HRV variable definitions
| HRV measure | Units | Definition |
| SDNN | ms | Standard deviation of RR intervals; Total variability, calculated by the SD of interbeat intervals |
| RMSSD | ms | Root mean square of successive RR interval differences, between adjacent RRs |
| pNN50 | % | The number of pairs of successive RRs that differ by more than 50 ms, divided by total number of RRs |
| HF | ms | High frequency; Index of parasympathetic influence on heart based on rhythmic respiration cycles; power spectral analysis calculated as high-frequency (0.15–0.40 Hz) band |
| HFnu | % | High frequency normalized units; [HF/(HF + LF)] x 100; index of modulation of the parasympathetic branch of the ANS as it influences the sinoatrial node of the heart |
HRV, heart rate variability.