Table 1.
Summary of time and frequency domain–HRV parameters
| HRV parameter (units) | Description |
|---|---|
| Time domain | |
| NN interval (ms) | Interval between R wave peaks for two normal successive beats |
| Average NN interval (ms) | Average time interval between normal successive R waves |
| RMSSD (ms) | Square root of the mean squared differences of successive NN intervals |
| pNN50 (%) | Proportion of NN intervals which differ by more than 50 ms |
| Triangular index (nu) | Integral of the density of the RR interval histogram divided by its height, a measure of total HRV |
| Frequency domain | |
| High frequency (ms2) | Power in the 0.15–0.40-Hz band |
| High frequency–normalised units (%) | HF power divided by power ≥ 0.04 Hz |
| Low frequency (ms2) | Power in the 0.04–0.15-Hz band |
| Low Frequency–normalised units (%) | LF power divided by power ≥ 0.04 Hz |
| Very low frequency (ms2) | Power less than 0.04 Hz |
| LF/HF ratio (nu) | Low frequency power divided by high frequency power |
| Total power | Power from 0 to Nyquist frequency |