ms | Time domain | A measure of shorter term variation through differences between adjacent R–R intervals. |

SD of successive differences | ms | Time domain | A longer term measure of variability through SD of differences between adjacent R–R intervals. |

SD of average R–R intervals | ms | Time domain | For longer signals, mRR is calculated on segments, often 5 min, and SD of these values are calculated. |

Number of R–R intervals over x ms | – | Time domain | Often R–R intervals over 50 ms, count of longer intervals to determine variability. |

Percentage of R–R intervals over x ms | % | Time domain | As above, normalised to the total number of intervals. Can be used when signal lengths vary. |

Total power (TP) | ms^{2} | Frequency domain | Total 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) | ms^{2} | Frequency domain | Power in the 0.003–0.04 Hz frequency band. |

Low frequency power (LF) | ms^{2} | Frequency domain | Power 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) | ms^{2} | Frequency domain | Power 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 domain | Power in each frequency band normalised to the total power. |

Low frequency to high frequency (LF/HF) ratio | – | Frequency domain | Ratio between LF and HF power bands. Often associated with sympathovagal balance in the literature; however, this interpretation is also debated. |

Sample entropy | – | Non-linear | Entropy measures periodic variations in the R–R interval signals not detectable using means and SD. |

Detrended fluctuation analysis exponent (α) | – | Non-linear | Finds long-term correlations in the signal, with the exponent giving a value of self-correlation of the signal. |

Poincaré standard deviations (SD1, SD2) | – | Non-linear | Poincaré 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 domain | Proportion of consecutive R-R intervals that differ by more than 50ms. Measure of parasympathetic activity. |

HRV, heart rate variability.