Version May 2024
| Variable | Units | Definition |
| Time Domain - Statistical measures | ||
| Night-day difference | ms | Difference between the average of all the normal RR intervals at night (24:00 to 05:00) and the average of all the normal RR intervals during the day (07:30 to 21:30). |
| SDNN | ms | Standard deviation of all normal RR intervals in the entire 24-hour ECG recording. |
| SDANN | ms | Standard deviation of the average normal RR intervals for all 288 5-minute segments of a 24-hour ECG recording (each average is weighted by the fraction of the 5 minutes that has normal RR intervals). |
| ASDNN | ms | Average of the standard deviations of normal RR intervals for all 288 5-minute segments of a 24-hour ECG recording. |
| r-MSSD | ms | Root mean square successive difference, the square root of the mean of the squared differences between adjacent normal RR intervals over the entire 24-hour ECG recording. |
| pNN50 | percent | Percent of differences between adjacent normal RR intervals that are greater than 50 ms computed over the entire 24-hour ECG recording. |
| NN50 | none | Number of adjacent normal RR intervals that are greater than 50 ms counted over the entire 24-hour ECG recording. |
| Time Domain - Geometric measures | ||
| HRV triangular index | none | Total number of NN intervals divided by the number of NN intervals in the modal bin of a histogram of all NN intervals with a bin width of 7.8125 msec (for a sampling rate of 128/sec). |
| TINN | ms | Baseline width of the minimum square difference triangular interpolation of the highest peak of the histogram of all NN intervals. |
| Frequency Domain Measures | ||
| Total power | ms2 | The energy in the heart period power spectrum up to 0.40 Hz. |
| Ultra low frequency (ULF) power | ms2 | The energy in the heart period power spectrum up to 0.0033 Hz. |
| Very low frequency (VLF) power | ms2 | The energy in the heart period power spectrum between 0.0033 and 0.04 Hz. |
| Low frequency (LF) power | ms2 | The energy in the heart period power spectrum between 0.04 and 0.15 Hz. |
| High frequency (HF) power | ms2 | The energy in the heart period power spectrum between 0.15 and 0.40 Hz. |
| LF/HF ratio | none | The ratio of low to high frequency power. |
| beta | none | Slope of log (power) on log (frequency) between 0.01 and 0.0001 Hz on a log-log plot. |
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