Changes in Dynamic Stability of Cardiovascular Time Series by Local Largest Lyapunov Exponent in the Time-Frequency Domain in Vasovagal Syncope

Orthostatic stress via a head-up tilt test (HUTT) has been clinically used to rule out vasovagal syncope (VVS), which is generated by an inappropriate autonomic nervous system (ANS) regulation. VVS has been evaluated mainly by hemodynamic variables and linear analysis of heart rate variability. In this study, the estimation of the local largest Lyapunov exponent (LLE[Formula: see text] was used to assess the predictability of the beat-to-beat interval (BBI), systolic (SBP), and diastolic blood pressure (DBP) times series to analyze in a nonlinear and time-varying fashion the impact of HUTT on ANS regulation in young females, healthy and patients with VVS. The estimation of LLE L of the time series was achieved in both the original domain (full-band) and narrow-band signals by the complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN). Results pointed out that during the orthostatic phase (OPH), the VVS patients showed: (1) significantly higher predictability (less complexity) of the BBI time series, i.e., significant altered cardiac vagal modulation, (2) less predictability of SBP variability suggesting altered sympathetic modulation to the vasculature, and (3) LLE L in conjunction with CEEMDAN evidenced an impairment of cardiovascular autonomic control.