Relationships between linear and nonlinear indexes of heart rate variability in obstructive sleep apnea syndrome

Heart rate variability (HRV) decrease has been described in patients with obstructive sleep apnea syndrome (OSAS). Nevertheless, time and frequency domain methods commonly used for HRV analysis are considered not completely reliable in case of respiratory and arrhythmic disorders. On the contrary, the nonlinear approach may supply powerful and promising tools to give a better insight of the autonomic control in OSAS. In this study, power-law β exponent, fractal dimension and approximate entropy parameters are calculated on the whole night recordings of 77 OSAS patients. The relationships between each pair of the parameters are quantified and discussed, also in comparison to the theoretical models.