time-domain
interbeat interval (IBI) - time period between successive heartbeats
time-domain indices – quantifications of the variability in interbeat interval measurements by discrete time buckets
values may be expressed in original units or as the natural logarithm (Ln) of original units to achieve a more normal distribution
time buckets
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short-term (ST) – about 5 minutes
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ultra-short-term (UST) – under 5 minutes
ST and UST values are NOT interchangeable with 24h values
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24 h – a full day
Relevant to processes with slower fluctuations such as circadian rhythms and the cardiovascular system’s response to a wider range of external stimuli, routines and workloads.
time-domain measures
| Parameter | Unit | Description |
|---|---|---|
| SDNN | ms | Standard deviation of NN intervals |
| SDRR | ms | Standard deviation of RR intervals |
| SDANN | ms | Standard deviation of the average NN intervals for each 5 min segment of a 24 h HRV recording |
| SDNN index (SDNNI) | ms | Mean of the standard deviations of all the NN intervals for each 5 min segment of a 24 h HRV recording |
| pNN50 | % | Percentage of successive RR intervals that differ by > 50ms |
| HR Max - HR min | bpm | Average difference between the highest and lowest heart rates during each respiratory cycle |
| RMSSD | ms | Root mean square of successive RR interval differences |
| HRV triangular index | Integral of interval histogram density divided by its height | |
| TINN | ms | Baseline width of the RR interval histogram |
frequency-domain
In a novel and comprehensive peer-reviewed journal on the statistical methods and measurements of HRV, The Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology divided heart rate (HR) oscillations into four bands.
frequency-domain measurements – estimates of the distribution of absolute or relative power into the following discrete frequency bands:
frequency bands
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ultra-low-frequency (ULF) – ≤0.003 Hz
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very-low-frequency (VLF) – 0.0033–0.04 Hz
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low-frequency (LF) – 0.04 Hz–0.15 Hz
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high-frequency (HF) – 0.15 Hz–0.4 Hz
frequency-domain measures
| Parameter | Unit | Description |
|---|---|---|
| ULF power | ms$^2$ | Absolute power of the ultra-low-frequency band (≤0.003 Hz) |
| VLF power | ms$^2$ | Absolute power of the very-low-frequency band (0.0033–0.04 Hz) |
| LF peak | Hz | Peak Frequency of the low-frequency band (0.04 Hz–0.15 Hz) |
| LF power | ms$^2$ | Absolute power of the low-frequency band (0.04 Hz–0.15 Hz) |
| LF power | nu | Relative power of the low-frequency band in normal units |
| LF power | % | Relative power of the low-frequency band (0.04 Hz–0.15 Hz) |
| HF peak | Hz | Peak frequency of the high-frequency band (0.15 Hz–0.4 Hz) |
| HF power | ms$^2$ | Absolute power of the high-frequency band (0.15 Hz–0.4 Hz) |
| HF power | nu | Relative power of the high-frequency band in normal units |
| HF power | % | Relative power of the high-frequency band (0.15 Hz–0.4 Hz) |
| LF/HF | % | Ratio of LF-to-HF power |
sources
https://www.frontiersin.org/articles/10.3389/fpubh.2017.00258/full