Table 4.
Summary of heart rate variability measures available in the PhysioZoo software.
| Measures | Units | Definition |
|---|---|---|
| Time domain | ||
| AVNN | [ms] | Average NN interval duration |
| SDNN | [ms] | Standard deviation of NN interval duration |
| RMSSD | [ms] | The square root of the mean of the sum of the squares of differences between adjacent NN intervals |
| pNNxx | [%] | Percent of NN interval differences greater than xx milliseconds |
| SEM | [ms] | Standard error of the mean NN interval |
| PIP | [%] | Percentage of inflection points in the NN interval time series |
| IALS | [n.u] | Inverse average length of the acceleration/deceleration segments |
| PSS | [%] | Percentage of short segments |
| PAS | [%] | The percentage of NN intervals in alternation segments |
| Frequency | ||
| Total power | [ms2] | Total power |
| VLF | [ms2] | Power in the very low frequency band |
| LF | [ms2] | Power in the low frequency band |
| HF | [ms2] | Power in the high frequency band |
| VLF norm∗ | [%] | Low frequency power in normalized units LF/(Total power) × 100 |
| LF norm | [%] | Low frequency power in normalized units LF/(Total power – VLF) × 100 |
| HF norm | [%] | High frequency power in normalized units HF/(Total power – VLF) × 100 |
| LF/HF | [n.u] | Low frequency band to high frequency band power ratio (LF/HF) |
| LF peak | [Hz] | Peak frequency in the low frequency band |
| HF peak | [Hz] | Peak frequency in the high frequency band |
| β | [n.u] | Slope of the linear interpolation of the spectrum in a log-log scale for frequencies below the upper bound of the VLF band |
| Non-linear | ||
| SampEn | [n.u] | Sample entropy Richman and Moorman, 2000 |
| SD1 | [ms] | NN interval standard deviation along the perpendicular to the line-of-identity |
| SD2 | [ms] | NN interval standard deviation along the line-of-identity |
| α1 | [n.u] | DFA low-scale slope Peng et al., 1995 |
| α2 | [n.u] | DFA high-scale slope |
VLF, very low frequency; LF, low frequency; HF, high frequency; DFA, detrended fluctuation analysis; NN, interval between normal beats. ∗Note that the normalization factor for the VLF norm measure (Total power) is different from the LF norm and HF norm ones (Total power-VLF). This is because the LF norm and HF norm measures were defined as such in the HRV standard. In the software we provide the option to change the normalization method to make it the same for all three frequency measures, i.e., divided by the total power.