Definition of HRV indices
| Parameter | Definition |
| SDNN | Standard deviation of all RR intervals |
| SDNNi | Mean of the SD of all RR intervals for all five minute segments |
| rMSSD | Square root of the mean of the sum of squares of differences between adjacent RR intervals |
| pNN50 | Percentage of differences between adjacent RR intervals that are greater than 50 msec |
| VLF | Very low frequency index (0.004–0.04 Hz) (msec) |
| LF | Low frequency index (0.04–0.15 Hz) (msec) |
| HF | High frequency index (0.15–0.4 Hz) (msec) |
| LF:HF | Ratio of LF to HF |
SD, standard deviation.