The noninvasive assessment of spontaneous temporal physiological parameters variations can provide valuable information about control systems involved in their complex regulation. Recently, in addition to conventional measures describing variability in tuna and frequency domain, measures based on nonlinear dynamics are increasingly used in physiology. The main drawback of these measures is the assumption of relatively long analysed record and therefore new measures for describing the hidden information in shorter tuna series are sought. Approximate Entropy (ApEn) quantifying predictability of tuna series and providing information about Complexity and integrity of biological control systems is ona of them. Taken together, ApEn is now mora frequently used in physiological research with perspective of the potential clinical application.

        Key words: nonlinear dynamics, heart rate variability, approximate entropy