Spectral analysis and interpretation of spectral components of heart rate variability

Requirements for a fragment of a random process taken for spectral analysis are formulated. It has been demonstrated that both the random process and its periodic components should be stationary within the fragment selected. Only in this case will the results of spectral analysis reflect the actual characteristics of the process studied. Variations in the depth of breathing and other factors have been shown to cause modulatory fluctuations of the HF and LF waves of the cardiac rhythm. The modulatory fluctuations are expressed as the VLF peak in the heart rate variability spectrum, the position of the peak on the frequency axis and its amplitude being determined by the frequency and depth of modulation, respectively. Depending on the modulation depth and the ratio between the modulation period and the length of the sample studied, spectral analysis of the same process may yield considerably different results. The ambiguity of the results of spectral analysis is the cause of discrepancy about the nature of individual spectral components of heart rate variability. © Pleiades Publishing, Inc., 2006.