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Heart rate variability

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Heart rate variability (HRV) is a measure of the beat-to-beat variations in heart rate. It is usually calculated by analyzing a time series of beat-to-beat intervals from the ECG, of beat-to-beat intervals derived from an arterial pressure tracing or of beat-to-beat intervals derived from a pulse wave signal measured by means of a photoplethysmograph (PPG).

Various measures of heart rate variability have been proposed, which can roughly be subdivided into time domain, frequency domain and geometric measures. HRV is regarded as an indicator of the activity of autonomic regulation of circulatory function, although controversy exists over whether this is an accurate metric for analyzing cardiovascular autonomic control. Alterations (mostly reductions) in HRV have been reported to be associated with various pathologic conditions such as hypertension, hemorrhagic shock, and septic shock. It also has some utility as a modest predictor of mortality after an acute myocardial infarction.

Time domain measures

A simple example of a time domain measure is the calculation of the standard deviation of beat-to-beat intervals. Other time domain measures include root mean square of the differences between heart beats (rMSSD), NN50 or the number of normal to normal complexes that fall within 50 milliseconds, and pNN50 or the percentage of total number beats that fall with 50 milliseconds.

Frequency domain measures

A common frequency domain method is the application of the discrete Fourier transform to the beat-to-beat interval time series. This provides an estimation of the amount of variation at specific 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 or the parasympathetic nervous system.
  • Low Frequency band (LF) between 0.04 and 0.15 Hz. LF derives from both parasympathetic 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 not well known, but it had been attributed to thermal regulation of the body's internal systems.
  • Ultra Low Frequency (ULF) band between 0 and 0.0033 Hz. The major background of ULF is day/night variation and therefore is only expressed in 24-hour recordings.
  • The ratio of low-to-high frequency spectra power(LF/HF) is has been proposed as an index of sympathetic to parasympathetic balance of heart rate fluctuation, but this is controversial because of the lack of understanding of the mechanisms for the LF component.

Geometric measures

The most commonly used non-linear method of analyzing heart rate variability is the Poincaré plot. The Poincaré plot fits heart rate data points to an ellipse that is fitted to two intersecting lines. SD1 and SD2, or the standard deviations of the data points have also been applied in the context of Poincaré analysis.

References

M. Malik and A. Camm, "Heart Rate Variability," Futura Publishing Company, 1995.

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