Complex demodulation of heart rate changes during orthostatic testing

Heart rate changes induced by the standing up of young healthy subjects are processed by a complex demodulation algorithm yielding time-local estimates of the amplitude of the heart-rate variability function. Complex demodulate of the high-frequency band (0.15 to 0.45 Hz) shows an abrupt decrease of amplitude immediately after standing up, presumably due into vagal withdrawal. Complex demodulation of a part of the low frequency band (0 to 0.055 Hz) shows a typical form of a bell-shaped curve. It is concluded that while the complex demodulation of the high-frequency band probably reflects parasympathetic activity, the complex demodulation of the low-frequency band does not seem to indicate the level of the sympathetic activity