The Theory

     behind spectral analysis of the heart rate variability is that the sympathetic and parasympathetic system influences can be extracted and attributed to one of the two systems. Incidentally (and ironically), the sympathetic system has been shown to modulate the heart rate at a lower frequency than the parasympathetic system, and it seems likely that aliasing is minimal given the frequency location and bandwidth of each of the signals. Thus, recent publications on the topic have dealt with the spectral and time-frequency analysis of the heart rate variability, in the hopes of being able to characterize the heart rate well enough to be able to diagnose illnesses in patients that cause suppression or amplification of the variability of the heart rate.