A healthy normal subject has beat-to-beat fluctuations of R-R intervals of electrocardiogram. This phenomenon is known as heart rate variability (HRV). The HRV attenuates in a diabetes patient with the autonomic neuropathy and completely disappears in a heart transplant patient. A heart with own automaticity contracts regurarily, but the HRV is produced when controlled by the autonomic nervous system. There are two significant peaks of power spectral density of HRV by using the spectral analysis. In the frequency domain, the power spectrum of HRV has been categorized into high- (HF; >0.15 Hz) and low- (LF; 0.04-0.15Hz) frequency bands. HF reflects the sympathetic nervous system activity, and LF reflects theinteraction between the sympathetic and the parasympathetic nervous system activities. There is a non-periodical oscillation (fractal nature) in the HRV. The spectral exponent of a healthy normal subject is approximately 1 busing the coarse graining spectral analysis of HRV. The HF and LF increases after life, having had a peak with a young people, and decreases with aging. The spectral exponent of a five-month baby is nearly equal to the one of adult subject. The complexity is disappeared in the elderly people. The detailed mechanism producing the HRV is unclear. Understanding of phenomenon is restricted only in approach from the scientific fields such as physiology and the the control electrical engineering based on the functional materialism.
