Second-order non-linearity of heart rate and blood pressure short-term variability

Heart rate (HR) and blood pressure (BP) variability reflect the function of the cardiovascular control system, which contains non-linear mechanisms. In the present study, we studied if these non-linearities are detectable in the HR and BP variability during the stationary epochs of few minutes length, in which conditions the signals are usually assumed to behave according to a linear small-signal model. HR, diastolic BP (DBP), and systolic BP (BP) time-series were recorded from healthy men in quiet supine and standing positions. The existence of the non-linearities was tested with Tsay´s test, which searches for the second order non-linearities. The results show that the variability of HR and BP reflect the non-linear nature of the cardiovascular control system even in the stable and short-term recordings.