Heart Rate Variability (HRV): Nonlinear HRV

The cardiovascular system is considered to be a nonlinear system. Therefore, an effort exists to study its signals, such as heart rate and blood pressure, by applying nonlinear dynamics tools in addition to the commonly used linear tools. This chapter provides a brief introduction to the nonlinear field in conjunction with the main results obtained in studying heart rate variability. The main nonlinear methods discussed here are phase space portrait, surrogate data test for nonlinearity, and the two most commonly used parameters: the correlation dimension and the largest Lyapunov exponents. In order to complete the view on nonlinear analysis of the cardiovascular system, nonlinearity in blood pressure is presented briefly. Keywords: heart rate variability; blood pressure; nonlinear dynamics; surrogate data test; correlation dimension; Lyapunov exponents