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Heart rate variability measures.
| Time Domain: |
| RR: time interval between successive ECG R-waves |
| SDNN: standard deviation of normal-to-normal RR intervals |
| SDNNI: mean of the standard deviation of RR intervals in 5-min segments |
| RMSSD: Root mean square of successive RR interval differences |
| Frequency Domain: |
| LF: low frequency (HRV frequency band set at 0.04–0.15 Hz |
| HF: high frequency (HRV frequency band set at 0.15–0.4 Hz) |
