Noise and determinism in cardiovascular dynamics

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. It has, for example, been variously described as chaotic, fractal, stochastic, and subject to 1/f fluctuations—and its true nature is still the subject of vigorous debate. We review and describe some recent experiments that illuminate the problem and discuss a combination of noise and almost periodic frequency modulation as a signature of the system dynamics.