Heart rate variability (HRV) is a measure of variations in the heart rate. It is usually calculated by analysing the time series of beat-to-beat intervals from ECG or arterial pressure tracings. Various measures of heart rate variability have been proposed, which can roughly be subdivided into time domain, frequency domain and phase domain measures. HRV is regarded by some as an indicator of the activity of autonomic regulation of circulatory function, although this is an area of some controversy. Alteration (mostly reduction) of HRV has been reported to be associated with various pathologic conditions like hypertension, hemorrhagic shock, and septic shock. It has found its role as a predictor of mortality after an acute myocardial infarction.

Time domain

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A simple example of a time domain measure is the calculation of the standard deviation of beat-to-beat intervals.

Frequency domain

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A common frequency domain method is the application of the discrete Fourier transform to the beat-to-beat interval time series. That expresses the amount of variation for different frequencies. Several frequency bands of interest have been defined in humans.

• High Frequency band (HF) between 0.15 and 0.4 Hz. HF is driven by respiration and appears to derive mainly from vagal activity.
• Low Frequency band (LF) between 0.04 and 0.15 Hz. LF derives from both vagal and sympathetic activity and has been hypothesized to reflect the delay in the baroreceptor loop.
• Very Low Frequency band (VLF) band between 0.0033 and 0.04 Hz. The origin of VLF is unknown, but it had been attributed to physical activity.
• Ultra Low Frequency (ULF) band between 0 and 0.0033 Hz. The major background of ULF is day/night variation.

Phase domain

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Various methods from dynamic systems theory have also been applied in the context of HRV analysis.