Table 1.
Conventional heart rate variability measurements.
| Variable | Units | Definition |
|---|---|---|
| TIME DOMAIN MEASURES | ||
| a. Statistical | ||
| SDNN | ms | SD of all normal R–R intervals |
| SDANN | ms | SD of the average normal R–R intervals calculated over short time periods (usually 5 min) for the entire recording period (usually 24 h) |
| RMSSD | ms | The square root of the mean squared differences between adjacent normal R–R intervals |
| SDNN index | ms | Mean of the SD of the normal R–R intervals calculated over short periods time (usually 5 min) for the entire recording period (usually 24 h) |
| NN50 | The number of pairs of adjacent normal R–R intervals that differ by more than 50 ms | |
| pNN50 | % | NN50 divided by the total number of normal R–R intervals × 100 |
| b. Geometrical | ||
| HRV triangular index | Number of normal R–R intervals divided by the height of the histogram of all the normal R–R intervals measured on discrete scale with bins of 1/128 s (7.8125 ms) | |
| TINN | ms | Baseline width of the minimum square difference of triangular interpolation of the highest peak of the histogram of all normal R–R intervals |
| FREQUENCY DOMAIN MEASURES | ||
| Total | ms2 | Area under the entire power spectral curve (usually ≤0.40), variance of all normal R–R intervals |
| ULF | ms2 | Ultra low frequency power (≤0.003 Hz) |
| VLF | ms2 | Very low frequency power (0.003–0.0.04 Hz) |
| LF | ms2 | Low frequency power (0.04–0.15 Hz) |
| HF | ms2 | High Frequency power (usually 0.15–0.40 Hz*) |
| LFnu | nu | Normalized low frequency power (LF/LF + HF) |
| HFnu | nu | Normalized high frequency power (HF/LF + HF) |
| LF/HF | Ratio of the low-to high frequency power | |
Nu, normalized units; *HF is shifted to higher ranges (0.24–1.04 Hz) in infants and exercising adults.