Signals derived from the human cardiovascular system are well known to exhibit highly complex, nearly periodic, oscillatory behaviour whose nature is something of an enigma and still the subject of vigorous debate. The variation of cardiac frequency with time, known as heart rate variability (HRV), has been intensively investigated using both deterministic and stochastic methods. It has, for example, been variously described as chaotic, fractal, stochastic, and subject to 1/f fluctuations and it was proposed that the state of the system can be classified by the slope of its power spectrum on a log-log plot.
We illustrate some problems in characterising slow modes in real measurements and show that oscillatory dynamics under the influence of strong noise, coupled with a limited time of observation, can lead to a -like behaviour. We review and describe some recent experiments that illuminate the problem and discuss a combination of almost periodic and stochastic frequency modulation as a signature of the system dynamics.
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In a healthy subject in repose a volume of blood equivalent to the total amount in the body returns to the heart every minute. Several oscillatory processes characterise cardiovascular dynamics within this circulation time. Cardiac and respiratory activities act on higher frequency scales, with characteristic frequencies of 1 Hz and 0.2 Hz, respectively. The lowest frequency component, at around 0.01 Hz, has been associated with the activity of the layer of endothelial cells forming the inner surfaces of all blood vessels.
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