Arterial pressure spontaneously oscillates in part as a consequence of sympathetic regulatory activity. The second-order oscillations are produced mechanically by respiratory activity. The third-order fluctuations, with a period of about 10 seconds, are related to vasomotion and are modulated by sympathetic activity; thus, they increase during the conditions associated with a sympathetic excitation, such as on standing. Therefore, the study of circulatory rhythms by spectrum analysis techniques may provide a valuable insight into the autonomic regulation of the cardiovascular system in health and disease. Power spectrum analysis on the basis of Fast Fourier Transform or autoregressive modeling provides the center frequency of rhythmic fluctuations, their time relation (phase), and amplitude, both in absolute and in normalized values. Absolute values are computed as the integral of each oscillatory component. Normalization overcomes the problems because of the marked changes in RR variance when comparing different subjects and experimental conditions. The low-frequency/high-frequency component of RR variability ratio that is independent of units of measure assesses the sympathovagal instantaneous relation (balance).
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