Spectral decomposition of RR-variability obtained by an open loop parametric model for the diagnosis of neuromediate syncope

The role of the cardiovascular regulatory mechanism in patients with neuromediate syncope (NS) is poorly understood. The aim of this study was to accomplish continuous non-invasive analysis of the baroreflex mechanism in patients during a head-tip tilt-table test (HTT) using an open-loop autoregressive model with exogenous input. The model describes the causal dependence of the RR interval on the systolic arterial pressure (SAP) variability. Thus, RR variability results as the linear composition of SAP-dependent (Pdep) and SAP-independent parts of the RR power (P). Further, the model allows the estimation of the baroreflex gain using the modulus of the transfer function (G) from SAP to RR in the low-frequency band. The results showed a significant decrease of both Pdep and G immediately before syncope. Thus, a reduced functionality of the baroreflex and of the sympathetic tone seemed to be the mechanism responsible for the NS event.