TABLE 1.
Selected heart rate variability (HRV) measures
| Parameter | Unit | Description | Frequency |
|---|---|---|---|
|
Time domain measures | |||
| SDNN | ms | SD of all normal R wave to R wave (N-N) intervals | |
| SDANN | ms | SD of 5 min average N-N intervals | |
| SDNN index | ms | Mean of the SDs for 5 min segments | |
| rMSSD | ms | Square root of the mean of the sum of squares of successive N-N interval differences. Reflects vagal modulation | |
| NN50 | ms | Number of pairs of adjacent N-N intervals differing by >50 ms. Reflects vagal modulation but is less commonly used in recent studies | |
| pNN50 | % | Number of successive N-N intervals differing by >50 ms divided by the total number of successive N-N intervals. Reflects vagal function | |
|
Frequency domain measures | |||
| HF | ms2 | Power in the high-frequency range. Reflects vagal function. Calculated for recordings ≥5 min | 0.15–0.40 Hz |
| LF | ms2 | Power in the low-frequency range. May reflect a combination of sympathetic and parasympathetic input. Calculated for recordings ≥5 min | 0.04–0.15 Hz |
| VLF | ms2 | Power in the very low-frequency range. Calculated for recordings ≥5 min | 0.0033–0.04 Hz |
| ULF | ms2 | Power in the ultra low-frequency range. Calculated for recordings ≥24 h | <0.0033 Hz |
| TP | ms2 | Variance of all N-N intervals. Calculated for recordings ≥24 h | ∼≤0.4 Hz |
| LF/HF ratio | Ratio of low-frequency to high-frequency. May reflect sympathovagal balance. Smaller ratios may represent parasympathetic dominance and larger ratios may represent sympathetic dominance | N/A | |
|
Spectral indexes | |||
| Vindex | ms2 | An average HF power derived from the most linear segment of HF power against the R-R interval regression curve. It is an HF spectral measure of vagal activity | |
| PLF | Hz | Analyzed from power spectra of all 5 min periods and represents the frequencies of all maximum peaks within the LF band detected at a resolution of 1/60 Hz averaged over an entire 24 h recording. A shift toward higher LF band frequencies is observed in high-risk patients (26) | |
|
Nonlinear indexes | |||
| α1 | Uses the detrended fluctuation analysis technique of HRV for short- and intermediate-term scaling properties. It detects the presence or absence of fractal-like scaling properties. It is a quantitative method for assessing the extent to which a time series is random at the one end and correlated at the other. Values range from 0.5 (random) to 1.5 (correlated). Values just over 1 are considered normal (8,15) | ||
| β | Assesses the power-law relationship of HRV for long-term fluctuations. Power-law analysis plots the log of spectral power against the log of frequency using spectral analysis data (15,17) | ||
| SD12 | Quantifies the shape of the Poincaré plot. The Poincaré plot corresponds graphically to the underlying pattern of the R-R interval time series. This nonlinear index plots the amount of time between two adjacent heart beats versus the amount of time between the subsequent two adjacent heart beats. SD12 is the ratio of the lengths of the axes of an imaginary ellipse. The hub of the ellipse is the average R-R interval of the time series. This ratio is fitted to the Poincaré plot. SD12 is the ratio of intermediate-term variability to short-term variability. It is considered to be an intermediate-term nonlinear measure (8,18) | ||
α1 Alpha-1; β Beta; N/A Not applicable