Table 3.
Main measures of HRV in frequency and time domains.
| Variable | Definition | ANS modulation and implication | |
|---|---|---|---|
| Frequency domain | Total power (ms2) | The variance of NN intervals over the temporal segment or 24 h (≤0.4 Hz) | Reflecting overall ANS activity |
| ULF (ms2) | Power in the ultra low-frequency range (≤0.003 Hz) | Only available in 24-h long-term HRV recording. Representing the influences of many uncontrolled factors | |
| VLF (ms2) | Power in the very low-frequency range (0.003–0.04 Hz) | Representing the influences of the peripheral vasomotor and renin–angiotensin systems, temperature regulation, and other uncontrolled factors | |
| LF (ms2) | Power in the low-frequency range (0.04–0.15 Hz) | Being mediated by a complex mixture of SNS and PNS modulation | |
| LF norm (n.u.) | LF power in normalized units: LF/(LF + HF) × 100% | Representing the relative value of LF in proportion to the sum of HF and LF and emphasizing the controlled and balanced behavior of the two branches of the ANS | |
| HF (ms2) | Power in the high-frequency range (0.15–0.4 Hz) | Being solely regulated by the PNS, with high HF power representing increased PNS activity | |
| HF norm (n.u.) | HF power in normalized units: HF/(LF + HF) × 100% | Representing the relative value of HF in proportion to the sum of HF and LF and emphasizing the controlled and balanced behavior of the two branches of the ANS | |
| LF/HF | Ratio of LF to HF power | Reflecting the balance of SNS and PNS functions | |
| HF + LF (ms2) | Power in the high- and low-frequency ranges (0.04–0.4 Hz) | May represent a more precise indicator of the overall ANS activity. A higher HF + LF value represents increased overall ANS activity, while a lower HF + LF value indicates decreased ANS activity | |
| Time domain | SDNN (ms) | SD of all NN intervals | Corresponding to total power |
| SDANN (ms) | SD of the average of NN intervals in all 5-min segments of the entire recording | Corresponding to ULF | |
| RMSSD (ms) | The square root of the mean of sum of the squares of differences between adjacent NN intervals | Corresponding to HF | |
| SDNN index (ms) | Mean of the SD of all NN intervals for all 5-min segments of the entire recording | Corresponding to mean of 5-min total power | |
| SDSD (ms) | SD of difference between adjacent NN intervals | Corresponding to HF | |
| NN50 count | Number of pairs of adjacent NN intervals differing by more than 50 ms in the entire recording | Corresponding to HF | |
| pNN50 (%) | NN50 count divided by total number of all NN intervals | Corresponding to HF | |
ANS, autonomic nervous system; HRV, heart rate variability; NN, normal – normal interval.