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6 Applications and Methods in Biosignal Processing

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Fig. 6.30: Example of a spectrum of heart rate with a pronounced peak of respiration at approxim-

ately 0.28 Hz.

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Another conspicuous component of HRV is the low frequency component (LF: low

frequency) in the range of 0.04 to 0.15 Hz. This component is almost exclusively

caused by the activity of the parasympathetic nervous system.

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The fluctuations below 0.04 Hz have been studied less intensively than at higher

frequencies. These frequency ranges are often divided into very low (VLF: very

low frequency) bands from 0.003 to 0.04 Hz and extremely low (ULF: ultra low

frequency) bands from 0 to 0.003 Hz. When HRV is measured for a short time,

the ULF band is usually also omitted. The low-frequency rhythms are attributed,

for example, to hormonal factors, thermoregulatory processes and the renin-

angiotensin-system, which also regulates blood pressure.

Determination of heart rate variability is a commonly used method when assessing

the functioning of the heart and autonomic regulation. It has been used in numer-

ous studies related to cardiovascular research. One of the main clinical areas in which

HRV has been found to be valuable is in the risk assessment of sudden cardiac death

following acute myocardial infarction. In addition, decreased HRV is generally con-

sidered an early warning sign of diabetic cardiovascular autonomic neuropathy, with

the most significant decrease in HRV associated with diabetes found within the first

5-10 years of diabetes.