Table 1.
Most common linear and non-linear HRV measures.
| Variability analysis | Parameters | Units | Description |
|---|---|---|---|
| Time domain | |||
| Statistical Geometric | SDNN SDANN RMSSD SDNN index SDSD NN50 pNN50 HRV triangular index TINN Differential index Logarithmic index | ms ms ms ms ms % ms ms ms−1 | Standard deviation of the NN intervala Standard deviation of average of NN intervals in 5 min epochs of entire recording Square of the root of the mean of the sum of square differences between adjacent NN intervals Mean of the standard deviations of all NN intervals for all 5 min segments Standard deviation of differences between adjacent NN intervals Number of pairs of adjacent NN intervals differing by more than 50 ms NN50 count divided by the total number of all NN intervals Total NN intervals divided by height of histogram of all NN intervals Baseline width of the minimum square difference triangular interpolation of the highest peak of the histogram of all NN intervals Difference between widths of the histograms of differences between adjacent NN intervals measured at selected heights |
| Frequency domain | |||
| Short-term recording (5 min) Long-term recording (24 h) | Total power VLF LF LF norm HF HF norm LF/HF α | ms2 ms2 ms2 n.u ms2 n.u | Variance of the NN intervals over the temporal segment Power of very low frequency range (0.0033 Hz–0.04 Hz) Power of the low frequency range (0.04 Hz–0.15 Hz) LF in normalised units Ratio of the LF to HF Power of the high frequency HF in normalised units (0.15 Hz–0.40 Hz) Slope of the linear interpolation of the spectrum in a log-log scale |
| Non-linear measures | |||
| Poincare analysis Entropy analysis Detrended fluctuation analysis | S SD1 SD2 SD1/SD2 ApEn SampEn DFAα1 DFAα2 D2 | ms ms ms % | Area of the ellipse, representing total variability Standard deviation perpendicular to the line of identity Standard deviation along the line of identity Ratio of SD1 to SD2 Approximate entropy, measures the regularity and complexity associated with a time series Sample entropy, measures the regularity and complexity associated with a time series Detrended fluctuation analysis that describes short term fluctuations (<10–11 beat) Detrended fluctuation analysis that describes long term fluctuations (>10 beat) Correlation dimension (estimates the minimum number of variables required to construct a model of system dynamics |
Note: Adapted from: Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, Standards of measurement of HRV guidelines.12
a
NN interval refers to RRi for normal R wave peaks, i.e. those that are not abnormal due to arrhythmia/interference/artefact. In practice, RRi and NN interval are used synonymously.